Thermochemical properties, bond energies and internal rotor potentials in methyl ethyl sulfide and oxygenated sulfur hydrocarbons intermediates for modeling its combustion and atmospheric chemistry

Small (1 to 4 carbon) hydrocarbon sulfides and thiols are formed in the biosphere by microorganisms and subsequently emitted into the lithosphere, hydrosphere and atmosphere. In the atmosphere they are oxidized by photochemical and radical reactions to intermediate hydrocarbon and to sulfur oxides. The oxides of these sulfur compounds and the intermediates from the oxidation process are known to form aerosols that can counteract the global warming green house effect. Recent studies also suggest that some aerosols can also contribute to global warming. Sulfur oxides are also major contributors to acid rain as the results of the atmospheric chemistry oxidation reactions on sulfur hydrocarbons and H2S involve SO2 formation. It is of great value to understand the thermochemistry and the elementary reaction processes of these sulfur compounds in order to better model atmospheric chemistry and global warming. The oxidation chemistry is also of value in model development for improvement of combustion processes and pollutant reduction. This study determines the structures, internal rotor potentials, bond energies and thermochemical properties (ΔfH°, S° and Cp(T)) of methyl ethyl sulfide (CH3SCH2CH3), a widely used sulfuric hydrocarbon, and its main partial oxidation products in the atmosphere (CH3SCH2CHO, CH3CH2SCHO and CH3SC(=O)CH3). At the same time their radicals after losing one H atom, and some of the main partial reaction intermediate molecules and their main radicals after loss of an H atom have also been studied. The molecular structure and H-molecule bond energy are determined using Density Functional B3LYP/6-31G (d,p) and B3LYP/6-31+G(2d,p) together with the higher level composite CBS-QB3. Enthalpies of formation (Hf) for stable species are calculated in the levels of B3LYP/6-31G (d,p), B3LYP/6-31+G(2d,p) and CBS-QB3 using work reactions that are presumed isodesmic. Internal rotation barriers have also been determined with some other DFT methods. Then, thermochemical parameters (S° and Cp(T)) are determined with the help of the Hf values and the data of moments of inertia and frequencies from the CBS-QB3 output files