Prof. Dr. Rochus Schmid

Computational Materials Chemistry, Inorganic Chemistry II
Ruhr-Universität Bochum


  • Configurational Entropy Driven High-Pressure Behaviour of a Flexible Metal–Organic Framework (MOF)
    Vervoorts, P. and Keupp, J. and Schneemann, A. and Hobday, C.L. and Daisenberger, D. and Fischer, R.A. and Schmid, R. and Kieslich, G.
    Angewandte Chemie - International Edition 60 (2021)
    Flexible metal–organic frameworks (MOFs) show large structural flexibility as a function of temperature or (gas)pressure variation, a fascinating property of high technological and scientific relevance. The targeted design of flexible MOFs demands control over the macroscopic thermodynamics as determined by microscopic chemical interactions and remains an open challenge. Herein we apply high-pressure powder X-ray diffraction and molecular dynamics simulations to gain insight into the microscopic chemical factors that determine the high-pressure macroscopic thermodynamics of two flexible pillared-layer MOFs. For the first time we identify configurational entropy that originates from side-chain modifications of the linker as the key factor determining the thermodynamics in a flexible MOF. The study shows that configurational entropy is an important yet largely overlooked parameter, providing an intriguing perspective of how to chemically access the underlying free energy landscape in MOFs. © 2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
    view abstract10.1002/anie.202011004
  • Identifying the Bottleneck for Heat Transport in Metal–Organic Frameworks
    Wieser, S. and Kamencek, T. and Dürholt, J.P. and Schmid, R. and Bedoya-Martínez, N. and Zojer, E.
    Advanced Theory and Simulations 4 (2021)
    Controlling the transport of thermal energy is key to most applications of metal–organic frameworks (MOFs). Analyzing the evolution of the effective local temperature, the interfaces between the metal nodes and the organic linkers are identified as the primary bottlenecks for heat conduction. Consequently, changing the bonding strength at that node–linker interface and the mass of the metal atoms can be exploited to tune the thermal conductivity. This insight is generated employing molecular dynamics simulations in conjunction with advanced, ab initio parameterized force fields. The focus of the present study is on MOF-5 as a prototypical example of an isoreticular MOF. However, the key findings prevail for different node structures and node–linker bonding chemistries. The presented results lay the foundation for developing detailed structure-to-property relationships for thermal transport in MOFs with the goal of devising strategies for the application-specific optimization of heat conduction. © 2020 The Authors. Advanced Theory and Simulations published by Wiley-VCH GmbH
    view abstract10.1002/adts.202000211
  • Influence of flexible side-chains on the breathing phase transition of pillared layer MOFs: A force field investigation
    Keupp, J. and Dürholt, J.P. and Schmid, R.
    Faraday Discussions 225 (2021)
    The prototypical pillared layer MOFs, formed by a square lattice of paddle-wheel units and connected by dinitrogen pillars, can undergo a breathing phase transition by a "wine-rack"type motion of the square lattice. We studied this behavior, which is not yet fully understood, using an accurate first principles parameterized force field (MOF-FF) for larger nanocrystallites on the example of Zn2(bdc)2(dabco) [bdc: benzenedicarboxylate, dabco: (1,4-diazabicyclo[2.2.2]octane)], and found clear indications for an interface between a closed and an open pore phase traveling through the system during the phase transformation [J. Keupp and R. Schmid, Adv. Theory Simul., 2019, 2, 1900117]. In conventional simulations in small supercells this mechanism is prevented by periodic boundary conditions (PBCs), enforcing a synchronous transformation of the entire crystal. Here, we extend this investigation to pillared layer MOFs with flexible side-chains, attached to the linker. Such functionalized (fu-)MOFs are experimentally known to have different properties with the side-chains acting as fixed guest molecules. First, in order to extend the parameterization for such flexible groups, a new parameterization strategy for MOF-FF had to be developed, using a multi-structure force based fit method. The resulting parameterization for a library of fu-MOFs is then validated with respect to a set of reference systems and shows very good accuracy. In the second step, a series of fu-MOFs with increasing side-chain length is studied with respect to the influence of the side-chains on the breathing behavior. For small supercells in PBCs a systematic trend of the closed pore volume with the chain length is observed. However, for a nanocrystallite model a distinct interface between a closed and an open pore phase is visible only for the short chain length, whereas for longer chains the interface broadens and a nearly concerted transformation is observed. Only by molecular dynamics simulations using accurate force fields can such complex phenomena can be studied on a molecular level. © 2021 The Royal Society of Chemistry.
    view abstract10.1039/d0fd00017e
  • Oxygenated PAH Formation Chemistry Investigation in Anisole Jet Stirred Reactor Oxidation by a Thermodynamic Approach
    Chen, B. and Kruse, S. and Schmid, R. and Cai, L. and Hansen, N. and Pitsch, H.
    Energy and Fuels 35 (2021)
    Oxygenated poly aromatic hydrocarbons (OPAH) are widely produced in biomass combustion. Recent studies suggest significantly higher toxicity for OPAH in comparison to PAH and soot. However, the present understanding of OPAH formation chemistry is rudimentary. Hence, fundamental knowledge on the formation pathways of OPAH is urgently required to develop predictive models for adequate emission control strategies on OPAH emission in biomass combustion. In this work, the OPAH formation from oxidation of anisole, a representative biomass surrogate, was studied in a jet stirred reactor (JSR). The reaction products were in-situ sampled by molecular beam (MB) and analyzed by time-of-flight mass spectrometry (TOF-MS) using synchrotron radiation as a photon ionization source. The unique experimental setup allows direct detection and identification of large OPAH molecules. Over 40 sum formula of OPAH species were detected and identified by experiments, and a computational thermodynamic approach was applied to deduce possible isomers of OPAH species. The thermodynamic modeling approach assumes that isomers with relatively lower Gibbs free energies are more likely to be present due to possible lower activation energies in the formation pathways. Furthermore, the formation pathways of elucidated OPAH structures are proposed by analogy to the literature based on the intermediate information. The joint study of OPAH by experiments and quantum chemistry advances the understanding of OPAH formation chemistry. © 2020 American Chemical Society.
    view abstract10.1021/acs.energyfuels.0c03829
  • Can small polyaromatics describe their larger counterparts for local reactions? A computational study on the H-abstraction reaction by an H-atom from polyaromatics
    Yonder, Ö. and Schmitz, G. and Hättig, C. and Schmid, R. and Debiagi, P. and Hasse, C. and Locaspi, A. and Faravelli, T.
    Journal of Physical Chemistry A 124 (2020)
    Hydrogen abstraction is one of the crucial initial key steps in the combustion of polycyclic aromatic hydrocarbons. For an accurate theoretical prediction of heterogeneous combustion processes, larger systems need to be treated as compared to pure gas phase reactions. We address here the question on how transferable activation and reaction energies computed for small molecular models are to larger polyaromatics. The approximate transferability of energy contributions is a key assumption for multiscale modeling approaches. To identify efficient levels of accuracy, we start with accurate coupled-cluster and density functional theory (DFT) calculations for different sizes of polyaromatics. More approximate methods as the reactive force-field ReaxFF and the extended semi-empirical tight binding (xTB) methods are then benchmarked against these data sets in terms of reaction energies and equilibrium geometries. Furthermore, we analyze the role of bond-breaking and relaxation energies, vibrational contributions, and post-Hartree-Fock correlation corrections on the reaction, and for the activation energies, we analyze the validity of the Bell-Evans-Polanyi and Hammond principles. First, we find good transferability for this process and that the predictivity of small models at high theoretical levels is way superior than any approximate method can deliver. Second, ReaxFF can serve as a qualitative exploration method, whereas GFN2-xTB in combination with GFN1-xTB appears as a favorable tool to bridge between DFT and ReaxFF so that we propose a multimethod scheme with employing ReaxFF, GFN1/ GFN2-xTB, DFT, and coupled cluster to cope effectively with such a complex reactive system. © 2020 American Chemical Society
    view abstract10.1021/acs.jpca.0c07133
  • Evaluating Computational Shortcuts in Supercell-Based Phonon Calculations of Molecular Crystals: The Instructive Case of Naphthalene
    Kamencek, T. and Wieser, S. and Kojima, H. and Bedoya-Martínez, N. and Dürholt, J.P. and Schmid, R. and Zojer, E.
    Journal of Chemical Theory and Computation 16 (2020)
    Phonons crucially impact a variety of properties of organic semiconductor materials. For instance, charge- and heat transport depend on low-frequency phonons, while for other properties, such as the free energy, especially high-frequency phonons count. For all these quantities one needs to know the entire phonon band structure, whose simulation becomes exceedingly expensive for more complex systems when using methods like dispersion-corrected density functional theory (DFT). Therefore, in the present contribution we evaluate the performance of more approximate methodologies, including density functional tight binding (DFTB) and a pool of force fields (FF) of varying complexity and sophistication. Beyond merely comparing phonon band structures, we also critically evaluate to what extent derived quantities, like temperature-dependent heat capacities, mean squared thermal displacements, and temperature-dependent free energies are impacted by shortcomings in the description of the phonon bands. As a benchmark system, we choose (deuterated) naphthalene, as the only organic semiconductor material for which to date experimental phonon band structures are available in the literature. Overall, the best performance among the approximate methodologies is observed for a system-specifically parametrized second-generation force field. Interestingly, in the low-frequency regime also force fields with a rather simplistic model for the bonding interactions (like the General Amber Force Field) perform rather well. As far as the tested DFTB parametrization is concerned, we obtain a significant underestimation of the unit-cell volume resulting in a pronounced overestimation of the phonon energies in the low-frequency region. This cannot be mended by relying on the DFT-calculated unit cell, since with this unit cell the DFTB phonon frequencies significantly underestimate the experiments. Copyright © 2020 American Chemical Society.
    view abstract10.1021/acs.jctc.0c00119
  • Experimental Evidence for the Incorporation of Two Metals at Equivalent Lattice Positions in Mixed-Metal Metal–Organic Frameworks
    Bitzer, J. and Otterbach, S. and Thangavel, K. and Kultaeva, A. and Schmid, R. and Pöppl, A. and Kleist, W.
    Chemistry - A European Journal 26 (2020)
    Metal–organic frameworks containing multiple metals distributed over crystallographically equivalent framework positions (mixed-metal MOFs) represent an interesting class of materials, since the close vicinity of isolated metal centers often gives rise to synergistic effects. However, appropriate characterization techniques for detailed investigations of these mixed-metal metal–organic framework materials, particularly addressing the distribution of metals within the lattice, are rarely available. The synthesis of mixed-metal FeCuBTC materials in direct syntheses proved to be difficult and only a thorough characterization using various techniques, like powder X-ray diffraction, X-ray absorption spectroscopy and electron paramagnetic resonance spectroscopy, unambiguously evidenced the formation of a mixed-metal FeCuBTC material with HKUST-1 structure, which contained bimetallic Fe−Cu paddlewheels as well as monometallic Cu−Cu and Fe−Fe units under optimized synthesis conditions. The in-depth characterization showed that other synthetic procedures led to impurities, which contained the majority of the applied iron and were impossible or difficult to identify using solely standard characterization techniques. Therefore, this study shows the necessity to characterize mixed-metal MOFs extensively to unambiguously prove the incorporation of both metals at the desired positions. The controlled positioning of metal centers in mixed-metal metal–organic framework materials and the thorough characterization thereof is particularly important to derive structure–property or structure–activity correlations. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/chem.201905596
  • Linking Fluid Densimetry and Molecular Simulation: Adsorption Behavior of Carbon Dioxide on Planar Gold Surfaces
    Tietz, C. and Sekulla, M. and Yang, X. and Schmid, R. and Richter, M.
    Industrial and Engineering Chemistry Research 59 (2020)
    Phase equilibria of fluid substances and their mixtures are important in numerous scientific as well as industrial applications and are, therefore, a major focus of thermophysical property research. For the development and improvement of thermophysical property models, reliable experimental data are crucial. However, measurements of thermophysical properties in the vicinity of the dew line can be substantially distorted by surface phenomena such as adsorption and capillary condensation on the quasi nonporous metal surfaces of the experimental apparatuses. To support the qualitative understanding of these phenomena on an atomistic level and to estimate their impact on experiments, we performed classical molecular dynamics (MD) simulations. As a first proof-of-concept investigation, we focused on pure CO2 on an idealized face-centered cubic (fcc) {111} gold surface. The results, in the form of an adsorption isotherm at T = 283.15 K, are compared to sorption measurements using a specially designed gold sinker incorporated in an optimized gravimetric sorption analyzer. This first comparison between atomistic MD simulations and gravimetric experiments helps in assessing the applicability of our simulation technique and paves the way for a deeper understanding of the relevant surface phenomena occurring in our apparatus. © 2020 American Chemical Society.
    view abstract10.1021/acs.iecr.0c01423
  • Ab Initio Derived Force Fields for Zeolitic Imidazolate Frameworks: MOF-FF for ZIFs
    Dürholt, J.P. and Fraux, G. and Coudert, F.-X. and Schmid, R.
    Journal of Chemical Theory and Computation 15 (2019)
    In this paper, we parametrized in a consistent way a new force field for a range of different zeolitic imidazolate framework systems (ZIF-8, ZIF-8(H), ZIF-8(Br), and ZIF-8(Cl)), extending the MOF-FF parametrization methodology in two aspects. First, we implemented the possibility to use periodic reference data in order to prevent the difficulty of generating representative finite clusters. Second, a new optimizer based on the covariance matrix adaptation evolutionary strategy (CMA-ES) was employed during the parametrization process. We confirmed that CMA-ES, as a state-of-the-art black box optimizer for problems on continuous variables, is more efficient and versatile for force field optimization than the previous genetic algorithm. The obtained force field was then validated with respect to some static and dynamic properties. Much effort was spent to ensure that the FF is able to describe the crucial linker swing effect in a large number of ZIF-8 derivatives. For this reason, we compared our force field to ab initio molecular dynamic simulations and found an accuracy comparable to those obtained by different exchange-correlation functionals. © 2019 American Chemical Society.
    view abstract10.1021/acs.jctc.8b01041
  • Ab initio molecular dynamics simulations of the ferroelectric-paraelectric phase transition in sodium nitrite
    Dürholt, J.P. and Schmid, R.
    Physical Review Materials 3 (2019)
    This paper reports on the first ab initio molecular dynamics study of the ferroelectric sodium nitrite, shedding light on its ferroelctric-paraelectric phase transition. The remnant polarization Pr was calculated using a Mulliken population analysis and maximally localized Wannier functions. Especially the Wannier based model is in outstanding agreement with experimental findings and previous Berry phase calculations. The simulations predict a ferroelectric Curie temperature Tc between 425 and 450K in excellent agreement with the experimental value of 437K. In addition, the anomalous lattice behavior (shrinking of the c axis) during the phase transition is reproduced. Furthermore, the analysis of the phase transition revealed a combined displacive and order-disorder mechanism. The crystal field effect in the material could be quantified by investigating the molecular dipoles based on the maximally localized Wannier functions and the intermolecular charge transfer by analyzing the Mulliken charges. In agreement with earlier experimental and theoretical findings, the polarization reversal mechanism was found to be dominated by a c-axis rotation of the nitrite ions. The molecular insight into such a simple and prototypical material serves as a basis for a further development of more complex crystalline ferroelectrics, using a design principle inspired by NaNO2. © 2019 American Physical Society.
    view abstract10.1103/PhysRevMaterials.3.094408
  • Assessing negative thermal expansion in mesoporous metal-organic frameworks by molecular simulation
    Evans, J.D. and Dürholt, J.P. and Kaskel, S. and Schmid, R.
    Journal of Materials Chemistry A 7 (2019)
    Most conventional materials display expansion in response to heating, so there is considerable interest in identifying materials that display the opposite behavior, negative thermal expansion (NTE). The current study investigated the temperature-induced contraction of seven mesoporous metal-organic frameworks (MOFs) of varying topology and composition, which exhibit outstanding porosity, using molecular simulation. We found exceptional NTE for the most porous MOFs, as well as a correlation between the coefficient of NTE and porosity. The large molecular subunits of the MOFs were further studied to find they intrinsically display NTE, corresponding to terahertz vibrational modes. As a result, NTE has a considerable effect on the mechanical properties of these MOFs and is an important consideration for understanding the mechanical stability of new extremely porous materials. © 2019 The Royal Society of Chemistry.
    view abstract10.1039/c9ta06644f
  • Development of a MOF-FF-compatible interaction model for liquid methanol and Cl− in methanol
    Siwaipram, S. and Bopp, P.A. and Soetens, J.-C. and Schmid, R. and Bureekaew, S.
    Journal of Molecular Liquids 285 (2019)
    If complex systems are to be studied in molecular simulation, one usually attempts to combine existing interaction models in order to describe the new system. This is, however, not always feasible. We thus propose here a new pairwise-additive interaction model for liquid methanol and solvated Cl− to be used to study the immersion of Metal-Organic Frameworks (MOFs) in methanol. Practically, it entails that all interactions must be written to be compatible with the family of MOF-FF models, which have been specifically developed and then widely employed in molecular simulations of such MOFs, in particular flexible ones. The new model for liquid methanol has been mostly tailored to provide densities and dielectric constants as close to experiment as possible in a large temperature domain. This is important since the flexible MOFs modify their shapes according to their loading with guest molecules of various types, and also according to the thermodynamic conditions. The model yields excellent agreement for the density-temperature, dielectric constant-temperature, and self-diffusion-temperature relationships, properties. Other properties such as e.g. the compressibilities or thermal expansion coefficients are of the correct order of magnitude. Since some MOF frameworks are electrically charged, counterions will be present in these cases. The interactions of Cl− with the liquid are thus also considered here. The solvation of this ion is also found to be satisfactory when compared to other MD studies. © 2019 Elsevier B.V.
    view abstract10.1016/j.molliq.2019.04.068
  • High Order Compact Multigrid Solver for Implicit Solvation Models
    Castañeda Medina, A. and Schmid, R.
    Journal of chemical theory and computation 15 (2019)
    The electrostatic problem defined by the continuum solvation models used in molecular mechanics and ab initio molecular dynamics is solved in real space through multiscale methods. First, the Poisson equation is rewritten as a stationary convection-diffusion equation and discretized by a general mesh size fourth-order compact difference scheme. Then, the linear system associated with such a discrete version of the elliptic partial differential equation is solved by a parallel (geometric) multigrid solver whose convergence rates and robustness are improved by an iterant recombination technique in which the multigrid acts as a preconditioner of a Krylov subspace method. The numerical tests performed on ideal and physical systems described by linear Poisson equations under different boundary conditions show the good performance of this accelerated multigrid solver. Furthermore, nonlinear Poisson equations, like the regular modified Poisson-Boltzmann equation, can also be solved by using in addition simple iterative schemes.
    view abstract10.1021/acs.jctc.8b00774
  • High Order Compact Multigrid Solver for Implicit Solvation Models
    Medina, A.C. and Schmid, R.
    Journal of Chemical Theory and Computation (2019)
    The electrostatic problem defined by the continuum solvation models used in molecular mechanics and ab initio molecular dynamics is solved in real space through multiscale methods. First, the Poisson equation is rewritten as a stationary convection-diffusion equation and discretized by a general mesh size fourth-order compact difference scheme. Then, the linear system associated with such a discrete version of the elliptic partial differential equation is solved by a parallel (geometric) multigrid solver whose convergence rates and robustness are improved by an iterant recombination technique in which the multigrid acts as a preconditioner of a Krylov subspace method. The numerical tests performed on ideal and physical systems described by linear Poisson equations under different boundary conditions show the good performance of this accelerated multigrid solver. Furthermore, nonlinear Poisson equations, like the regular modified Poisson-Boltzmann equation, can also be solved by using in addition simple iterative schemes. © 2019 American Chemical Society.
    view abstract10.1021/acs.jctc.8b00774
  • Molecular Dynamics Simulations of the “Breathing” Phase Transformation of MOF Nanocrystallites
    Keupp, J. and Schmid, R.
    Advanced Theory and Simulations 2 (2019)
    The displacive phase transformation of metal-organic frameworks (MOFs), referred to as “breathing,” is computationally investigated intensively within periodic boundary conditions (PBC). In contrast, the first-principles parameterized force field MOF-FF is used to investigate the thermal- and pressure-induced transformations for non-periodic nanocrystallites of DMOF-1 (Zn2(bdc)2(dabco); bdc: 1,4-benzenedicarboxylate; dabco: 1,4-diazabicyclo[2.2.2]octane) as a model system to investigate the effect of the PBC approximation on the systems' kinetics and thermodynamics and to assess whether size effects can be captured by this kind of simulation. By the heating of differently sized closed pore nanocrystallites, a spontaneous opening is observed with an interface between the closed and open pore phase moving rapidly through the system. The nucleation temperature for the opening transition rises with size. By enforcing the phase transition with a distance restraint, the free energy can be quantified via umbrella sampling. The apparent barrier is substantially lower than for a concerted process under PBC. Interestingly, the barrier reduces with the size of the nanocrystallite, indicating a hindering surface effect. The results demonstrate that the actual free energy barriers and the importance of surface effects for the transformation under real conditions can only be studied beyond PBC. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adts.201900117
  • Retrofitting metal-organic frameworks
    Schneider, C. and Bodesheim, D. and Keupp, J. and Schmid, R. and Kieslich, G.
    Nature Communications 10 (2019)
    The post-synthetic installation of linker molecules between open-metal sites (OMSs) and undercoordinated metal-nodes in a metal-organic framework (MOF) — retrofitting — has recently been discovered as a powerful tool to manipulate macroscopic properties such as the mechanical robustness and the thermal expansion behavior. So far, the choice of cross linkers (CLs) that are used in retrofitting experiments is based on qualitative considerations. Here, we present a low-cost computational framework that provides experimentalists with a tool for evaluating various CLs for retrofitting a given MOF system with OMSs. After applying our approach to the prototypical system CL@Cu3BTC2 (BTC = 1,3,5-benzentricarboxylate) the methodology was expanded to NOTT-100 and NOTT-101 MOFs, identifying several promising CLs for future CL@NOTT-100 and CL@NOTT-101 retrofitting experiments. The developed model is easily adaptable to other MOFs with OMSs and is set-up to be used by experimentalists, providing a guideline for the synthesis of new retrofitted MOFs with modified physicochemical properties. © 2019, The Author(s).
    view abstract10.1038/s41467-019-12876-1
  • Rhodium-Catalyzed ortho-Arylation of (Hetero)aromatic Acids
    Weber, P. and Rank, C.K. and Yalcinkaya, E. and Dyga, M. and van Lingen, T. and Schmid, R. and Patureau, F.W. and Gooßen, L.J.
    Advanced Synthesis and Catalysis 361 (2019)
    Rhodium acetate effectively promotes the carboxylate-directed ortho-arylation of (hetero)aromatic carboxylates with aryl bromides. The main advantage of this phosphine-free, redox-neutral method arises from its efficiency in assembling biologically meaningful electron-rich arylpyridines, which are problematic substrates in known C−H arylations using Pd, Ru, and Ir catalysts. (Figure presented.). © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adsc.201900596
  • Solution of high order compact discretized 3D elliptic partial differential equations by an accelerated multigrid method
    Medina, A.C. and Schmid, R.
    Journal of Computational and Applied Mathematics 350 (2019)
    A robust and efficient numerical solver for three-dimensional linear elliptic partial differential equations without cross derivative terms, but variable coefficients is presented. The proposed algorithm is based on a general meshsize fourth-order compact finite-difference scheme and an accelerated geometric multigrid solver. The former not only increases the accuracy of the discretization, but also does not affect the bandwidth of the common second-order finite-difference scheme, which is relevant for distributed memory parallel implementations of the multigrid preconditioner. The acceleration technique, on the other hand, allows for a dramatic reduction of the number of cycles and smoothing steps required to achieve convergence. Furthermore, it is found that depending on the coefficients of the elliptic partial differential equation, the multigrid preconditioned solver can be the only way to obtain a reliable numerical solution or attain convergence. © 2018 Elsevier B.V.
    view abstract10.1016/
  • Tuning the Electric Field Response of MOFs by Rotatable Dipolar Linkers
    Dürholt, J.P. and Jahromi, B.F. and Schmid, R.
    ACS Central Science 5 (2019)
    Recently the possibility of using electric fields as a further stimulus to trigger structural changes in metal-organic frameworks (MOFs) has been investigated. In general, rotatable groups or other types of mechanical motion can be driven by electric fields. In this study we demonstrate how the electric response of MOFs can be tuned by adding rotatable dipolar linkers, generating a material that exhibits paraelectric behavior in two dimensions and dielectric behavior in one dimension. The suitability of four different methods to compute the relative permittivity κ by means of molecular dynamics simulations was validated. The dependency of the permittivity on temperature T and dipole strength μ was determined. It was found that the herein investigated systems exhibit a high degree of tunability and substantially larger dielectric constants as expected for MOFs in general. The temperature dependency of κ obeys the Curie-Weiss law. In addition, the influence of dipolar linkers on the electric field induced breathing behavior was investigated. With increasing dipole moment, lower field strengths are required to trigger the contraction. These investigations set the stage for an application of such systems as dielectric sensors, order-disorder ferroelectrics, or any scenario where movable dipolar fragments respond to external electric fields. Copyright © 2019 American Chemical Society.
    view abstract10.1021/acscentsci.9b00497
  • Computational Structure Prediction of (4,4)-Connected Copper Paddle-wheel-based MOFs: Influence of Ligand Functionalization on the Topological Preference
    Impeng, S. and Cedeno, R. and Dürholt, J.P. and Schmid, R. and Bureekaew, S.
    Crystal Growth and Design 18 (2018)
    The effect of linkers with extended π-system on the topological preference of (4,4)-connected copper paddle-wheel-based metal-organic frameworks (MOFs) was investigated using the reverse topological approach (RTA) in which a genetic algorithm (GA) and the DFT-derived force field MOF-FF were used for ranking and predicting the most stable phase. Three tetracarboxylate linkers bearing different functionality, namely, phenylene (L1), naphthalene (L2), and anthracene (L3) groups, were studied. All potential topologies including nbo-b, ssa, ssb, pts, and lvt-b were considered. The computational results reveal that nbo-b is the most stable topology for all three investigated linkers. However, L2 is also formed in ssb according to experimental findings. Our simulation results show that the CH-π interactions with a Y-shaped configuration between naphthalene moieties of L2 stabilize the ssb framework. Unlike L2, CH-π interactions are not favorable for L1 and L3 because of unsuitable size of the π-system. The results of the RTA predictions are in agreement with experimentally reported data, suggesting the capability of RTA for accurate structural predictions of MOFs. More importantly, this work shows the exemption of reticular chemistry in which linker functionalization can result in alteration of the resulting topology, as found in the case of linker L2. © 2018 American Chemical Society.
    view abstract10.1021/acs.cgd.8b00238
  • Influence of biomass torrefaction parameters on fast pyrolysis products under flame-equivalent conditions
    Pielsticker, S. and Möller, G. and Gövert, B. and Kreitzberg, T. and Hatzfeld, O. and Yönder, Ö. and Angenent, V. and Hättig, C. and Schmid, R. and Kneer, R.
    Biomass and Bioenergy 119 (2018)
    Pretreating raw biomass via torrefaction changes fuel specific properties like grindability, volatile content, energy density and biochemical stability and thus enables an enhanced fuel replacement for pulverized fossil fuel fired furnaces. In this study, the influence of torrefaction temperature on devolatilization behavior is investigated in a small-scale fluidized bed reactor approximating flame-equivalent conditions. Therefore the pyrolysis products of two different biofuels with varying degree of torrefaction are determined via ex-situ FTIR gas analysis in an N2 atmosphere in the temperature range from 873 to 1473 K. Furthermore, the mass fraction of residual char particles is determined by adding O2 to the fluidizing gas and analyzing the burnout products. Char fraction and volatile composition are used to estimate the energy release distribution between homogeneous volatile combustion and heterogeneous char burnout. The experiments revealed enlarging char yields at the expense of volatile yields with increasing degree of torrfaction at all investigated pyrolysis temperatures. Furthermore, torrefaction favors higher fractions of CO2 and lower fractions of CO and C2Hx in the light gas. Further on, no significant impact of torrefaction conditions on the tar composition could be identified. The calculation of higher heating value (HHV) based on char yield and gas composition reveals an overall increase of HHV, while the relative contribution from the volatile fraction decreases with increasing degree of torrefaction. Following this, an increase of torrefaction degree will shift combustion from a high intense volatile combustion in the near burner region towards a less intense but prolonged char conversion in the far burner region. © 2018 Elsevier Ltd
    view abstract10.1016/j.biombioe.2018.08.014
  • Molecular Dynamics Investigation of the Dielectric Decrement of Ion Solutions
    Pache, D. and Schmid, R.
    ChemElectroChem 5 (2018)
    Molecular dynamics simulations, using a classical force field model, have been used to determine the dependence of the static relative dielectric constant of ion solutions with respect to the nature and concentration of the ions and the field strength. The experimentally observed effect of a reduction of the dielectric permittivity due to solvated ions is known as dielectric decrement. We used both the polarization fluctuation at zero field and the constant dielectric displacement method for finite fields to determine the dielectric constant of the bulk solution. All the experimentally observed tendencies of the dielectric decrement could be qualitatively reproduced. The analysis of different solute solvent radial distribution functions indicate that the dielectric decrement arises from the competition between the macroscopic electric field and the local water-ion interaction. The results suggest that the electric field eventually manages to overcome the local molecular interactions, breaking up the structure of the solvation shell and thus lowering the ion's effect on the dielectric constant. This effect seems to correlate with the solvation energy of the individual ions as well as the type of counter ion, indicating that also long range interactions might play a role. The results can be used to improve especially continuum electrolyte models, used to study electrochemical interfaces, where currently the dielectric decrement is generally not included. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.201800158
  • TopoFF: MOF structure prediction using specifically optimized blueprints
    Keupp, J. and Schmid, R.
    Faraday Discussions 211 (2018)
    For the structure prediction of MOFs and related crystalline framework materials we have proposed the Reversed Topological Approach (RTA), where the default embedding of a topology is used as a blueprint. The optimal rotational insertion of the building blocks (BBs) at the fixed vertex positions of the blueprint is performed by minimizing the target function of the average angle deviation (AAD). Here we extend this idea by pre-optimizing the maximum symmetry embedding of a topology in order to minimize the overall mean AAD for the given set of BBs. By this fast and essentially parameter-free topoFF method, the vertex positions and cell parameters of the blueprint are further optimized in order to fit the structural needs of the BBs, which speeds up the overall search for the most energetically favorable structure. In addition, different topologies can be ranked in a quantitative and intuitive way. The definition and implementation of topoFF is explained and its application for the RTA-based structure prediction of MOFs is demonstrated with a number of instructive examples. © The Royal Society of Chemistry.
    view abstract10.1039/c8fd00051d
  • An Electric Field Induced Breath for Metal-Organic Frameworks
    Schmid, R.
    ACS Central Science 3 (2017)
    view abstract10.1021/acscentsci.7b00162
  • Electrode potential dependent desolvation and resolvation of germanium(100) in contact with aqueous perchlorate electrolytes
    Niu, F. and Schulz, R. and Castañeda Medina, A. and Schmid, R. and Erbe, A.
    Physical Chemistry Chemical Physics 19 (2017)
    The electrode potential dependence of the hydration layer on an n-Ge(100) surface was studied by a combination of in situ and operando electrochemical attenuated total reflection infrared (ATR-IR) spectroscopy and real space density functional theory (DFT) calculations. Constant-potential DFT calculations were coupled to a modified generalised Poisson-Boltzmann ion distribution model and applied within an ab initio molecular dynamics (AIMD) scheme. As a result, potential-dependent vibrational spectra of surface species and surface water were obtained, both experimentally and by simulations. The experimental spectra show increasing absorbance from the Ge-H stretching modes at negative potentials, which is associated with an increased negative difference absorbance of water-related OH modes. When the termination transition of germanium from OH to H termination occurs, the surface switches from hydrophilic to hydrophobic. This transition is fully reversible. During the switching, the interface water molecules are displaced from the surface forming a "hydrophobic gap". The gap thickness was experimentally estimated by a continuum electrodynamic model to be ≈2 Å. The calculations showed a shift in the centre of mass of the interface water by ≈0.9 Å due to the surface transformation. The resulting IR spectra of the interfacial water in contact with the hydrophobic Ge-H show an increased absorbance of free OH groups, and a decreased absorbance of strongly hydrogen bound water. Consequently, the surface transformation to a Ge-H terminated surface leads to a surface which is weakening the H-bond network of the interfacial water in contact. © the Owner Societies 2017.
    view abstract10.1039/c6cp08908a
  • Multiscale modeling of the HKUST-1/polyvinyl alcohol) interface: From an atomistic to a coarse graining approach
    Semino, R. and Durholt, J.P. and Schmid, R. and Marin, G.
    Journal of Physical Chemistry C 121 (2017)
    We present a computational multiscale study of a metal-organic framework (MOF)/polymer composite combining micro- and mesoscopic resolution, by coupling atomistic and coarse grained (CG) force field-based molecular dynamics simulations. As a proof of concept, we describe the copper paddlewheel-based HKUST-1 MOF/poly(vinyl alcohol) composite. Our newly developed CG model reproduces the salient features of the interface in excellent agreement with the atomistic model and allows the investigation of substantially larger systems. The polymer penetrates into the open pores of the MOF as a result of the interactions between its OH groups and the O and Cu atoms in the pores, suggesting an excellent MOF/polymer compatibility. Polymer structure is affected by the MOF surface up to a distance of ∼2.4 times its radius of gyration. This study paves the way toward understanding important interfacial phenomena such as aggregation and phase separation in these mixed matrix systems. © 2017 American Chemical Society.
    view abstract10.1021/acs.jpcc.7b07090
  • Verlet-like algorithms for Car-Parrinello molecular dynamics with unequal electronic occupations
    Castañeda Medina, A. and Schmid, R.
    Journal of Chemical Physics 147 (2017)
    The ab initio molecular dynamics simulations of metallic, charged, and electrochemical systems require, in principle, the inclusion of unequally occupied electronic states. In this contribution, the general approach to work with fixed but arbitrary occupations within the Car-Parrinello molecular dynamics scheme is revisited, focusing on the procedure which is required to maintain the orthonormality constraints in the commonly used position-Verlet integrator. Expressions to constrain also the orbital velocities, as it is demanded by a velocity-Verlet integrator, are then derived. The generalized unequal-occupation SHAKE algorithm is compared with the standard procedure for damped dynamics (energy optimization) of systems including fully unoccupied electronic states. In turn, the proposed unequal-occupation RATTLE algorithm is validated by the corresponding microcanonical ensemble simulations. It is shown that only with the proper orthogonalization method, a correct ordering of states and energy conserving dynamics can be achieved. © 2017 Author(s).
    view abstract10.1063/1.4987005
  • Coarse graining of force fields for metal-organic frameworks
    Dürholt, J.P. and Galvelis, R. and Schmid, R.
    Dalton Transactions 45 (2016)
    We have adapted our genetic algorithm based optimization approach, originally developed to generate force field parameters from quantum mechanic reference data, to derive a first coarse grained force field for a MOF, taking the atomistic MOF-FF as a reference. On the example of the copper paddle-wheel based HKUST-1, a maximally coarse grained model, using a single bead for each three and four coordinated vertex, was developed as a proof of concept. By adding non-bonded interactions with a modified Buckingham potential, the resulting MOF-FF-CGNB is able to predict local deformation energies of the building blocks as well as bulk properties like the tbo vs. pto energy difference or elastic constants in a semi-quantitative way. As expected, the negative thermal expansion of HKUST-1 is not reproduced by the maximally coarse grained model. At the expense of atomic resolution, substantially larger systems (up to tens of nanometers in size) can be simulated with respect to structural and mechanical properties, bridging the gap to the mesoscale. As an example the deformation of the [111] surface of HKUST-1 by a "tip" could be computed without artifacts from periodic images. © The Royal Society of Chemistry 2016.
    view abstract10.1039/c5dt03865k
  • Dioxygen binding to Fe-MOF-74: Microscopic insights from periodic QM/MM calculations
    Moeljadi, A.M.P. and Schmid, R. and Hirao, H.
    Canadian Journal of Chemistry 94 (2016)
    Accurate MOF-FF parameter sets were determined for the ferrous and ferric forms of an iron-based metal-organic framework (MOF) called Fe-MOF-74. For this purpose, density functional theory (DFT) calculations were applied to truncated cluster models of Fe-MOF-74, and the DFT-calculated geometries and energy derivatives were used for the force-field parameterization. The resultant parameter sets performed remarkably well in reproducing the experimentally determined structure of the MOF. We also performed periodic quantum mechanics (QM) / molecular mechanics (MM) calculations employing a subtractive scheme called ONIOM, with the optimized MOF-FF parameters used for the MM calculations, in an attempt to evaluate the binding energies between O2 and several Fe-MOF-74 variants. The calculated binding energy for Fe-MOF-74 agreed very well with the experimental value, and QM/MM geometry optimization calculations confirmed that the O2-bound complex has a side-on geometry. Our calculations also predicted that, when the two neighboring iron ions around the O2-binding site are replaced with other metal ions (Mg2+, Ni2+, Zn2+, Co2+, or Mn2+), there are noticeable variations in the binding energy, indicating that these substituted metal ions affect the O2 binding indirectly. © 2016 Published by NRC Research Press.
    view abstract10.1139/cjc-2016-0284
  • Influence of pore dimension on the host-guest interaction in metal-organic frameworks
    Amirjalayer, S. and Schmid, R.
    Journal of Physical Chemistry C 120 (2016)
    Atomistic simulations were performed in order to investigated the effect of pore dimension on the interaction of guest molecules with the inner surface of metal-organic frameworks (MOFs). In these systems, which only differ in the metric of their open structure, the chemical nature is conserved, and less impact on the host-guest interaction is expected compared to chemically funcionalized MOFs. However, by performing molecular dynamics simulations of benzene loaded MOF-5 derivatives (IRMOFs), which differ just in the length of the organic linker, it can be shown that impacts are present. The influence of the soft-modification can be explained only by a detailed analysis of the free energy topology and the diffusion mechanism. Note that the calculated self-diffusivity of benzene Dself shows no change with respect to the elongation of the linkers. The apparent contradiction between the macroscopic observable Dself and the microscopic free energy landscape could be resolved by introducing a hopping model for the diffusion process and subsequent Monte Carlo simulations. This study demonstrates the importance of atomistic simulations and the need to understand the host-guest interaction in MOFs in a multiscale fashion. © 2016 American Chemical Society.
    view abstract10.1021/acs.jpcc.6b08609
  • Model Study of Thermoresponsive Behavior of Metal-Organic Frameworks Modulated by Linker Functionalization
    Alaghemandi, M. and Schmid, R.
    Journal of Physical Chemistry C 120 (2016)
    The temperature-responsive behavior of functionalized metal-organic frameworks (fu-MOF) with the general formula [Zn2(fu-L)2dabco]n has been investigated using molecular dynamics simulations (fu-L = alkoxy-functionalized 1,4-benzenedicarboxylate, dabco = 1,4-diazabicyclo[2.2.2]octane). The studied frameworks show a narrow pore (np) form at low temperatures, while at higher temperatures, large pore (lp) structures can be observed. The transition temperature is controlled by the chemical nature of the linker's side chains as well as their length. In general, enhancing the side chain length decreases the transition temperature. On the other hand, more polar linkers shift the transition temperature to higher values. The so-called opening process of the narrow pores is caused by the thermally induced motion of the alkoxy side chains of the functionalized linkers. For qualitative comparison, the difference in internal energy as well as entropy between two forms (np and lp) was calculated for all studied linker types. (Figure Presented). © 2016 American Chemical Society.
    view abstract10.1021/acs.jpcc.5b12331
  • Pentlandite rocks as sustainable and stable efficient electrocatalysts for hydrogen generation
    Konkena, B. and Puring, K.J. and Sinev, I. and Piontek, S. and Khavryuchenko, O. and Dürholt, J.P. and Schmid, R. and Tüysüz, H. and Muhler, M. and Schuhmann, W. and Apfel, U.-P.
    Nature Communications 7 (2016)
    The need for sustainable catalysts for an efficient hydrogen evolution reaction is of significant interest for modern society. Inspired by comparable structural properties of [FeNi]-hydrogenase, here we present the natural ore pentlandite (Fe 4.5 Ni 4.5 S 8) as a direct rock' electrode material for hydrogen evolution under acidic conditions with an overpotential of 280 mV at 10 mA cm -2. Furthermore, it reaches a value as low as 190 mV after 96 h of electrolysis due to surface sulfur depletion, which may change the electronic structure of the catalytically active nickel-iron centres. The rock' material shows an unexpected catalytic activity with comparable overpotential and Tafel slope to some well-developed metallic or nanostructured catalysts. Notably, the rock' material offers high current densities (≤650 mA cm -2) without any loss in activity for approximately 170 h. The superior hydrogen evolution performance of pentlandites as rock' electrode labels this ore as a promising electrocatalyst for future hydrogen-based economy.
    view abstract10.1038/ncomms12269
  • The Impact of Mesopores on the Mechanical Stability of HKUST-1: A Multiscale Investigation
    Dürholt, J.P. and Keupp, J. and Schmid, R.
    European Journal of Inorganic Chemistry 2016 (2016)
    Correlated missing node defects can lead to mesoporosity in MOFs. In order to investigate the effect of such cavities in the microporous crystalline material, spherical pores of increasing size were cut into HKUST-1, and the resulting inner surface was completely saturated. Our recently developed coarse-grained force field was used to relax these systems and to assess the mechanical stability by computing the bulk moduli. In a multiscale fashion, atomistic models were generated systematically from the coarse-grained relaxed structures in order to determine void fraction, surface area, and the relative mesoporous void volume. Despite the presence of mesopores with large radii of above 20 Å, the lattice parameters shrink only by a negligible amount. The bulk moduli are reduced, but overall a sufficient mechanical stability is found, consistently with experimental observations. Interestingly, for a fixed amount of defect degree, and thus mesoporous void volume, larger cavities lead to higher mechanical stability. For small mesopores in the size range below that of a HKUST-1 unit cell we observe unexpected trends due to the additional surface area generated by saturating the inner surface of the mesopore. With a large number of small mesopores, this can lead to an effective increase in the gravimetric surface area. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/ejic.201600566
  • Isoreticular isomerism in 4,4-connected paddle-wheel metal-organic frameworks: Structural prediction by the reverse topological approach
    Bureekaew, S. and Balwani, V. and Amirjalayer, S. and Schmid, R.
    CrystEngComm 17 (2015)
    The theoretical structure prediction for a series of 4,4-connected copper paddle-wheel metal-organic frameworks has been performed by using the reverse topological approach, starting from the nbo-b topology. Since the rectangular-shaped tetracarboxylate linkers have a lower symmetry than the square vertices in nbo-b, two alternative insertion modes are possible for each linker. This leads, in principle, to the formation of multiple isoreticular isomers, which have been screened by a genetic global minimum search algorithm, using the first principles parameterized force field MOF-FF for structure optimization and ranking. It is found that isoreticular isomerism does, in this case, not lead to disorder but to a number of well-defined but structurally distinct phases, which all share the same network topology but have substantially different pore shapes and properties. In all cases, the experimentally observed structure is correctly predicted, but in addition a number of other slightly less stable phases are observed. Only one of these phases has been synthesized yet. The theoretical analysis of the molecular model systems of the pore cages revealed the reasons for the trends in conformational energy. This proof-of-concept study demonstrates that screening of isoreticular isomerism using an efficient but accurate force field allows prediction of the atomistic structure of even complex and flexible frameworks. © The Royal Society of Chemistry.
    view abstract10.1039/c4ce01574f
  • QuickFF: A program for a quick and easy derivation of force fields for metal-organic frameworks from ab initio input
    Vanduyfhuys, L. and Vandenbrande, S. and Verstraelen, T. and Schmid, R. and Waroquier, M. and Van Speybroeck, V.
    Journal of Computational Chemistry 36 (2015)
    QuickFF is a software package to derive accurate force fields for isolated and complex molecular systems in a quick and easy manner. Apart from its general applicability, the program has been designed to generate force fields for metal-organic frameworks in an automated fashion. The force field parameters for the covalent interaction are derived from ab initio data. The mathematical expression of the covalent energy is kept simple to ensure robustness and to avoid fitting deficiencies as much as possible. The user needs to produce an equilibrium structure and a Hessian matrix for one or more building units. Afterward, a force field is generated for the system using a three-step method implemented in QuickFF. The first two steps of the methodology are designed to minimize correlations among the force field parameters. In the last step, the parameters are refined by imposing the force field parameters to reproduce the ab initio Hessian matrix in Cartesian coordinate space as accurate as possible. The method is applied on a set of 1000 organic molecules to show the easiness of the software protocol. To illustrate its application to metal-organic frameworks (MOFs), QuickFF is used to determine force fields for MIL-53(Al) and MOF-5. For both materials, accurate force fields were already generated in literature but they requested a lot of manual interventions. QuickFF is a tool that can easily be used by anyone with a basic knowledge of performing ab initio calculations. As a result, accurate force fields are generated with minimal effort. © 2015 Wiley Periodicals, Inc.
    view abstract10.1002/jcc.23877
  • Rotational and translational dynamics of CO2 adsorbed in MOF Zn2(bdc)2(dabco)
    Peksa, M. and Burrekaew, S. and Schmid, R. and Lang, J. and Stallmach, F.
    Microporous and Mesoporous Materials 216 (2015)
    The dynamics of adsorbed CO2 in the metal-organic framework Zn2(bdc)2 dabco (DMOF-1) was investigated using molecular dynamics (MD) simulations and 13C NMR spectroscopy. The statistical analysis of the MD trajectories suggest a preferred localization of the CO2 molecules in the Zn2(bdc)4 corners of the DMOF-1 lattice. The adsorbed molecules retain a high but anisotropic rotational and translational mobility in the channel system. Based on these MD-results, the residual chemical shift anisotropy 〈Δδ〉MD = -114 ppm and the diffusion anisotropy (D∥D⊥)MD = 9.8 ± 0.5 were calculated. They are found to be in reasonable agreement with the experimental NMR data of 〈Δδ〉NMR=-(55 ± 2) ppm and (D∥D⊥)NMR = 3 respectively. © 2015 Elsevier Inc. All rights reserved.
    view abstract10.1016/j.micromeso.2015.02.043
  • Almost enclosed buckyball joints: Synthesis, complex formation, and computational simulations of pentypticene-extended tribenzotriquinacene
    Henne, S. and Bredenkötter, B. and Alaghemandi, M. and Bureekaew, S. and Schmid, R. and Volkmer, D.
    ChemPhysChem 15 (2014)
    We report the synthesis of a tribenzotriquinacene-based (TBTQ) receptor (3) for C60 fullerene, which is extended by pentiptycene moieties to provide an almost enclosed concave ball bearing. The system serves as a model for a self-assembling molecular rotor with a flexible and adapting stator. Unexpectedly, nuclear magnetic resonance spectroscopic investigations reveal a surprisingly low complex stability constant of K1= 213±37m-1 for [C60⊂3], seemingly inconsistent with the previously reported TBTQ systems. Molecular dynamics (MD) simulations have been conducted for three different [C60⊂TBTQ] complexes to resolve this. Because of the dominating dispersive interactions, the binding energies increase with the contact area between guest and host, however, only for rigid host structures. By means of free-energy calculations with an explicit solvent model it can be shown that the novel flexible TBTQ receptor 3 binds weakly because of hampering entropic contributions. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cphc.201402475
  • Ferrocene in the metal-organic framework MOF-5 studied by homo- and heteronuclear correlation NMR and MD simulation
    Wehring, M. and Magusin, P.C.M.M. and Amirjalayer, S. and Schmid, R. and Stallmach, F.
    Microporous and Mesoporous Materials 186 (2014)
    Advanced solid-state 2D NMR spectroscopy and molecular dynamics computation are employed to investigate the interaction between adsorbed ferrocene molecules and the MOF-5 lattice. Relayed 13C-1H heteronuclear correlation (HETCOR) 2D NMR spectra clearly indicate short-distance contacts between the ferrocene guests and the benzene-1,4-dicarboxylic-acid linkers, mediated via intermolecular 1 H spin diffusion. By use of 2D 1H-1H correlation spectroscopy the distances between 1H nuclei in the guests and the linkers are estimated to be shorter than 0.5 nm. MD computer simulations support the interpretation of the 2D solid state NMR studies. Moreover, they suggest a wide distribution of intermolecular distances in this host-guest system with the shortest intermolecular hydrogen-hydrogen distances of 0.15 nm. © 2013 Elsevier Inc. All rights reserved.
    view abstract10.1016/j.micromeso.2013.11.045
  • Structural complexity in metal-organic frameworks: Simultaneous modification of open metal sites and hierarchical porosity by systematic doping with defective linkers
    Fang, Z. and Dürholt, J.P. and Kauer, M. and Zhang, W. and Lochenie, C. and Jee, B. and Albada, B. and Metzler-Nolte, N. and Pöppl, A. and Weber, B. and Muhler, M. and Wang, Y. and Schmid, R. and Fischer, R.A.
    Journal of the American Chemical Society 136 (2014)
    A series of defect-engineered metal-organic frameworks (DEMOFs) derived from parent microporous MOFs was obtained by systematic doping with defective linkers during synthesis, leading to the simultaneous and controllable modification of coordinatively unsaturated metal sites (CUS) and introduction of functionalized mesopores. These materials were investigated via temperature-dependent adsorption/desorption of CO monitored by FTIR spectroscopy under ultra-high-vacuum conditions. Accurate structural models for the generated point defects at CUS were deduced by matching experimental data with theoretical simulation. The results reveal multivariate diversity of electronic and steric properties at CUS, demonstrating the MOF defect structure modulation at two length scales in a single step to overcome restricted active site specificity and confined coordination space at CUS. Moreover, the DEMOFs exhibit promising modified physical properties, including band gap, magnetism, and porosity, with hierarchical micro/mesopore structures correlated with the nature and the degree of defective linker incorporation into the framework. © 2014 American Chemical Society.
    view abstract10.1021/ja503218j
  • Surface termination of the metal-organic framework HKUST-1: A theoretical investigation
    Amirjalayer, S. and Tafipolsky, M. and Schmid, R.
    Journal of Physical Chemistry Letters 5 (2014)
    The surface morphology and termination of metal-organic frameworks (MOF) is of critical importance in many applications, but the surface properties of these soft materials are conceptually different from those of other materials like metal or oxide surfaces. Up to now, experimental investigations are scarce and theoretical simulations have focused on the bulk properties. The possible surface structure of the archetypal MOF HKUST-1 is investigated by a first-principles derived force field in combination with DFT calculations of model systems. The computed surface energies correctly predict the [111] surface to be most stable and allow us to obtain an unprecedented atomistic picture of the surface termination. Entropic factors are identified to determine the preferred surface termination and to be the driving force for the MOF growth. On the basis of this, reported strategies like employing "modulators" during the synthesis to tailor the crystal morphology are discussed. © 2014 American Chemical Society.
    view abstract10.1021/jz5012065
  • Tribenzotriquinacene receptors for C60 fullerene rotors: Towards C3 symmetrical chiral stators for unidirectionally operating nanoratchets
    Bredenkötter, B. and Grzywa, M. and Alaghemandi, M. and Schmid, R. and Herrebout, W. and Bultinck, P. and Volkmer, D.
    Chemistry - A European Journal 20 (2014)
    The synthesis of a stereochemically pure concave tribenzotriquinacene receptor (7) for C60 fullerene, possessing C3 point group symmetry, by threefold condensation of C2-symmetric 1,2-diketone synthons (5) and a hexaaminotribenzotriquinacene core (6) is described. The chiral diketone was synthesized in a five-step reaction sequence starting from C2h-symmetric 2,6-di-tert-butylanthracene. The highly diastereo-discriminating Diels-Alder reaction of 2,6-di-tert-butylanthracene with fumaric acid di(-)menthyl ester, catalyzed by aluminium chloride, is the relevant stereochemistry introducing step. The structure of the fullerene receptor was verified by 1H and 13C NMR spectroscopy, mass spectrometry and single crystal X-ray diffraction. VCD and ECD spectra were recorded, which were corroborated by ab initio DFT calculations, establishing the chiral nature of 7 with about 99.7 % ee, based on the ee (99.9 %) of the chiral synthon (1). The absolute configuration of 7 could thus be established as all-S [(2S,7S,16S,21S,30S,35S)-(7)]. Spectroscopic titration experiments reveal that the host forms 1:1 complexes with either pure fullerene (C60) or fullerene derivatives, such as rotor 1'-(4-nitrophenyl)-3'-(4-N,N- dimethylaminophenyl)-pyrazolino[4',5':1,2][60]fullerene (R). The complex stability constants of the complexes dissolved in CHCl3/CS 2 (1:1 vol. %) are K([C60-7])=319(±156) M -1 and K([R-7])=110(±50) M-1. With molecular dynamics simulations using a first-principles parameterized force field the asymmetry of the rotational potential for [R-7] was shown, demonstrating the potential suitability of receptor 7 to act as a stator in a unidirectionally operating nanoratchet. Going through the motions: The synthesis of a stereochemically pure concave tribenzotriquinacene receptor (1) for C 60 fullerenes is described. Spectroscopic titration experiments reveal that the host forms 1:1 complexes with fullerenes. Molecular dynamics simulations show the asymmetry of the rotational potential for [R-1], demonstrating the potential suitability of receptor 1 to act as a stator in a unidirectionally operating nanoratchet (see figure). © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/chem.201304980
  • A cryogenically flexible covalent organic framework for efficient hydrogen isotope separation by quantum sieving
    Oh, H. and Kalidindi, S.B. and Um, Y. and Bureekaew, S. and Schmid, R. and Fischer, R.A. and Hirscher, M.
    Angewandte Chemie - International Edition 52 (2013)
    Pyrdine incorporation into the covalent organic framework COF-1 resulted in a highly dense packing structure in which the pyridine occupies the hexagonal pore space between the COF layers. This optimizes pore aperture for quantum sieving of hydrogen isotopes and introduces flexibility at cryogenic temperatures into the system. The separation factor (S D 2/H 2) is about 10 at 22K, which is the highest reported to date. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/anie.201307443
  • CO adsorption on a mixed-valence ruthenium metal-organic framework studied by UHV-FTIR spectroscopy and DFT calculations
    Noei, H. and Kozachuk, O. and Amirjalayer, S. and Bureekaew, S. and Kauer, M. and Schmid, R. and Marler, B. and Muhler, M. and Fischer, R.A. and Wang, Y.
    Journal of Physical Chemistry C 117 (2013)
    The mixed-valence metal-organic framework [Ru3 II,III(btc)2Cl1.5] (Ru-MOF) was synthesized by the controlled SBU approach and characterized by combined powder XRD, XPS, and FTIR methods. The interaction of CO molecules with Ru-MOF was studied by a novel instrumentation for ultra-high-vacuum (UHV) FTIR spectroscopy. The high-quality IR data demonstrate the presence of two different CO species within the framework: a strongly bonded CO showing a low-lying band at 2137 cm-1 and a second CO species at 2171 cm-1 with a lower binding energy. It was found that these IR bands cannot be assigned in a straightforward manner to CO molecules adsorbed on the coordinatively unsaturated RuII site (CUS) and RuIII site connected to an additional Cl- ion for charge compensation. The accurate DFT calculations reveal that the structural and electronic properties of the mixed-valence Ru-MOF are much more complex than expected. One of the Cl- counterions could be transferred to a neighboring paddle-wheel, forming an anionic SBU blocked by two Cl- counterions, whereas the other positively charged paddle-wheel with a Ru2 II,III dimer exposes two "free" CUS, which can bind two CO molecules with different frequencies and binding energies. © 2013 American Chemical Society.
    view abstract10.1021/jp3056366
  • Erratum: A novel method to measure diffusion coefficients in porous metal-organic frameworks (Physical Chemistry Chemical Physics (2010) 12 (8093-8098) DOI: 10.1039/B927601G)
    Zybaylo, O. and Shekhah, O. and Wang, H. and Tafipolsky, M. and Schmid, R. and Johannsmann, D. and Wöll, C.
    Physical Chemistry Chemical Physics 15 (2013)
    view abstract10.1039/c3cp90155f
  • Erratum: Highly soluble energy relay dyes for dye-sensitized solar cells (Physical Chemistry Chemical Physics (2013) 15 (11306-11312) DOI: 10.1039/C3CP51018B)
    Margulis, G.Y. and Lim, B. and Hardin, B.E. and Unger, E.L. and Yum, J.-H. and Feckl, J.M. and Fattakhova-Rohlfing, D. and Bein, T. and Grätzel, M. and Sellinger, A. and McGehee, M.D.
    Physical Chemistry Chemical Physics 15 (2013)
    view abstract10.1039/c3cp90155f
  • Hypothetical 3D-periodic covalent organic frameworks: Exploring the possibilities by a first principles derived force field
    Bureekaew, S. and Schmid, R.
    CrystEngComm 15 (2013)
    A scheme to predict as yet unknown, hypothetical covalent organic frameworks (COFs) from scratch by screening the possible space of supramolecular isomers is presented. This is achieved by extending our currently developed first principles derived force field MOF-FF with a parametrization for the boroxin unit. We considered four non-tetrahedral monomers with four boronic acid groups inspired by the corresponding carboxylate linkers known from metal-organic frameworks, and investigated the potential 3,4-connected topologies with edge-transitivity (ctn, bor, pto and tbo) or transitivity 32 (ofp, tfj, fjh, iab and nju). Due to the partly lower symmetry of the building blocks with respect to the vertex, beyond topological isomerism also isoreticular isomers are formed. We have used our reverse topological approach to construct the fictitious structures and employed an automated genetic algorithm based global minimum search approach to screen the vast configurational space of isoreticular isomerism and predicted a series of hypothetical 3D-COFs. All structures are completely relaxed by including the lattice parameters. From the atomistic structures, the accessible surface areas were determined, and, because of the isomer screening procedure, the question of crystallographic disorder could also be answered. Beyond the examples of hypothetical 3D-COFs serving as a lead for future synthetic investigations, this work is intended in particular to introduce the efficient predictive modeling method which can be applied to any kind of hypothetical COF system. © 2013 The Royal Society of Chemistry.
    view abstract10.1039/c2ce26473k
  • MOF-FF - A flexible first-principles derived force field for metal-organic frameworks
    Bureekaew, S. and Amirjalayer, S. and Tafipolsky, M. and Spickermann, C. and Roy, T.K. and Schmid, R.
    Physica Status Solidi (B) Basic Research 250 (2013)
    In this contribution the development, definition and selected applications of a new force field (FF) for metal-organic frameworks MOF-FF is presented. MOF-FF is fully flexible and is parameterized in a systematic and consistent fashion from first principles reference data. It can be used for a variety of different MOF-families and in particular - due to the reparametrization of a variety of organic linkers - also to explore isoreticular series of systems. The history of the development, leading to the final definition of MOF-FF is reviewed along with the application of the previous incarnations of the FF. In addition, the parametrization approach is explained in a tutorial fashion. The currently parametrized set of inorganic building blocks is constantly extended. Formate models of currently covered inorganic building blocks. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.201248460
  • Adsorption of hydrocarbons in metal-organic frameworks: A force field benchmark on the example of benzene in metal-organic framework 5
    Amirjalayer, S. and Schmid, R.
    Journal of Physical Chemistry C 116 (2012)
    For the application of metal-organic frameworks (MOFs), the understanding of host-guest interactions on a molecular level is crucial, and often only theoretical methods allow such an insight. However, to obtain quantitative information from such calculations, a validation of the applied methods is indispensable. In this work we investigate for the first time the physisorption of benzene, as a probe molecule for larger guest molecules, within the matrix of MOF-5 using high-level quantum mechanical methods. The calculations reveal a large contribution of dispersion effects on the host-guest interaction. The importance of both reliable and efficient quantum mechanical techniques, which are able to properly cover these effects, is discussed. We find that in particular a double-hybrid functional together with an empirical long-range dispersion correction is an accurate and robust method for the rather large model systems. In addition, our quantum mechanical results enabled us to benchmark for the first time the performance of force fields to describe the interaction of hydrocarbons with the periodic framework. This kind of bottom-up validation of host-guest interactions strengthens the predictive power of theoretical methods in the area of MOF research. © 2012 American Chemical Society.
    view abstract10.1021/jp302713m
  • Advanced buckyball joints: Synthesis, complex formation and computational simulations of centrohexaindane-extended tribenzotriquinacene receptors for C 60 fullerene
    Henne, S. and Bredenkötter, B. and Dehghan Baghi, A.A. and Schmid, R. and Volkmer, D.
    Dalton Transactions 41 (2012)
    The synthesis of a structurally optimized tribenzotriquinacene receptor 9 is described, which is extended by centrohexaindane moieties to give rise to a half-round concave ball bearing, with optimum shape complementarity towards C 60 fullerene. Spectroscopic investigations reveal that this novel host forms a 1:1 host-guest complex with C 60 with a complex stability constant of K 1 = 14550 ± 867 M -1, which is considerably higher than those reported for structurally related tribenzotriquinacene hosts reported previously. Both the suppression for binding of a second receptor (i.e. formation of a 2:1 host-guest complex) as well as the increase of complex stability of the 1:1 complex can be rationalized in terms of multiple additive van der Waals and π-π interactions between C 60 and the aromatic groups of the receptor, as revealed by DFT + D and force-field calculations. Combining results from spectroscopic and theoretical investigations leads to predictions in light of future receptor designs, which - apart from shape complementarity - will have to consider an optimized electronic match (i.e. partial charge transfer) between the receptor and the fullerene host. © 2012 The Royal Society of Chemistry.
    view abstract10.1039/c2dt12435a
  • Low-Temperature CO Oxidation over Cu-Based Metal-Organic Frameworks Monitored by using FTIR Spectroscopy
    Noei, H. and Amirjalayer, S. and Müller, M. and Zhang, X. and Schmid, R. and Muhler, M. and Fischer, R.A. and Wang, Y.
    ChemCatChem 4 (2012)
    view abstract10.1002/cctc.201200164
  • Orbital directing effects in copper and zinc based paddle-wheel metal organic frameworks: The origin of flexibility
    Bureekaew, S. and Amirjalayer, S. and Schmid, R.
    Journal of Materials Chemistry 22 (2012)
    We have used density functional theory calculations to study non-periodic model systems for the ubiquitous layer-pillar metal organic frameworks built from paddle-wheel building blocks. Experimentally, these porous materials show nearly identical structures for both copper and zinc forming the paddle-wheel, but differ depending on the type of the metal center in their properties. Our theoretical results clearly reveal orbital directing effects for the d 9 Cu(ii) center, enforcing a square planar conformation, to be the main reason for the difference in contrast to the flexible d 10 Zn(ii) system. Surprisingly, this difference is directly visible in the structure of the bare vertex model without axial ligands, whereas in the case of pyridine coordination both copper and zinc complexes are structurally nearly indistinguishable. However, in the vibrational normal modes the higher degree of flexibility for the zinc-based systems is still noticeable, explaining the higher flexibility of the corresponding periodic MOFs. © 2012 The Royal Society of Chemistry.
    view abstract10.1039/c2jm15778k
  • Prediction of structure and properties of boron-based covalent organic frameworks by a first-principles derived force field
    Amirjalayer, S. and Snurr, R.Q. and Schmid, R.
    Journal of Physical Chemistry C 116 (2012)
    We present a method to theoretically predict structures in arbitrary network topologies for all currently known boron based covalent organic frameworks (COFs). This is particularly useful because these materials are accessible experimentally only as polycrystalline powders. The method is based on a new fully flexible molecular mechanics force field. The consistent parameter set is derived by a genetic algorithm optimization approach from first-principles reference computed data. To achieve high accuracy, the convergence with respect to the level of theory is carefully controlled for this reference. The force field is used to investigate the relative stability of the two high symmetry topologies ctn and bor. Interestingly, for all systems, the ctn topology is found to be energetically more stable. This preference is observed experimentally, too, with the single exception of COF-108, which forms the bor topology. This exception can thus be attributed to the different synthesis conditions, demonstrating that other topologies might be accessible in principle for all COFs. The force field is further used to compute first benchmark surface areas for ideal systems, thermal expansion coefficients, elastic constants, and CO 2 adsorption isotherms for all systems in both topologies, which are experimentally unavailable. Our force field opens the way for theoretical structure prediction and prescreening of properties for these fascinating materials. © 2012 American Chemical Society.
    view abstract10.1021/jp211280m
  • Exploring network topologies of copper paddle wheel based metal-organic frameworks with a first-principles derived force field
    Amirjalayer, S. and Tafipolsky, M. and Schmid, R.
    Journal of Physical Chemistry C 115 (2011)
    We have applied an accurate molecular mechanics force field, parametrized with respect to first-principles calculated reference data, for copper paddle wheel (Cu2(O2C)4) based metal organic frameworks to investigate possible systems with a 3,4-connected network topology. The results explain why the well-known HKUST-1 forms a tbo net, whereas for an extended linker, as in MOF-14, the pto topology is preferred. In particular, the complex structure of the latter system, consisting of two deformed and "interwoven" nets, is accurately predicted, and the necessary deformation energy can be quantified. In this context also all possible forms of interpenetration were considered. Finally, by designing a bromine-substituted extended linker the system can be forced back into the more open tbo topology. This first molecular mechanics investigation of the relative strain energies of MOF network topologies demonstrates that the structure is to a large extent defined by the intrinsic conformational preferences of the building blocks. Our approach allows to analyze and understand the reasons for this preference and can be used as a computational tool for the design of specific topologies. © 2011 American Chemical Society.
    view abstract10.1021/jp200123g
  • Flexibility and sorption selectivity in rigid metal-organic frameworks: The impact of ether-functionalised linkers (Chemistry - A European Journal (2010) (16))
    Henke, S. and Schmid, R. and Grunwaldt, J.-D. and Fischer, R.A.
    Chemistry - A European Journal 17 (2011)
    view abstract10.1002/chem.201190003
  • NMR studies of benzene mobility in metal-organic framework MOF-5
    Hertel, S. and Wehring, M. and Amirjalayer, S. and Gratz, M. and Lincke, J. and Krautscheid, H. and Schmid, R. and Stallmach, F.
    EPJ Applied Physics 55 (2011)
    The concentration and temperature dependence of the self-diffusion of benzene adsorbed in the metal-organic framework MOF-5 (IRMOF-1) is studied by pulsed field gradient (PFG) NMR spectroscopy. When increasing the loading from 10 to 20 molecules per unit cell of MOF-5, the experimental diffusion data drop by a factor of about 3 while current molecular dynamic (MD) simulations predict slightly increasing diffusion coefficients for this range of loadings. The observation is rationalized using the recently predicted clustering of adsorbate molecules in microporous systems for temperatures well below the adsorbate critical temperature. Necessary improvements of molecular simulation models for predicting diffusivities under such conditions are discussed. © EDP Sciences, 2011.
    view abstract10.1051/epjap/2011100370
  • Surface chemistry of metal-organic frameworks at the liquid-solid interface
    Zacher, D. and Schmid, R. and Wöll, C. and Fischer, R.A.
    Angewandte Chemie - International Edition 50 (2011)
    MOFs on surfaces: Many parameters need to be considered in the formation of metal-organic frameworks (MOFs; see structures) at the liquid-solid interface. The methods and growth mechanisms for the layer-by-layer deposition of MOFs on functional materials, the homo- and heteroepitaxial deposition of MOF heterocrystals, and the coordination modulation of MOF surfaces are reviewed. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/anie.201002451
  • A novel method to measure diffusion coefficients in porous metal-organic frameworks
    Zybaylo, O. and Shekhah, O. and Wang, H. and Tafipolsky, M. and Schmid, R. and Johannsmann, D. and Wöll, C.
    Physical Chemistry Chemical Physics 12 (2010)
    We present a novel method to determine diffusion constants of small molecules within highly porous metal-organic frameworks (MOFs). The method is based on the recently proposed liquid-phase epitaxy (LPE) process to grow MOF thin films (SURMOFs) on appropriately functionalized substrates, in particular on organic surfaces exposed by thiolate-based self-assembled monolayers (SAMs). By applying the LPE-method to SAM-coated quartz crystals, the time-dependence of the mass-uptake of the MOF when exposing it to a gas is measured by a quartz-crystal microbalance (QCM). The homogenous nature of the SURMOFs together with their well-defined thickness allow to analyze the QCM-data using Fickian diffusion to yield the diffusion constant. We demonstrate the potential of this method for the case of pyridine diffusion within HKUST-1 (Cu3(BTC) 2) MOF, for which the diffusion coefficient at room temperature is found to amount to 1.5 × 10-19 m2 s-1. Assuming a Fickian diffusion and a hopping mechanism, we yield a binding energy of 0.78 eV of the pyridine to the Cu2+ sites within the HKUST-1 MOF, a value in good agreement with the results of precise ab initio quantum chemistry calculations. © the Owner Societies 2010.
    view abstract10.1039/b927601g
  • Atomistic theoretical models for nanoporous hybrid materials
    Tafipolsky, M. and Amirjalayer, S. and Schmid, R.
    Microporous and Mesoporous Materials 129 (2010)
    In this contribution, available atomistic theoretical models for the new class of functional porous hybrid materials are critically overviewed. These hybrid materials (including covalent- and metal-organic frameworks, COFs and MOFs) are characterized by organic linkers, allowing for conformational flexibility and connecting nodes formed by a wide variety of elements and coordination modes. The flexibility and variability of these porous materials represent one of their potentials for application, but also afford that theoretical methods, tailored for more traditional materials like zeolites, need to be extended or modified. The current status of both periodic and non-periodic quantum mechanic, as well as molecular mechanic models are considered, focusing on the peculiarities of an application on hybrid frameworks. Atomistic models are used for structural prediction, computation of materials properties and in particular for the modeling of host-guest interactions. © 2009 Elsevier Inc. All rights reserved.
    view abstract10.1016/j.micromeso.2009.07.006
  • First-principles-derived force field for copper paddle-wheel-based metal-organic frameworks
    Tafipolsky, M. and Amirjalayer, S. and Schmid, R.
    Journal of Physical Chemistry C 114 (2010)
    We present a fully flexible and ab initio-derived molecular mechanics force field for the ubiquitous copper paddle-wheel building block Cu2 (O2C)4 in metal-organic frameworks. The force field expression is based on the established MM3 force field, extended by additional cross terms and specific bond-stretching and anglebending terms for the square-planar CuO4 coordination environment. Using reference data computed at the DFT level for nonperiodic reference systems, the parametrization is performed using an automated genetic algorithm optimization strategy in order to reproduce structure and low normal modes of the model systems. It is validated on the much investigated Cu-btc (HKUST-1) metal-organic framework. Beyond the structure, lattice-dynamic-dependent properties such as the bulk modulus and the observed negative thermal expansion effect of Cu-btc are quantitatively predicted by the force field without recourse with respect to experimental data. In connection with available parametrizations of various organic linkers, it can be useful for aiding the structure determination of known MOFs, but it also paves the way for the computational prescreening of yet unknown copper paddle-wheel-based frameworks. © 2010 American Chemical Society.
    view abstract10.1021/jp104441d
  • Flexibility and Sorption Selectivity in Rigid Metal-Organic Frameworks: The Impact of Ether-Functionalised Linkers
    Henke, S. and Schmid, R. and Grunwaldt, J. D. and Fischer, R. A.
    Chemistry-a European Journal 16 (2010)
    The functionalisation of well-known rigid metal organic frameworks (namely, [Zn4O(bdc)(3)](n), MOF-5, IRMOF-1 and [Zn-2(bdc)(2)(dabco)](n); bdc = 1,4-benzene dicarboxylate, dabco=diazabicyclo[2.2.2]octane) with additional alkyl ether groups of the type -O-(CH2)(n)-O-CH3 (n = 2-4) initiates unexpected structural flexibility, as well as high sorption selectivity towards CO2 over N-2 and CH, in the porous materials. These novel materials respond to the presence/absence of guest molecules with structural transformations. We found that the chain length of the alkyl ether groups and the substitution pattern of the bdc-type linker have a major impact on structural flexibility and sorption selectivity. Remarkably, our results show that a high crystalline order of the activated material is not a prerequisite to achieve significant porosity and high sorption selectivity.
    view abstract10.1002/chem.201002341
  • atomistic simuations

  • density functional theory

  • hybrid materials

  • metal organic framework

  • molecular dynamics

  • multiscale modelling

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