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.
†Centro de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal.
Filipe
F
Agapito
†Centro de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal.
Rui C
RC
Santos
†Centro de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal.|‡Institute for Biotechnology and Bioengineering, Center for Molecular and Structural Biomedicine, Universidade do Algarve, Campus de Gambelas, 8005-139 Faro, Portugal.
Rui M
RM
Borges dos Santos
†Centro de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal.
José A
JA
Martinho Simões
D034242:Cyclooctanes / Q000031:analogs & derivatives* / Q000138:chemical synthesis / Q000737:chemistry*; D011789:Quantum Theory; D013816:Thermodynamics
D034242:Cyclooctanes