Intermolecular potentials and rovibrational energy levels of the Ar complexes with HCN and HCCH

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Abstract
The intermolecular potential surfaces for ArHCN and ArHCCH are computed by Mo/ller–Plesset perturbation theory at the fourth-order approximations (MP4) with a large basis set containing bond functions. Rovibrational energies and spectroscopic constants of the two systems are computed from the intermolecular potentials using the collocation method. The intermolecular potential for ArHCN at the MP4 level has a single minimum at the collinear Ar−H−C−N configuration (R=4.56 Å, θ=0°) with a minimum potential energy of Vm=−135.9 cm−1. The bending frequencies, rotational constants, and centrifugal distortion constants of ArHCN and ArDCN calculated using the MP4 potential are in good agreement with experiment. Rovibrational energies with J=0 through 6 arising from j=0 and j=1 levels of HCN are calculated and compared with the experimental transition frequencies. The intermolecular potential surface for ArHCCH has a symmetric double minimum near the T-shaped configuration. The minimum positions at the MP4 level are (R=4.05 Å, θ=60° and 120°) and the minimum potential energy is Vm=−110.9 cm−1. The rotational constants and bending frequency of ArHCCH arising from the MP4 potential are calculated and compared with experiment. The anisotropy of the MP4 potential is slightly underestimated. The effects of monomer bending vibration on the ArHCN and ArHCCH potentials are studied by additional calculations. The potential anisotropy of ArHCN decreases, whereas that of ArHCCH increases as the monomer vibration is taken into account. This might be partially responsible for the discrepancies between the theoretical predictions and experiment.