Microsolvation of the ammonia cation in argon: I. Ab initio and density functional calculations of NH 3+–Ar n ( n=0–5)

Abstract

The intermolecular interaction and microsolvation process of the ammonia cation (NH 3 +, 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 NH 3 +–Ar dimer calculated at the MP2 level corresponds to a planar proton-bound H 2NH +–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 R e =2.22 A ̊ . 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 D e =866 cm −1 and D 0=672 cm −1 , and a N–Ar separation of R e =2.95 A ̊ . The planar side-bound structure, in which the Ar ligand is attached to the side of the NH 3 + triangle (C 2v symmetry, D e =813 cm −1 , D 0=726 cm −1 , R N– Ar =3.24 A ̊ ) , 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 NH 3 + ion is dominated almost exclusively by induction forces for all angular orientations. The most stable structures of larger NH 3 +–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 NH 3 + 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 XH 3 + ion by nonpolar spherical ligands. Comparison between XH 3 +–Ar n (X=C, N, O) and NH k +–Ar n ( k=2–4) reveals the influence of the electron density in the 2p z orbital of XH 3 + and the number of equivalent protons in NH k + on the dimer PES and the microsolvation process.