Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan.
Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan.
Phys Chem Chem Phys. 2018 Jun 6;20(22):14971-14991. doi: 10.1039/C7CP08072G.
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.
由于醇的氢键配位性质，其可能的氢键网络结构被归类为仅少数几种类型。因此，醇的气相团簇可以作为一个非常简单的模型体系来研究氢键网络的性质，例如随着团簇尺寸和温度的变化而发生的结构发展。在这个视角下，我们专注于质子化短链醇团簇的结构研究，其过量质子（电荷）使大小选择性光谱与质谱技术相结合成为可能。大小选择性红外光谱和理论多尺度异构体搜索被应用于甲醇（短链醇的原型）的质子化团簇，详细阐明了它们的氢键网络发展。量子谐波叠加近似预测了随着温度升高完全异构体种群的转换，这种异构体的转换通过观察到的红外光谱中显著的温度（内部振动能）依赖性得到证实。将质子化甲醇的温度依赖性特征与水和中性甲醇的温度依赖性特征进行了比较。此外，基于对质子化甲醇的研究结果，讨论了甲醇化离子的可能氢键网络。惰性气体标记的团簇内部能量的逐步变化得到了证明。还讨论了大团簇中氢键网络向块状网络的收敛。通过比较它们的红外光谱与质子化甲醇团簇的红外光谱，确定了更长的正烷基链醇（乙醇、1-丙醇、1-丁醇和 1-戊醇）的质子化簇的氢键结构。结果表明，尽管也发现了一些例外情况，但正烷基链对最稳定的氢键结构只有轻微的干扰。这些例外情况为进一步的理论和计算研究提供了很好的模型体系。
