Internal rotation in the spectrum of acetaldehyde

The geometrical structure of acetaldehyde from the microwave spectrum and torsional transitions from the far infrared spectrum have been fitted with a semirigid model in order to obtain the torsional parameters V″₃ = 415.0 and V″₆ = 22.3 cm^?1 and the torsional energy levels of the isotopic species CH₃CHO, CH₃CDO, CD₃CHO, and CD₃CDO. These have been used in fitting torsional sequences in the 182-nm system of the electronic spectra of these species to obtain the excited state parameters V′₃ = 880 and V′₆ = 77 cm^?1. Both the ground and excited state parameters are in good agreement with ab initio predictions.