Migration of the proton in the strong O-H...O hydrogen bond in urea-phosphoric acid (1/1)

Chick C. Wilson, Acta Cryst. (2001). B57, 435-439

(a) ORTEPII (Johnson, 1971) plot of the urea–phosphoric acid structure at 150 K, showing the atomic numbering and the strong O–H...O hydrogen bond. Note that partial transfer of the H atom (elongation of O5–H4) is indicative of signicant formation of the salt uronium phosphate. Probability ellipsoids are drawn at the 50% level. (b) The increasingly symmetric location of the proton in the short, strong hydrogen bond in urea–phosphoric acid is illustrated by the convergence of the 'bonded' (O5–H4; open triangles) and non-bonded (H4…O4; lled circles) distances as the temperature increases. Also shown on this plot (as lled squares inside circles— error bars omitted for clarity) are the values found by Harkema at 100 K (unpublished work) and by Kostansek & Busing (1972) at room to the eye and the dotted line indicates the 'centred' situation found here above ~300 K.

The author is an expert in single-crystal neutron diffraction using time-of-flight techniques and has developed a method for using multiple samples of molecular crystals. This can lead to a reduction in data collection times by the use of several rather smaller, deliberately misaligned crystals, mounted together in the cryorefrigerator. Previous experiments attempted in this area are reviewed and compared with this latest method used on the SXD single-crystal machine at ISIS, which was developed primarily for examining H-atom parameters in a variety of molecular crystals. The title compound provides an example of a rather short O-H...O interaction, whose complex nature cannot be described from X-ray data alone, namely whether there's a complete or partial proton transfer to form a salt or whether the H atom remains localized as in a simple adduct.

The relatively low data-to-parameter ratios in the GSAS refinements (Larson & Von Dreele, 1994) are justified by the successful outcomes reported, i.e., the normal appearance of anisotropic displacement parameters and the acceptable standard uncertainties of the atomic coordinates. The model from a previous, conventional neutron diffraction study at 150 K was used as the overall starting point and each new refinement at one temperature was used as the starting model for refinement at each subsequent temperature. The geometry of the hydrogen bond is very clearly temperature dependent in this short O-H...O interaction, showing that the proton migrates towards the centre of the O...O separation at the higher temperature (see figure). By contrast, two other O-H...O 'normal' intermolecular hydrogen bonds in the crystal structure show no such marked H-atom migrations, just a slight increase in the H...O non-bonded separation over the same temperature range and both values are significantly longer than those under discussion here (1.5-1.7 Å cf. 1.18-1.25 Å).

References
Harkema, S. (1993). Unpublished work, presented in ISIS Annual Report 1993, p. 18. Rutherford Appleton Laboratory Report RAL-93–050. Oxfordshire, UK.
Johnson, C.K. (1971). ORTEPII. Report ORNL-3794, revised. Oak Ridge National Laboratory, TN, USA. (Program implemented in GSAS, 1994.)
Kostansek, E.C. & Busing, W.R. (1972). Acta Cryst. B28, 2454–2459.
Larson, A.C. & Von Dreele, R.B. (1994). GSAS, General Structure Analysis System. Report LAUR-86–748. Los Alamos National Laboratory, NM, USA.