Infra-Red Spectrum of C_nD_{2n + 2} and the 'Long-Chain Frequency' in Paraffins

Abstract

THE most characteristic feature in the infra-red spectrum of any long-chain paraffin (C_nH_{2n + 2}) is a strong band near 14µ (725 cm.^-1). It was first noted by Coblentz¹, but Lecomte² and many other workers have directed attention to it, while an extensive investigation of hydrocarbons at Oxford and Cambridge³ during the War showed that any molecule containing the group —(CH₂)_n.CH₃, in which n is equal to or greater than 3, possesses this band. So far as we are aware, no satisfactory interpretation of this 'long-chain frequency' has yet been given⁴, although it has been frequently assumed that it arises from a 'skeletal' vibration of the carbon atoms in the chain. The difficulty about this assignment is that the magnitude of the frequency is rather low for it to belong to a stretching mode of the carbon chain, and distinctly too high for it to be a deformation vibration. Recently, it has been suggested⁵ that it is due to the overtone of a deformation vibration.