The jet-cooled electronic spectrum of acetaldehyde and deuterated derivatives at rotational resolution

Abstract

The laser induced fluorescence excitation spectrum (à 1A″←X̃ 1A′) of jet-cooled CH3CHO, CH3CDO, CD3CHO, and CD3CDO at 0.5–2 K have been recorded up to E′vib ≂1500 cm−1. The electronic origin is unequivocally assigned for CH3CHO (29 771 cm−1), CH3CDO (29 820 cm−1), CD3CHO (29 756 cm−1), and CD3CDO (29 808 cm−1). Three Franck–Condon active vibrations in the à A″ state are identified: ν10 the in-plane C–C–O bend; ν′14 the out-of-plane aldehyde hydrogen bend; and ν15 the methyl torsion. Evidence is seen for anharmonic coupling of ν′14 and ν15. The height of the threefold barrier to methyl rotation (∼660 cm−1) is fairly insensitive to deuterium substitution as is the frequency of ν′10. An analysis of the rotational structure reveals that all of three possible band polarizations are present in this spectrum, i.e., A, B, and C type. The polarization of some bands is apparently anomalous. The C=O stretch (ν4) could not be assigned due to a high degree of congestion above Evib =1000 cm−1, although it is expected to have a strong Franck–Condon intensity.