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Materials for hydrogen storage: structure and dynamics of borane ammonia complex
Vencislav M Parvanov1, Gregory K Schenter, Nancy J Hess
1Fundamental and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, WA 99352, USA.
Dalton Transactions (Cambridge, England : 2003)
|August 14, 2008
Summary
This study analyzes ammonia borane rotations using electronic structure theory. It reveals that BH(3) group rotation is strongly correlated with NH(3) rotation in the solid state.
Area of Science:
- Solid-state chemistry
- Computational chemistry
- Molecular dynamics
Background:
- Ammonia borane is a key molecule in materials science and hydrogen storage.
- Understanding molecular rotations is crucial for predicting material properties.
- Previous studies on ammonia borane rotations lacked detailed theoretical analysis.
Purpose of the Study:
- To computationally determine the activation energies for molecular rotations in ammonia borane.
- To characterize the factors influencing these rotational dynamics.
- To elucidate the correlation between BH(3) and NH(3) group rotations.
Main Methods:
- Synthesis of perdeuterated, (11)B-enriched ammonia borane ((11)BD(3)ND(3)).
- Structure refinement using neutron powder diffraction at 175 K.
- Electronic structure calculations on a molecular cluster model.
Main Results:
- Calculated activation energy for independent NH(3) rotation: 12.7 kJ/mol, influenced by conformational torsion.
- Calculated activation energy for independent BH(3) rotation: 38.3 kJ/mol, due to torsion and intermolecular repulsion.
- Calculated activation energy for correlated rotation: 31.1 kJ/mol, indicating coupled motion.
Conclusions:
- BH(3) rotation in ammonia borane is not independent but strongly correlated with NH(3) rotation.
- This correlated rotation explains previously observed BH(3) rotation barriers.
- The findings provide insights into the solid-state dynamics of ammonia borane.
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