Proton Affinity of Dimethyl Sulfoxide and Relative Stabilities of C₂H₆OS Molecules and C₂H₇OS⁺ Ions. A Comparative G2(MP2) ab Initio and Density Functional Theory Study

Relative energies of several C 2 H 6 OS molecules and C 2 H 7 OS + ions were obtained by ab initio and density functional theory calculations. Geometry optimizations with Becke's hybrid functional (B3LYP) and the 6-31+G(d,p) basis set gave good-quality equilibrium structures which did not improve significantly by B3LYP calculations using the larger 6-311+G(2df,2p) basis set. Among C 2 H 6 OS isomers with S−O bonds, dimethyl sulfoxide (DMSO, 1 ) was the most stable species, followed by methyl methanesulfenate ( 3 ), S -hydroxy- S -methyl- S -methylenesulfurane (DMSO enol, 2 ), S -methoxy- S -methylene- S -(H)sulfurane ( 4 ), and ( S, S -H 2 )2-oxa-1-thietane ( 5 ). O-protonated dimethyl sulfoxide ( 1aH + ) was the most stable C 2 H 7 OS + isomer of those studied. Ion 1aH + was separated from the less stable isomers by substantial potential energy barriers. The G2(MP2) proton affinity of DMSO (885 kJ mol -1 ) was in excellent agreement with the value from equilibrium measurements. DMSO-enol ( 2 ) was predicted to be a strong gas-phase base for protonation at the methylene and hydroxyl groups. S-protonation in 3 was 44 kJ mol -1 more exothermic than O-protonation. In general, proton affinities were overestimated by B3LYP calculations and underestimated by MP2 calculations with all basis sets used. Empirical averaging of the B3LYP and MP2 values, obtained from calculations with the 6-311+G(2df,2p) basis set, provided an improved agreement with experimental or accurate G2(MP2) proton affinities.