INTRAMOLECULAR VIBRATIONAL COUPLING IN THE GROUND ELECTRONIC STATE (S0) OF TRANS-STILBENE

Solution vibrational spectra of trans-stilbene and its 12 isotopomers have been recorded. Several key vibrations of the ground electronic state (S0) which are sensitive to trans-cis isomerization are analyzed in detail in terms of the potential energy distribution (PED) and/or harmonic mode scrambling matrices (HMS). Generally, the vibrational mixing is weak for the majority of ring vibrations. Those for which it is strong are determined by the closeness of their and the bridging groups' intrinsic frequencies. The observed changes in the distribution of vibrational intensities in the 1600-1200-cm-1 region are explained in terms of vibrational mixing. For isotopomers with the C5H=CeH (CeH=CeD) moiety, the normal mode associated with the 1639 (1621)-cm-1 band reveals not only that the major contribution is made by the Ce=Ce stretching coordinate but also that there is significant coupling with ethylenic bending vibrations. In derivatives with the C6D=CeD or 13CeH=13CeH group the Ce=Ce stretching vibration is strongly mixed with two ring vibrations around 1600 cm-1 which gives rise to the three normal modes with equal PED contributions of the Ce=Ce stretching coordinate. For the Raman-active Ce-H in-plane deformation modes found in the 1300-cm-1 region both spectral and normal-coordinate analyses indicate extensive vibrational mixing with various ring vibrations depending on the modifications introduced by isotopic substitution. Purer modes are observed for the CeD=CeD derivatives. The corresponding infrared-active modes appear at lower wavenumbers. The Ph-Ce stretchings around 1200 cm-1 are strongly coupled with other neighboring phenyl coordinates, and their positions depend dominantly on the degree of deuteration in the phenyl rings and not in the ethylenic group. The Raman-active bands have been observed at lower wavenumbers than their infrared counterparts. The relevance of the reported results to the excited state (S1) vibrational transitions is discussed. © 1990 American Chemical Society.