A FORCE-FIELD CALCULATION FOR TRANS-STILBENE ION RADICALS

Resonance Raman spectra of isotopomers (parent compound, six symmetrically and six unsymmetrically deuterated and alpha,alpha'-C-13(2) derivatives) Of trans-stilbene anion and cation radicals are assigned and valence force fields calculated. The C(e)=C(e) stretching frequency for both radical ions is significantly downshifted relative to its value for the neutral molecule. The shift amounts 86 cm-1 for the parent anion radical and about 75 cm-1 for the parent cation radical. The latter difference is, however, less certain due to the strong mixing of group vibrations. The force fields for anion and cation radicals are calculated within the planar C2h model obtained from AM1 calculations and on the basis of the empirical force field of neutral trans-stilbene. Major geometrical changes on going from a neutral molecule to an ion radical take place in the ethylenic bridge. The C(e)=C(e) bond is slightly of the C(e)=C(e) bond and shortening of the Ph-C(e) bond have been calculated for both radical ions. The C(e)=C(e) bond is slightly shorter in the anion than in the cation, while the opposite holds for the Ph-C(e) bond. These differences are within 0.01 angstrom for bond lengths and 1.2-degrees for angles, meaning that the two ion radicals have strikingly different spectra solely due to the different electronic charge distributions. The sensitivity of frequencies on changes in geometry and potential energy parameters is estimated. By change of only force constants governing ethylenic modes, a good agreement of calculated frequencies and experimental values could be attained. However, a lack of experimental data (infrared frequencies of ethylenic moiety would be especially helpful) has led to a variety of solutions reproducing the set of observed Raman frequencies equally good even under severe approximations which had to be introduced.