Local mode and normal mode models for molecules with two non equivalent C-H bonds
Résumé
For molecules with C-H bonds, a local mode description of the bonds is often adopted to interpret the C-H stretching overtone spectra, although a representation in terms of normal mode is also possible. Equivalence between the two alternative representations demands simple numerical equations between the normal mode anharmonicity constants xrs and the quartic anharmonic coupling constants Krstu , the so-called x-K relations. The relations appropriate to low symmetry molecules with two non-equivalent C-H bonds (ν1 , ν2), such as XHC=CHY, are derived in this paper. The consequences of these relations are illustrated with a detailed analysis of the C-H stretching overtone region, where the naive application of the relations obtained for more symmetric molecules could lead to a different interpretation of the overtones 2ν1 , 2ν2 and of the combination band ν1 + ν2 . The relations are tested with theoretical calculations for difluoro- and chlorofluoro-substituted ethenes, propadiene, and butatrienes.
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