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. Pure Appl. Chem. 1990, 62, 17) was revised on the basis of recent experimental and theoretical (G2 calculations) results. A new anchoring based 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 compounds is given. Correlations between gas-phase basicities toward lithium cation and proton were examined. Though a general trend is discernible, fair correlations are obtained provided that separate lines are drawn for homogeneous families. The differences in slopes are traced back to the different sensitivities to structural effects. Large deviations are explained by either a different attachment center for Li+ and H+ or a chelation effect toward Li+. G2 and G2(MP2) calculations of LCBs for a wide selection of 37 compounds and density functional theory (B3LYP/6-311+G**) calculations of LCBs of 63 compounds of different classes were carried out. In most cases the performed calculations adequately describe the gas-phase lithium cation basicities of a wide variety of bases of different chemical origin and LCB range. The results of the calculations were also used for explaining the largest deviations from correlation lines between gas-phase lithium cation and proton basicities.