The Spectroscopic Features of Ionized Water Medium: Theoretical Characterization and Implication Using (H2O)n+, n=3–4, Cluster Model

Abstract

AbstractA model using (H2O)n+, n=3–4, clusters was adopted to study the spectroscopic features of ionized water medium. The finite clusters were characterized using a systematic searching algorithm to locate the stable minimum structures. The electronic transition features of the minimum structures were calculated using Symmetry Adapted Cluster/Configuration Interaction (SAC‐CI) and Time‐Dependent Density Functional Theory (TDDFT) approaches. A new type of minimum structure – SolCat, a cationic water monomer being directly solvated by two proton‐acceptor solvent molecules, was identified in addition to the proton transfer (PT) and hemibonding (Hm) isomers. CCSD(T)/aug‐cc‐pVTZ//CCSD/aug‐cc‐pVDZ was adopted to characterize the minimum structures of (H2O)n+, n=2–4, clusters, and this theory level provided the converged energetics for (H2O)2+ clusters and showed consistent results as the early EOM‐IP‐CC(2,3)//EOM‐IP‐CCSD calculations reported by Pieniazek et al. [J. Phys. Chem. A, 112, 6159–6170 (2008)]. For describing the larger size of clusters, BH&HLYP and BNL functionals were recommended for the economical geometry searching and sampling. The electronic transition features predicted by BH&HLYP and BNL functional gave the best vertical excitation energy with respect to the SAC‐CI method. Taking into account the population probability and the oscillator strength, the electronic transition of the transient but spectroscopically‐sensitive Hm and SolCat species could not be neglected at high temperature condition.