SINGLET-TRIPLET SPECTROSCOPY OF BUTADIENE

Author Institution: Department of Chemistry, Purdue UniversityThe $T_{1}-S_{0}$ absorption spectrum of gas phase 1,3-butadiene was recorded over the region from 20,500 to $23,000 cm^{-1}$ using cavity ring-down spectroscopy. The origin is located at $20,777 cm^{-1}$, and several vibronic bands are observed. These bands are assigned to vibrations of a planar triplet structure based on Franck-Condon calculations. This structure has been calculated to be about 3 kcal $mol^{-1}$ higher in energy than the twisted structure, in which one $CH_{2}$ group is oriented perpendicular to the plane of the rest of the molecule. A two-dimensional torsional calculation involving the rotation of the two terminal $CH_{2}$ groups suggests that the planar structure is located in a shallow minimum at the top of a barrier between two twisted structures