The interaction of water with sulfonium ions and the effects of hydration on the energetics of methyl group transfer: An ab initio molecular orbital study of the hydration of (CH3)3S+ and (CH3)2S+CH2CO2 −

Abstract

The interactions of the sulfonium ions (CH3)3S+, (CH3)2S+CH2CO2−, and (CH3)2S+-CH2CH2CO2− with up to four water molecules have been studied by ab initio molecular orbital methods. Complexes of (CH3)3S+ with one to three water molecules involve strong electrostatic sulfur-oxygen interactions; in contrast, the sulfide (CH3)2S interacts with water molecules via weak S-H hydrogen bonds, suggesting that methyl-group transfer from (CH3)3S+ in aqueous solution involves a significant alteration of the hydration pattern around the sulfur atom. Two conformers of (CH3)2S+CH2CO2− were found that display sulfur-oxygen distances which are approximately 0.3 a less than the sum of the sulfur and oxygen van der Waals radii, indicating a strong intramolecular electrostatic interaction. For the complexes (CH3)2S+CH2CO2−·nH2O(n =1–4), water interacts primarily with the carboxylate group via hydrogen bonds, rather than electrostatically with the sulfur atom, although in complexes with the three- and four-water complexes, the proximity of the positively charged sulfur atom to the carboxylate group significantly alters the hydration pattern compared to that in the corresponding of complexes CH3SCH2CO2−· Thus, methyl transfer from (CH3)2S+CH2CO2− to an acceptor in aqueous solution also involves substantial changes in the hydration pattern around the carboxylate group.