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 protontransferred 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.