Structures,Energetics, and Infrared Spectra of the Cationic Monomethylamine-Water Clusters

The structures, energetics, and infrared (IR) spectra of the cationic monomethylamine-water clusters, (CH\begin{document}$_3$\end{document}NH\begin{document}$_2$\end{document})(H\begin{document}$_2$\end{document}O)\begin{document}$_n$\end{document}]\begin{document}$^+$\end{document} (\begin{document}$n$\end{document}=1\begin{document}$-$\end{document}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 CH\begin{document}$_2$\end{document}NH\begin{document}$_3$\end{document}\begin{document}$^+$\end{document} 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\begin{document}$\cdots$\end{document}HN hydrogen bonds with the positively charged NH\begin{document}$_3$\end{document}\begin{document}$^+$\end{document} moiety of CH\begin{document}$_2$\end{document}NH\begin{document}$_3$\end{document}\begin{document}$^+$\end{document}, whose motif is retained in the larger clusters. The CH\begin{document}$_3$\end{document}NH\begin{document}$_2$\end{document}\begin{document}$^+$\end{document} 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 CH\begin{document}$_2$\end{document}NH\begin{document}$_3$\end{document}\begin{document}$^+$\end{document} and CH\begin{document}$_3$\end{document}NH\begin{document}$_2$\end{document}\begin{document}$^+$\end{document} 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 (CH\begin{document}$_3$\end{document}NH\begin{document}$_2$\end{document})(H\begin{document}$_2$\end{document}O)\begin{document}$_n$\end{document}]\begin{document}$^+$\end{document} complexes.