Decoupled isotopomer vibrational frequencies in cubic ice: A simple unified view of the Fermi diads of decoupled H2O, HOD, and D2O

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Abstract
The existing infrared spectroscopic data for the isotopomers of water [H2O, D2O, (HOD)2, and HOD] decoupled in cubic ice at 90 K are reviewed and combined with new results to complete the infrared data for the internal vibrational modes. An assignment of the observed absorption bands, including the perturbed Fermi diads for νs in resonance with 2ν2, that largely follows established views is offered. This assignment is shown to be internally self-consistent by the analysis of the Fermi diads within a single framework based on the simplest representation of the effects of Fermi resonance and using the Burneau–Corset value for the Fermi-interaction parameter appropriate to cubic ice (60 cm−1 for H2O). It is shown that the inclusion of the decoupled-HOD diads significantly lowers the estimated value of νs for H2O and, consequently, allows the downshift of this mode to the observed frequency value (3225 cm−1), through resonance with 2ν2 (where ν2=1735 cm−1), to be closely modeled using the relatively small Fermi-interaction parameter of Burneau and Corset.