The lowest triplet state of 1,3,5-hexatrienes: Quantum chemical force field calculations and experimental resonance Raman spectra

Abstract

Theoretical and Raman spectroscopic studies are presented of E and Z-1,3,5-hexatriene and their 3,4- and 2,5-dideuteriated analogs in ground and excited triplet states. The T1 potential energy surface is calculated from extended SCF-LCAO-MO-CI theory. Energy minima and equilibrium geometries are determined in T1 . Frequencies and normal modes of vibration are calculated for the minima of the T1 and S0 states. Energies of higher triplet levels are computed and oscillator strengths for the transitions from T1 to Tn are determined. The displacements in equilibrium geometries between the T1 and the Tn level corresponding to the strongest T1→Tn transitions are calculated and are used to estimate the intensities of the resonance Raman spectra of the T1 state under the assumption of a predominant Franck–Condon scattering mechanism. The results indicate that the planar E and Z forms of hexatriene and its analogs are the only ones contributing substantially to the T1→Tn absorption and the T1 resonance Raman spectra found in the present experiments. The existence of a twisted form in the T1 state cannot be ruled out, but its contribution to the resonance Raman spectra corresponding to an electronic T1→Tn transition around 315 nm is likely to be much weaker than that of the E or Z forms. Satisfactory agreement is found between the calculated and experimentally determined resonance Raman spectra. An assignment is obtained for the experimentally determined vibrational modes in T1. The theoretical results indicate a substantial rotation of normal modes from S0 to T1.