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 D_2h structure. A planar D_2h 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 D_2h geometry shown in Figure 4.1c. Geometry optimization of C₁₆H₄ that is started from an initial structure with a planar D_2h 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.