Microsolvation of the water cation in argon: I. Ab initio and density functional calculations of H2O+-Ar-n (n=0-4)

The intermolecular interaction and microsolvation process of the water cati on in its B-2(1) ground electronic state with up to four Ar ligands are inv estigated with quantum chemical ab initio and density functional calculatio ns at the unrestricted HF, MP2, and B3LYP levels of theory using a basis se t of aug-cc-pVTZ quality. The intermolecular potential energy surface (PES) of the H2O+-Ar dimer calculated at the MP2 level features a planar proton (H)-bound H-O-H-Ar global minimum. The slightly translinear ionic hydrogen bond is characterized by a binding energy, D-0 similar to 2200 cm(-1), an H -Ar separation, R-e similar to 1.92 Angstrom, and a bond angle, phi (e) sim ilar to 176 degrees. The p-bound structure, with the Ar atom attached in a T-shaped fashion to the partially filled 2p(y) orbital of oxygen, is a loca l minimum with D-0 similar to 1300 cm(-1) and an O-Ar separation, R-e simil ar to 2.47 Angstrom. The attraction in the H-bound structure is dominated b y induction forces, whereas charge transfer from Ar to the 2p(y) orbital of H2O+ provides a significant contribution to the stabilization energy of th e p-bound isomer. In the most stable structures of H2O+-Ar-n (n = 1-4) the first two Ar ligands occupy H-bound sites and the next two ligands are loca ted at the p-bound sites leading to geometries with C-s(n = 1,3) and C-2 nu symmetry (n = 2,4), respectively.