A definitive heat of vaporization of silicon through benchmark ab initio calculations on SiF4

Abstract

To resolve a significant uncertainty in the heat of vaporization of silicon-a fundamental parameter in gasphase thermochemistry-Delta H degrees(f,o) [Si(g)] has been determined from a thermochemical cycle involving the precisely known experimental heats of formation of SiF4(g) and F(g) and a benchmark calculation of the total atomization energy (TAE(o)) of SiF4 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), scaler 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(o) = 565.89 +/- 0.22 kcal/mol leads to Delta H degrees(f,o) [Si(g)] = 107.15 +/- 0.38 kcal/mol (Delta H degrees(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.