Ammonia triborane: Theoretical study of the mechanism of hydrogen release

High-level electronic structure calculations have been used to predict the thermodynamic stability of ammonia triborane B3H7NH3 and the molecular mechanism of H-2 elimination from various isomeric forms in the gas phase. Geometries of stationary points were optimized at the second-order perturbation theory MP2 level, and total energies were computed at the coupled-cluster CCSD(T) theory with the aug-cc-pVnZ (n = D, T, Q) basis sets and extrapolated to the complete basis set limit. Heats of formation for the structures considered in the gas phase were evaluated at both 0 and 298 K. The lowest-energy process for H-2 release from the most stable isomer of B3H7NH3 is a 1,3-elimination characterized by an energy barrier of 28.9 kcal/mol. Although the barrier height for H-2 release from B3H7NH3 is slightly smaller than the B-N bond cleavage energy of 30.7 kcal/mol yielding B3H7 + NH3, the calculated rate coefficients predict that bond cleavage is faster than H-2 release by 3 orders of magnitude at 298 K and 1 atm. We predict the heat of formation for the most stable isomer of B3H7 to be Delta H-f (0 K) = 37.1 +/- 0.8 kcal/mol and Delta H-f (298 K) = 32.5 +/- 0.8 kcal/mol, and for the most stable isomer of B3H7NH3 to be Delta H-f (0 K) = 0.4 +/- 1.0 kcal/mol and Delta H-f (298 K) = -7.1 +/- 1.0 kcal/mol.