Benchmark computational investigations for the basic model of the salt-water complex: NaCl(H 2 O) and its anion NaCl(H 2 O) .

The microsolvation of salts in water is a fundamental physicochemical process. In this work, the aqueous salt complex NaCl(HO) and its anion NaCl(HO) were investigated using comprehensive calculations, including the costly and accurate CCSD(T)-F12a and focal point analysis (FPA) methods. For the neutral NaCl(HO), three isomers exist, two of which are mirror-symmetric with almost identical structures and their corresponding anions are also mirror-symmetric. For the NaCl(HO) anion, there are four isomers. Several transition states are found for the first time. The structural rearrangements of neutral NaCl(HO) and NaCl(HO) 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 HO and NaCl.