Conformational dependence of the isolated C-D stretching and CHD rocking vibrations of selectively monodeuterated alkyl chains as proved by conformational polymorphism of 1-bromopentane

Abstract

The Raman and infrared spectra of three selectively monodeuterated species of 1-bromopentane (1-bromopentane-1-d 1, -2-d 1 and -3-d 1) have been studied to examine the correlation between the isolated CD stretching and CHD rocking vibrations and the associated local conformation of the alkyl chain. The conformational polymorphism exhibited by this compound in the crystalline solid state makes unambiguous conformational assignments of the observed bands possible. The observed C-D stretching wavenumbers have been correlated with the C-D bond lengths calculated by the ab-initio MO method, and a linear relationship has been obtained between them. The present results on monodeuterated 1-bromopentanes show that the isolated CD stretching vibrations give different wavenumbers in the 2130–2230 cm −1 region, depending on the position of the deuterium substitution and the local conformation in the vicinity of the C-D bond. The isolated C-D stretching vibrations have great potential to be used as a probe for studying the conformational state of alkyl chains. The CHD rocking vibrations also provide useful key bands to study the chain conformation.