Microwave Spectrum, Structure, and Dipole Moment of s-trans Acrolein

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Abstract
The microwave spectrum of acrolein has been studied in the 19–37 kmc region. A series of R‐type transitions was identified for the ground and three excited torsional states of the s‐trans isomer and these spectra were successfully fitted in the rigid rotor approximation. Using the ground state moments and additional electron diffraction data a reasonable though not unique structure has been obtained. The torsional frequency was found to be 200±50 cm—1 from relative intensity measurements, and a lower limit of about 2300 cal was estimated for the barrier hindering trans‐cis conversion. The dipole moment for the ground state was found to be 3.11±0.04 d, oriented about 14° from the carbonyl group. The results obtained and the absence of a strong s‐cis spectrum establish the s‐trans isomer as the predominant form.