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# Solving Centrosymmetric Crystal Structures in Non-Centrosymmetric Space Groups - PowerPoint PPT Presentation

Solving Centrosymmetric Crystal Structures in Non-Centrosymmetric Space Groups. Michael Shatruk September 12, 2011. Steps in X-ray single crystal experiment. Crystal selection (including unit cell determination). Data collection. Data processing:

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Solving Centrosymmetric Crystal Structures in Non-Centrosymmetric Space Groups

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Solving Centrosymmetric Crystal Structures in Non-Centrosymmetric Space Groups

Michael Shatruk

September 12, 2011

Steps in X-ray single crystal experiment Non-Centrosymmetric Space Groups

• Crystal selection (including unit cell determination).

• Data collection.

• Data processing:

• Absorption correction (to match intensities of equivalent reflections)

• Space group determination (XPREP)

• Structure solution (SHELXS)

• Structure refinement (SHELXL)

• Structure validation (Platon, IUCR checkcif)

Step Non-Centrosymmetric Space Groups3 – Data Processing

After integration and absorption correction, the space group is determined with XPREP, which also writes SHELXS input files.

• # reflections total

• # reflections (strong)

• Step a.

• Determine the lattice type

• The options in square brackets are default ones.

• Step b. Non-Centrosymmetric Space Groups

• Search for higher metric symmetry

• The table of possible higher symmetries

• Desired R(sym) < 0.10

• Step c. Non-Centrosymmetric Space Groups

• Determine the space group

• Space group determination options

• In most cases, we use option [S].

• Sometimes, we will choose to input the space group.

• Step c. Non-Centrosymmetric Space Groups

• Determine the space group

• XPREP determines the space group

• Lattice type

• Centrosymmetric?

• Systematically absent reflections allow locating specific symmetry elements

• All the information gathered above is used to derive the possible space groups

• Desired CFOM < 5 (the lower, the better)

• Step d.

• Molecular formula

• This is painful!

• Do this ahead of time in your experimental description in APEX, and you won’t have to enter the formula here!

• Again, we skipD and proceed directly to F

• Option F sets up the input file for structure solution in SHELXS.

• Step e. proceed directly to option C

• SHELXS input

• Choose an appropriate filename (e.g., I prefer to use the corresponding space group symbol)

• This is the SHELXS input file

• Usually we overwrite the hkl file (intensity data)

• Now we can quit, although it is possible to go back, if necessary

• Step c. proceed directly to option C

• Determine the space group

• Now we choose to input the space group

• The symbol is not case-sensitive

• The systematic absences statistics calculated for the input space group

• After this, choose option F again to set up the SHELXS input file with the newly defined symmetry

Important Points Regarding XPREP proceed directly to option C

• In most cases, use defaults commands, but always beware of what they are. Don’t just hit “Enter” without thinking.

• Sometimes XPREP might now show you the correct space group, and you might need to change the space group later on, as you move on with the structure refinement.

• It is better to set up all the possible space groups right away, to save time and don’t need to use XPREP again.