Infra-red spectroscopic studies of rotational isomerism in the polymethylene halides

Infra-red spectra between 450 and 1500 cm-1 are presented for a series of polymethylene halides in the liquid and crystalline solid states. The spectra are discussed in terms of the different rotational isomeric forms of these molecules. A comparison of the observed carbon-halogen stretching frequencies with those of the n-propyl halides is used to determine the configurations of the C—C—C— X groups (X = halogen atom). The analysis shows that the solid-state isomers of a number of the tri-, tetra- and pentamethylene halides have non-planar skeletal structures. Crystalline trimethylene iodide exists in two forms, one of which is metastable; it is concluded that the metastable form has a planar skeleton (symmetry C2v) and the stable form a non-planar skeleton (symmetry C2). The solid-state isomers of the decamethylene halides probably have a fully extended planar zigzag configuration.