Increasing stability of the fullerenes with the number of carbon atoms:: The experimental evidence

The values of the molar standard enthalpies of formation, Delta H-f(m)degrees (C-76, cr) = (2705.6 +/- 37.7) kJ center dot mol(-1), Delta H-f(m)degrees (C-78, cr) = (2766.5 +/- 36.7) kJ center dot mol(-1), and Delta H-f(m)degrees (C-84,cr) = (2826.6 +/- 42.6) kJ center dot mol(-1), were determined from the energies of combustion, measured by microcombustion calorimetry on a high-purity sample of the D-2 isomer of fullerene C-76, as well as on a mixture of the two most abundant constitutional isomers of C-78 (C-2v-C-78 and D-3-C-78) and C-84 (D-2-C-84, and D-2d-C-84). These values, combined with the published data on the enthalpies of sublimation of each cluster, lead to the gas-phase enthalpies of formation, Delta H-f(m)degrees (C-76, cr) = (2911.6 +/- 37.9) kJ center dot mol(-1); Delta H-f(m)degrees (C-78, g) = (2979.3 +/- 37.2) kJ center dot mol(-1), and Delta H-f(m)degrees (C-84, g) = (3051.6 +/- 43.0) kJ center dot mol(-1), results that were found to compare well with those reported from density functional theory calculations. Values of enthalpies of atomization, strain energies, and the average C-C bond energy were also derived for each fullerene. A decreasing trend in the gas-phase enthalpy of formation and strain energy per carbon atom as the size of the cluster increases is found. This is the first experimental evidence that these fullerenes become more stable as they become larger. The derived experimental average C-C bond energy EC-C = 461.04 kJ center dot mol(-1) for fullerenes is close to the average bond energy EC-C = 462.8 kJ center dot mol(-1) for polycyclic aromatic hydrocarbons (PAHs).