Single Crystal X-ray Structure Determination
Optical screening is the first and the most important step for sorting suitable for X-ray structure determination single crystal. Polarization Microscope is the most convenient tool for analyzing a quality and size of crystals. In most cases, single crystal has to be at least 0.1 mm in all three dimensions( if the substance has heavy atoms) or 0.2 (only light atoms as C, H, O, N, F). Normally, a good single crystal is well-shaped with visible planar faces. If the crystal is optically transparent it is possible to use polarized light to find all impurities, defects or twinning problem. It should be mentioned, that passing this stage of testing can not guarantee the data suitable for successful structure solution. However, at this point we can definitely sort out the samples which are not suitable for analysis in principle.
Rotation Photo is required for analyzing scattering capacity of a sample. To obtain rotation photo crystal rotates in a X-ray beam. Liquid, powder and single crystal produce absolutely different rotation patterns , therefore it is possible to sort out all non- single crystalline samples and weak diffracting crystals.
Cell parameters determination can be done only for single crystal. During the routine about 25 frames in three perpendicular directions of reciprocal space should be collected and positions of reflection have to be analyzed. The cell parameters in first approximation are the metric values for each particular substance ( to be more correct, for each crystal modification of substance, as many of individual substances can crystallize in two or more polymorphic modifications). Analyzing the profiles of reflections it is possible to check quality of single crystal. This stage is very important for twinned samples as some kinds of twins give excellent rotation photos, but can not be used for X-ray structure determination.
Data Collection is automatic routine to file positions and intensities of reflections in reciprocal space. The standard procedure is to collect the full-sphere data for better redundancy and absorption correction. However, for high-symmetry crystals and unstable crystals it is possible to collect hemi-sphere data. The duration of full-sphere data collection vary from 9 hours to some days depending on the "scattering power" of a sample
The laboratory uses almost exclusively SHELXTL PLUS V4.21 for structure solution and refinement. SHELXTL PLUS is a commercial version, distributed by Bruker-AXS, of George Sheldrick's SHELX-76 and SHELX-86 solution and refinement programs. Refinement using SHELXL-93 is also available. SHELXTL PLUS includes routines for processing of collected intensity data. It also includes routines for the production of tables and figures for publication.
Powder Pattern Analysis Software:
For analysis of powder patterns obtained from the Scintag, the Diffraction Management Software (DMS) distributed by Scintag has a number of routines in it for background correction and peak searching. The peak search routines try to match the relevant phases based on the International Center for Diffraction Data's PDF-1 database. Currently the laboratory has up to Set 42 of the JCPDS database available.
The DMS software also has routines for Rocking Curve analysis and some crystallographic routines.
All solution software and computer time on the Windows XP cluster are available to the certified users. Data sets may be collected and processed on the users own systems using the software of their choice.