Positional disorder in ammonia borane at ambient conditions

TL;DR

This paper explains the apparent fourfold symmetry in crystalline ammonia borane at room temperature as a result of rapid rotational dynamics, resolving a long-standing experimental discrepancy.

Contribution

It demonstrates that fast molecular rotations cause the observed symmetry, providing a dynamic explanation rather than static disorder.

Findings

Rotations at 2 rev/ps explain the fourfold symmetry.

Dynamic rotational motion accounts for experimental observations.

Static disorder is not necessary to explain the symmetry.

Abstract

We solve a long-standing experimental discrepancy of NH$_3$BH$_3$, which---as a molecule---has a threefold rotational axis, but in its crystallized form at room temperature shows a fourfold symmetry about 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.