S1←S0 vibronic spectrum and structure of 2,2-difluoroethanal in the S1 state

The vibronic spectrum of the 2,2-difluoroethanal vapor was recorded using a multipass optical cell with an optical length of at least 140 m. The spectrum in the region of 300-364 nm was assigned to the S-1<--S-0 electronic transition (from the ground S-0 to the first excited singlet S-1 electronic state) the vibrational structure of the spectrum was analyzed. The spectrum bands were assigned to two systems of vibronic transitions, namely, transitions between the levels of the cis-conformer (S-0) and of the S-1 conformers, with the origins (000 transitions between the zero vibrational levels of conformers) at 29192 and 29087 cm(-1), respectively. Analysis of the spectrum showed that the S-1<--S-0 electronic excitation of the cis-conformer was followed by rotation of the CHF2 top and pyramidal distortion of the carbonyl fragment. A number of fundamental frequencies were found for S, conformers, ill particular, torsion and inversion energy levels. The experimental data arc in satisfactory agreement with the results of quantum-chemical calculations for the 2,2-difluoroethanal molecule in the S-0 and S-1 states.