An experimental thermochemical and theoretical study of triquinacene:: Definitive disproof of its neutral homoaromaticity

The enthalpy of formation, (Delta H-f(o)(g) = 57.51 +/- 0.70 kcal/mol) of triquinacene (1), newly determined by measuring its energy of combustion in a microcalorimeter, is about 4 kcal/mol higher than that previously reported and corresponds to ab initio and density functional theory computational results. As a consequence, the previously derived homoaromatic stabilization energy (claimed to be 4.5 kcal/mol) from enthalpy of hydrogenation measurements is not present in 1. The lack of homoaromaticity in 1 is supported by evaluation of geometric, energetic, and magnetic criteria. In contrast, the isomerization transition state from diademane (5) to 1 is highly aromatic on the basis of the same criteria. The enthalpy of isomerization of 5 to 1 was experimentally determined by differential scanning calorimetry (DSC) to be -29.4 +/- 0.3 kcal/ mol (measured at 368.2 K). The enthalpy of activation for this rearrangement as determined from the DSC measurements (28.4 +/- 0.2 kcal/mol) is 2.5 kcal/mol higher than the value computed at B3LYP/6-311+G**ZPE.