Microsolvation of the Water Cation in Argon: I. Ab Initio and Density Functional Calculations of H2O+−Arn (n = 0−4)

Abstract

The intermolecular interaction and microsolvation process of the water cation in its 2B1 ground electronic state with up to four Ar ligands are investigated with quantum chemical ab initio and density functional calculations at the unrestricted HF, MP2, and B3LYP levels of theory using a basis set 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 HOHAr global minimum. The slightly translinear ionic hydrogen bond is characterized by a binding energy, D0 ∼ 2200 cm-1, an H−Ar separation, Re ∼ 1.92 Å, and a bond angle, φe ∼ 176°. The p-bound structure, with the Ar atom attached in a T-shaped fashion to the partially filled 2py orbital of oxygen, is a local minimum with D0 ∼ 1300 cm-1 and an O−Ar separation, Re ∼ 2.47 Å. The attraction in the H-bound structure is dominated by induction forces, whereas charge transfer from Ar to the 2py orbital of H2O+ provides a significant contribution to the stabilization energy of the p-bound isomer. In the most stable structures of H2O+−Arn (n = 1−4) the first two Ar ligands occupy H-bound sites and the next two ligands are located at the p-bound sites leading to geometries with Cs (n = 1,3) and C2v symmetry (n = 2,4), respectively.