Deprotonating Molecules and Free Radicals to Form Carbon-Centered Anions: A G2 ab Initio Study of Molecular and Free Radical Acidity

Abstract

Molecules CH3X and free radicals •CH2X can be deprotonated to form carbon-centered anions CH2X- and radical anions HCX•-, respectively. We have studied the geometric and thermochemical changes that accompany such deprotonation processes for a variety of substituents X including the π-donor groups NH2, OH, OCH3, PH2, SH, F, Cl, and Br and the π-acceptor groups BH2, AlH2, CHO, NO2, CN, and NC. Thermochemical properties calculated and discussed include the gas-phase acidities of the molecules and free radicals, the electron affinities of the •CH2X free radicals, various dissociation energies, and the heats of formation of all species. The acidities of •CH2X radicals are predicted to be greater than those of CH3X for π-donor substituents but less for π-acceptor substituents (except CN and NC). The changes that are predicted to occur upon deprotonation in C−X bond lengths, C−X homolytic and heterolytic bond dissociation enthalpies, C−H homolytic bond dissociation enthalpies, and radical stabilization energies may be understood by examining the orbital interactions that take place in each species.