Positional disorder in ammonia borane at ambient conditions

We solve a long-standing experimental discrepancy of NH3BH3, which - as a molecule - has a threefold rotational axis, but in its crystallized form at room temperature shows a fourfold symmetry around the same axis, creating a geometric incompatibility. To explain this peculiar experimental result, we study the dynamics of this system with ab initio Car-Parrinello molecular dynamics and nudged-elastic band simulations. We find that rotations, rather than spatial static disorder, at angular velocities of 2 rev/ps - a time scale too small to be resolved by standard experimental techniques - are responsible for the fourfold symmetry.