School of EPS: Current Staff & Students

EPS Current Students and Staff

X-Ray Crystallography & Computing Service


Single crystal diffraction is the definitive method for determining the identity and structure of both new and known compounds and is therefore vital to chemical research, from mineralogy to pharmaceuticals, catalysis to biological macromolecules such as peptides and proteins.

The majority of structures determined are from research groups within the department. However some crystals of vital compounds are too small to be studied using a conventional laboratory X-ray source and in these cases data are collected at the Diamond Light Source through the National X-ray crystallographic service.

New structures are compared with existing structures, using the Cambridge Structural Database. Private databases compiled using the CSD facility PREQUEST enable concurrent searching of the CSD with structures determined in the department that are not yet published.


Sample Submission Forms

If you wish to submit a sample for single crystal structure determination, use the single crytal structure form.

Samples for powder X-ray diffraction should be submitted in the holder outside WP G.05 with this form along with a completed powder diffraction form.


Facilities:

Hardware


Since January 2004 we have a Bruker Nonius X8-Apex2 CCD diffractometer with an Oxford Cryosystems Cryostream. This has greatly improved crystallographic support to the research in the department, whereby chemists can have the structure determined within a day, from presenting the crystals to a picture of their molecule. We have had our Siemens (now Bruker AXS) P4 4-circle X-ray diffractometer since April 1995 when the Crystallography Service at Heriot-Watt (run by Dr Georgina Rosair) became operational, working in conjunction with synthetic research groups in organic, inorganic and materials chemistry in the department.

We have had low temperature facilities since September 1997 an Oxford Cryosystems Cryostream was installed allowing us to collect data at low temperature (typically 100 K) making it possible to collect data on thermally unstable, air and solvent-loss sensitive samples as well as improving the quality of data collected on weakly diffracting crystals.

 

The X8Apex2 CCD X-ray Diffractometer (left) and the "Molecule of the Month" (right).

Software


The X8 is used with the Apex2 software suite incorporating COSMO and SAINT data collection strategies and data reduction respectively

A monitor within the cabinet can display recently measured frames during data collection and also the position of the crystal using information from the coloured video-microscope.


An artificial precession image reconstituted from frames collected during a standard data collection run.
This shows the symmetry of the diffraction pattern and helps in determining the Laue symmetry in cases where this is unclear.


The P4 diffractometer is controlled by the XSCANS program which has a graphical interface and shows the profile of the most recently measured reflection with a visual representation of 2-dimensional portions of the reciprocal lattice as well as the intensities of the standard reflections relative to the start of data collection.

Structure solution and Refinement is carried out with the SHELXTL suite of programs running on Pentium II 350 MHz Windows NT4 workstations.


Online Advice

There are some help sheets available for local users which cover the following topics:

 

Absorption correction on the X8


Some correction packages (e.g. SADABS) will provide only one 'relative-correction-factor'. In such cases, Tmax should be given as Tmax-expected and Tmin = relative-correction-factor * Tmax.
>From the .abs file
Ratio of minimum to maximum apparent transmission: 0.737895
>From running checkcif on your CIF, there’s a table at the top which gives: Tmin,Tmax 0.769,0.916
Tmax = 0.916
Tmin = (0.916 x 0.737895)
Enter these manually into the Cif file
_exptl_absorpt_correction_type 'multi-scan'
_exptl_absorpt_correction_T_min ?
_exptl_absorpt_correction_T_max ?
_exptl_absorpt_process_details 'Bruker SADABS'


BCA Spring Meeting 2001:


Talk "Structural Chemistry from the Edge(s): An Introduction"
Click here to download the powerpoint version of this talk.

 


And finally, here are some

Useful Crystallographic Links:


Please contact Dr Georgina Rosair for further information.


Links to the other X-Ray Crystallography web pages are also listed here for your convenience:-