Structural and theoretical investigations of short hydrogen bonds: neutron diffraction and plane-wave DFT calculations of urea–phosphoric acid

Low temperature neutron diffraction and high level computational methods have been applied to investigate the short hydrogen bond in urea–phosphoric acid. It is found that isolated molecule calculations predict a `normalʹ O–H⋯O hydrogen bond, in strong disagreement with the very short, 3c–4e hydrogen bond found from the neutron diffraction. Extending these calculations into a periodic environment using plane-wave DFT methods give much improved agreement with experiment, with a much shorter, stronger hydrogen bond, and significant elongation of the O–H `covalentʹ bond.