Structure of Ethylene Oxide and Cyclopropane

A. D. WALSH¹ has suggested that the properties of cyclopropane and ethylene oxide are better accounted for by the formulæ I (three equivalent resonance forms) and II than by III and IV:

One consequence of this suggestion is that, in the CH-bonds, the carbon orbitals should be of the sp² hybrid type, rather than the sp³ hybrid suggested by III and IV. In 1945 I obtained the force constants of a number of CH-bonds and found the following: methane, 4.97 (in the usual units); ethane, 4.81; ethylene (sp² bonds), 5.1². Fox and Martin³ have suggested that in paraffins the CH-force constant is: CH₄, 5.04; CH₃ group, 4.75; CH₂ group, 4.56; CH group, 4.56. The force constant of the CH-link in cyclopropane and ethylene oxide might, therefore, prove valuable in deciding between the above structures.

From the Raman⁴ and infra-red⁵ spectra of cyclopropane it has been decided that the CH-valency vibration frequencies are: A₁', 3,000; E', 3,000; A₂", 3,050; E", 3,080 cm.^-1. The derived force constants are: 5.04, 5.04, 4.91 and 5.02. The mean is 5.0. (The same values of the fundamental constants have been used as in my previous paper².) The Raman⁴ and infra-red⁵ spectra of ethylene oxide indicate that the CH-valency vibration frequencies are A₁, 3,000; B₁, 3,000; B₂, 3,062; A₂, 3,062 cm.^-1. The derived force constants are 5.04, 5.04, 4.95 and 4.95. The mean is again 5.0. Ethylene sulphide is also of interest. Thompson and Dupré, having examined the Raman and infra-red spectra, suggested for the CH-valency vibrations: A₁, 2990; B₁, 2,990; A₂, 3,080; and B₂, 3,080 cm.^{-1 6}. The derived force constants are 5.00, 5.00, 5.02 and 5.02. The mean may be taken as 5.0. In all these three compounds the CH-force constant is 5.0.

For the CH-bond in a methylene group in an ordinary paraffin chain, we would expect the force constant to be about 4.6 (Fox and Martin), and certainly not greater than 4.8 (CH₃-group). The force constant in the methylene groups in these ring compounds is certainly significantly greater than this. It is, moreover, nearer the ethylene figure of 5.1 than to the figure for a methylene group in a paraffin. (It seems proper to make the comparison with the CH₂-group in a paraffin chain, though it may be noted that the CH-force constant in these ring compounds is not appreciably different from that in methane itself.) The conclusion to be made from these calculations must be that the force constants indicate that the CH-links in these ring compounds approach those in ethylene. So the calculations provide a measure of support for Walsh's suggestion. Of course, the CH-force constant might change simply as a result of the distortion in the angle between the bonds which form the ring. However, such a change is hypothetical, and it is, therefore, rather unsatisfactory as an explanation of these results.

The possible usefulness of the above calculations became apparent to me after hearing a lecture by Dr. Walsh on this and related topics.

J.W. LINNETT