Isomer dependence of HF vibrational frequency shift for ArnHF (n=4–14) van der Waals clusters: Quantum five-dimensional bound state calculations

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Abstract
The HF vibrational frequency shifts for ArnHF van der Waals (vdW) clusters with n=4–14 are predicted to be strongly isomer-specific, providing distinct spectroscopic signatures for different cluster isomers. This represents an extension of our recent studies of the size dependence of the vibrational frequency shift for ArnHF clusters [J. Chem. Phys. 101, 6359, 10 181 (1994)]. The HF vibrational frequency shifts calculated for the two or three lowest-energy isomers of each cluster size considered differ by at least a couple of wave numbers. Their relative magnitudes directly reflect the number of Ar atoms that each ArnHF isomer has in the first solvation shell around HF. The calculations are performed on pairwise additive intermolecular potential energy surfaces constructed from spectroscopically accurate Ar–Ar and anisotropic Ar–HF potentials. In the frequency shift calculations, the Arn subunit is treated as rigid, frozen in the geometry of one of the global or local ArnHF minima found previously by simulated annealing [J. Chem. Phys. 100, 7166 (1994)]. The 5D coupled intermolecular vibrational levels of what is now effectively a floppy Arn–HF dimer, are calculated highly accurately by the quantum 5D bound state methodology which is described in detail. The 5D vdW vibrational zero-point energy of the ArnHF cluster affects significantly the energy gap between various isomers.