Electron‐Diffraction Study of the Structures of C₂H₄ and C₂D₄

The structures of ethylene and deuteroethylene were investigated by electron diffraction to resolve apparent discrepancies between earlier diffraction and spectroscopic studies. Satisfactory agreement with the previous diffraction study was found for five of the six molecular parameters determined but a difference of five standard deviations was encountered for the C–H bond length. Mean bond lengths and standard errors for C₂H₄ obtained in the present investigation were r_g(CH)=1.103₀±0.001₈ Å and r_g(CC)=1.336₉±0.001₆ Å. Corresponding lengths for C₂D₄ were r_g(CD)=1.099±0.003 Å and r_g(CC)=1.338±0.003 Å. Bond angles, corrected for shrinkage effects, were ∠CCH=121.4±0.6° and ∠CCD=121.4±0.8°. The new diffraction results are compared with mean bond lengths and angles calculated from spectroscopic rotational constants taking rotation—vibration interactions into account. Excellent agreement is found. Significant differences exist, however, between the conventional bond‐length parameters derived by spectroscopy and diffraction. Root‐mean‐square amplitudes of vibration were also determined by diffraction, and these agreed satisfactorily with amplitudes calculated from vibrational frequencies.