Transferability of Urey-Bradley Force Constants. III. The Vinylidene Halides

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Abstract
Force constants have been calculated by a least‐squares method for vinylidene bromide and vinylidene fluoride using complete deuterium isotopic data and a Urey‐Bradley potential function. The results of this work show that the ``C=C stretching'' and ``CH2 scissors'' modes are not pure vibrations but exhibit appreciable coupling between C=C stretching and CH2 bending coordinates. Further, the molecules within the vinylidene‐halide series do not seem to have constant normal coordinates and hence the frequency differences are not entirely associated with changes in the force field. It follows that in molecules of this type, it is dangerous to infer changes in bond strength from observed frequency shifts except with the aid of a complete normal‐coordinate analysis.