Charge density studies involve the direct experimental measurement of the electron distribution in molecules, and hence provide detailed information about the chemical bonding between atoms. They require data of the highest quality, with observed reflections collected out to high scattering angles (preferably 0.5-0.4 Å), and also with very high redundancy of the dataset, preferably of 10 or more. Very high quality crystals are required to meet these requirements; growing appropriate crystals is very often the most challenging part of a charge density experiment.

Typical data collections using CCD diffractometers require collection of overlapping datasets using different collection times. Long data collection times are required for the weaker higher angle reflections, whereas the low angle reflections require much shorter times to avoid overloading the CCD chip.

Using an imaging plate system for charge density data collections has a number of advantages; the greater dynamic range of the imaging plate means that it is a simple matter to collect both the strong low-angle reflections and the high-angle reflections at an exposure time that gives good counting statistics for the weaker high angle reflections.

The IUCr electron density standard, oxalic acid dihydrate, was mounted on an RAPID II integrated with the Rigaku ultraX 18 rotating anode generator with Mo radiation and a graphite monochromator at low temperature.

A contour map based on a Fourier synthesis using only the lower-resolution data was calculated in the plane of the four oxalate O atoms. In the plot, the lowest contour level is 0.15 e/ų and the highest is 0.45 e/ų. 

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The fact that bonding density shows up using only the standard atomic model is evidence of the high quality of the data. There is clear indication of bonding density in the C-C and C-O bonds. The lone pairs on atom O2 are clearly indicated, as is their participation in the hydrogen bonds to the water molecules. (Note that in published work describing the experimental observation of bonding density in oxalic acid, the lone pairs on the oxygen atoms are not visible in the difference map, but become apparent only after multipole refinement).

Atoms O3B and O3C are essentially in the calculation plane, and their contribution of a lone pair to the hydrogen bond to the acidic H atom is clearly visible. Atoms O3 and O3A are about 0.33 A out of the plane, and the O3-H3...O2 hydrogen bond is apparent. The O2...H2B-(O3) hydrogen bond is also indicated. The lone pairs on O1 are out of the plane, and so do not appear in this section of the Fourier.