Spectroscopic determination of the intermolecular potential energy surface for Ar–NH3

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Abstract
The three‐dimensional intermolecular potential energy surface (IPS) for Ar–NH3 has been determined from a least‐squares fit to 61 far infrared and microwave vibration–rotation–tunneling (VRT) measurements and to temperature‐dependent second virial coefficients. The three intermolecular coordinates (R,θ,φ) are treated without invoking any approximations regarding their separability, and the NH3 inversion–tunneling motion is included adiabatically. A surface with 13 variable parameters has been optimized to accurately reproduce the spectroscopic observables, using the collocation method to treat the coupled multidimensional dynamics within a scattering formalism. Anisotropy in the IPS is found to significantly mix the free rotor basis functions. The 149.6 cm−1 global minimum on this surface occurs with the NH3 symmetry axis nearly perpendicular to the van der Waals bond axis (θ=96.6°), at a center‐of‐mass separation of 3.57 Å, and with the Ar atom midway between two of the NH3 hydrogen atoms (φ=60°). The position of the global minimum is very different from the center‐of‐mass distance extracted from microwave spectroscopic studies. Long‐range (R≳3.8 Å) attractive interactions are greatest when either a N–H bond or the NH3 lone pair is directed toward the argon. Comparisons with ab initio surfaces for this molecule as well as the experimentally determined IPS for Ar–H2O are presented.

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