Investigations of infra-red spectra. Determination of C-H frequencies (~3000 cm. -1 ) in paraffins and olefins, with some observations on “polythenes”

Abstract

In a previous communication (1938) we described the results of an investigation into the infra-red absorption in the region of 3 µ of a number of hydrocarbons dissolved in carbon tetrachloride, with special reference to the absorption of ⟩CH 2 groups in different molecules. It was found that in many simple compounds the CH 2 group gave rise to two frequencies, essentially C-H valency vibrations, about 2857 and 2927 cm. -1 , and that from one compound to another these frequencies varied by only a few wave numbers. The lower frequency was identified with the mode of vibration in which the hydrogen atoms move in phase, while the other frequency was taken as the unsymmetrical mode of vibration. This assignment was substantiated by calculations with potential functions for molecules of the general type CH 2 — X , where X represents the rest of the molecule and is attached to the CH 2 group by single bonds. It was found that the CH frequencies of a CH 2 group are but little affected by the nature of X in saturated compounds, but that when the CH 2 group is attached to X by a double bond the CH frequencies are some 150 cm. -1 higher. In ethylene each CH 2 group has two CH valency modes of vibration, and since the CH 2 groups themselves can vibrate in or out of phase with one another, four CH frequency modes are possible for the C 2 H 4 molecule, two being Raman active and two infra-red active. In many molecules containing several CH 2 groups, similar coupling effects are important, and frequently four infra-red CH frequencies are observed.