A candidate for an ion pair in the vapor phase: Proton transfer in complexes R₃N - HX

Large-scale ab initio calculations have been performed on the complexes NH₃·HCl, NH₃·HBr, CH₃NH₂·HCl and CH₃NH₂·HBr. Two-dimensional energy surfaces as a function of R_N---X and R_HX have been scanned in order to explore the possibility for the formation of stable vapor-phase ion-pair complexes. While the complexes NH₃·HCl, NH₃·HBr and CH₃NH₂·HCl are still of the neutral type, the ionic form CH₃NH₃⁺·Br⁺ is energetically slightly more favourable than the neutral-type complex. Further increase in base strength of the amine will result in stable ionic amine HX complexes in the vapor phase.