Temperature and Size Dependence of Characteristic Hydrogen-Bonded Network Structures with Ion Core Switching in Protonated (Methanol)_6–10–(Water)₁ Mixed Clusters: A Revisit

Hydrogen-bonded network structures and preferential ion core in the protonated methanol–water mixed clusters, H⁺(methanol)_n–(water)₁ (n = 6–10), were explored by a combination of infrared spectroscopy and theoretical calculations. Infrared spectra of the OH stretch region of the clusters were measured at the two different temperature ranges by using Ar-tagging. Stable isomer structures of the clusters were searched by the multiscale modeling approach and temperature dependent infrared spectra were simulated based on the statistical populations of the isomers. The combined experimental and theoretical studies revealed that the characteristic multiring structures begin to form at n = 7 under the low temperature condition and they are preferential at the wide temperature range in n ≥ 8. It was also demonstrated that the preferential ion core type changes from methanol (MeOH₂⁺) to water (H₃O⁺) with increasing cluster size. In n ≤ 8, the observed infrared spectral features partly depend on the monitoring vibrational predissociation channel, and weak correlations between the hydrogen-bonded network structure and preferential dissociation channels were suggested. However, the ion core type does not necessarily correlate to the preferential dissociation channel. This implies that large rearrangement of the hydrogen-bonded network structure occurs prior to the dissociation.