Probing three-body intermolecular forces: Near-infrared spectroscopy of Ar2HF and Ar2DF van der Waals modes

Four intermolecular vibrational states of the weakly bound complexes Ar2HF and Ar2DF have been studied via high-resolution infrared spectroscopy. The vibrations are accessed as combination bands built on the v=1 HF or DF intramolecular stretch. These van der Waals vibrational states correlate adiabatically with j=1 motion of a hindered HF/DF rotor, corresponding to librational motion either in, or out of, the molecular plane. The vibrational origins of the Ar2HF in-plane and out-of-plane bends are 4008.9665(24) and 4035.174 41(86) cm−1, respectively, which are 62.374 and 88.582 cm−1 above the origin of the intermolecular ground state in the vHF=1 manifold. For Ar2DF, the in-plane and out-of-plane origins are 2939.836 63(4) and 2967.101 29(5) cm−1, respectively, which correspond to intermolecular bending frequencies in the vDF=1 manifold of 44.852 and 72.117 cm−1. Two-dimensional angular calculations are presented which solve for the hindered rotor HF/DF eigenfunctions and eigenvalues on a pairwise additive potential constructed using a rigid Ar2 framework; the predicted bending frequencies reproduce the correct energy ordering of the vibrational levels, but are systematically greater than experimentally observed. Rigorous full five-dimensional theoretical calculations of the intermolecular vibrational frequencies by Ernesti and Hutson [Phys. Rev. A 51 239 (1995)] on the full pairwise additive surface are found to be as much as 11% higher than the experimental values, indicating the presence of three-body repulsive contributions to the true angular potential. Inclusion of conventional three-body dispersion and induction terms can only account for a minority (≊1/3) of the observed discrepancies. The majority (≊2/3) of the vibrational shifts can be attributed to three-body ‘‘exchange’’ effects, i.e., the strongly anisotropic interaction of the HF/DF dipole with an exchange quadrupole formed by Ar–Ar. Inclusion of all three nonadditive terms (dispersion, induction, and exchange) improves the agreement with experiment by up to an order of magnitude.