Influence of Resonance on the Acidity of Sulfides, Sulfoxides, Sulfones, and Their Group 16 Congeners

The influence of resonance on the acidities of dimethyl sulfide (DMS), dimethyl sulfoxide (DMSO), and dimethyl sulfone (DMSO2) and their group 16 congeners (DMXO(n) for X = Se, Te, Po and n = 0-2) is examined using ab initio methods and the natural bond orbital (NBO) and natural resonance theory (NRT) analyses. Gas-phase acidities are evaluated using B3LYP-optimized geometries with coupled cluster energies and complete basis set extrapolation. The acidity of the DMSO(n) molecules increases with increasing coordination of the central S atom. Acidity also tends to increase when the central atom is substituted by a heavier group 16 atom. NRT analysis reveals significant resonance delocalization in the DMXO(n) molecules and their anions. On deprotonation, the DMXO(n) molecules undergo structural changes that are consistent with changes in the resonance character of the calculated charge densities. However, resonance cannot account for the trends in the deprotonation energies. Whereas the DMX- anions are more strongly resonance stabilized than their parent molecules DMX, the DMXO2(-) anions and DMXO2 molecules are nearly equally resonance stabilized. Thus, there appears to be no extra stabilization of DMXO2(-) compared to that of DMX- that would account for the enhanced acidity of DMXO2 relative to DMX.