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 CH 3 OH 2 + (CH 3 OH) 3 and linear CH 3 OH 2 + (CH 3 OH) 4 or delocalized between two methanol molecules in linear C 2 H 9 O 2 + (CH 3 OH) 2 and cyclic C 2 H 9 O 2 + (CH 3 OH) 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.