Solvent effect on the protonation of acetylene and ethylene - Continuum solvent quantum chemical calculations

The protonation of acetylene and ethylene (yielding the vinyl and ethyl cat ion, respectively) was investigated computationally by ab initio calculatio ns [B3LYP/6-31G(d,p)], in the gas phase and in water, as modeled by the IPC M and SCIPCM continuum methods. The structures and NBO atomic charges were thus;determined for the neutral bases and their protonated forms, while the comparison of gas-phase and aqueous basicities afforded the hydration ener gies of the protonated bases. It was found that the aqueous protonation of acetylene is more endothermic than that of ethylene by 5 kcal/mol, owing to the lower intrinsic basicity of the former (by 7.4 kcal/mol), which is onl y partly compensated for by the more exothermic hydration (by 3.8 kcal/mol) of the vinyl cation.