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

The vibrational frequencies of both isomers of stilbene have been comp uted at the ab initio 3-21G level by using the scaled quantum mechanic al force field methodology. First, the force fields of benzene and eth ylene have been computed and scaled, and then the so obtained scale fa ctors have been transferred to trans- and cis-stilbene. The excellent agreement between the a priori computed frequencies and the experiment al ones has allowed for the reassignment of the spectrum of trans-stil bene and proposal for the first time of-a general assignment of cis-st ilbene. The main differences between the spectra of both isomers are d iscussed on the basis of the respective molecular structures.