Infrared spectroscopy of hydrated benzene cluster cations, [C6H6-(H2O)n]+ (n = 1–6): Structural changes upon photoionization and proton transfer reactions

Abstract

Infrared (IR) spectra of benzene–(water)_n cluster cations (Bz–W_n)⁺ (n = 1–6) in the OH and CH stretching vibrational region were observed in order to investigate their structure and reactivity. The cluster cations were prepared by two different production methods: one is due to collision between bare benzene cations and water clusters; and the other utilizes resonance enhanced multiphoton ionization (REMPI) of neutral clusters. The former method prefers the production of the most stable isomer cluster cations, while the latter would reflect the Franck-Condon restriction in the ionization process. The structures of the n = 1 and n = 2 clusters were determined on the basis of the comparison between the IR spectra and density functional theory (DFT) calculations. In the n = 1 cluster cation, the oxygen atom of the water molecule is located in the benzene ring plane and coordinates to the benzene moiety by two identical CH–O hydrogen bonds. The IR spectra of the n = 2 cluster cation showed absorption bands arising from two different types of isomers: one has a hydrogen-bonded water dimer interacting with the benzene cation; in the other isomer two water molecules are independently bound to the benzene cation. The production ratio between the isomers was found to strongly depend on the cluster ion preparation methods. Except for the case of the n = 2 cluster, the cluster cations prepared by the two different methods gave identical IR spectra. This means that quite extensive rearrangements of the cluster structure occur upon ionization of the neutral clusters, leading to the most stable form of the cluster cations. The spectral features of the n = 3 cluster cation are very similar to the n = 2 cluster, suggesting similar structures among these clusters. Higher clusters larger than the n = 3 cluster showed quite different IR spectra from those of the n ≤ 3 clusters, but their spectral features are very similar to those of hydrated clusters of protonated species, X–H⁺–(H₂O)_n, indicating that proton transfer reactions from the benzene cation to the water moiety occur in the larger clusters than those with n = 3.