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 comb ustion in a microcalorimeter, is about 4 kcal/mol higher than that pre viously reported and corresponds to ab initio and density functional t heory computational results. As a consequence, the previously derived homoaromatic stabilization energy (claimed to be 4.5 kcal/mol) from en thalpy of hydrogenation measurements is not present in 1. The lack of homoaromaticity in 1 is supported by evaluation of geometric, energeti c, and magnetic criteria. In contrast, the isomerization transition st ate from diademane (5) to 1 is highly aromatic on the basis of the sam e 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 t his 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.