Far-Ultraviolet Spectra of Fluoroethylenes

The vacuum ultraviolet spectra of the complete series of fluoroethylenes were measured from 2000 to 1150 Å under moderate resolution. The π*←π transitions are easily identified in all the spectra. In most cases their maxima are only slightly displaced from their position in ethylene, but a very large hypsochromic shift is found in going from the tri- to the tetrasubstituted derivative. Three Rydberg series are observed in all the spectra, converging to the π ionization potential. They all exhibit vibrational fine structure dominated by C=C stretching and twisting vibrations. In the highly fluorine substituted ethylenes the first Rydberg band is well separated from the π*←π band (at about 1900 Å) as in the highly alkyl substituted derivatives. There is no evidence for transitions of σ electrons in this part of the spectrum except perhaps at the high frequency end of the range (1200 Å).