Hydrogen‐Bond Rearrangement and Intermolecular Proton Transfer in Protonated Methanol Clusters

Abstract

AbstractVibrational predissociation spectra (VPS) of hydrogen‐bonded and non‐hydrogen‐bonded (free) OH stretches within the frequency range of 2700–3800 cm−1 are closely analyzed to determine the isomeric structures and proton location in protonated methanol clusters, H+(CH3OH)n. The present report emphasizes protonated methanol tetramers and pentamers, which are predicted by ab initio calculations to exist in linear and cyclic forms. Only one isomer with linear structure was identified in the VPS of protonated methanol tetramers in a supersonic expansion. Both linear and cyclic configurations were found for protonated methanol pentamers, and an isomeric transition between these two structures due to hydrogen‐bond rearrangement was observed at a cluster temperature of 190 K. Ab initio calculations indicated that the excess proton in these clusters can be either localized on one methanol unit, as in cyclic H+(CH3OH)4 and linear H+(CH3OH)5 isomers, or delocalized between two methanol molecules, as in linear H+(CH3OH)4 and cyclic H+(CH3OH)5 isomers. Such unique proton delocalization behavior is revealed in the VPS of cyclic H+(CH3OH)5, which is recognized as a symmetric five‐membered pentamer by its characteristic free‐OH stretching absorption at 3647 cm−1 and hydrogen‐bonded OH stretches at 3448 and 3461 cm−1. Using protonated methanol pentamers as a model system, a proton transfer mechanism is suggested to involve intracluster proton transfer mediated by a sequence of hydrogen‐bond breaking and reforming processes. This appealing mechanism can be closely associated with the exceptionally high proton mobility in liquid methanol.