PNNL

Abstract

The activation energies for rotations in low temperature orthorhombic ammonia borane were analyzed and characterized in terms of electronic structure theory. The perdeuterated, 11B- enriched ammonia borane 11BD3ND3 sample was synthesized and the structure was refined from neutron powder diffraction data at 175 K. This temperature has been chosen as median of the range of previously reported NMR measurements of these rotations. A representative molecular cluster model was assembled from the refined geometry and the activation energies were calculated and characterized by analysis of the environmental factors that control the rotational dynamics. The barrier for independent NH3 rotation, Ea = 12.7 kJ/mol, largely depends on the molecular conformational torsion in the solid state geometry. The barrier for independent BH3 rotation, Ea = 38.3 kJ/mol, results from the summation of the effect of molecular torsion and large repulsive intermolecular hydrogen-hydrogen interactions. However, a barrier of Ea = 31.1 kJ/mol was calculated for rotation with preserved molecular conformation. Analysis of the barrier heights and the corresponding rotational pathways shows that rotation of the BH3 group involves strongly correlated rotation of the NH3 end of the molecule. This observation suggests that the barrier from previously reported measurement of BH3 rotation, corresponds to H3B—NH3 correlated rotation. Support for this work by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences Division is gratefully acknowledged. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.