The thermodynamic stabilities of tricyclic tetraene C12H12 hydrocarbons

The thermodynamic stabilities and heats of formation df tricyclic C12H12 te traenes 1-6 as well as of a truncated tetrahedron hydrocarbon isomer 7 were computed by various density functional methods in conjunction with a polar ized double-zeta; basis set. As the DFT stabilities of 1-7 differ significa ntly from the MM2, MM3, MM4 and AM1 results, we conclude that these empiric al and semiempirical methods are inappropriate to study such polycyclic hyd rocarbons. Compound 6 with only endocyclic double bonds, a potential synthe tic precursor of 7, is found to be less favorable energetically than the ot her isomers. Although the C-sp3-C-sp3 single bonds in 1-6 are rather long ( 1.601 to 1.620 Angstrom) due to a combination of ring strain and hyperconju gation, 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 t his is due to the cumulative diatropic influence of the four cyclopropane r ings.