Isomeric Transitions between Linear and Cyclic H+(CH3OH)4,5: Implications for Proton Migration in Liquid Methanol

Abstract

Vibrational predissociation spectroscopy of protonated methanol clusters (tetramers and pentamers) reveals linear and cyclic structural isomers in a supersonic expansion. The cyclic pentamer, containing a five-membered ring, is identified by its characteristic free-OH stretch at 3647 cm-1 and hydrogen-bonded OH stretches at 3448 and 3461 cm-1. Ab initio calculations indicate that the excess proton in these clusters can be either localized on one methanol unit in cyclic CH3OH2+(CH3OH)3 and linear CH3OH2+(CH3OH)4 or delocalized between two methanol molecules in linear C2H9O2+(CH3OH)2 and cyclic C2H9O2+(CH3OH)3. Dynamic intracluster proton transfer can occur upon repeated ring opening and closing. The association of this process with the anomalously high proton mobility in liquid methanol is discussed.