Studies on determination of degree of coupling of Fermi resonance

Fermi resonance is a complicating factor in analyzing polyatomic vibrational spectra since it obscures the sequencing for assignments. In this Letter we propose a new method to decouple Fermi resonant wavefunctions back to ‘pure’ vibrational wavefunctions. The method utilizes the observation that among the linear combinations of Fermi resonant wavefunctions, only ‘pure’ wavefunctions can reach the extension extremes (both maxima and minima) in the configurational space. Besides tested in a model system, the method is also applied to the hydroperoxyl radical, HO2, system for which the accurate coupling coefficients are identified and compared with the previous reports.