Ab initio and vibrational predissociation studies on methylammonium-(water) 4 complex: Evidence for multiple cyclic and non-cyclic hydrogen-bonded structures

Abstract

The combined ab initio and vibrational predissociation (VP) spectroscopic studies on methylammonium-(water)4 complex aimed at understanding the hydration behavior of an amphiphilic ion core are described. The ab initio calculations predicted eleven low-energy isomers forming cyclic, tripod, chain, and caged structures, and their relative stabilities, total hydration energies and thermodynamic functions at 298 K and 150 K. The excellent correlation between the observed VP spectra and ab initio spectra for bonded N-H, bonded O-H and free O-H stretches suggested co-existence of five cyclic isomers and two non-cyclic isomers in ion beam at 150 K, consistent with the trends of calculated Gibbs free energies.