Chemical Crystallography Resolution Requirements

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Last updated: Sept 25, 2024 by AS MX Wiki
2 min read

 

The basic requirements for chemical crystallographic data are laid out by IUCr, this is the requirements as specified for 
Acta Crystallographica Section C
STRUCTURAL CHEMISTRY

https://journals.iucr.org/c/issues/2002/01/00/me0150/

 

Given that there are these requirements there is validation that must be carried out on almost all submissions for publication.  
https://checkcif.iucr.org/

RESOLUTION REQUIREMENTS

Of particular interest is the resolution of the data, when using a large area detector on axis of the beam as resolution is limited by how close the detector can get to the beam.

 

_diffrn_reflns_theta_max

Theθmaxof measured reflections should be such that sin θmax/λexceeds 0.6 Å−1(i.e.θmax > 25° for MoKα; θmax > 67° for CuKα). It is assumed that all unique reflections out to the specifiedθmaxare measured. This provides the minimum number of reflections recommended for an average structural study. If intensities are consistently weak at the recommendedθmax, low-temperature measurements may be needed unless a study at a specific temperature (or pressure) is being reported.

Data resolution on the Beamline

The resolution of your data depends on 2 things:

  1. X-ray energy

    • Higher energy → Higher resolution

  2. Detector distance from sample

    • Longer distance → Lower resolution

This is why for CX samples, if you want the highest resolution possible you should collect at high energy (generally 17.44 keV) with the detector at closest approach (105 mm for MX1, 110 mm for MX2).

Reasons why you might not do this:

  • Spots are so close they are overlapping. This can happen with very large cells. You can move the detector out slightly to separate your spots.

  • Unfavourable energy. Depending on your sample, 17.44 keV might not be the best energy. You can change the energy to something more favourable. Although talk to beamline staff about accessible energies.

 

Since resolution is calculated in terms of angle, this means there are rings of resolution on the rectangular detector. So the resolution on the edge is worse than at the edge of the detector. But you will not have complete data to the resolution at the corner.

Resolution .png

If you want to calculate the resolution you can achieve at a specific energy or detector distance, you can do that with this handy spreadsheet!

 



 

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