Rotationally resolved structures in the fifth and sixth torsional states of $Ã 1A″$ acetaldehyde: Internal rotation above the torsional barrier Available to Purchase

The fluorescence excitation spectrum of acetaldehyde in its transition $Ã 1A″–X̃ 1A′$ 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 $140−.$