Structural and thermochemical studies on CH3SCH2CHO, CH3CH2SCHO, CH3SC(═O)CH3, and radicals corresponding to loss of H atom

Abstract

AbstractCH3SCH2CHO, CH3CH2SCHO, and CH3SC(═O)CH3 are intermediates during the partial oxidation of CH3SCH2CH3 in the atmosphere and in combustion processes. Thermochemical properties (ΔHfo, So and Cp(T)), structures, internal rotor potentials, and C─H bond dissociation energies of the parent molecules and their radicals formed after loss of a hydrogen atom are of value in understanding the oxidation processes of methyl ethyl sulfide. The lowest energy molecular structures were initially determined using the density functional B3LYP/6‐311G/(2d,d,p) level of theory. Standard enthalpies of formation (ΔHfo298) for the radicals and their parent molecules were calculated using the density functional B3LYP/6‐31G(d,p), B3LYP/6‐31 + G(2d,p), and the composite CBS‐QB3 ab initio methods using isodesmic reactions. Internal rotation potential energy diagrams and internal rotation barriers were investigated using B3LYP/6‐31 + G(d,p) level calculations. The contributions for So298 and Cp(T) were calculated using the rigid rotor harmonic oscillator approximation on the basis of the structures and vibrational frequencies obtained by the density functional calculations, with contributions from torsion frequencies replaced by internal rotor contributions from the method of Pitzer‐Gwinn. The recommended values for enthalpies of formation of the most stable conformers of CH3SCH2CHO, CH3CH2SCHO, and CH3SC(═O)CH3 are −34.6 ± 0.8, −42.4 ± 1.2, and ‐49.7 ± 0.8 kcal/mol, respectively. The structural and thermochemical data presented for CH3SCH2CHO, CH3CH2SCHO, and CH3SC(═O)CH3 and their radicals are of value in understanding the mechanism and kinetics of methyl ethyl sulfide oxidation under varied temperatures and pressures. Group additivity values are developed for estimating properties of structurally similar, larger sulfur‐containing compounds.