W0072

Absorption Correction of Area Detector Data from a VERY Large Crystal. Ward, Donald L., Dept. of Chemistry, Michigan State Univ., East Lansing, MI.

Faced with a very large crystal, and having acquired a long history of failed attempts to cut other crystals of this particular compound to a reasonable size, it was decided to take this opportunity to demonstrate the truth (or, the lack of truth) about rumors of the ability of the SADABS absorption correction program to correct the diffraction intensity measurements for errors due to the crystal extending beyond the limits of the X-ray beam.

The situation usually mentioned in these rumors was that of a needle crystal, having only its long dimension larger (and, then only slightly larger) than the X-ray beam diameter, so that the crystal would only extend beyond the limits of the X-ray beam for a small fraction of the intensity measurements.

The size of the crystal used in this demonstration, in relation to the 0.5 mm diameter collimator and X-ray beam, was VERY large at 0.8 mm x 1.3 mm x 1.8 mm. This meant, that for every possible orientation of the crystal, a large fraction of the crystal extended beyond the limits of the X-ray beam. Only a small fraction of the crystal was illuminated by the incident X-ray beam while a larger fraction of the crystal was positioned to absorb the diffracted X-ray beams. And, of course, the diffracting and absorbing fractions of the crystal varied as the orientation of the crystal changed throughout the data collection. All in all, a rather severe situation to use to demonstrate whether SADABS could correct for a crystal extending beyond the X-ray beam.

Using SAINT v5.0 (Bruker AXS, 1998) and SADABS (Sheldrick, 1999), both run with default settings and parameters, intensity data were obtained and corrected from which a reasonable crystal structure was solved and refined. Further work, using SAINT v6.0 and the beta test release of SADABS v.3, is underway to explore whether an even better structure can be obtained from what was, initially, severely flawed intensity data.