How Does Microsolvation of Protonated Methanol Clusters by Aprotic Molecules Converge to Solvation in Solutions?: Infrared Spectroscopy of H+(Methanol)n-(Benzene)m (n = 2–5, m = 2 and 3) Clusters in the Gas Phase

Infrared spectroscopy of protonated methanol clusters H⁺(methanol)_n in benzene solution has been studied by Stoyanov et al. (Chem. Eur. J. 2008, 14, 3596–3604). In H⁺(methanol)_n, one-dimensional hydrogen-bonded chains extend in two directions from the central protonated site, and the OH group at each end of the hydrogen-bonded chains forms a π–hydrogen bond with the solvent benzene. It has been reported that the OH stretching frequencies of these π–hydrogen bonded ends change with the cluster size (n), reflecting the influence of the excess proton at the center of the cluster. To investigate the convergence process from the gas phase to the liquid phase through the progression of microsolvation, we performed infrared spectroscopy of gas-phase H⁺(methanol)_n-(benzene)_m (n = 2–5, m = 2, 3) clusters and compared the results with our previously reported results for m = 1 and with those in the benzene solutions. The π–hydrogen bonded OH stretching frequencies show a significant difference from the benzene solutions for m = 1, but the difference is greatly reduced for m = 2, where both the ends of the methanol hydrogen-bonded chains are solvated, indicating that anticooperative effects play a significant role until the completion of full solvation of the hydrogen bond network. The third benzene molecule directly solvates the protonated site, but there is still a non-negligible difference from the benzene solutions, suggesting that at least two benzene molecules are effectively involved in the direct solvation of the protonated site of the cluster in the benzene solutions.