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.