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 nuclear magnetic resonance spectroscopy 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-1, largely depends on the molecular conformational torsion in the solid-state geometry. The barrier for independent BH3 rotation, Ea = 38.3 kJ mol-1, 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-1 was calculated for internally correlated 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.
Original language | English |
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Pages (from-to) | 4514-4522 |
Number of pages | 9 |
Journal | Dalton Transactions |
Issue number | 33 |
DOIs | |
State | Published - 2008 |
Externally published | Yes |