A-PRIORI SCALED QUANTUM-MECHANICAL VIBRATIONAL-SPECTRA OF TRANS-STILBENE AND CIS-STILBENE

The vibrational frequencies of both isomers of stilbene have been computed at the ab initio 3-21G level by using the scaled quantum mechanical force field methodology. First, the force fields of benzene and ethylene have been computed and scaled, and then the so obtained scale factors have been transferred to trans- and cis-stilbene. The excellent agreement between the a priori computed frequencies and the experimental ones has allowed for the reassignment of the spectrum of trans-stilbene and proposal for the first time of-a general assignment of cis-stilbene. The main differences between the spectra of both isomers are discussed on the basis of the respective molecular structures.