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 rg(CH)=1.1030±0.0018 Å and rg(CC)=1.3369±0.0016 Å. Corresponding lengths for C2D4 were rg(CD)=1.099±0.003 Å and rg(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.

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