✍ Scribed by Prof. Dr. Gerd Kaupp; Dipl.-Chem. Jürgen Boy
No coin nor oath required. For personal study only.
Covalent C-C single bonds are typically 1.54 A long. This fact has been confirmed over and over again by X-ray crystal structure analyses (Cambridge Database, Ian Castleden) and other techniques. Larger deviations from the standard value are observed only in structures having exceptional ring strain, steric congestion, or electronic peculiarities. Numerous efforts have been directed at finding very long C-C single bonds. Thus, semiempirical calculations of the polycycle 1['1 predicted a particularly long diphenyl-substituted bond (PM3 1.642 8,; MNDO 1.652 A); however, it did not appear worthwhile to conduct an X-ray structure analysis, as much longer C-C single bonds had already been described. A presumed world record was the length of the central bonds in the photoisomer 2 of [2.2]tetrabenzoparacyclophane 3, which was determined by an X-ray analysis and reported by Ehrenberg as 1.77 8,.['] However, in this study no evident means of avoiding the thermal reaction leading to the golden yellow isomer 3 were taken.[Ib. 'I 2 3
Ar,C-CAr, 4
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