The absorption spectra of the cyclic dienes in the vacuum ultra-violet

The absorption spectra of cyclohexadiene, cyclopentadiene, pyrrole, thiophene and furan have been investigated in the vacuum ultra-violet. The spectra obtained are due to the excitation of a π electron from the conjugated double-bond electrons (X2 1 x2 2 )• The ionization potentials of the molecules deduced from Rydberg series are respectively 8.4, 8.58, 8.9, 8.91 and 9.01V, these values referring to the outer X2 shell of π electrons. A second less certain ionization potential of about 10.8V was obtained for furan. Some discussion of the electronic structure of the molecules on the basis of their spectra is given, and it is concluded that only in furan is there marked tendency towards homocyclic conjugation. It is also concluded that ‘hyperconjugation’ does not play so large a part in the interpretation of the spectra of cyclohexadiene and cyclopentadiene as previously supposed.