RESONANCE RAMAN-SCATTERING OF BUTADIENE - VIBRONIC ACTIVITY OF A B(U) MODE DEMONSTRATES THE PRESENCE OF A 1AG 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 2v24 to v9, the a(g) symmetry CCC chain deformation mode, demonstrates that enhancement of 2v24 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.