Heats of formation of platonic hydrocarbon cages by means of high-level thermochemical procedures

Hydrocarbon cages are key reference materials for the validation and parameterization of computationally cost-effective procedures such as density functional theory (DFT), semiempirical molecular orbital theory, and molecular mechanics. We obtain accurate total atomization energies (TAEs) and heats of formation (Δ_fH°₂₉₈) for platonic and prismatic hydrocarbon cages by means of the Wn-F12 explicitly correlated thermochemical protocols. We consider the following kinetically stable (CH)_n polycyclic hydrocarbon cages: (i) platonic hydrocarbons (tetrahedrane, cubane, and dodecahedrane), (ii) prismatic hydrocarbons (triprismane, cubane, and pentaprismane), and (iii) one truncated tetrahedrane (octahedrane). Our best theoretical heat of formation for cubane (144.8 kcal mol^-1) suggests that the experimental value adopted by the NIST thermochemical database (142.7 ± 1.2 kcal mol^-1) should be revised upwards by ∼2 kcal mol^-1. Our best heat of formation for dodecahedrane (20.2 kcal mol^-1) suggests that the semiexperimental value (22.4 ± 1 kcal mol^-1) should be revised downward by ∼2 kcal mol^-1. We use our benchmark Wn-F12 TAEs to evaluate the performance of a variety of computationally less demanding composite thermochemical procedures. These include the Gaussian-n (Gn) and the complete basis set (CBS) methods. The CBS-QB3 and CBS-APNO procedures show relatively poor performance with root-mean-squared deviations (RMSDs) of 4.2 and 2.5 kcal mol^-1, respectively. The best performers of the Gn procedures are G4 and G3(MP2)B3 (RMSD = 0.5 and 0.6 kcal mol^-1, respectively), while the worst performers are G3 and G4(MP2)-6X (RMSD = 2.1 and 2.9 kcal mol^-1, respectively). Isodesmic and even homodesmotic reactions involving these species are surprisingly challenging targets for DFT computations.