Bond-switch defects in carbon allotropes: Stone-Wales and connected exchange

By analogy with the Stone–Wales rearrangement in sp2-hybridized carbon allotropes that involves turning one CC bond by 90° in a graphene sheet and reconnecting it cross-wise with two carbon atoms, the similar rearrangement for the diamond lattice was called connected exchange. In this case one C–C bond in the diamond lattice involving sp3-hybridized atoms is switched such that two anti bonds exchange one of their endpoints. As a result of this connected exchhange one obtains a defect in the diamond lattice with four 5-membered and four 7-membered rings. For the hydrocarbon pyracyclene with sp2-hybridized atoms, the Stone–Wales rearrangement is degenerate (automerization). By analogy, one can imagine a hydrocarbon having sp3-hybridized atoms that could undergo a degenerate connected exchange.