Abstract:
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 AlH_3NH_3(g), AlH_3PH_3(g), [AlH_4^-][NH_4^+](s), [AlH_4^-][PH_4^+](s), and [BH_4^-][PH_4^+](s) can potentially serve as hydrogen storage systems, in addition to BH_3NH_3 and [BH_4^-][NH_4^+](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 π-bond energy is defined as the rotational barrier between the ground state and C_s transition state structures, the intrinsic π-bond energy as the adiabatic rotational barrier corrected for inversion, and σ-bond energy, as the adiabatic dissociation energy minus the adiabatic π-bond energy. Within the substituted boranes H_(3-n)BX_n (X = F, Cl, Br, I, NH_2, 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 BI_3, lowest for B(NH_2)_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 PF_xO and SF_xO compounds are employed to explain trends in their stepwise BDEs. The adiabatic BDE for removal of fluorine from stable closed-shell SF_6 to give the unstable SF_5 radical is 2.8 times the BDE for removal of fluorine from the unstable SF_5 radical to give stable closed-shell SF_4. Simlar principles govern the BDEs of the phosphorous fluorides and the phosphoro and sulfur oxofluorides.
