Molecular structure and dynamics in the low temperature (orthorhombic) phase of NH3BH3

Variable temperature. H-2 NMR experiments on the orthorhombic phase of selectively deuterated NH3BH3 spanning the static to fast exchange limits of the borane and amine motions are reported. New values of the electric field gradient (EFG) tensor parameters have been obtained from the static H-2 spectra of V-zz = 1.652 (+/- 0.082) x 10(21) V/m(2) and eta = 0.00 +/- 0.05 for the borane hydrogens and V-zz = 2.883 ( +/- 0.144) x 10(21) V/m(2) and q = 0.00 0.05 for the amine hydrogens. The molecular symmetry inferred from the observation of equal EFG tensors for the three borane hydrogens and likewise for the three amine hydrogens is in sharp contrast with the C-s symmetry derived from diffraction studies. The origin of the apparent discrepancy has been investigated using molecular dynamics methods in combination with electronic structure calculations of NMR parameters, bond lengths, and bond angles. The computation of parameters from a statistical ensemble rather than from a single set of atomic Cartesian coordinates gives values that are in close quantitative agreement with the H-2 NMR electric field gradient tensor measurements and are more consistent with the molecular symmetry revealed by the NMR spectra.