Cherniak and Costain measured the microwave spectra of 10 isotopomers to determine an accurate structure for acrolein which was found to adopt a planar anti conformer.¹ They used 8 mono-substituted species and two di-substituted species. Because two of the C atoms and two the H atoms have nearly equivalent positions, there was some ambiguity in assigning the mono-substituted structures. This ambiguity was removed by examining two di-substituted species and thus creating a new coordinate system. The other structural parameters that they obtained were close to the values predicted from the structures of similar molecules. The length of the C-C single bond was of special interest because the measurement would test the prediction of VB resonance and MO models that the bond possesses significant double-bond character. They found the C-C bond length to equal 1.470 Å, a result consistent with these models. The predicted planarity of anti-acrolein was also examined. They found a negative value for the ground state inertial defect which indicated that the molecule might not be planar. However, they concluded that trans-acrolein was planar because there was a very harmonic nature to the torsional energy levels. An examination of the microwave transitions was done to characterize excited torsional states and to find cis and gauche conformations. This determination of the potential function from the microwave data was unsuccessful and no evidence for the other conformers was found. It was concluded that these species must be located at least 1000cm^-1 above the anti ground state.

Marit Traetteberg measured acrolein’s electron diffraction at 2 camera distances.² His results improved upon the previously published electron diffraction results of Kuchitsu, Fukuyama, and Morino³ by decreasing the uncertainties for the r_s structure. Traetteberg’s technique required him to make several assumptions about CH bond lengths and angles. While Traetteberg’s results were less complete, his reported uncertainties for bond length were lower than the microwave structure bond length uncertainties. The uncertainties for bond angles were, on the other hand, lower for the microwave structure. Traetteberg also considered non-planrity of acrolein allowing non-planarity and and found that a torsion angle of 11.2° gives slightly better results than the planar model. However, because of the small size of the deviations and by taking into account other effects of the torsion angle, Traetteberg concluded confidently that acrolein does not deviate significantly from a planar trans conformation.

1) Cherniak, E.A. and C.C. Costain, "Microwave Spectrum and Molecular Structure of trans-Acrolein", J. Chem. Phys., 45, 104-110 (1966).

2) Tratteberg, Marit, "The Single and Double Bonds between sp²-Hybridized Carbon Atoms, as Studied by the Gas Electron Diffraction Method", Acta Chem. Scand., 24, 373-375 (1970).

3) Kuchitsu, K., Fukuyama, T. and Y. Morino "Average Structures of Butediene, Acrolein, and Glyoxal determined by Gas Electron Diffraction and Spectroscopy", J. Mol. Struct., 1, 463-79 (1968).