Rotationally resolved spectra of transitions involving motion of the methyl group of acetaldehyde in the system à 1A″−X̃ 1A′

Abstract

Fluorescence excitation spectra, at resolution 0.02 cm−1, in the system à 1A″−X̃ 1A′ were recorded for acetaldehyde in a supersonic jet. We performed full rotational analysis of bands 1400+150n and 1400−150n, for n=0–4, in which 140+ and 140− denote the two inversion tunneling components of the aldehyde hydrogen out of plane bending, in the vibrational ground state of à 1A″. Torsional levels from near the methyl torsional barrier to beyond that barrier are assigned. Sublevels of torsional symmetry A below the barrier are fitted as an asymmetric rotor, but the resulting values of the rotational parameter A are affected significantly by the torsional motion. For the E sublevels, K doublet states split significantly with torsional quantum number vt. Anomalous transitions (ΔKa=0, ΔKc=0) to A sublevels are observed for bands 1400+1504 and 1400−1503, which cannot be simply explained from the Coriolis-type interaction of torsion and rotation. The positions of A and E sublevels in 140−15n cannot be fitted with a program involving only interaction of torsion and rotation, because for n=0–2 states the A/E splitting is reversed from those in 140+15n. Interaction with inversion evidently varies the splitting of torsional sublevels and the K structure.