Ab initio study of the energetics of protonation and cationic homoconjugation of trimethylamine and its N-oxide

Ab initio methods at the RHF (Restricted Hartree Fock) and MP2 (Moller-Plesset) levels were used to study the energetics of protonation of trimethylamine and its derivative trimethylamine N-oxide, as well as the energetics of formation of hydrogen bonded (N...H...N)(+) and (O...H...O)(+) type, respectively, homocomplexed cations. The Gaussian functional basis sets 3-21G, 6-31G, 6-311G, 6-31G* and 6-31+G* were employed to calculate energy and Gibbs free energy of protonation and cationic homoconjugation in the gas phase and with the inclusion of solvation effects (using PCM method). The calculated energetic parameters in the gas phase and in solution, as well as experimental values of equilibrium constants of the acid dissociation and cationic homoconjugation reaction for trimethylamine and trimethylamine N-oxide systems provided a basis for a comparison of the basicity and tendency towards cationic homoconjugation of both compounds under study. Consequently, the acid-base properties of aliphatic bases have been compared with those of heterocyclic bases containing both oxygen and nitrogen.