A Definitive Heat of Vaporization of Silicon through Benchmark ab Initio Calculations on SiF₄

To resolve a significant uncertainty in the heat of vaporization of silicon a fundamental parameter in gas-phase thermochemistry Δ H ° f,0 [Si(g)] has been determined from a thermochemical cycle involving the precisely known experimental heats of formation of SiF 4 (g) and F(g) and a benchmark calculation of the total atomization energy (TAE 0 ) of SiF 4 using coupled-cluster methods. Basis sets up to [8s7p6d4f2g1h] on Si and [7s6p5d4f3g2h] on F have been employed, and extrapolations for residual basis set incompleteness applied. The contributions of inner-shell correlation (−0.08 kcal/mol), scalar relativistic effects (−1.88 kcal/mol), atomic spin−orbit splitting (−1.97 kcal/mol), and anharmonicity in the zero-point energy (+0.04 kcal/mol) have all been explicitly accounted for. Our benchmark TAE 0 = 565.89 ± 0.22 kcal/mol leads to Δ H ° f,0 [Si(g)] = 107.15 ± 0.38 kcal/mol (Δ H ° f,298 [Si(g)] = 108.19 ± 0.38 kcal/mol): between the JANAF/CODATA value of 106.5 ± 1.9 kcal/mol and the revised value proposed by Grev and Schaefer [ J. Chem. Phys. 1992, 97, 8389], 108.1 ± 0.5 kcal/mol. The revision will be relevant for future computational studies on heats of formation of silicon compounds. Among standard computational thermochemistry methods, G2 and G3 theory exhibit large errors, while CBS-Q performs relatively well, and the very recent W1 theory reproduces the present calibration result to 0.1 kcal/mol.