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Analysis of previous calculations indicates that the observations of an irregular overtone/combination spectrum implies increasingly close local-mode near-degeneracies in the higher overtone states. This behaviour is accurately reproduced by a model of two harmonically coupled anharmonic oscillators, when the anharmonicity parameter exceeds the coupling strength, the effect being independent of the momentum or potential origin of the coupling. The same model reproduces familiar normal mode behaviour in the opposite limit. Applications of the model indicate local-mode behaviour for H2O and C2H2 and normal mode features for C2D2 and SO2, the dominant inter-bond coupling in all cases except H2O being due to cross-terms in the kinetic-energy operator. In H2O such momentum coupling is combined with an approximately equal potential coupling contribution.
Child et al. (Thu,) studied this question.