Quasielastic neutron scattering of -NH3 and -BH3 rotational dynamics in orthorhombic ammonia borane

Energy barriers for rotation of -NH3 and -BH3 in the orthorhombic phase of ammonia borane, NH3BH3, were determined using quasielastic neutron scattering (QENS). QENS confirms the 3-site jump model of rotational diffusion and yields barrier heights of 23.6 +/- 1.0 kJ/mol and 14.8 +/- 0.4 kJ/mol for the borane and amine groups, respectively, which are comparable to barrier heights determined by recent H-2 and N-15 NMR studies suggesting no significant isotope effect on rotational motion in the orthorhombic phase of ammonia borane. Published by Elsevier B.V.