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

Abstract

The vibronic absorption spectrum of fluoral vapor was studied in the region of the S1←S0 electronic transition (313–360 nm). The origin O00+) of the transition (29419 cm−1) and a number of fundamental frequencies in the S0 and S1 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 S1 state has pyramidal structure (in contrast, the carbonyl fragment of the fluoral molecule in the S0 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 S1 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 kJ mol−1), respectively. The conformational changes caused by S1←S0 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.