Rotationally resolved structures in the fifth and sixth torsional states of à 1A″ acetaldehyde: Internal rotation above the torsional barrier

Abstract

The fluorescence excitation spectrum of acetaldehyde in its transition à 1A″–X̃ 1A′ is analyzed for torsional states above the barrier. States with torsional vibrational quantum numbers vt=5A, 5E, and 6A at term energies 660–927 cm−1 are assigned. This region is 100–370 cm−1 above the torsional barrier. These states lie between the limits of torsional vibrational motion and free internal rotor motion, so that the close-lying 5A2 and 6A1 states mix for K>0, and K states in the E sublevel are widely split. From an analysis of calculated eigenfunctions, the composition of K=1 sublevels reaches nearly equal proportions of odd (A2) and even (A1) torsional wave functions when levels exceed the top of the torsional barrier, whereas the K=1 eigenfunctions of vt=4A1 lying near consists of mostly even (A1) functions. Avoided crossings with ΔK=0 and Δm=0 mod 3 selection rules are observed between K=2, E sublevels of 140+154 and 140+155 and between K=2, E sublevels of 140−154 and 140−155. The K energy-level structure of 140+155 deviates significantly from the expected parabolic shape. The K structures and the K state at the lowest energy differ appreciably for the two inversion doublets 140+ and 140−.