Structures of cyclic mono- and dialkynes

In Table 8-1 we have listed the lengths of the triple bonds and the angles at the sp centers for cyclic monoalkynes as derived by X-ray and electron diffraction measurements [6, 33, 36-41], It is seen that bending the triple bond even to 150° does not have a significant effect on the bond lengths. The deviation from 1.20 A - the value of the C-C triple bond in unstrained systems - is small. 

Bond angles at sp centers. Cisoid deformation at the triple bonds. Electron diffraction data. 

There is a correlation between the C chemical shift of the sp center and the angle at this center (see Fig. 8-3). This has been pointed out by Meier for monocyclic acetylenes [3 b]. 

In the case of cyclic diynes more structural data are available, probably due to the fact that diynes crystallize better. The most relevant structural parameters of cyclic diacetylenes have been collected in Tables 8-2-8-4. The most strained diynes are those with an eight-membered ring, 1,5-cyclooctadiyne (56) [25] and 5,6,ll,12-tetradehydrodibenzo[fl,e]cyclooctene (98) [42]. The sp centers in both hydrocarbons are strongly bent like those listed in Table 8-1. The trans-annular distances between the triple bonds amount to 2.597 A (56) and 2.617 A (98), respectively (Table 8-2). A further consequence of the imposed strain is an elongation of the other bonds the bonds between the sp atoms in 56 are stretched to 1.57 A, those between the sp centers in 98 to 1.43 A. In the tetrasila compound 99 most of the strain is relieved in the long Si-Si bonds [43]. Consequently the bond angles at the sp centers deviate only 11-16° from linearity. 

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