Heats of Formation of Beryllium, Boron, Aluminum, and Silicon Re-examined by Means of W4 Theory

Benchmark total atomization energies (TAE0 values) were obtained, by means of our recent W4 theory [Karton,
A.; Rabinowitz, E.; Martin, J. M. L.; Ruscic, B. J. Chem. Phys. 2006, 125, 144108], for the molecules Be2,
BeF2, BeCl2, BH, BF, BH3, BHF2, B2H6, BF3, AlF, AlF3, AlCl3, SiH4, Si2H6, and SiF4. We were then able to
deduce “semi-experimental” heats of formation for the elements beryllium, boron, aluminum, and silicon by
combining the calculated TAE0 values with experimental heats of formation obtained from reactions that do
not involve the species Be(g), B(g), Al(g), and Si(g). The elemental heats of formation are fundamental
thermochemical quantities that are required whenever a molecular heat of formation has to be derived from
a calculated binding energy. Our recommended

[A(g)] values are Be 76.4 ± 0.6 kcal/mol, B 135.1 ±
0.2 kcal/mol, Al 80.2 ± 0.4 kcal/mol, and Si 107.2 ± 0.2 kcal/mol. (The corresponding values at 298.15 K
are 77.4, 136.3, 80.8, and 108.2 kcal/mol, respectively.) The Be value is identical to the CODATA
recommendation (but with half of the uncertainty), while the B, Al, and Si values represent substantial revisions
from established earlier reference data. The revised B and Si values are in agreement with earlier semi-ab
initio derivations but carry much smaller uncertainties.