Model stretching overtone eigenvalues for SF6, WF6, and UF6

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Abstract
A four parameter potential model for the stretching modes of XY6 octahedra is shown to give an excellent fit to the published experimental vibrational eigenvalues of SF6, WF6, and UF6. A new technique for the automatic computation of symmetrized local mode basis functions is used to generate eigenvalues, converged to within 1 cm−1, for overtone and combination states up to vtot=v1+v2+v3=6 for WF6 and UF6. Similar convergence for SF6 is possible only up to vtot=4 due to axial momentum transfer arising from the relatively low central mass, which renders a local mode basis less convenient for this molecule. The most striking result for both WF6 and UF6 is that the highest component of the nν3 manifold for n≤6 appears to lie within 1 cm−1 of n multiples of the ν3 fundamental. The continuation of such a pattern up to dissociation would imply that no special anharmonic resonances are needed to account for the infrared multiphoton dissociation.