Thermodynamic Properties of Molecular Borane Amines and the [BH4-][NH4+] Salt for Chemical Hydrogen Storage Systems from ab Initio Electronic Structure Theory

Abstract

The heats of formation for the borane amines BH₃NH₃, BH₂NH₂, and HBNH, tetrahedral BH₄^-, and the BN molecule have been calculated by using ab initio molecular orbital theory. Coupled cluster calculations with single and double excitations and perturbative triples (CCSD(T)) were employed for the total valence electronic energies. Correlation consistent basis sets were used, up through the augmented quadruple-ζ, to extrapolate to the complete basis set limit. 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 (in kcal/mol) at 0 K in the gas phase are ΔH_f(BH₃NH₃) = −9.1, ΔH_f(BH₂NH₂) = −15.9, ΔH_f(BHNH) = 13.6, ΔH_f(BN) = 146.4, and ΔH_f(BH₄^-) = −11.6. The reported experimental value for ΔH_f(BN) is clearly in error. The heat of formation of the salt [BH₄^-][NH₄⁺](s) has 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 BH₃NH₃(g) and [BH₄^-][NH₄⁺](s) can serve as good hydrogen storage systems which release H₂ in a slightly exothermic process. The hydride affinity of BH₃ is calculated to be 72.2 kcal/mol, in excellent agreement with the experimental value at 298 K of 74.2 ± 2.8 kcal/mol.