Cf. Lee et al., THE STRUCTURES OF QUADRICYCLANONE AND ITS 3-CYCLOPENTADIENYLIDENE DERIVATIVE, Journal of the Chinese Chemical Society, 42(6), 1995, pp. 943-946
The crystal structures of quadricyclanone (2) and 3-(cyclopentadienyli
dene)quadricyclane (3) are solved by X-ray diffraction analyses. Their
bonding parameters cannot be accurately estimated by semiempirical mo
dels AM1 and PM3.