Structures, energetics, and infrared spectra of the cationic monomethylamine-water clusters^†

The structures, energetics, and infrared (IR) spectra of the cationic monomethylamine-water clusters, [(CH₃NH₂)(H₂O)_n]⁺ (n=1–5), have been studied using quantum chemical calculations at the MP2/6-311+G(2d,p) level. The results reveal that the formation of proton-transferred CH₂NH₃⁺ ion core structure is preferred via the intramolecular proton transfer from the methyl group to the nitrogen atom and the water molecules act as the acceptor for the O⋯HN hydrogen bonds with the positively charged NH₃⁺ moiety of CH₂NH₃⁺, whose motif is retained in the larger clusters. The CH₃NH₂⁺ ion core structure is predicted to be less energetically favorable. Vibrational frequencies of CH stretches, hydrogen-bonded and free NH stretches, and hydrogen-bonded OH stretches in the calculated IR spectra of the CH₂NH₃⁺ and CH₃NH₂⁺ type structures are different from each other, which would afford the sensitive probes for fundamental understanding of hydrogen bonding networks generated from the radiation-induced chemical processes in the [(CH₃NH₂)(H₂O)_n]⁺ complexes.