title: Heats of formation of platonic hydrocarbon cages by means of high-level thermochemical procedures.
abstract: 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 (Δf H°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.
authors.affiliation: School of Chemistry and Biochemistry, The University of Western Australia, Perth, Western, 6009, Australia.
authors.forename: Amir
authors.initials: A
authors.lastname: Karton
authors.affiliation: Institute of Organic Chemistry, Justus-Liebig University, Heinrich-Buff-Ring 58, 35392, Giessen, Germany.
authors.forename: Peter R
authors.initials: PR
authors.lastname: Schreiner
authors.affiliation: Department of Organic Chemistry, Weizmann Institute of Science, Reḥovot, Israel.
authors.forename: Jan M L
authors.initials: JM
authors.lastname: Martin
keywords: CCSD(T); computational thermochemistry; explicitly correlated coupled cluster; platonic hydrocarbon cages; prismatic hydrocarbon cages