A theoretical study of the condensation reactions of methyl cation with ammonia,water, hydrogen fluoride and hydrogen sulphide

Ab initio molecular orbital calculations have been used to study the condensation reactions of CH₃^? with NH₃, H₂O, HF and H₂S. Geometry optimization has been carried out at the Hartree—Fock (HF) level with the split-valence plus d-polarization 6-31G* basis set and improved relative energies obtained from calculations which employ the split-valence plus dp-polarization 6-31G** basis set with electron correlation incorporated via Moller—Plesset perturbation theory terminated at third order (MP3). Zero-point vibrational energies have also been determined and taken into account in deriving relative energies. The structures of the intermediates CH₃XH^? (X = NH₂, OH, F and SH) have been obtained and dissociation of these intermediates into CH₂X⁺ + H₂ on the one hand, and CH₃^? + HX on the other, has been examined. It is found that for those species for which the methyl condensation reaction is observed to have an appreciable rate (X = NH₂ and SH), the transition structure for hydrogen elimination from CH₃XH^? lies significantly lower in energy than the reactants CH₃^? + HX (by 75 and 70 kJ mol^?1 respectively). On the other hand, for those species for which the methyl condensation reaction is not observed (X = OH and F), the transition structure for H₂ elimination lies higher in energy than CH₃^? + HX (by 6 and 87 kJ mol^?1 respectively).