RESONANCE RAMAN EXCITATION PROFILES OF 1,3-BUTADIENE IN VAPOR AND SOLUTION PHASES

Resonance Raman spectra of 1,3-butadiene have been obtained in the vapor phase at six excitation wavelengths between 2231 and 2140 angstrom and in cyclohexane solution at six wavelengths between 2310 and 2179 angstrom. Absolute scattering cross sections have also been measured in the vapor at 2150 angstrom and in solution at three wavelengths. The experimental absorption spectra and Raman excitation profiles are compared with the results of numerical simulations that explicitly incorporate 18 vibrational modes and include excited-state frequency changes and Duschinsky rotation but consider only a single resonant electronic state (the lowest allowed state of B-1(u)+ symmetry). The effect of the nearby forbidden 2(1)A(g)- state is also explored in separate simulations that properly treat the coupling of the two states by nu24, the lowest frequency vibration of b(u) symmetry, but explicitly consider only three vibrational modes at a time. It is concluded that the forbidden state has a relatively small effect on the resonance Raman intensities of most transitions not involving excitation of nu24. However, the sensitivity of the spectra to small changes in the Duschinsky rotation parameters leads to considerable ambiguity in efforts to experimentally determine excited-state vibrational frequencies and geometry changes and solvent effects on these quantities. The data are most consistent with a model in which the extreme diffuseness of the absorption spectrum arises from rapid depopulation of the initially excited B-1(u)+ state without any large distortion from planarity in that state.