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Isolated CH Stretching Frequencies: An Aid to Differentiating Isomeric Species

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Abstract:

"Isolated" CH stretching frequencies [ν(CH)is], obtained from the gas-phase spectra of partially deuterated compounds or molecules which contain only one H atom, can be used to predict CH bond lengths, bond strengths, dissociation energies, and the equilibrium atomic charges on the H atom. The ability to predict these parameters with confidence arises from the gradients of the correlation plots, the fit of the data, and the precision and accuracy with which ν(CH)is can be obtained. In addition to ν(CH)is values, the number of absorption bands in the CH stretching region, together with their rotational contours, enables isolated CH vibrations to be used as a means of differentiating isomeric species, i.e., as a probe of molecular architecture. This possibility relies on the sum of the substituent effects being different for the isomers concerned and on the effects of conformation which can give rise to more than one CH stretching vibration in certain cases. For example, the potential barrier opposing internal rotation of the CHF2 group in CHF2CF2Cl is sufficiently large to produce two conformers, one in which the H atom is trans to the Cl and one in which it is trans to a F atom. These conformers give rise to two isolated CH stretching vibrations, as shown in Fig. 1. The type-C band at 2999.6 cm−1 is assigned to the conformer where the H atom is trans to a F atom and the type-A band, whose main peak is at 2986.4 cm−1, where it is trans to a Cl atom. However, CHFClCF3 produces only one isolated CH stretching vibration at 3001.3 cm−1, since the H is always trans to a F atom (Fig. 2).

Keywords: Infrared spectroscopy; Isolated CH stretching frequencies; Molecular conformation

Document Type: Short Communication

DOI: http://dx.doi.org/10.1366/0003702884430173

Affiliations: Research and Technology Department, Imperial Chemical Industries PLC, P.O. Box 90, Wilton, Middlesbrough, Cleveland, TS6 8JE, England

Publication date: August 1, 1988

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