MIT5_067F09_lec1_data

MIT5_067F09_lec1_data - From Crystal to Diffraction Pattern...

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Initial Immages 1 • Judging the Quality of a Crystal. • Determining the Best Exposure Time. • Data Collection Strategy. From Crystal to Diffraction Pattern Take two immages 90° appart. Judging the Quality of the X-tal Figure by MIT OpenCourseWare.
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Initial Immages The Rocking Curve Analyze Rocking Number of frames in both directions Size of the box halfwidth = mosaicity Judging the Quality of the X-tal Take two immages 90° appart. Judging the Quality of the X-tal 2 Figure by MIT OpenCourseWare. Figure by MIT OpenCourseWare.
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Determining the Exposure Time 20 seconds is better n 10 seconds. seconds not better n 20 sconds. r exposure time is seconds per frame. tha 30 tha Ou 20 compare Collect Dark Current Detector Dark Current Now you are ready for data collection What is the best strategy? Collect a new dark current at your exposure time or at least load an older (but not too old!) dark current taken at your exposure time. 3 Figure by MIT OpenCourseWare. Figure by MIT OpenCourseWare.
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ACA 2005 in Orlando FL: Session 09:01 Data Collection Strategies MoO is No Schmu – Why Less is Not Always More. Peter Müller MIT Roland Boese distinguishes between unavoidable errors ( aka artifacts), avoidable errors, and really avoidable errors. Artifacts Libration. C-C and C-N triple bonds determined too short. Inaccurately determined hydrogen positions. Fourier truncation effects. Avoidable Errors Wrong unit-cell. Twin refined as disorder. Wrong atom type assignment. Incorrect space group. Fourier truncation peaks mistaken for hydrogen atoms. 4
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Roland Boese distinguishes between unavoidable errors ( aka artifacts), avoidable errors, and really avoidable errors. Really Avoidable Errors Typos in unit-cell parameters. Misadjustment of the diffractometer (zero points, etc. ). No absorption correction. Data collection at room temperature. Wrong data collection strategy . The Standard Data Collection Strategy for the Platform geometry as recommended by Bruker for many years is still used in many labs: Three 180° ω -scans, φ = 0°, 90° and 180°; Scan width: 0.3° Is this good enough? In most cases probably yes. But can we do better? In any case! 5
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What is the optimal Scan Width ? The narrow frame algorithm in SAINT allows for 3D profile fitting, which is a good thing. For this to work, the scan width needs to be smaller than the mosaicity, but not much smaller. 0.3° is fine, but 0.5° suffices in almost any case. Going from a scan width of 0.3° to 0.5° saves memory and time. These gains can be used to collect more frames, which will increase the quality of the dataset. Z. Dauter (1999), Acta Cryst. D55 , 1703-1717. What is
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This note was uploaded on 11/27/2011 for the course CHEMICAL E 20.410j taught by Professor Rogerd.kamm during the Spring '03 term at MIT.

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MIT5_067F09_lec1_data - From Crystal to Diffraction Pattern...

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