Comparison of the Electronic Spectra of Geometric Isomers. III. β-Substituted Styrenes

The ultraviolet absorption spectra of cis and trans isomers of various β-substituted styrenes have been measured both in the gaseous state and in n-hexane solution. The excitation energies observed for cis isomers are either smaller or greater than those of the trans, depending on the type of β-substituents. The isomeric difference in the energy of the lowest allowed transition was found to be linearly correlated with Taft’s steric substituent constants. The electronic states of the various isomeric pairs were calculated by the LCAO–SCF–CI method of the Pariser-Parr-Pople type. The results were found to be entirely in line with the observed spectral features.