Room-Temperature Structure of Ammonia Borane

Abstract

Structural determinations of ammonia borane (BH3NH3) have been carried out for the orthorhombic (at 90 K) and tetragonal (at 298 K) modifications using single-crystal X-ray data. The orthorhombic structure (space group Pmn21) agreed with a previously published neutron determination, while the tetragonal structure (I4mm) exhibited halos of hydrogen atom occupancy around both the nitrogen and boron atoms. The bond angles to the regions of hydrogen occupancy are consistent with the expected tetrahedral geometry for –BH3 and –NH3 groups. A new model for tetragonal BH3NH3 was constructed which accounts for the hydrogen disorder in the I4mm structure while introducing only weak new diffraction peaks. These peaks could not be found, however, and it is likely that the hydrogen disorder in tetragonal BH3NH3 arises from either rotations of higher than 3-fold order, or from random orientations of hydrogen-containing groups.

Positional disorder in ammonia borane at ambient conditions

“…Heated above 225 K, it undergoes a phase transition to a body-centered tetragonal structure with space group I4mm. It is this room-temperature phase that exhibits unexpected experimental results: While the molecule itself has a threefold symmetry about the B-N axis, neutron [11,14] and x-ray [12,15,16] diffraction on the solid reveal a fourfold symmetry about the same axis, creating a geometric incompatibility within the structure. Investigating the dynamics of the system with ab initio methods, we find that the individual halos are rotating with angular velocity on the order of 0.7 • /fs ≈ 2 rev/ps, such that standard experiments can only probe the time-averaged positions, leading to the tetragonal host structure with fourfold symmetry.…”

“…Ammonia borane NH 3 BH 3 has drawn significant interest in recent years because of its potential as a hydrogen storage material, with a gravimetric storage density of 19.6 mass% [1][2][3][4][5][6][7]. The structure of its solid phase has been explored previously [8][9][10][11][12][13], but the literature does not agree about the hydrogen behavior at room temperature. The molecule consists of a dative B-N bond and a trio of H atoms (henceforth referred to as a "halo") bonded to each of those two atoms, forming an hourglass shape, visible in Fig.…”

“…The structure of its solid phase has been explored previously [8][9][10][11][12][13], but the literature does not agree about the hydrogen behavior at room temperature. The molecule consists of a dative B-N bond and a trio of H atoms (henceforth referred to as a "halo") bonded to each of those two atoms, forming an hourglass shape, visible in Fig.…”

“…The CPMD simulations used an electronic convergence of 10 −8 eV, a fictitious electron mass of 400 a.u., and a time step of 5 a.u. We further used a 2 × 2 × 2 supercell, accommodating 16 molecules, and started from the experimental lattice constants [12] at 297 and 90 K for tetragonal and orthorhombic phases, respectively. Similar calculations have been done previously [24], but with a different functional and at much higher temperature.…”

High Pressure and Low Temperature Study of Ammonia Borane and Lithium Amidoborane

“…It transforms from the disordered I4mm structure to the ordered orthorhombic (Pmn2 1 ) structure at the temperature of ~ 225 K [69,70,71,72]. The transition temperature from disordered tetragonal (I4mm) structure to the ordered orthorhombic (Pmn2 1 ) structure increases with pressure, which indicates the Clapeyron slope is positive [73].…”