Single-crystal neutron diffraction of urea-phosphoric acid: evidence for H-atom migration in a short hydrogen bond between 150 K and 350 K

The structure of urea-phosphoric acid has been refined using single-crystal neutron diffraction data collected at seven temperatures in the range 150K to 350 K. The structure is orthorhombic, space group Pbca, z = 8, a = 17.453(5)-17.679(8), b = 7.454(3)-7.483(4), c = 8.963(3)-8.992(4) Angstrom, V = 1166.0-1189.6 Angstrom (3). The molecules are linked by a short intermolecular O . . .H . . .O hydrogen bond, with O . . .O approximate to 2.4 Angstrom and angleO . . .H . . .O approximate to 170 degrees. At 150 K, this bond is markedly asymmetric, with O . . .H = 1.168(4) Angstrom and H . . .O = 1.252(4) Angstrom. However, at 300 K and above, the H-atom is essentially centred between the two O-atoms. This systematic shift in position of the H-atom (up to approximate to0.04 Angstrom) is significant in relation to the esd's of the measurements. H-atom migration along a hydrogen bond with temperature has been observed previously with other species, including quinolinic acid, and the hydrogen phthalate anions of lithium hydrogen phthalate monohydrate. There is no evidence to support the notion that the H-atom within the O . . .H . . .O hydrogen bond in urea-phosphoric acid is disordered, and the diffraction data are consistent with the previously reported hypothesis of a single-minimum potential energy surface for the short hydrogen bond.