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

Abstract

Atomization energies at 0 K and heats of formation at 0 and 298 K are predicted for diammoniosilane, H(4)Si(NH(3))(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 NH(3) molecules, so that the diammoniosilane complex is only 25.6 kcal/mol at 298 K above the separated reactants SiH(4) + 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 H(3)Si(NH(2))(NH(3)) and H(2)Si(NH(2))(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 H(2)Si(NH(2))(2) consequently yielding HN=SiH(NH(2)) 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 H(4)Si(NH(3))(2) would be almost thermoneutral at 0 K.