Knowledge Bank

The potential energy surfaces for the double rotation of the methyl groups in biacetyl in its singlet ground and first triplet excited states were determined theoretically. For this purpose the energy of seven conveniently chosen conformations were calculated at the RHF and UHF levels with 6-31G(d,p) basis and full optimization of the geometry. These seven energy values were fitted to a symmetry adapted potential as a function of the two torsion angles. With this potential energy function, the double $C3v$ problem was solved and the transition energies and Franck - Condon intensities were obtained. The location of the $S0\to S1$, system origin and the gearing and antigearing modes were made from a comparison of the laser fluorescence excitation recorded under cold-jet and warm-jet conditions with the calculated spectrum.