A quantum chemical anaylsis of the time-resolved resonance Raman spectrum of 2,5-dimethyl–2,4-hexadiene in the T1 state

Abstract

Theoretical calculations are presented on 1,3-butadiene and 2,5-dimethyl–2,4-hexadiene (tetramethyl–butadiene or TMB) in ground and excited triplet states. The T1 potential energy surfaces are calculated from extended self-consistent-field–linear combination of atomic orbits–molecular orbital–configuration interaction (SCF-LCAO-MO-CI) theory. Energy minima and equilibrium geometries are determined in T1. Frequencies and normal modes of vibration are calculated for planar geometries in S0 and T1 and for a geometry in T1 twisted 90° around one of the formal C■C double bonds. 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 Tn levels corresponding to the strongest T1→Tn transitions are calculated and are used to estimate the intensities of the resonance Raman spectra in T1 under the assumption of a predominant Franck–Condon scattering mechanism. For TMB, the calculated resonance Raman spectra are compared with the experimentally observed one. Satisfactory agreement is found between the calculated and observed spectra. An assignment is obtained for the experimentally observed vibrational modes of TMB in T1. From this analysis, it is concluded that for TMB in T1, the main contribution to the observed resonance Raman spectrum is arising from molecules in the planar form.