PRELIMINARY ANALYSIS OF CCD DATA FOR OXALIC ACID DIHYDRATE AT 100 K. A. A. Pinkerton, Department of Chemistry, University of Toledo, Toledo, Ohio 43606, C. F. Campana and M. R. Pressprich, Siemens Energy and Automation, Inc., Analytical Instrumentation, 6300 Enterprise Lane, Madison, Wisconsin 53719-1173, U.S.A.

We have used a standard Siemens CCD-based SMART diffractometer system equipped with a Mo-target sealed-tube X-ray source (operated at 2000 watts) to collected an 86.3 hour data set on a single-crystal specimen of oxalic acid dihydrate, C₂H₂O₄[[dotaccent]]2H₂O, at 100 K. The detector was placed at a distance of 4.984 cm. from the crystal. This data set consisted of three overlapping shells of data to maximum 2[[theta]] angle of 142.76[[ordmasculine]] (0.376 Å resolution). A total of 8400 frames of data were collected with 30 second exposure times and 0.3[[ordmasculine]] frame widths in [[omega]].

The frames were integrated with the Siemens SAINT^(c) software package using a narrow-frame integration algorithm. The integration of the data using a primitive monoclinic unit cell yielded a total of 15657) reflections, of which 4675 were independent (redundancy 3.35) and 3858 (82.5%) were greater than 4[[sigma]](F). The final cell constants for a P2₁/n unit cell of dimensions a = 6.1039(3) Å, b = 3.4988(2) Å, c = 11.9576(5) Å, [[beta]] = 105.785(5)[[ordmasculine]], volume = 245.74(6) ų, were obtained from the refinement of the XYZ-centroids of 8192 reflections above 20 [[sigma]](I).

These preliminary data have been analyzed and the structure refined using several commonly used refinement packages, in order to evaluate the suitability of CCD data for charge density studies. These results will be compared with the previously reported analyses¹. The evaluation of this preliminary data will help us to optimize experimental parameters for more careful measurements to be carried out in the near future.

¹Coppens et al., Acta Cryst., (1984), A40, 184-195.