The S1 ← S0 fluorescence excitation spectrum and structure of propanal in the S1 excited electronic state

We have obtained and analyzed the S-1 <- S-0 fluorescence excitation spectra of jet-cooled propanal-h(1) (CH3CH2CHO) and -d(1) (CH3CH2CDO). Using the results of theoretical studies of the structure of propanal molecule in the S-1 lowest excited singlet electronic state, we have assigned the bands of both spectra to the vibronic transitions of the cis conformer (in the S-0 ground electronic state) to the 1 and 3 conformers (in the S-1 state) differed by the angle of the C2H5 ethyl group rotation around the central C-C bond. The origins of the 1 <- cis and 3 <- cis electronic transitions have been observed at 29 997 and 30 075 cm(-1) for propanal-h(1) and at 30 040 and 30 115 cm(-1) for propanal-d(1), respectively. The high activity of torsional (C2H5 ethyl groups) and inversional (CCHO/CCDO carbonyl fragments) vibrations and the intensity distribution of the bands in torsional sequences (passing through maximum) are in agreement with the theoretical prediction that the S-1 <- S-0 electronic excitation of the cis conformer causes (after geometrical relaxation) the pyramidalization of carbonyl fragments and the rotation of ethyl groups around the central C-C bond. A number of energy levels have been found for torsional and inversional vibrations, and also fundamentals of nu(10) (CCO bend) and v1(3) (CCC bend) for the both 1 and 3 conformers of propanal-h(1) and -d(1) have been found. Then the "experimental'' potential functions of inversion for the pair of the 1 and 3 conformers have been determined. The heights of potential barriers to inversion and the angle values corresponding to the minima of potential functions of inversion are 900 cm(-1) and 351 for propanal-h(1) and 820 cm(-1) and 34 degrees for propanal-d(1), respectively.