Relations between Electronic Absorption Spectra and Spatial Configurations of Conjugated Systems. V. Stilbene

Abstract

Abstract (1) The quantitative relations between the degree of deviation from the coplanarity of the molecular configuration and the ultraviolet absorption spectrum as well as the quantities such as the extra-resonance energy and the π-bond orders have been determined for the stilbene-type compounds by calculations based on the simple LCAO molecular orbital method. (2) The ultraviolet absorption spectra of trans-stilbene in the crystalline state, in the solution state, and in the vapor state have been discussed in reference to the spatial configurations of the molecule in the various states. The conjugation band in the vapor spectrum is at considerably shorter wavelengths than that in the solution spectra as well as the crystal spectra, a fact which has been considered as probably indicating that the most likely configuration of the molecule in the vapor state deviates considerably from coplanarity. The magnitude of the red-shift of the conjugation band in the crystal spectra measured by the pressed KCl-disk technique relative to the band in the spectrum of the n-heptane solution has been found to be smaller than the value anticipated for the rigid molecule. From this fact it has been inferred that the most probable configuration of trans-stilbene in solution does not significantly differ from the one in the crystalline state and is planar or nearly planar. (3) On the basis of the above inference, the calculations have been applied to cis and trans-stilbenes, and the interplanar angles in the most probable configuration of cis-stilbene have been estimated at about 28° from the position of the conjugation band, in fairly good agreement with the value estimated from the scale model. The results of the calculations have been discussed. (4) In addition, the solvent effects of various solvents on the spectra of trans-stilbene and its p-methoxy- and p-nitroderivatives have been discussed with special reference to the effect of benzene which has been found to cause a large bathochromic displacement of the conjugation band and a remarkable redistribution of intensity among the vibrational bands of the conjugation band of each compound.