Conformational analysis of propionaldehyde (propanal) by two-photon spectroscopy of the 3s ← n Rydberg transition

The two-photon 3s ← n Rydberg spectra of propionaldehyde have been recorded under cell and free-jet expansion conditions. The occurrence of two separate electronic systems for the cis and disfavoured gauche conformers has been confirmed. In-phase electronic transitions are found in both formyl and methyl torsional coordinates, and the analysis of torsional sequences has provided new information on conformational potential functions. The cis (n, 3s) conformer is stabilised, the energy gap between cis and gauche conformers increases to 1260 cm−1 and the methyl internal rotation barriers increase for both conformers on electronic excitation.