Equilibrium structures and approximate HF vibrational red shifts for ArnHF (n=1–14) van der Waals clusters

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Abstract
This paper presents a theoretical study of the size evolution of equilibrium structures and approximate HF vibrational red shifts for ArnHF van der Waals clusters, with n=1–14. Pairwise additive ArnHF intermolecular potential energy surfaces were constructed from spectroscopically accurate Ar–Ar and anisotropic Ar–HF potentials. The latter depend on vibrational excitation of the HF monomer. The global and energetically close‐lying local minima of ArnHF, n=1–14, for HF v=0 and v=1, were determined using simulated annealing followed by a direct minimization scheme. For ArnHF clusters with n≤8, the lowest‐energy structure always has HF bound to the surface of the Arn subunit. In contrast, for n≥9, the global minimum of ArnHF corresponds to HF inside a cage. Ar12HF has the minimum‐energy configuration of an HF‐centered icosahedron, which appears to be unusually stable. Size dependence of the HF vibrational red shift in ArnHF (n=1–14) clusters was investigated by means of a simple approximation, where the red shift was represented by the energy difference between the global minima of a cluster obtained for HF v=0 and v=1, respectively. The approximation reproduced rather accurately the experimentally determined variation of the ArnHF red shift with the number of Ar atoms, for n=1–4, although it overestimated their magnitude. For larger ArnHF clusters, 4<n≤14, a nonmonotonic, step‐like dependence of the red shift on the cluster size is predicted, which can be interpreted in terms of changes in the minimum‐energy cluster geometries. The predicted red shift for the icosahedral Ar12HF, where the first solvation shell is full, is 44.70 cm−1, which is only 5.4% higher than the experimental HF vibrational red shift in an Ar matrix, of 42.4 cm−1.