Screening the crystal

Ahead of full data collection, a brief screening process is carried out on the mounted crystal to gain some preliminary information on the crystals and check they are suitably diffracting. Short bursts of X-rays, typically upto 30 seconds are used, with the power steadily being ramped up until a suitable diffraction pattern is observed. The crystal is kept in a single position during this step, unlike the full data collection where the crystal has data collected from many orientations.

The crystallographer is looking for a good number of diffraction spots, over a range of angles, especially at high angles, with the spots having good intensity (bright spots), with a concise form (not elongated or streaked)

Peaks fit -> info

How many of the spots give this same unit cell. Rare to be 100%

Determining unit cell

From the initial screening, this will generate a unit cell. Obtaining the shape and dimensions (a,b,c and alpha, beta gamma) and the volume.

This unit cell data is then matched to any entries in the CSD and to locally collected data.

Checking that these are chemically meanigful if matching, coincidence of unit cell v same compound.

From the volume, and the crystal shape, this gives a rough indication of how many atoms are likely to be in this unit cell. Either one molecule, or n repeat, and could include solvents of crystallisation.

Preexperiment

Calculating how many frames (measurements) required in order to obtain a complete dataset.

Frames is each individual location of detector in relation to the X-ray beam.

Symmetry could rule out needing to collect many angles, as this data would already be available from data collected from earlier places.

Computing data collection strategy

The crystallographer can then alter the minimum level of parameters which have been calculated for data collection. The crystallographer could then override:

• Time of data collection per frame (0.05 seconds is the fastest, to a maximum of 1 second being generally the upper limit

• Symmetry v full sphere

• Whether to collect for resolution of chirality (changing hkl and -hkl from being equivalent)

• Setting parameters where a crystal is strongly suspected of being a twinned crystal

• Options to collect additional redundant data

Altering any of the above will affect the data collection time, and these need to be balanced.

Inputting information on the expected compound is also provided at this stage, which allows the programme AutoChemSolve to attempt to determine the structure during the data collection stage. This can be really helpful especially with longer data collection strategies to give an indication if the data is worth continuing with the collection.

Crystal visual recording

The crystal is slowly rotated and photographed at every angle, allowing determination of the physical dimensions of the mounted crystal.

Collecting frames

The detector then rotates to each calculated location and a frame of data is collected for each position. Whilst this is ongoing, AutoChem will attempt to solve the data, with the structure improving as each additional frame is collected. AutoChem knows nothing about the potential structure at this stage?

Output

At the end of all the frames being recroded we have:

• A movie of the crystal

• An image file for each individual frame rodhypix 

• Unit cell data

• HKL file

• The AutoChem RES file 

• Plus the saved preexperiment data