RESONANCE RAMAN INVESTIGATION OF THE RADICAL-CATION OF 1,3,5-HEXATRIENE

The resonance Raman spectrum of the 1,3,5-hexatriene radical cation ge nerated by gamma-irradiation in a Freon glass is reported. The spectru m is excited at 395 nm in resonance with the second absorption band. I dentical spectra are obtained from ionized (E)- and (Z)-1,3,5-hexatrie ne. The presence of more than one rotamer has to be assumed to account for all the observed resonance Raman bands. The bands are assigned to a mixture of the two rotamers, calculated at lowest energy, the ttt a nd tct rotamers, on the basis of ab initio ROHF/6-31G calculated harmo nic frequencies. The spectrum indicates that the ttt and tct rotamers are formed in a ratio in the range 0.4/0.6-0.6/0.4. Two possible mecha nisms are proposed to explain the presence of these two rotamers.