COMPARISON OF 5 HIGH RESOLUTION X-RAY DATA SETS OF DL-ASPARTIC ACID MEASURED AT 20 K. Dieter Zobel1, Ralf Flaig1, Peter Luger1, Hans-Georg Krane2, 1Inst. f. Krist., FU Berlin, Takustr. 6, D- 14195 Berlin, 2Mineral. Inst. d. Uni. Würzburg, Am Hubland, D-97074 Wurzburg
Based on Bader's formalism /1/ topological properties of selected amino acids are being investigated. To obtain charge density information very accurate high resolution data sets at sin [[theta]]/[[lambda]] >= 1.0 Å-1 have to be collected at the lowest possible temperature. For DL-aspartic acid (C2/c, V=1057.4 Å3), our first example in that course, we measured a total of five data sets, each one at 20 K, to get an impression on optimum experimental conditions:
1) Mo-X-ray tube (sin [[theta]]/[[lambda]] <= 1.07Å-1), Nb-[[beta]]-filter, szintil. counter.
2) Mo-X-ray tube (sin [[theta]]/[[lambda]] <= 1.07Å-1), Si(Li)-solid state detector.
3) Ag-X-ray tube (sin [[theta]]/[[lambda]] <= 1.37Å-1), Pd-[[beta]]-filter, szintil. counter.
4) Ag-X-ray tube (sin [[theta]]/[[lambda]] <= 1.37Å-1), Si(Li)-solid state detector.
5) Synchrotron radiation (sin [[theta]]/[[lambda]] <= 1.01Å-1), monochr. szintil. counter.
The use of the solid state detector (measurement 2 and 4) with an energy resolution of better than 200 eV avoids the attenuation of [[alpha]]-radiation by conventional monochromators or filters and results in a peak-to-background improvement. Critical comparison of the measurements represented by I/[[sigma]](I) will be discussed. In all cases our 20 K He closed-cycle refrigerator /2/ was used for cooling the sample.
Full order refinements resulted in R=0.028 for 7047 unique reflexions of measurement 4. Further refinements on the basis of multipole expansion (program XD /3/) reached R=0.020 giving the experimental basis for charge density calculations and related properties.
[1] Bader, R. C. W., "Atoms in Molecules"Clarendon Press,Oxford,1990
[2] Zobel,D., Luger,P. & Dreißig,W. 1992. Acta Cryst. B 48.835-848.
[3] XD: "A Program for refinement and analysis of charge density", T. Koritsanszky et al.(l995), FU Berlin.