Revised and expanded scale of gas-phase lithium cation basicities. An experimental and theoretical study

The previously reported gas-phase lithium cation basicity (LCB) scale (Taft et al. Pore Appl. Chem. 1990, 62, 17) was revised on the basis of recent e xperimental and theoretical (G2 calculations) results. A new anchoring base d on the experimental LCA value for H2O is suggested (all earlier reported values of LCB should be reduced by 2.6 kcal/mol). New LCBs for 28 compounds were measured using FT-ICR, and a revised LCB scale now extended to 205 co mpounds is given. Correlations between gas-phase basicities toward lithium cation and proton were examined. Though a general trend is discernible, fai r correlations are obtained provided that separate Lines are drawn for homo geneous families. The differences in slopes are traced back to the differen t sensitivities to structural effects. Large deviations are explained by ei ther a different attachment center for Li+ and H+ or a chelation effect tow ard Li+. G2 and G2(MP2) calculations of LCBs for a wide selection of 37 com pounds and density functional theory (B3LYP/6-311+G**) calculations of LCBs of 63 compounds of different classes were carried out. In most cases the p erformed calculations adequately describe the gas-phase lithium cation basi cities of a wide variety of bases of different chemical origin and LCB rang e. The results of the calculations were also used for explaining the larges t deviations from correlation lines between gas-phase lithium cation and pr oton basicities.