The intermolecular potential of NH4+-Ar I. Calculations for the internal rotor structure of the nu3 band

The intermolecular potential energy surface of the electronic ground state of the ammonium-argon ionic dimer, NH4+-Ar, is calculated by ab initio methods using different levels of theory (MP2, MP4) and basis sets (aug-cc-pVXZ, X = D/T/Q). The deformation of the ammonium ion in the complex is shown to be small and its geometry is therefore fixed in these calculations to the tetrahedral structure optimized for the bare ion. The global minimum of the potential corresponds to a proton-bound structure with C3upsilon symmetry (Re approximately 3:4 A, De approximately 950cm-1) and the barrier to internal rotation between the four equivalent minima is around 200cm-1. The threedimensional potential is expanded in tetrahedral harmonics whose radially dependent coefficients, Vi (R), are compared for the considered levels of theory. The rotationintermolecular vibration Hamiltonian is solved using a two-dimensional fixed-R representation of the calculated potentials, Vi equals Vi(Reff), where the effective intermolecular separation, Reff, is determined from the experimental rotational constants of the complex. The accuracy of these parametrized potential energy surfaces is judged by their ability to reproduce the hindered rotor subband structure in the experimental upsilon3(t2) infrared band of the complex. The simulations using the potentials calculated at the MP2/aug-cc-pVTZ or higher levels of theory reproduce the coarse structure of the experimental spectrum well. Further improvement could be achieved by least-squares fitting the potential parameters to the observed subband positions. The fitted V3 and V4 parameters remain in close agreement with those determined from the ab initio calculations but the anisotropy of the potential is significantly different from that in a previous least-squares fit of V3 alone.