Thermodynamic Properties of Molecular Borane Phosphines, Alane Amines, and Phosphine Alanes and the [BH4-][PH4+], [AlH4-][NH4+], and [AlH4-][PH4+] Salts for Chemical Hydrogen Storage Systems from ab Initio Electronic Structure Theory

J. Phys. Chem. A, 109 (44), 10138 -10147, 2005. 10.1021/jp054152y S1089-5639(05)04152-6
Web Release Date: October 15, 2005

Thermodynamic Properties of Molecular Borane Phosphines, Alane Amines, and Phosphine Alanes and the [BH₄^-][PH₄⁺], [AlH₄^-][NH₄⁺], and [AlH₄^-][PH₄⁺] Salts for Chemical Hydrogen Storage Systems from ab Initio Electronic Structure Theory

Daniel J. Grant and David A. Dixon*

Department of Chemistry, University of Alabama, P.O. Box 870336, Tuscaloosa, Alabama 35487-0336

Received: July 27, 2005

In Final Form: September 1, 2005

Abstract:

The heats of formation for the molecules BH₃PH₃, BH₂PH₂, HBPH, AlH₃NH₃, AlH₂NH₂, HAlNH, AlH₃PH₃, AlH₂PH₂, HAlPH, AlH₄^-, PH₃, PH₄, and PH₄⁺, as well as the diatomics BP, AlN, and AlP, have been calculated by using ab initio molecular orbital theory. The coupled cluster with single and double excitations and perturbative triples method (CCSD(T)) was employed for the total valence electronic energies. Correlation consistent basis sets were used, up through the augmented quadruple-, to extrapolate to the complete basis set limit. Additional d core functions were used for Al and P. Core/valence, scalar relativistic, and spin-orbit corrections were included in an additive fashion to predict the atomization energies. Geometries were calculated at the CCSD(T) level up through at least aug-cc-pVTZ and frequencies were calculated at the CCSD(T)/aug-cc-pVDZ level. The heats of formation of the salts [BH₄^-][PH₄⁺](s), [AlH₄^-][NH₄⁺](s), and [AlH₄^-][PH₄⁺](s) have been estimated by using an empirical expression for the lattice energy and the calculated heats of formation of the two component ions. The calculations show that both AlH₃NH₃(g) and [AlH₄^-][NH₄⁺](s) can serve as good hydrogen storage systems that release H₂ in a slightly exothermic process. In addition, AlH₃PH₃ and the salts [AlH₄^-][PH₄⁺] and [BH₄^-][PH₄⁺] have the potential to serve as H₂ storage systems. The hydride affinity of AlH₃ is calculated to be -70.4 kcal/mol at 298 K. The proton affinity of PH₃ is calculated to be 187.8 kcal/mol at 298 K in excellent agreement with the experimental value of 188 kcal/mol. PH₄ is calculated to be barely stable with respect to loss of a hydrogen to form PH₃.

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