Are "Bright-State" Models Appropriate for Analyzing Fermi-Coupled Bands in Molecular Vibrational Spectra?

Bright-state models are often applied to "deperturb" Fermi-coupled bands in molecular vibrational spectra, in cases where a harmonically forbidden transition "borrows" intensity from an energetically nearby allowed transition. However, forbidden transitions can also acquire intensity through anharmonic couplings on the potential energy surface ("mechanical anharmonicity") or dipole moment surface ("electrical anharmonicity") that are not accounted for within the bright-state model. In this work, we compare deperturbation shifts obtained by analysis of experimental data with those predicted using the bright-state model, for a series of discrete encapsulated chloride hydrate isotopomers. Predicted band center shifts and Fermi coupling matrix elements obtained using the bright-state model are larger than those estimated from experimental data.