Quantitative Temperature Dependence of the Microscopic Hydration Structures Investigated by Ultraviolet...

To discuss the temperature effect on microscopic hydration structures in clusters, relative populations of the isomers having different hydration structures at well-defined temperatures are quite important. In the present study, we measured ultraviolet photodissociation spectra of the temperature-controlled hydrated phenol cation [PhOH(H₂O)₅]⁺ trapped in the 22-pole ion trap. Two isomers having a distinct hydration motif with each other are identified in the spectra, and a clear change in the relative populations is observed in the temperature range from 30 to 150 K. This behavior is quantitatively interpreted by statistical mechanical estimation based on density functional theory calculations. A ring with tail-type hydration motif is dominant in cold conditions, whereas a chain-like motif is dominant in hot conditions. The present study provides very quantitative information about the temperature effect on the microscopic hydration structures.