<P> Unusually long bond lengths do exist . In one compound, tricyclobutabenzene, a bond length of 160 pm is reported . The current record holder is another cyclobutabenzene with length 174 pm based on X-ray crystallography . In this type of compound the cyclobutane ring would force 90 ° angles on the carbon atoms connected to the benzene ring where they ordinarily have angles of 120 ° . </P> <P> The existence of a very long C--C bond length of up to 290 pm is claimed in a dimer of two tetracyanoethylene dianions, although this concerns a 2 - electron - 4 - center bond . This type of bonding has also been observed in neutral phenalenyl dimers . The bond lengths of these so - called "pancake bonds" are up to 305 pm . </P> <P> Shorter than average C--C bond distances are also possible: alkenes and alkynes have bond lengths of respectively 133 and 120 pm due to increased s - character of the sigma bond . In benzene all bonds have the same length: 139 pm . Carbon--carbon single bonds increased s - character is also notable in the central bond of diacetylene (137 pm) and that of a certain tetrahedrane dimer (144 pm). </P> <P> In propionitrile the cyano group withdraws electrons, also resulting in a reduced bond length (144 pm). Squeezing a C--C bond is also possible by application of strain . An unusual organic compound exists called In - methylcyclophane with a very short bond distance of 147 pm for the methyl group being squeezed between a triptycene and a phenyl group . In an in silico experiment a bond distance of 136 pm was estimated for neopentane locked up in fullerene . The smallest theoretical C--C single bond obtained in this study is 131 pm for a hypothetical tetrahedrane derivative . </P>

The carbon carbon bond length in benzene is