Aqueous solvation of bihalide anions

Second-order Møller-Plesset perturbation theory (MP2) is used to perform geometry optimizations on XHX(-).(H(2)O)(n) for X = Br, I, with n = 1 to 6 water molecules. Of particular interest is the manner in which the solvent molecules orient themselves around the solute and which configurations are lowest in energy. Although for most values of n, water molecules may donate all of their hydrogen atoms for hydrogen bonding to the solute, this type of structure is the lowest in energy only for n = 0 to 2 and is only a local minimum for n = 3, 4, and 6. For n = 5, this type of structure is a saddle point. Coupled cluster single-point calculations at the MP2 geometries are used to obtain accurate relative energies for all stationary points.