Theoretical study of ion–molecule potentials for He+ and Li+ with N2

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Abstract
High-level ab initio calculations have been carried out on the lowest charge-transferexcited state of [ HeN 2 ] + and the [ LiN 2 ] + ground state, over a range of intermolecular distances R of 3–20a 0 at a fixed N 2 bond length r e =2.074 30a 0 for three orientation angles γ=0°, 45°, and 90°. The calculations employed extended atomic basis sets, chosen to represent accurately the electrical properties of the interacting partners; for N 2 the key properties α ∥ , α ⊥ , and Θ are within 1.7%, 2.7%, and 2.1% of the best experimental values. All interaction energies were corrected for basis-set superposition error by the counterpoise method, and fitted by analytic forms incorporating the proper long-range expansion through R −7 . Our value for the tetrahexacontapole (2 6 -pole) moment of N 2 is −15.95 a.u. The most stable geometries for both systems occur for linear (γ=0°) complexes, with minima −D e of −7.00 (−12.65) kcal/mol located at R e =3.048 (2.610) Å for He + (Li + )+N 2 ; the Li + values are in good agreement with previous theoretical results. At small R the splitting ΔV(R, γ ) between the He + and Li + surfaces is found to have a radial dependence close to the prediction of the hydrogenic charge-transfer coupling model, while its anisotropy is similar to that of the surfaces themselves.