Rearrangement of hydrogen bonds in the protonated methanol-water cluster ion H + (CH 3 OH) 4 H 2 O is analyzed. The analysis, based on ab initio calculations performed at the B3LYP/aug-cc-pVTZ//6-31+G* and MP4/ 6-311+G*//B3LYP/6-31+G* levels of computation, provides information about potential minima, transition states, and pathways for the hydrogen bond rearrangement processes. Results of the analysis are compared systematically to the experimental measurements for H + (CH 3 OH) 4 H 2 O, where two distinct charge-centered (H 3 O + and CH 3 OH 2 + ) isomers have been identified in a supersonic expansion by fragment-dependent vibrational predissociation spectroscopy (Chaudhuri et al. J. Chem. Phys. 2000, 112, 7279). Revealed by the calculations, the lowest energy pathway for the transition from an open noncyclic hydronium-centered isomer [H 3 O + (CH 3 OH) 4 ] to a linear methyloxoium-centered isomer [CH 3 OH 2 + (CH 3 OH) 3 H 2 O] involves three stable intermediates and four transition states. The transition can go through either all four-membered ring isomers or a mixture of four-membered and five-membered ring intermediates. The latter is an energetically more favorable process because of less strain involved in the five-membered ring formation. A barrier height of <2.5 kcal/mol (after zero-point energy corrections) is predicted, suggesting that rapid interconversions among different isomers can occur at room temperature for this particular cluster cation.

Competitive solvation of the excess proton in protonated mixed methanol−water clusters [H+(CH3OH) m (H2O) n , m + n = 4] has been characterized by vibrational predissociation spectroscopy in combination with density functional theory calculations. The solvation topology of the clusters can be classified as (1) the closed shell, in which a hydronium ion H3O+ is fully solvated by three neutral molecules forming a complete solvation shell, and (2) the open chain, where the excess proton is tugged between two mixed subunits in a linear chain. The existence of these two types of isomer is verified from a close examination of the characteristic free-OH and hydrogen-bonded-OH stretching modes in the spectra. It is found that sequential replacement of the water molecule in H+(H2O)4 by methanol redistributes the population between the closed-shell and the open-chain isomers. While the excess proton is preferentially taken by methanol (instead of water) in the chain configuration, it can be either localized as CH3OH2 + or delocalized as CH3OH···H+···CH3OH at m ≥ 2, depending sensitively on the number of the methanol molecules and the symmetry of the cluster isomers. In contrast to that of NH4 +(NH3) m (H2O) n , m + n = 4, previously studied, this work provides a clear picture of competitive solvation of a charge between the constituent solvent molecules within a cluster.