how many (quasi) equivalent positions do you think the chiral compound can adopt in the organo-metal framework? Or the organo-metal framework in the crystal? If it's two, you can probably do alternate conformations - if there are more, it could become impossible, even in P1.
Mark J van Raaij Lab 20B Dpto de Estructura de Macromoleculas Centro Nacional de Biotecnologia - CSIC c/Darwin 3 E-28049 Madrid, Spain tel. (+34) 91 585 4616 http://www.cnb.csic.es/~mjvanraaij On 13 Aug 2014, at 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
