Aäron G. Vandeputte

Modeling the Gas‐Phase Thermochemistry of Organosulfur Compounds

Key to understanding the involvement of organosulfur compounds in a variety of radical chemistries, such as atmospheric chemistry, polymerization, pyrolysis, and so forth, is knowledge of their thermochemical properties. For organosulfur compounds and radicals, thermochemical data are, however, much less well documented than for hydrocarbons. The traditional recourse to the Benson group additivity method offers no solace since only a very limited number of group additivity values (GAVs) is available. In this work, CBS-QB3 calculations augmented with 1D hindered rotor corrections for 122 organosulfur compounds and 45 organosulfur radicals were used to derive 93 Benson group additivity values, 18 ring-strain corrections, 2 non-nearest-neighbor interactions, and 3 resonance corrections for standard enthalpies of formation, standard molar entropies, and heat capacities for organosulfur compounds and organosulfur radicals. The reported GAVs are consistent with previously reported GAVs for hydrocarbons and hydrocarbon radicals and include 77 contributions, among which 26 radical contributions, which, to the best of our knowledge, have not been reported before. The GAVs allow one to estimate the standard enthalpies of formation at 298 K, the standard entropies at 298 K, and standard heat capacities in the temperature range 300-1500 K for a large set of organosulfur compounds, that is, thiols, thioketons, polysulfides, alkylsulfides, thials, dithioates, and cyclic sulfur compounds. For a validation set of 26 organosulfur compounds, the mean absolute deviation between experimental and group additively modeled enthalpies of formation amounts to 1.9  kJ  mol(-1). For an additional set of 14 organosulfur compounds, it was shown that the mean absolute deviations between calculated and group additively modeled standard entropies and heat capacities are restricted to 4 and 2 J  mol(-1)  K(-1), respectively. As an alternative to Benson GAVs, 26 new hydrogen-bond increments are reported, which can also be useful for the prediction of radical thermochemistry.

Kinetic modeling of hydrogen abstractions involving sulfur radicals.

One of the requisites for the development of detailed reaction networks is the availability of accurate kinetic data. Group additivity based models linking the Arrhenius parameters to structural characteristics of the transition state have proven to be a valuable tool to obtain those data. In this work, group additivity values are presented to allow a broad range of CH and SH hydrogen abstraction reactions by S radicals to be modeled. Rate coefficients in the temperature range from 300 to 1500 K are obtained by using the CBS-QB3 method in the high-pressure limit and are corrected for tunneling and anharmonicity of rotation about the transitional bond. A total of 149 reactions are studied. From these reactions, a total of 52 group additivity values and 35 resonance corrections are derived. The general applicability of the group additivity method is demonstrated for a test set containing 25 reactions. At 300 K, rate coefficients are on average reproduced within a factor of 2.8. The mean absolute deviations on the Arrhenius parameters are 2 kJ mol(-1) for the activation energy and 0.38 for log A in which A is the pre-exponential factor.

Kinetics of Homolytic Substitutions by Hydrogen Atoms at Thiols and Sulfides

Thermodynamic and kinetic data in the temperature range 300-1500 K are calculated for 94 homolytic substitution reactions by a hydrogen atom at thiols and sulfides with the CBS-QB3//BMK/6-311G(2d,d,p) method. The studied reactions were found to proceed according to a one-step mechanism. A group additivity (GA) method is presented to model the Arrhenius parameters of this reaction family. The required GA values were derived from data obtained for a set containing 58 reactions. By using the developed GA scheme, rate coefficients at 300 K for 26 substitution reactions by the hydrogen atom are reproduced within a factor of 2.2. Mean absolute deviations on the activation energy and pre-exponential factor are limited to 1.1 kJ mol(-1) and 0.19, respectively. Rate coefficients for the reverse reactions, that is, substitution reactions by C- and S-centered radicals with expulsion of a hydrogen atom, are reproduced within a factor of 6 by using thermodynamic consistency. At 1000 K, group additive and calculated rate coefficients for the forward and reverse reactions agree within a factor of 1.8 and 4, respectively. Experimental rate coefficients in the temperature range 300-400 K are reproduced within a factor of 5. Discrepancies between calculated and experimental data are discussed.

Automatic mechanism generation for pyrolysis of di-tert-butyl sulfide.

The automated Reaction Mechanism Generator (RMG), using rate parameters derived from ab initio CCSD(T) calculations, is used to build reaction networks for the thermal decomposition of di-tert-butyl sulfide. Simulation results were compared with data from pyrolysis experiments with and without the addition of a cyclohexene inhibitor. Purely free-radical chemistry did not properly explain the reactivity of di-tert-butyl sulfide, as the previous experimental work showed that the sulfide decomposed via first-order kinetics in the presence and absence of the radical inhibitor. The concerted unimolecular decomposition of di-tert-butyl sulfide to form isobutene and tert-butyl thiol was found to be a key reaction in both cases, as it explained the first-order sulfide decomposition. The computer-generated kinetic model predictions quantitatively match most of the experimental data, but the model is apparently missing pathways for radical-induced decomposition of thiols to form elemental sulfur. Cyclohexene has a significant effect on the composition of the radical pool, and this led to dramatic changes in the resulting product distribution.