The Thermochemistry of Cubane 50 Years after Its Synthesis: A High-Level Theoretical Study of Cubane and Its Derivatives

The gas-phase enthalpy of formation of cubane (603.4 ± 4 kJ mol–1) was calculated using an explicitly correlated composite method (W1-F12). The result obtained for cubane, together with the experimental value for the enthalpy of sublimation, 54.8 ± 2.0 kJ mol–1, led to 548.6 ± 4.5 kJ mol–1 for the solid-phase enthalpy of formation. This value is only 6.8 kJ mol–1 higher than the 50-year-old original calorimetric result. The carbon–hydrogen bond dissociation enthalpy (C–H BDE) of cubane (438.4 ± 4 kJ mol–1), together with properties relevant for its experimental determination using gas-phase ion thermochemistry, namely the cubane gas-phase acidity (1704.6 ± 4 kJ mol–1), cubyl radical electron affinity (45.8 ± 4 kJ mol–1), cubane ionization energy (1435.1 ± 4 kJ mol–1), cubyl radical cation proton affinity (918.8 ± 4 kJ mol–1), cubane cation appearance energy (1099.6 ± 4 kJ mol–1), and cubyl ionization energy (661.2 ± 4 kJ mol–1), were also determined. These values were compared with those calculated for unstrained hydrocarbons (viz., methane, ethane, and isobutane). The strain energy of cubane (667.2 kJ mol–1) and cubyl radical (689.4 kJ mol–1) were independently estimated via quasihomodesmotic reactions. These values were related via a simple model to the C–H BDE in cubane. Taking into account the accuracy of the computational method, the comparison with high-precision experimental results, and the data consistency afforded by the relevant thermodynamic cycles, we claim an uncertainty better than ±4 kJ mol–1 for the new enthalpy of formation values presented.