Computational and experimental evidence on reaction mechanisms of oxidized sulfur-containing compounds in ground and excited states

This dissertation concerns the elucidation of certain reaction mechanisms of sulfoxides, sulfinyl derivatives, and sulfones by means of thermal chemical studies, photochemical studies, and computational quantum chemical studies. The transition states for the syn-elimination (Ei) of sulfoxides, sulfinyl derivatives, and suffones to form olefins were investigated via gas-phase kinetics and by high-level ab initio methods. Several substituent effects were evaluated for the sulfoxide elimination. Sulfones and sulfinate esters were new molecules shown to undergo the Ei reaction. Deuterium kinetic isotope effects (KIE) were investigated experimentally and computationally for methyl 3-phenylpropyl sulfoxide (240°–300°C) and methyl 3-phenylpropyl sulfone (500°–550°C). For both the sulfoxide and sulfone, a significant KIE was determined (k_H/k_D ∼ 2.0 for both molecules). The barriers for elimination are in the order sulfones > sulfinate esters ∼ sulfoxides. The ab initio methods were shown to accurately reproduce experimental activation barriers.; The excited state potentials of H₂SO and DMSO were evaluated computationally to better understand the photostereomutation process. Multiconfigurational ab initio methods (CASSCF and MCQDPT) with large basis sets were employed for this study. Several excited state structures were found to be stationary points on the lowest two excited potentials. A T-shaped geometry without chirality at sulfur was found. It is lower in energy than the vertical geometry and may be reached without barrier. This possible mechanism for stereomutation is reminiscent of olefin photoisomerization. This suggests that sulfoxide photo-racemization can occur without C-S bond cleavage and recombination.; The photo-assisted reduction of alkyl aryl and diaryl sulfoxides to produce the corresponding sulfides was examined. The effect of electron donors (methoxide, amines, carbazoles, phosphines, and others), solvent (protic versus aprotic), ions (halides) was studied. Methoxide, aryl amines, carbazoles and alkyl phosphines in methanol were found to be efficient in reducing sulfoxides. The reduction did not occur with halide ions. Aprotic solvents tended to stop or slow severely the reduction. The reduction was discovered to also occur by directing the light absorption into carbazoles and anilines. Yields of up to 80% of the sulfide are obtained and quantum yields are in the 0.3–0.4 range for the above systems.