Isolated C–H stretching vibrations of <i>n</i>-alkanes: Assignments and relation to structure

Abstract

The conformational/configurational dependence of the frequencies of the deuterium-isolated C–H stretching modes of the gas-phase alkanes C1, C2, n-C3, n-C4(t and g), n-C5(tt and gt), cyclo-C6, iso-C4, and neo-C5 are reported. Most of the isolated C–H frequencies were obtained from Raman spectra of specifically and randomly protonated deuterohydrocarbons. An extraordinarily precise correlation is found between the observed isolated C–H frequencies and the corresponding ab initio calculated C–H bond lengths. In the case of the n-alkanes, the observed C–H frequencies tend to fall in clusters that are regularly spaced with an average separation of about 14.5±1 cm−1. The clustering occurs because the isolated C–H stretching frequencies are determined by the structure of the n-alkane in the immediate vicinity of the C–H bond. The relation between frequency and local structure can be expressed in a simple way and used to predict the effect of conformational change.