Gas-Phase Cation Basicities for Sulfuryl Species from Calculation and Experiment

Gas-phase Brønsted basicity (GB) and lithium cation basicity (LCB) for sulfuryl compounds have been measured using FT-ICR. In addition, GB, LCB, and sodium cation basicity (SCB) have been estimated from MO and DFT calculations for a family of nine sulfuryl compounds including sulfoxides, sulfones, sulfinate, sulfonate, sulfite, and sulfate. The newer DFT-hybrid methods, based upon B3PW91/6-31++G*, provided better correlation with experimental results than MP2 and HF methods. Both the calculated and experimental data provided good linear free energy relationships (LFERs) between the three sets of values, GB, LCB, and SCB, across the series of eight diverse sulfuryl species. Experimental data were in accord with the conclusion from calculations that chelation of lithium provides little if any stabilization over "linear" complexation. The electrostatic interaction of the cation with the sulfuryl bond dipole is indicated to be dominant, based upon (i) previous rationalizations of similar LFERs observed for other families of compounds and (ii) the good linear correlations observed between GB, LCB, SCB, and the calculated S=O bond dipole moment or S=O bond length. Remarkably accurate predictions of basicity are possible simply from S=O bond lengths calculated at the DFT level.