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.