Microsolvation of the ammonia cation in argon:: I. Ab initio and density functional calculations of NH3+-Arn (n=0-5)

The intermolecular interaction and microsolvation process of the ammonia cation (NH3+, ammoniumyl) in its (2)A(2)" ground electronic state with up to five Ar ligands are investigated by quantum chemical ab initio and density functional calculations at the unrestricted HF, MP2, and B3LVYP levels of theory using a basis set of aug-cc-pVTZ quality. The global minimum of the intermolecular potential energy surface (PES) of the NH3+-Ar dimer calculated at the MP2 level corresponds to a planar proton-bound H2NH+-Ar structure with C-2v symmetry. The linear N-H-Ar bond is characterized by binding energies of D-e = 1133 cm(-1) and D-0 = 897 cm(-1) and an intermolecular H-Ar separation of Re = 2.22 Angstrom. The p-bound structure, in which the Ar ligand is attached to the partially filled 2p(z) orbital of nitrogen along the C-3 rotation axis, is a slightly less stable local minimum with C-3v symmetry, dissociation energies of De = 866 cm(-1) and Do = 672 cm(-1), and a N-Ar separation of Re = 2.95 Angstrom. The planar side-bound structure, in which the Ar ligand is attached to the side of the NH3+ triangle (C-2v symmetry, D-e = 813 cm(-1), D-0 = 726 cm(-1), RN-Ar = 3.24 Angstrom), is a transition state for in-plane internal rotation connecting two equivalent H-bound global minima with a low barrier of V-b = 320 cm(-1). Analysis of the charge distributions shows that the attraction between Ar and the open-shell NH3+ ion is dominated almost exclusively by induction forces for all angular orientations. The most stable structures of larger NH3+ -Ar-n clusters (n = 1-5) are highly symmetric and form two distinct subshells within the first solvation shell. The first three Ar ligands form equivalent (nearly) linear proton bonds to the three protons of NH3+ leading to structures with C-2v and D-3h symmetry. The next two Ar ligands are attached to opposite sides of the 2p(z) orbital of N leading to geometries with C-3v and D-3h symmetry, respectively. The intermolecular proton bonds significantly destabilize the intramolecular N-H bonds, whereas the p-bonds strengthen them slightly. Noncooperative effects are observed for this prototype solvation of a XH3+ ion by nonpolar spherical ligands. Comparison between XH3+ -Ar-n (X = C, N, O) and NHk+ -Ar-n (k = 2-4) reveals the influence of the electron density in the 2p(z) orbital of XH3+ and the number of equivalent protons in NHk+ on the dimer PES and the microsolvation process. (C) 2002 Elsevier Science B.V. All rights reserved.