Dear Kristof, Lijun is right, space group Pc is not compatible with chiral molecules. Maybe diffraction of your non-chiral metal structure overwhelmed the chiral contribution of your organic framework. Why not use a trick from protein crystallography: process and solve your structure in P1? According to the international tables there are two asymmetric units in Pc, so the crystallographic problem should remain manageable.
Best, Herman Von: CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK] Im Auftrag von Lijun Liu Gesendet: Mittwoch, 13. August 2014 11:59 An: CCP4BB@JISCMAIL.AC.UK Betreff: Re: [ccp4bb] Twinning in space group Pc 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" <kristofrg.vanhe...@gmail.com<mailto:kristofrg.vanhe...@gmail.com>> 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
