Lowest excited triplet state of 2,5-dimethyl-1,3,5-hexatriene: resonance Raman spectra and quantum chemical calculations

Theoretical and Raman spectroscopic studies are presented of the ground and lowest triplet states of (E)- and (Z)-2,5-dimethyl-1,3,5-hexatriene and their 3,4-dideuterio and 3-deuterio isotopomers. The T{sub 1} and S{sub 0} states. Energies of higher triplet levels are computed and oscillator strengths for the T{sub 1} {yields} T{sub n} transitions are determined. The displacements in equilibrium geometries between the T{sub 1} and the T{sub n} level corresponding to the strongest T{sub 1} {yields} T{sub n} transition are calculated and are used to estimate the intensities of the resonance Raman spectra of the T{sub 1} state under the assumption of a predominant Franck-Condon scattering mechanism. The influence of the ground-state conformation around C-C single bonds on T{sub 1} resonance Raman spectra is considered in detail for the two isomers. It is found that for the E isomers the tEt and for the Z isomers the tEc forms are the predominant ones in the T{sub 1} state. The Z forms are at considerable higher energy than the E forms in the E forms in the T{sub 1} state due to nonbonded interaction. A good agreement is found between theoretically calculated and experimental spectra. The results are compared with previously published data for 1,3,5-hexatriene.