A symmetric hydrogen bond revisited: potassium hydrogen maleate by variable temperature, variable pressure neutron diffraction and plane-wave DFT methods.

The symmetric hydrogen bond in potassium hydrogen maleate has been re-investigated by diffraction and computational methods. Single crystal neutron diffraction under varying temperature (30–295 K) and pressure (ambient-4 kbar) confirmed the crystallographically-constrained symmetric disposition of the hydrogen atom in the short, strong hydrogen bond for each of eight sets of (p,T) conditions. Plane-wave DFT calculations show the hydrogen atom still to occupy this symmetric position even when crystallographic symmetry is removed, in stark contrast to the results from isolated molecule ab initio calculations. These results demonstrate that the symmetric (or close to symmetric) nature of this hydrogen bond is intrinsic.