The S-1 <- S-0 vibronic spectrum and the structure of the 2-chloroethanal molecule in the S-1 state

The vibronic absorption spectrum or 2-chloroethanal vapor was obtained in a multipass cell with a large optical path (up to 120 m), The spectrum in th e region 303-347 mn was assigned to tile S-1 <-- S-0 electronic transition (from the ground So to the first excited singlet S, electronic state). The vibrational structure of the spectrum was studied. An analysis showed that the S-1 <-- S-0 excitation of 2-chloroethanal conformers was accompanied by rotation of tops and pyramidal distortion of carbonyl fragments, which are planar in the S-0 state. The spectral bands were assigned to three systems of vibronic transitions: between trans conformer (S-0) levels and the leve ls of conformers 1, 11, and III (S-1) with "origins" (0(0)(0) transitions b etween zero-point vibrational levels) at 29070 +/- 200 (estimated), 29883 /- 1, and 29830 +/- 200 cm(-1) (estimated), respectively. Several fundament al frequencies were identified for all conformers and, in particular, torsi onal and inversion vibrational levels. The obtained data closely agree with the results of quantum-mechanical calculations of the 2-chloroethanal mole cule in the S-0 and S-1 states.