The onset of dissociation in the aqueous LiOH clusters: a solvation study with the effective fragment potential model and quantum mechanics methods

Abstract

The first systematic study on the solvation of the LiOH(H 2O) n ( n=1–6,8) clusters is herein presented by using both the molecular-mechanics effective fragment potential (EFP) model and full quantum mechanics methods. Aqueous clusters are sequentially calculated first with the LiOH solute molecule described by a restricted Hartree–Fock (RHF) wavefunction and the H 2O solvent molecules by the EFP model (EFP/RHF), and then with the full quantum mechanics RHF and Møller–Pleset second-order perturbation (MP2) theories. Calculated properties include equilibrium geometries, Mulliken charges, bond lengths and orders, and relative energies inter alia. Present results indicate that at the least six H 2O molecules are necessary to be added to one LiOH molecule to cause its spontaneous dissociation into a Li +/OH − separated ion pair. Another instance of dissociation is also observed in one of the LiOH(H 2O) 8 isomers. EFP/RHF reasonably reproduces RHF structural properties and dissociation patterns but discrepancies arise in the solute charge description. Comparisons of the present theoretical results with previous EFP studies and with a few experimental data are also discussed.