Diammoniosilane: Computational Prediction of the Thermodynamic Properties of a Potential Chemical Hydrogen Storage System

Atomization energies at 0 K and heats of formation at 0 and 298 K are predicted for diammoniosilane, H4Si(NH3)(2), and its dehydrogenated derivates at the CCSD(T) and G3(MP2) levels. To achieve near chemical accuracy (+/- 1 kcal/mol), three corrections were added to the complete basis set binding energies based on frozen core coupled cluster theory energies: a correction for core-valence effects, a correction for scalar relativistic effects, and a correction for first-order atomic spin-orbit effects. Vibrational zero-point energies were computed at the CCSD(T) or MP2 levels. The edge inversion barrier of silane is predicted to be 88.9 kcal/mol at 298 K at the CCSD(T) level and a substantial amount, -63.6 kcal/mol, is recovered upon complexation with 2 NH3 molecules, so that the diammoniosilane complex is only 25.6 kcal/mol at 298 K above the separated reactants SiH4 + 2NH(3). The complex is a metastable species characterized by all real frequencies at the MP2/aV(T+d)Z level. We predict the heat of reaction for the sequential dehydrogenation of diammoniosilane to yield H3Si(NH2)(NH3) and H2Si(NH2)(2) to be exothermic by 33.6 and 12.2 kcal/mol at 298 K at the CCSD(T) level, respectively. The cumulative dehydrogenation reaction yielding two molecules of hydrogen at 298 K is -45.8 kcal/mol at the CCSD(T) level. The sequential release of H-2 from H2Si(NH2)(2) consequently yielding HN=SiH(NH2) and HN=Si=NH are predicted to be largely endothermic reactions at 45.3 and 55.7 kcal/mol at the CCSD(T) level at 298 K. If the endothermic reaction for the third step and the exothermic reactions for the release of the first two H-2 were coupled effectively, loss of three H-2 molecules from H4Si(NH3)(2) would be almost thermoneutral at 0 K.