Dear,
I’m struggling with the following (small molecule) problem:
We are trying to solve the structure of a metal-organic framework containing a
chiral compound.
The space group is most probably Pc, but when refining, SHELX gives the error
“Possible racemic twin or wrong absolute structure - try TWIN refinement”.
As we know our compound is enantiopure, a racemic twin is very unlikely. In
this regard, also a centro-symmetric space group is not possible (although
CrysAlisPro always gives P2/c as the proper space group). As a matter of fact,
trying different space groups is not solving the problem.
The second problem is that half of the structure is visible, but the other half
is completely not clear. Refinement is not possible at all (R-value of 33%).
When running TwinRotMat (Platon), I get the following possible 2-fold twin axes:
2-axis ( 0 1 -1 ) [ -2 5 -4 ], Angle () [] = 2.31 Deg, Freq = 14
*************
(-0.992 -0.019 0.015) (h1) (h2) Nr Overlap = 84
(-0.430 0.075 -0.860) * (k1) = (k2) BASF = 0.96
( 0.459 -1.146 -0.083) (l1) (l2) DEL-R =-0.064
2-axis ( 0 1 1 ) [ 2 5 4 ], Angle () [] = 2.31 Deg, Freq = 15
*************
(-0.992 0.019 0.015) (h1) (h2) Nr Overlap = 229
( 0.430 0.075 0.860) * (k1) = (k2) BASF = 0.94
( 0.459 1.146 -0.083) (l1) (l2) DEL-R =-0.050
2-axis ( 1 0 -2 ) [ 3 0 -2 ], Angle () [] = 0.54 Deg, Freq = 19
*************
(-0.136 0.000 -0.576) (h1) (h2) Nr Overlap = 992
( 0.000 -1.000 0.000) * (k1) = (k2) BASF = 0.86
(-1.703 0.000 0.136) (l1) (l2) DEL-R =-0.030
2-axis ( 1 2 -1 ) [ 5 5 -1 ], Angle () [] = 0.39 Deg, Freq = 13
*************
(-0.380 0.620 -0.124) (h1) (h2) Nr Overlap = 854
( 1.256 0.256 -0.251) * (k1) = (k2) BASF = 0.88
(-0.617 -0.617 -0.877) (l1) (l2) DEL-R =-0.019
However, none of these do actually improve the refinement.
Has anyone encountered possible twinning/twin laws in Pc please?
Or any other suggestions are most welcome?
Thank you very much
Kristof
