Effects of Na+ and Cl- on hydrated clusters by ab initio study

A comprehensive genetic algorithm is used to perform a global search for Cl-(H2O)1-9 and NaCl(H2O)1-9. The structural optimization, energy calculations, vibrational characteristics, and charge distribution were performed at an ab initio high-level theory. Combined with the calculation results of Na+(H2O)1-6 by Wang et al. [Front. Chem. 7, 624 (2019)] in our group, we systematically investigate these three systems at the same theoretical level. A comparison of bond lengths reveals that in Cl-(H2O)n, the inclusion of Na+ to form NaCl(H2O)n reduces the average distance between Cl- and H2O, indicating that Na+ has a stabilizing effect on ionic hydrogen bonds. Conversely, in Na+(H2O)n, the introduction of Cl- weakens the interactions between Na+ and H2O. In the NaCl(H2O)1-9 structures searched by the genetic algorithm, the ground-state configurations correspond to contact ion pairs, and the solvent-separated ion pair structures appear when n = 7. Furthermore, the anharmonic corrected infrared spectra of Cl-(H2O)1-5 and NaCl(H2O)1-4 exhibit good agreement with the experimental results. According to charge analysis of NaCl(H2O)n, it is observed that charge transfer primarily occurs from Cl- to H2O, resulting in the presence of negative charges on the water molecules. These findings are helpful to understand the effects of Na+ and Cl- on hydrated clusters at the molecular level.