Ab initio MO study of protonation of carbamic acid, methyl carbamate and methyl N-methylcarbamate

Abstract

The ab initio SCF method was applied to the protonation of the carbonyl group in carbamic acid and its methyl derivatives, viz. methyl carbamate and methyl N-methylcarbamate. Complete geometry optimization was accomplished for these compounds and their protonated species using the MINI-1, 3-21 G, and 6-31 G* bases and the proton affinities were calculated at the MINI-1, 3-21 G, 6-31 G*, and 6-31 G** levels. 2nd and 3rd order Moller-Plesset perturbation calculations were also performed for examining the effect of the correlation energy on the calculated protonation energies. The carbonyl protonation energies were found to increase in order carbamic acid < methyl carbamate < methyl N-methylcarbamate. The absolute values of calculated gas phase proton affinities depend on the basis used and way of evaluating the correlation energy. The results are discussed with respect to the theoretical proton affinities of structurally related amides and to related available theoretical gas phase proton affinities.