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

A 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.