Solving centrosymmetric crystal structures in non centrosymmetric space groups
This presentation is the property of its rightful owner.
Sponsored Links
1 / 10

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


  • 71 Views
  • Uploaded on
  • Presentation posted in: General

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:

Download Presentation

Solving Centrosymmetric Crystal Structures in Non-Centrosymmetric Space Groups

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript


Solving centrosymmetric crystal structures in non centrosymmetric space groups

Solving Centrosymmetric Crystal Structures in Non-Centrosymmetric Space Groups

Michael Shatruk

September 12, 2011


Steps in x ray single crystal experiment

Steps in X-ray single crystal experiment

  • Crystal selection (including unit cell determination).

  • Data collection.

  • Data processing:

  • Integration (to assign intensity to each observed hkl spot)

  • Absorption correction (to match intensities of equivalent reflections)

  • Space group determination (XPREP)

  • Structure solution (SHELXS)

  • Structure refinement (SHELXL)

  • Structure validation (Platon, IUCR checkcif)


Step 3 data processing

Step 3 – 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.


Solving centrosymmetric crystal structures in non centrosymmetric space groups

  • Step b.

  • Search for higher metric symmetry

  • The table of possible higher symmetries

  • Desired R(sym) < 0.10


Solving centrosymmetric crystal structures in non centrosymmetric space groups

  • Step c.

  • Determine the space group

  • Space group determination options

  • In most cases, we use option [S].

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


Solving centrosymmetric crystal structures in non centrosymmetric space groups

  • Step c.

  • 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)


Solving centrosymmetric crystal structures in non centrosymmetric space groups

  • In most cases, the option D can be skipped, and we can proceed directly to option C

  • 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.


Solving centrosymmetric crystal structures in non centrosymmetric space groups

  • Step e.

  • 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


Solving centrosymmetric crystal structures in non centrosymmetric space groups

  • Step 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

Important Points Regarding XPREP

  • 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.


  • Login