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 B 3 H 7 NH 3 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 B 3 H 7 NH 3 is a 1,3-elimination characterized by an energy barrier of 28.9 kcal/mol. Although the barrier height for H 2 release from B 3 H 7 NH 3 is slightly smaller than the B−N bond cleavage energy of 30.7 kcal/mol yielding B 3 H 7 + NH 3, 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 B 3 H 7 to be Δ H f (0 K) = 37.1 ± 0.8 kcal/mol and Δ H f (298 K) = 32.5 ± 0.8 kcal/mol, and for the most stable isomer of B 3 H 7 NH 3 to be Δ H f (0 K) = 0.4 ± 1.0 kcal/mol and Δ H f (298 K) = −7.1 ± 1.0 kcal/mol.