Nets with Both sp2- and sp3-Hybridized Carbon Atoms

INFINITE NETS WITH BOTH sp2 - and sp3 -HYBRIDIZED CARBON ATOMS 

For converting graphite into diamond, one has to connect carbon atoms belonging to different graphene sheets. Theoretical data were reported by Hoffmann et al. [22], and practical data are summarized by Bundy [18] and De Vries [19]. If two connections involve oi tho- situated carbon atoms (we call them 1,2-connections) one would obtain 4-membered rings (Fig. 8) if they involve para- situated carbon atoms (1,4-connections) one would obtain 8-membered rings (Fig. 9) [23]. 

A more favorable situation is an ortho- plus para- connection which leads to the formation of a 6-membered ring by a (1,2 + l,4)-connection (Fig. 10) however, with 1,3-connections one may obtain either the hexagonal or the cubic diamond lattice, as shown for the latter situation in Fig. 11, in which three graphene sheets are shown, still with their double bonds at the margins [23]. 

The graphitization of diamond may involve thermally-allowed six re-electron concerted processes an electrocyclic reaction converts a six-membered ring in the diamond lattice into three isolated double bonds as shown in Fig. 12. Subsequently, each of the three cyclohexene rings can undergo a retro-Diels-Alder reaction, multiplying the number of double bonds in a triple cascade reaction 23]. 

A few theoretical and experimental references relevant to the present discussion on transitions between diamond and graphite (or the B,N-analogs) and vice-versa follow [24-26). 

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