lines are partially blended. Thus, it appears that Chiu's equations give very reasonable predictions for the intensities of this mixed magnetic-dipole-e1ectric-quadrupole transition.Wilkinson and Mulliken,2 using relatively poor uncalibrated photographic data on the a lIIgf-X ll;g+ system and employing line-strength formulas developed only for a ll;g+f-ll;g+ transition 9 (admittedly very tentatively), estimated Q/D=0.15, which is of the order of magnitude of the value obtained here. This presumably more precise value of 0.33 leads to a value of X in Eq. (1) of 9 a.u. instead of 4.5 a.u. given previously.The above determined ratio is certainly not in disagreement with the crude theoretical prediction of Q/ D"-'0.1 mentioned earlier. Nevertheless, Condon 7 has studied such transitions for p2, p3, and p4 configura-9 H. M. James and A. S. Coolidge, Astrophys. J. 85, 438 (1938).THE JOURNAL OF CHEMICAL PHYSICS 2683 tions in atoms, and has found quite variable values for this ratio. It does appear that a careful theoretical study of the vibronic matrix elements for this particular transition would be most helpful.It is perhaps worth mentioning that the line-intensity study reported here is one of the few experimental tests on intensities in forbidden transitions in diatomic molecules and only the fourth in the vacuum ultraviolet. lO ACKNOWLEDGMENTSThe authors wish to express their appreciation to Mr.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 C2H, obtained in the present investigation were rg (CH)=1.1030±0'()()l s A and r g (CC)=1.3369± 0.0016 A. Corresponding lengths for C2D, were r g (CD)=1.099±0.003 A and r g (CC)=1.338±0.003 A.Bond angles, corrected for shrinkage effects, were L CCH= 121.4±0.6° and L 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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