Resonance Raman investigation of the radical cation of 1,3,5-hexatriene

The resonance Raman spectrum of the 1,3,5-hexatriene radical cation generated by gamma-irradiation in a Freon glass is reported. The spectrum is excited at 395 nm in resonance with the second absorption band. Identical spectra are obtained from ionized (E)- and (Z)-1,3,5-hexatriene. 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 and tct rotamers, on the basis of ab initio ROHF/6-31G calculated harmonic 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 mechanisms are proposed to explain the presence of these two rotamers.