Resonance Raman scattering of butadiene: Vibronic activity of a b u mode demonstrates the presence of a 1A g symmetry excited electronic state at low energy

Abstract

Resonance Raman spectra of buta-1,3-diene-d0 and buta-1,3-diene-1,1,4,4-d4 have been obtained with ultraviolet excitation from 239.5 to 199.9 nm. Activity of the first overtone of mode 24, the bu symmetry CCC chain deformation mode, is observed with excitation energy below the origin of the 1 1Bu state. This vibronic activity of a nontotally symmetric mode is shown to be evidence of resonance with the 2 1Ag state of butadiene. A quantitative analysis of the ratio of intensities of 2ν24 to ν9, the ag symmetry CCC chain deformation mode, demonstrates that enhancement of 2ν24 cannot be due to resonance with the 1 1Bu state. The resonance enhancement behavior of this overtone band also shows that it is of vibronic origin rather than Franck–Condon allowed. The intensity pattern seen for the modes of bu symmetry is fully consistent with the results of a quantitative calculation of vibronic activity for the eight bu symmetry modes. The 2 1Ag electronic state is estimated to be ca. 0.25 eV below the 1 1Bu electronic state. Overtones of out-of-plane C–H bending and CH2 twisting modes are seen with excitation radiation near the peak of the transition to the 2 1Ag state, indicating that the 2 1Ag state of butadiene has appreciably lower resistance to deformation along out-of-plane coordinates than does the ground electronic state. This is consistent with the expectations of semiempirical calculations.