Ion pair formation in water clusters: a theoretical study

Two hydrated ion pairs of the formula (H3O+)(H2O)3 (OH−) are studied using Hartree-Fock and MP2 wavefuctions with a 6-311 + + G∗∗ basis set. Two neutral water pentamers are also studied for comparison. Thermodynamics, IR spectra, dipole moments, rotational constants and spatial extent of the four clusters are examined in detail. Agreement of calculated thermodynamic quantities and IR spectra with experimental values is significant. The hydrated ion pairs are stablized more by electron correlation than the neutral species. This can be partially explained in terms of the strained configuration of these hydrated ion pairs. The hydrated ion pairs also have a suprisingly small calculated dipole moment.