Substituent Effects in Thiophenes

He I and He II photoelectron spectra (UPS) of 2-acetyl and 2-cyano thiophene have been measured. The electronic strucutre of the title compounds and, in particular, the effect of substituents on the thiophene % orbitals is discussed on the basis of band widths, He I/He II intensities and semi-empirical MO calculations. Definite assignments are given for the two molecules, thus completing the previous work. The available results on 2- and 3-monosubstituted thiophenes have been compiled and rationalized