The absorption spectra of ethylene, deutero-ethylene and some alkyl-substituted ethylenes in the vacuum ultra-violet

Abstract

Because of the great importance to organic chemistry of the double C=C bond and the resonance effects which arise from the conjugation of double bonds, the electronic spectra of the molecules ethylene and butadiene (see following paper) are of very considerable interest. Unfortunately it is not possible to obtain these spectra in emission since an electrical discharge tends to disrupt polyatomic molecules into diatomic fragments. However, the spectra can be obtained in absorption. It is found that the first strong bands appear in the neighbourhood of 2000 A and that they extend towards shorter wave-lengths far into the vacuum ultra-violet. The instruments which have been employed in this present investigation are two vacuum grating spectrographs. These were used in conjunction with a source of the Lyman continuum which enabled the absorption to be followed down to about 1000 A (Rathenau. 1933; Collins and Price 1934: details of the experimental technique are given in these articles). A certain amount of earlier work has been done on ethylene and butadiene by Scheibe and Grieneisen (1934). As these authors employed the molecular hydrogen continuum in conjunction with fluorite spectrographs in their investigations, their measurements were limited to the region above 1650 A and under the circumstances no attempts at analysis were made. The extension of the investigations to shorter wave-lengths by the present methods has given a more complete picture of the absorption. From this and with the help of more recent ideas on the structure of these molecules it has been possible to arrive at a satisfactory analysis of the bands. A preliminary account of the spectrum of ethylene together with a tentative analysis has been given by Price (1935). By obtaining the spectrum of tetra-deutero-ethylene (and also C 2 H 3 D) to compare with that of C 2 H 4 we have been able to put this analysis on a firm basis, to correct minor defects and also to correlate a great deal of additional data.