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Graphite and diamond are typical carbon allotropes. In recent years, fullerenes, carbon nanotubes and graphene have been further discovered as newer forms of carbon allotropes. These carbon allotropes are mostly composed of six-membered rings, although other types of rings are also included in some fullerenes and nanotubes. Is it possible to build new carbon allotropes by using a four-membered carbon ring as a key building block instead of a six-membered carbon ring? Are they too unstable to be detected even if they can be produced?

Cyclobutadiene has a typical four-membered carbon ring, and is a planar D2h structure. A planar D2h structure can also be seen in butalene, which is shown in Figure 4.1 together with cyclobutadiene. These structures can be found for some hydrocarbons; cyclobutadiene was found as an equilibrium (EQ) structure EQ4 for C4H4 in Figure 4.10, and butalene may also be found as an EQ for C6H4 on the PES.1,2  Based on these findings for cyclobutadiene and butalene, one may imagine a ladder structure for C16H4 with a planar D2h geometry shown in Figure 4.1c. Geometry optimization of C16H4 that is started from an initial structure with a planar D2h geometry shown in Figure 4.1c will yield rounded forms as minima, such as those in Figure 4.1d and e, since the planar ladder form (Figure 4.1c) is unstable.

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