S1←S0 vibronic spectrum and structure of fluoral in the S1 state

The vibronic absorption spectrum of fluoral vapor was studied in the region of the S-1<--S-0 electronic transition (313-360 nm). The origin (0(0)(0+)) of the transition (29419 cm(-1)) and a number of fundamental frequencies in the S-0 and S-1 states were determined. The character of intensity distribution in the spectral bands indicates that the electronic excitation leads to significant change of the CF3 group orientation relative to the molecular frame. Moreover, it was found that the carbonyl fragment of the molecule in the S-1 slate has pyramidal structure tin contrast, the carbonyl fragment of the fluoral molecule in the S-0 state is planar). The experimental torsion and inversion energy levels were used for the calculation of internal rotation and inversion potential functions of fluoral molecule in the S-1 state. The potential barriers to internal rotation and inversion were found to be 1270 cm(-1) (15.2 kJ mol(-1)) and 550 cm(-1) (6.6 kT mol(-1)), respectively. The conformational changes caused by S-1<--S-0 electronic excitation in the fluoral molecule are similar to those observed in acetaldehyde and biacetyl molecules and differ from the conformational behavior of hexafluorobiacetyl molecule.