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 = 17.4 53(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 intermolec ular O . . .H . . .O hydrogen bond, with O . . .O approximate to 2.4 Angstr om 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 ce ntred between the two O-atoms. This systematic shift in position of the H-a tom (up to approximate to0.04 Angstrom) is significant in relation to the e sd's of the measurements. H-atom migration along a hydrogen bond with tempe rature has been observed previously with other species, including quinolini c acid, and the hydrogen phthalate anions of lithium hydrogen phthalate mon ohydrate. 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, a nd the diffraction data are consistent with the previously reported hypothe sis of a single-minimum potential energy surface for the short hydrogen bon d.