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

Abstract

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.