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Markham GD, Bock CW
Structural and Thermodynamic Properties of Sulfonium Ions - an Abinitio Molecular-Orbital Study
Journal of Physical Chemistry (1993) 97:5562-5569.
Abstract
Transfers of alkyl groups from sulfonium centers to various oxygen, nitrogen, and sulfur acceptors are widespread processes in biological systems. Relatively little is known, however, about the energetics of these reactions. The molecular structures and energies of 14 sulfonium ions and 13 related thiols and thioethers have been computed using ab initio molecular orbital methods with full geometry optimization at the RHF/6-31G* level; total molecular energies have been calculated with the inclusion of electron correlation at the MP2/6-31G**//RHF/6-31G* level or higher. The neutral molecules considered range from hydrogen sulfide (H2S) to the amino acid methionine (CH3SCH2CH2CH(NH+3)CO2-). The sulfonium compounds ranged from H3S+ to methionine protonated at sulfur (CH3S+(H)(CH2)2CH(NH3+)CO2-). The structural consequences of replacing hydrogens of the simplest sulfonium ion, H3S+, and the corresponding thiol H2S, with a variety of alkyl groups have been examined. The effects of alkyl substitution on the enthalpy changes accompanying proton or alkyl transfer from these sulfonium centers have been determined, and a linear correlation is found between substituent effects on proton affinity and methyl cation affinity. The calculated enthalpy changes are in excellent agreement with published gas-phase values. In these reactions, the sulfonium ions are stabilized by approximately 15 kcal/mol when a methyl group replaces a proton on the sulfur and to a slightly greater extent when an ethyl group is the substituent. In cases where an anionic group is attached to the sulfur (CH2CO2- or CH2CH2CO2-), the sulfonium ion is nearly 100 kcal/mol more stable than when sulfur is attached to uncharged alkyl group substituents. Comparison of calculated (gas-phase) enthalpy changes in methyl-transfer reactions with experimental values for reactions in aqueous solutions reveals that solvation plays a crucial role in determining the energetics of methyl group transfer in biological systems.
Note
Publication Date: 1993-05-27.
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