R. A. DURIE AND A. G. GAYDON
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There are still one or two peculiar features, not fully comprehended. We shall conclude by reference to one of them. It has recently been shown by C r ~ i c k s h a n kwho , ~ ~ refined some earlier experimental work .of Jeffrey, that in dibenzyl the bond lengths of the C-C bonds which connoect the two rings have values 1.523 and 1.510 A. as in the figure. These are both distigctly less than the normal paraffinic value 1.544 A. It has been sugH
H
gested that hyperconjugation is responsible for this shortening. But if by this it is implied that structures symbolically represented by H2=C2C3=& are effective, then the explanation can hardly be correct, since such structures occur in every paraffin chain, where the standard C-C length is 1.54 A. The explanation of the shortening of the bond CICZseems to be that C1is a trigonal atom and (24) D. W. J. Cruickshank, Acta Cryslall., a, 65 (1949).
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Cz a tetrahedral one, so that the analysis of section (2) applies. The shortening is of the right order of magnitude. The explanation of the shortening of the central bond Cs-C3 is a little more complicated. Partly it arises from the angle ClC&3 which is larger than tetrahedral (for steric reasons, presumably) ; and this affects the hybridization ratios and leads to a small shortening of C2C3and C2Cl at the expense of the CH bonds. (These latter should correspondingly be a little weaker.) Partly also it may be related to the fact that if we regard dibenzyl as derived from a paraffin chain by replacing the alkyl groups a t C1 and Cq by phenyl groups, then the greater electron affinity of the phenyl groups will remove a-electrons from Cz and Ca toward CI and C4. This, in turn, will increase the electron affinity of Cz and Ct, and so strengthen the bond; it mill also remove electrons which are not paired with each other away from C2 and C3 and therefore reduce the repulsive forces in C2-C3 which always exist between non-paired electrons.25 It seems as if the best that we can say a t the moment is that all three effects cooperate in reducing the length of Cz-C3. But further study of this and other similar systems is clearly desirable. (25) A. D. Walsh, Trans. Faraday Soc., 43, BO (1947); I
