PNNL

Abstract

High-level electronic structure calculations have been used to predict the thermodynamic stability of ammonia triborane B3H7NH3 and the molecular mechanism of H2 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 H2 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 H2 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 H2 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 ?H sub f (0 K) = 37.1 plus or minus 0.8 kcal/mol and ?H sub f (298 K) = 32.5 plus or minus 0.8 kcal/mol, and for the most stable isomer of B3H7NH3 to be ?H sub f (0 K) = 0.4 plus or minus 1.0 kcal/mol and ?H sub f (298 K) = -7.1 plus or minus 1.0 kcal/mol.