Benchmark computational investigations for the basic model of the salt-water complex: NaCl(H₂O) and its anion...

The microsolvation of salts in water is a fundamental physicochemical process. In this work, the aqueous salt complex NaCl(H₂O) and its anion NaCl(H₂O)^- were investigated using comprehensive calculations, including the costly and accurate CCSD(T)-F12a and focal point analysis (FPA) methods. For the neutral NaCl(H₂O), three isomers exist, two of which are mirror-symmetric with almost identical structures and their corresponding anions are also mirror-symmetric. For the NaCl(H₂O)^- anion, there are four isomers. Several transition states are found for the first time. The structural rearrangements of neutral NaCl(H₂O) and NaCl(H₂O)^- anions are mainly caused by breaking and forming of the hydrogen bonds and the enhancement and weakening of interactions between Na and O atoms. The distributions of the anion complexes from 15-300 K are computed and compared to recent experimental results. The analysis of the intermolecular weak interactions shows the weak van der Waals interactions between Na and O atoms, as well as hydrogen bonding between H and Cl. Moreover, the theoretically predicted anion photoelectron spectra are assigned and analyzed in detail, and they agree with experimental spectra satisfactorily. The Na-Cl stretching vibrational mode dominates the vibrational structure in both anion spectra with some minor contributions from the intermolecular motions between H₂O and NaCl.