Dr. Holger Somnitz

Theoretical Chemistry
University of Duisburg-Essen

Author IDs

  • Direct rate-constant measurements and theoretical insight into the mechanism of the reactions H + hexamethyldisiloxane and H + tetramethyldisiloxane*
    Somnitz, H. and Peukert, S. and Schäffer, R. and Fikri, M. and Schulz, C.
    Molecular Physics (2021)
    Shock-tube experiments were conducted to determine rate-coefficient data of the H-atom abstraction reactions H + HMDSO (hexamethyldisiloxane) → H2 + HMDSO−H and H + TMDSO (tetramethyldisiloxane) → H2 + TMDSO−H. The experiments comprised a temperature range of 1109–1240 K and pressures between 1.2 and 1.5 bar. In all experiments, C2H5I was used as a precursor for H atoms. H-atom concentrations were measured with atomic resonance absorption spectrometry (ARAS) and rate-coefficient data were obtained from pseudo-first-order analysis of temporal H-atom concentration profiles. Experimental total rate-coefficients were well represented by the following Arrhenius equations: k total,HMDSO(T) = 10−9.49±0.36 exp(−32.4 ± 8.1 kJmol−1/RT) cm3molecule−1s−1 for H + HMDSO and k total,TMDSO(T) = 10−8.00±0.36 exp(−53.3 ± 7.9 kJ mol−1/RT) cm3molecule−1s−1 for H + TMDSO. Multiple reaction channels were explored by computations at G4//B3lyp-D3BJ level of theory. © 2021 Informa UK Limited, trading as Taylor & Francis Group.
    view abstract10.1080/00268976.2021.1963871
  • Kinetics of the Thermal Decomposition of Ethylsilane: Shock-Tube and Modeling Study
    Sela, P. and Peukert, S. and Somnitz, H. and Janbazi, H. and Wlokas, I. and Herzler, J. and Fikri, M. and Schulz, C.
    Energy and Fuels (2021)
    The thermal decomposition of ethylsilane (H3SiC2H5, EtSiH3) is investigated behind reflected shock waves and the gas composition is analyzed by gas chromatography/mass spectrometry (GC/MS) and high-repetition-rate time-of-flight mass spectrometry (HRR-TOF-MS) in a temperature range of 990-1330 K and pressure range of 1-2.5 bar. The unimolecular decomposition of EtSiH3 is considered to be initiated via a molecular elimination of H2 (H3SiC2H5 → H2 + HSiC2H5) followed by reactions of cyclic silicon-containing species. The main observed stable products were ethylene (C2H4) and silane (SiH4). Measurements are performed with a large excess of a silylene scavenger (C2H2) to suppress bimolecular reactions caused by silylene (SiH2) and to extract unimolecular rate constants. A kinetics mechanism accounting for the gas-phase chemistry of EtSiH3 is developed, which consists of 24 Si-containing species, 31 reactions of Si-containing species, and a set of new thermochemical data. The derived unimolecular rate constant is represented by the Arrhenius expression kuni(T) = 1.96 × 1012 s-1 exp(-205 kJ mol-1/RT). The experimental data is reproduced very well by simulations based on the mechanism of this work and is in very good agreement with literature values. It is shown that EtSiH3 is a promising precursor for the synthesis of SiC nanoparticles. © 2021 American Chemical Society.
    view abstract10.1021/acs.energyfuels.0c03425
  • Carbon Isotope Fractionation of Substituted Benzene Analogs during Oxidation with Ozone and Hydroxyl Radicals: How Should Experimental Data Be Interpreted?
    Willach, S. and Lutze, H.V. and Somnitz, H. and Terhalle, J. and Stojanovic, N. and Lüling, M. and Jochmann, M.A. and Hofstetter, T.B. and Schmidt, T.C.
    Environmental Science and Technology 54 (2020)
    Oxidative processes frequently contribute to organic pollutant degradation in natural and engineered systems, such as during the remediation of contaminated sites and in water treatment processes. Because a systematic characterization of abiotic reactions of organic pollutants with oxidants such as ozone or hydroxyl radicals by compound-specific stable isotope analysis (CSIA) is lacking, stable isotope-based approaches have rarely been applied for the elucidation of mechanisms of such transformations. Here, we investigated the carbon isotope fractionation associated with the oxidation of benzene and several methylated and methoxylated analogs, namely, toluene, three xylene isomers, mesitylene, and anisole, and determined their carbon isotope enrichments factors (ϵC) for reactions with ozone (ϵC = -3.6 to -4.6 ‰) and hydroxyl radicals (ϵC = 0.0 to -1.2‰). The differences in isotope fractionation can be used to elucidate the contribution of the reactions with ozone or hydroxyl radicals to overall transformation. Derivation of apparent kinetic isotope effects (AKIEs) for the reaction with ozone, however, was nontrivial due to challenges in assigning reactive positions in the probe compounds for the monodentate attack leading to an ozone adduct. We present several options for this step and compare the outcome to quantum chemical characterizations of ozone adducts. Our data show that a general assignment of reactive positions for reactions of ozone with aromatic carbons in ortho-, meta-, or para-positions is not feasible and that AKIEs of this reaction should be derived on a compound-by-compound basis. © 2020 American Chemical Society.
    view abstract10.1021/acs.est.0c00620
  • Synthesis of Furan-Annelated BINOL Derivatives: Acid-Catalyzed Cyclization Induces Partial Racemization
    Octa-Smolin, F. and Van Der Vight, F. and Yadav, R. and Bhangu, J. and Soloviova, K. and Wölper, C. and Daniliuc, C.G. and Strassert, C.A. and Somnitz, H. and Jansen, G. and Niemeyer, J.
    Journal of Organic Chemistry 83 (2018)
    In this account, we describe the synthesis of a series of BINOL-based bis- and trisphosphoric acids 11d/e/f, which commonly feature an unusual phosphoric acid monoester motif. This motif is generated by an acid-catalyzed 5-endo-dig cyclization of the 3-alkynyl-substituted BINOL precursors to give the corresponding Furan-annelated derivatives, followed by phosphorylation of the remaining phenolic alcohols. In the cyclization reaction, we observed an unexpected partial racemization in the bis- and tris-BINOL scaffolds, leading to mixtures of diastereomers that were separated and characterized spectroscopically and by X-ray crystal structure analyses. The cyclization and racemization processes were investigated both experimentally and by DFT-calculations, showing that although the cyclization proceeds faster, the barrier for the acid-catalyzed binaphthyl-racemization is only slightly higher. © 2018 American Chemical Society.
    view abstract10.1021/acs.joc.8b02353
  • Experimental investigation and modeling of the kinetics of CCl4 pyrolysis behind reflected shock waves using high-repetition-rate time-of-flight mass spectrometry
    Aghsaee, M. and Drakon, A. and Eremin, A. and Dürrstein, S.H. and Böhm, H. and Somnitz, H. and Fikri, M. and Schulz, C.
    Physical Chemistry Chemical Physics 15 (2013)
    The pyrolysis kinetics of CCl4 behind reflected shock waves was studied with high-repetition-rate time-of-flight mass spectrometry. For modeling, quantum mechanical calculations were performed to evaluate the dissociation energies of CCl bonds for the different CClx (x = 1 to 4) radicals. Good agreement with the JANAF thermochemical table was found. With the reaction mechanism developed for CCl4 decomposition satisfactory agreement with experimental results was obtained. The investigations show the importance of C2Cl2 formation for understanding the processes of carbon cluster growth leading to carbonaceous particle formation.© 2013 the Owner Societies.
    view abstract10.1039/c2cp42574b
  • DFT calculations suggest a new type of self-protection and self-inhibition mechanism in the mammalian heme enzyme myeloperoxidase: Nucleophilic addition of a functional water rather than one-electron reduction
    Sicking, W. and Somnitz, H. and Schmuck, C.
    Chemistry - A European Journal 18 (2012)
    The mammalian heme enzyme myeloperoxidase (MPO) catalyzes the reaction of Cl- to the antimicrobial-effective molecule HOCl. During the catalytic cycle, a reactive intermediate "Compound I" (Cpd I) is generated. Cpd I has the ability to destroy the enzyme. Indeed, in the absence of any substrate, Cpd I decays with a half-life of 100 ms to an intermediate called Compound II (Cpd II), which is typically the one-electron reduced Cpd I. However, the nature of Cpd II, its spectroscopic properties, and the source of the additional electron are only poorly understood. On the basis of DFT and time-dependent (TD)-DFT quantum chemical calculations at the PBE0/6-31G* level, we propose an extended mechanism involving a new intermediate, which allows MPO to protect itself from self-oxidation or self-destruction during the catalytic cycle. Because of its similarity in electronic structure to Cpd II, we named this intermediate Cpd IIa'. However, the suggested mechanism and our proposed functional structure of Cpd IIa' are based on the hypothesis that the heme is reduced by charge separation caused by reaction with a water molecule, and not, as is normally assumed, by the transfer of an electron. In the course of this investigation, we found a second intermediate, the reduced enzyme, towards which the new mechanism is equally transferable. In analogy to Cpd II′, we named it FeIIa'. The proposed new intermediates Cpd IIa' and FeIIa' allow the experimental findings, which have been well documented in the literature for decades but not so far understood, to be explained for the first time. These encompass a) the spontaneous decay of Cpd I, b) the unusual (chlorin-like) UV/Vis, circular dichroism (CD), and resonance Raman spectra, c) the inability of reduced MPO to bind CO, d) the fact that MPO-Cpd II reduces SCN- but not Cl-, and e) the experimentally observed auto-oxidation/auto-reduction features of the enzyme. Our new mechanism is also transferable to cytochromes, and could well be viable for heme enzymes in general. Heme mechanisms explained: Direct visual comparison with Cpd II demonstrates that Cpd II′ is a one-electron reduced intermediate with respect to the heme system. In both cases an electron is transferred: in Cpd II from an external donor, and in Cpd II′ through charge separation caused by reaction with a water molecule (see figure). Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/chem.201103477
  • High temperature shock-tube study of the reaction of gallium with ammonia
    Fikri, M. and Bozkurt, M. and Somnitz, H. and Schulz, C.
    Physical Chemistry Chemical Physics 13 (2011)
    The gas-phase reaction of Ga atoms with NH3 was studied behind reflected shock waves in the temperature range of 1380 to 1870 K at pressures of 1.4 to 4.0 bar. Atomic-resonance-absorption spectroscopy (ARAS) at 403.299 nm was applied for the time-resolved determination of the Ga-atom concentration. Trimethylgallium (Ga(CH3)3) was used as a precursor of Ga atoms. After the initial increase in Ga concentration due to Ga(CH 3)3 decomposition, the Ga concentration decreases rapidly in the presence of NH3. For the simulation of the measured Ga-atom concentration profiles from the studied reaction, additional knowledge about the thermal decomposition of Ga(CH3)3 is required. The rate coefficient k4 of the reaction Ga + NH3 → products (R4) was determined from the Ga-atom concentration profiles under pseudo-first-order assumption and found to be k4(T) = 10 14.1±0.4 exp(-11900 ± 700 K/T) cm3 mol -1 s-1 (error limits at the one standard deviation level). No significant pressure dependence was noticeable within the scatter of the data at the investigated pressure range. © the Owner Societies 2011.
    view abstract10.1039/c0cp01373k
  • density functional theory

  • first-principles calculation

  • kinetics

  • modelling and simulation

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