The role of multiple observations in small-molecule single-crystal service X-ray structure determination

Abstract

herein are an extension of studies carried out by AEW as an undergraduate project student in 2002, since when attempts to make contact have been unsuccessful. In her absence, ALT and DJW wish to acknowledge her contribution and inspiration, understanding that responsibility for any errors is entirely theirs.In order to gain a better understanding of how to improve the quality of smallmolecule single-crystal X-ray diffraction data achievable in a finite time, a study was carried out to investigate the effect of varying the multiplicity, acquisition time, detector binning, maximum resolution and completeness. The results suggest that, unless there are strong arguments for a different strategy, a good routine procedure might be to optimize the conditions necessary to get the best data from single scans, and then choose a multiplicity of observations (MoO) to utilize the available time fully. Different strategies may be required if the crystal is highly absorbing, is larger than the incident beam, is enclosed in a capillary tube or is unusual in some other way. The signal-to-noise ratio should be used with care, as collecting data for longer or at higher multiplicity appears to give a systematic underestimate of the intensity uncertainties. Further, the results demonstrate that including poor-quality data in a refinement may degrade the result and, in the general case, the accidental omission of reflections has a very small impact on the refinement as long as they are omitted at random. Systematic omission of reflections needs a convincing procedural justification.