Dear Kristof,
Have you tried to solve it with the new SHELXT? You can force it to
consider only chiral (Sohnke) space groups by putting -c on the command
line.
Best wishes, George
On 13.08.2014 11:00, Kristof Van Hecke wrote:
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
--
Prof. George M. Sheldrick FRS
Dept. Structural Chemistry,
University of Goettingen,
Tammannstr. 4,
D37077 Goettingen, Germany
Tel. +49-551-39-33021 or -33068
Fax. +49-551-39-22582
