Two-Dimensional H2O−Cl2 and H2O−Br2 Potential Surfaces:  An Ab Initio Study of Ground and Valence Excited Electronic States

All electron ab initio calculations for the interaction of H2O with Cl2 and Br2 are reported for the ground state and the lowest triplet and singlet Π excited states as a function of both the X−X and O−X bond lengths (X = Cl or Br). For the ground state and lowest triplet state, the calculations are performed with the coupled cluster singles, doubles, and perturbative triple excitation level of correlation using an augmented triple-ζ basis set. For the 1Π state the multireference average quadratic coupled cluster technique was employed. For several points on the potential, the calculations were repeated with the augmented quadruple-ζ basis set. The ground-state well depths were found to be 917 and 1183 cm-1 for Cl2 and Br2, respectively, with the triple-ζ basis set, and they increased to 982 and 1273 cm-1 for the quadruple-ζ basis set. At the geometry of the ground-state minimum, the lowest energy state corresponding to the unperturbed 1Π states of the halogens increases in energy by 637 and 733 cm-1, respectively, relative to the ground-state dissociation limit of the H2O−X2 complex. Adding the attractive ground-state interaction energy to that of the repulsive excited state predicts a blue-shift, relative to that of the free halogen molecules, of ≈1600 cm-1 for H2O−Cl2 and ≈2000 cm-1 for H2O−Br2. These vertical blue-shifts for the dimers are greater than the shift of the band maximum upon solvation of either halogen in liquid water.