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

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 b_u symmetry CCC chain deformation mode, is observed with excitation energy below the origin of the 1 ¹B_u state. This vibronic activity of a nontotally symmetric mode is shown to be evidence of resonance with the 2 ¹A_g state of butadiene. A quantitative analysis of the ratio of intensities of 2ν₂₄ to ν₉, the a_g symmetry CCC chain deformation mode, demonstrates that enhancement of 2ν₂₄ cannot be due to resonance with the 1 ¹B_u 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 b_u symmetry is fully consistent with the results of a quantitative calculation of vibronic activity for the eight b_u symmetry modes. The 2 ¹A_g electronic state is estimated to be ca. 0.25 eV below the 1 ¹B_u 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 ¹A_g state, indicating that the 2 ₁A_g 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.