Hi folks
Pc *must* have both enantiomers, since it's got a glide plane ( =
mirror + translation parallel to mirror).
So the sample *cannot* be enantiopure if the space group is Pc (or P2/
c)...
BTW, Pc isn't a centrosymmetric space group.
Unless I'm wrong...
On 13 Aug 2014, at Wed13 Aug 12:37, Eleanor Dodson wrote:
Twin laws are possible if there are 2 ways to index your cell, and
non-merefedral twinning is possible in any system depending on the
cell.
I am not sure of the small molecule tools to check twinning though.
Eleanor Dodson
On 13 August 2014 05:58, Lijun Liu <[email protected]> wrote:
Hi, Pc may not be the space group for your crystal, if the molecule
is chiral. Seems like the data were forced to be reduced to a
mirror_related SG. Lijun
On Aug 13, 2014 2:00 AM, "Kristof Van Hecke"
<[email protected]> 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
Harry
--
** note change of address **
Dr Harry Powell, MRC Laboratory of Molecular Biology, Francis Crick
Avenue, Cambridge Biomedical Campus, Cambridge, CB2 0QH
Chairman of European Crystallographic Association SIG9
(Crystallographic Computing)
