The thermodynamic stabilities of tricyclic tetraene C12H12 hydrocarbons
The thermodynamic stabilities and heats of formation of tricyclic C12H12 tetraenes 1–6 as well as of a truncated tetrahedron hydrocarbon isomer 7 were computed by various density functional methods in conjunction with a polarized double-ζ basis set. As the DFT stabilities of 1–7 differ significantly from the MM2, MM3, MM4 and AM1 results, we conclude that these empirical and semiempirical methods are inappropriate to study such polycyclic hydrocarbons. Compound 6 with only endocyclic double bonds, a potential synthetic precursor of 7, is found to be less favorable energetically than the other isomers. Although the Csp3–Csp3 single bonds in 1–6 are rather long (1.601 to 1.620 Å) due to a combination of ring strain and hyperconjugation, the small nucleus-independent chemical shift (NICS) values of 1 and 6 confirm the expectation that cyclic electron delocalization is lacking. In contrast, NICS is unusually large in the cage center (–14.6) of 7, but this is due to the cumulative diatropic influence of the four cyclopropane rings.