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