Structure, heats of formation, and bond dissociation energies of group IIIA-group IVA-group VA molecules for chemical hydrogen storage systems

The potential of Group IIIA.IVA.VA compounds for chemical hydrogen storage have been evaluated from thermodynamic properties, heats of formation and bond dissociation energies (BDEs), from CCSD(T) calculations in conjunction with correlation consistent basis sets extrapolated to the complete basis set limit, including additional core-valence, scalar-relativistic, and atomic spin-orbit corrections. Geometry optimizations and frequencies were computed at the CCSD(T)/MP2 levels. Diatomic distances, frequencies, and anharmonic constants were obtained from a potential energy curve fit at the CCSD(T) level. Calculations show that AlH3NH3(g), AlH3PH 3(g), [AlH4-][NH4+](s), [AlH4-][PH4+](s), and [BH4-][PH4+](s) can potentially serve as hydrogen storage systems, in addition to BH3NH3 and [BH4-][NH4+](s). Dehydrogenation of methyl-substituted ammonia boranes is most favorable across B-N where methylation at N reduces the reaction exothermocity, becoming more thermoneutral. The adiabatic pi-bond energy is defined as the rotational barrier between the ground state and Cs transition state structures, the intrinsic pi-bond energy as the adiabatic rotational barrier corrected for inversion, and sigma-bond energy, as the adiabatic dissociation energy minus the adiabatic pi-bond energy. Within the substituted boranes H 3-nBXn (X = F, Cl, Br, I, NH2, OH, and SH), fluorines have the largest BDEs while the second and third largest are for hydroxyl and amino. Hydride and fluoride affinities have been predicted to judge the Lewis acidities with the highest affinities found for BI3, lowest for B(NH2) 3, and within the boron trihalides, the acidity increases down the periodic table. Although the sequential dehydrogenation of diammoniosilane is exothermic, further dehydrogenation is largely endothermic, requiring an effective coupling process to remove three hydrogen molecules thermoneutrally. Except for methyliodosilane, methyl and halide substitution increases the Si-X and Si-C BDEs compared to the halosilanes and methylsilane, respectively. The differences in the adiabatic and diabatic BDEs in the PFxO and SFxO compounds are employed to explain trends in their stepwise BDEs. The adiabatic BDE for removal of fluorine from stable closed-shell SF6 to give the unstable SF5 radical is 2.8 times the BDE for removal of fluorine from the unstable SF5 radical to give stable closed-shell SF 4. Simlar principles govern the BDEs of the phosphorous fluorides and the phosphoro and sulfur oxofluorides.