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

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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 T 1potential energy surface is calculated from extended SCF‐LCAO‐MO‐CI theory. Energy minima and equilibrium geometries are determined in T 1 . Frequencies and normal modes of vibration are calculated for the minima of the T 1 and S 0 states. Energies of higher triplet levels are computed and oscillator strengths for the transitions from T 1 to T n are determined. The displacements in equilibrium geometries between the T 1 and the T n level corresponding to the strongest T 1→T n transitions are calculated and are used to estimate the intensities of the resonance Raman spectra of the T 1 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 T 1→T n absorption and the T 1 resonance Raman spectra found in the present experiments. The existence of a twisted form in the T 1 state cannot be ruled out, but its contribution to the resonance Raman spectra corresponding to an electronic T 1→T n 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 T 1. The theoretical results indicate a substantial rotation of normal modes from S 0 to T 1.

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