Because of the hydrogen bond coordination properties of alcohols, their possible hydrogen bond network structures are categorized into only a few types. Therefore, gas phase clusters of alcohols can be a very simple model system to examine the properties of hydrogen bond networks, such as structural development with cluster size and temperature dependence. In this perspective, we focus on the structural study of protonated short-chain alcohol clusters, whose excess protons (charge) enable size-selective spectroscopy in combination with mass spectrometric techniques. Size-selective infrared spectroscopy and a theoretical multi-scale isomer search were applied to protonated clusters of methanol, which is a prototype of short-chain alcohols, and their hydrogen bond network development is elucidated in detail. Complete isomer population switching with increasing temperature was predicted by the quantum harmonic superposition approximation and this isomer switching was evidenced by the remarkable temperature (internal vibrational energy) dependence of the observed infrared spectra. The characteristics of the temperature dependence of protonated methanol were compared with those of water and neutral methanol. In addition, possible hydrogen bond networks of methanolated ions were discussed on the basis of the results for protonated methanol. Stepwise changes in the internal energy of clusters with inert gas tagging are demonstrated. Convergence of the hydrogen bond network to the bulk-like network in large clusters is also discussed. The hydrogen bond structures of the protonated clusters of longer normal alkyl chain alcohols (ethanol, 1-propanol, 1-butanol, and 1-pentanol) are determined by comparison of their infrared spectra with those of the protonated methanol clusters. It is demonstrated that the normal alkyl chain interferes only slightly with the most stable hydrogen bond structure, although a few exceptional cases were also found. These exception cases serve as good model systems for further theoretical and computational studies.

Structural isomer population of a hydrogen-bonded cluster generally depends on temperature. Therefore, determination of an isomer population profile in a wide temperature range is important to understand the nature of hydrogen bond networks of the cluster. To explore an isomer population profile, stepwise changes of internal vibrational energy of a protonated hydrogen-bonded cluster are performed by inert gas tagging. We observe infrared spectra of the protonated methanol pentamer with various tag species. The bare protonated methanol pentamer practically has only two possible isomer types. With the tagging, the relative population of the two isomer types changes according to the binding energy with the tag species. The observed relative population follows its theoretically predicted temperature dependence.

Although many studies have been reported on structures of neutral water clusters, most of experimental information has been restricted to their most stable structures. With elevation of temperature, however, transient structures as well as higher energy stable structures can be formed, as recently demonstrated in small-sized neat water clusters (Zischang, J.; Suhm, M. A. J. Chem. Phys. 2014, 140, 064312). In the present study, we performed infrared spectroscopy of warm phenol-(H2O)2, which is an analogue of (H2O)3 concerning the hydrogen-bond structure, in order to overcome the size uncertainty in the neat water cluster study. The strict size selection was achieved by infrared-ultraviolet double-resonance spectroscopy combined with mass spectrometry. The temperature control of the cluster was accomplished by the reduction of the stagnation pressure of the jet expansion and the internal energy selective detection of the cluster. A remarkable shift to higher frequency of the phenolic OH stretch band was observed with elevation of temperature, suggesting deformation of the cluster from the most stable cyclic structure to the transient chain-type structure.