Absorption of the CH 2 group in the region of 3μ

Abstract

We have previously recorded a few observations on the CH vibrations of some organic compounds in the 3μ region (Fox and Martin 1937), and indicated that while in general a CH group gave rise to a single C—H vibration band, and a CH 2 group to two such bands, in many molecules, especially those containing a benzene ring, the C—H units interacted in such a way as to give several bands, and in particular benzene itself and many mono-substituted benzenes gave three strong C—H bands which could be regarded as characteristic of the phenyl group. Coblentz (1905), in his extensive investigations of infra-red spectra, found that all compounds which contained C—H linkages had strong absorption in the region of 3μ. The persistence of frequencies characteristic of certain groups throughout a wide range of compounds has also been recognized from Raman spectra, and this persistency has received considerable emphasis from the work of Wulf and others (1935, 1936) on the characteristic infra-red bands of OH and NH groups in the region of 1·5μ. The bands observed were the first overtones of the fundamental vibrations, and it was found that the rest of the molecule, however complicated, had surprisingly little effect on the characteristics of the OH and NH bands. In a general way the explanation given was that each molecule contained a number of oscillating units, OH, CH, C=C, all loosely coupled together, the persistence of a given frequency through a variety of compounds indicating the looseness of this coupling.