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 (Delta H-f degrees(298)) for platonic and prismatic hydrocarbon cages by means of the Wn-F12 explicitly correlated thermochemical protocols. We consider the following kinetically lstable (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 similar to 2 kcal mol(-1). Our best heat of formation for dodecahedrane (20.2 kcal mol(-1)) suggests that the semiex-perimental value (22.4 +/- 1 kcal mol(-1)) should be revised downward by similar to 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. (C) 2015 Wiley Periodicals, Inc.