Reliable Predictions of the Thermochemistry of Boron-Nitrogen Hydrogen Storage Compounds: BxNxHy, x=2,3
Thermochemical data calculated using ab initio molecular orbital theory are reported for 16 BxNxHy compounds with x = 2, 3 and y ≥ 2x. Accurate gas phase heats of formation were obtained using coupled cluster with single and double excitations and perturbative triples (CCSD(T)) valence electron calculations extrapolated to the complete basis set (CBS) limit with additional corrections including core/valence, scalar relativistic, and spin-orbit corrections to predict the atomization energies and scaled harmonic frequencies to correct for zero point and thermal energies and estimate entropies. Computationally cheaper calculations were also performed using the G3MP2 and G3B3 variants of the Gaussian-3 method, as well as density functional theory (DFT) using the B3LYP functional. The G3MP2 heats of formation are too positive by up to ~6 kcal/mol as compared to CCSD(T)/CBS values. The more expensive G3B3 method predicts heats of formation that are too negative as compared to the CCSD(T)/CBS values by up to 3 to 4 kcal/mol. Density functional theory (DFT) using the B3LYP functional and 6-311+G** basis set predict isodesmic reaction energies to within a few kcal/mol compared to the CCSD(T)/CBS method so isodesmic reactions involving BN compounds and the analogous hydrocarbons can be used to estimate heats of formation. Heats of formation of cyclo-B3N3H12 and cyclo-B3N3H6 are, in kcal/mol at 298 K, -95.5 and -115.5, respectively using our best calculated CCSD(T)/CBS approach. The experimental value for cyclo-B3N3H6 appears to be ~7 kcal/mol too negative. Enthalpies, entropies and free energies are calculated for many dehydrocoupling and dehydrogenation reactions that convert BNH6 to alicyclic and cyclic oligomers and H2(g). Generally, the reactions are highly exothermic, and exorgonic as well, due to the release of 1 or more equivalents of H2(g). For cyclo-B3N3H12 and cyclo-B3N3H6, available experimental data for sublimation and vaporization lead to estimates of their condensed phase 298 K heats of formation: ΔHf°[cyclo-B3N3H12(s)] = -124 kcal/mol and ΔHf°[cyclo-B3N3H6(l)] = -123 kcal/mol. The reaction thermochemistry for dehydrocoupling of BNH6(s) to cyclo-B3N3H12(s) and the dehydrogenation of cyclo-B3N3H12(s) to cyclo-B3N3H6(l) are much less exothermic compared to the gas phase reactions due to intermolecular forces which decrease in the order BNH6 > cyclo-B3N3H12 > cyclo-B3N3H6. The condensed phase reaction free energies are less negative compared to the gas phase reactions, but still too favorable for BNH6 to be regenerated from either cyclo-B3N3H12 or cyclo-B3N3H6 by just an overpressure of H2. Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Environmental Molecular Sciences Lab. (EMSL)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 912501
- Report Number(s):
- PNNL-SA-53979; 9601; EB4202000; TRN: US200801%%1055
- Journal Information:
- Journal of Physical Chemistry A, 111(20):4411-4421, Vol. 111, Issue 20
- Country of Publication:
- United States
- Language:
- English
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