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
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