Solving Centrosymmetric Crystal Structures in Non-Centrosymmetric Space Groups Michael Shatruk September 12, 2011

2 Steps in X-ray single crystal experiment 1.Crystal selection (including unit cell determination). 2.Data collection. 3.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)

3 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) The options in square brackets are default ones. Step a. Determine the lattice type

4 The table of possible higher symmetries Step b. Search for higher metric symmetry Desired R(sym) < 0.10

5 In most cases, we use option [S]. Sometimes, we will choose to input the space group. Step c. Determine the space group Space group determination options

6 Systematically absent reflections allow locating specific symmetry elements XPREP determines the space group Desired CFOM < 5 (the lower, the better) Step c. Determine the space group Lattice type Centrosymmetric? All the information gathered above is used to derive the possible space groups

7 In most cases, the option D can be skipped, and we can proceed directly to option C This is painful! Do this ahead of time in your experimental description in APEX, and you won’t have to enter the formula here! Step d. Molecular formula Again, we skip D and proceed directly to F Option F sets up the input file for structure solution in SHELXS.

8 Choose an appropriate filename (e.g., I prefer to use the corresponding space group symbol) Step e. SHELXS input This is the SHELXS input file Now we can quit, although it is possible to go back, if necessary Usually we overwrite the hkl file (intensity data)

9 Now we choose to input the space group Step c. Determine the space group The systematic absences statistics calculated for the input space group The symbol is not case-sensitive After this, choose option F again to set up the SHELXS input file with the newly defined symmetry

10 Important Points Regarding XPREP 1.In most cases, use defaults commands, but always beware of what they are. Don’t just hit “Enter” without thinking. 2.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. 3.It is better to set up all the possible space groups right away, to save time and don’t need to use XPREP again.