Heteroatomic Deprotonation of Substituted Methanes and Methyl Radicals: Theoretical Insights into Structure, Stability, and Thermochemistry

High-level W1w ab initio calculations have been used to investigate the structural and thermochemical changes that result from heteroatomic deprotonation of CH(3)YH molecules and (•)CH(2)YH radicals (YH = BH(2), CH(3), NH(2), OH, AlH(2), SiH(3), PH(2), and SH). The thermochemical quantities considered include gas-phase acidities, various bond dissociation enthalpies, and heats of formation. The high-level ab initio results are compared with available experimental data and generally show excellent agreement. In a small number of cases in which we find discrepancies that persist at even higher theoretical levels (e.g., W4), we suggest that the experimental data should be re-examined. We find that the C-Y bond lengths of (•)CH(2)YH contract upon deprotonation, whereas for CH(3)YH, the predicted effect, in general, is a lengthening of the C-Y bond. These structural changes, for the most part, are reflected in the changes to the C-Y bond dissociation enthalpies. The CH(3)YH molecules are calculated to be 50-200 kJ mol(-1) less acidic in the gas phase than the corresponding (•)CH(2)YH radicals, indicating relative stabilization of the (•)CH(2)Y(-) radical anions. The structural and thermochemical changes are rationalized using a combination of resonance and orbital interaction arguments.