The infra-red absorption spectrum of methylene chloride

Abstract

The absorption spectrum of methylene chloride has been studied over limited regions in the infra-red by several workers (Ellis 1926; Lecomte 1933; Corin 1936; Emschwiller and Lecomte 1937). So far as we are aware it has never been investigated between 2-5 and 7 μ , although this is a very important region. The present paper describes a study of the absorption of this substance in the liquid state between 2 and 12 μ , which was undertaken with the following objects in view. First, it was hoped that it might be possible to resolve certain difficulties in the correlation of the infra-red with the Raman spectrum of this molecule. Thus Trumpy (1934) from a very careful examination of the Raman spectrum had made an assignment of the nine fundamental frequencies which did not admit of a reasonable explanation of a very strong absorption observed in the infra-red by Lecomte (1933) near 8 μ . Before any reinterpretation of existing data could be attempted it was obviously necessary to complete the map of the spectrum between 2∙5 and 7 μ and to confirm Lecomte’s work at 8 μ It was also important to observe as many overtone and combination bands as possible to provide data for verifying and testing any new assignment of the fundamental frequencies. Secondly, it was chosen as a suitable molecule for the further study of the assumption of independent groups (Sutherland and Dennison 1935; Mecke 1932) in computing the frequencies of a polyatomic molecule from a simplified but still very general potential function. Thus it is well known that certain groups (such as CH 2 or CH 3 ) in a molecule give rise to certain characteristic frequencies in the infra-red and Raman spectra of these molecules, being apparently practically in­dependent of the rest of the molecule. Such frequencies are easy to identify in analysing the vibration spectrum of a polyatomic molecule but the frequencies which arise from the vibrations of those semi-independent groups as wholes are not at all easy to assign and it is just those “inter­-group” frequencies as opposed to the “intra-group” frequencies for which we have found a new method of treatment in this case. Yet the method which depends on an application of the isotope effect, is a very general one and should be applicable to many classes of polyatomic molecules.