USE OF X-RAY CRYSTALLOGRAPHY 687 CIF/PLATON to ensure the overall validity of the radiation of different wavelength.synthesis of a derivative structure determination.For bsote-contion deter containing resonant-sc attering atoms.and synthesis of a ins 0.00:No stan on the Flack pa the no hic ch ical e ssess the to be plete rubbish as with this sac c)Thisis that of() .1②but ith n o ter un taint implying that b stru 9 del sh and the ref m ted is the =0.05(2).space group Pra2:Absolute-configura- in the form of the glide ort th rystal FI for the beer en the rac mate Absolute the crystal is p ed by inye sion contai Du sible in this cas and 19%0 tedoa0 en( =0052,spa and XRD.Ho ver() paran er is less tha 0.10.and ing from a partial-pola achieved. Moreo er,as the spac tains onl hic evid ity of centrosy obta ained for an th he m as impurity nation has achie he meling tempe compour tography is neces y to make the raphers curse is bad softwa oupC2.good Friedel covera tion co diatel sib ed by which For me surements on a SDO tals would ex(u) ≈0.05② and the othe 50% che CIF/PLATON that a center of s needs to b to the insure tha matography the single crystal used for the diffraction dat ement er ha -O.1②：The value0.2 of the standard uncer trosymm etric ure mmetri group.No the ap n det n are n tal in ndicat is within thre dard und ertainties o ckCIF/PLATON that a centerof nmetry needs Possi. re This is a ver mds to achieve smaller sta dard uncertainty or ments,me ement a low rate is either e are indicatio PLAT or achiral:If ther del Friedel inter to he added to the Chirality DOI 10.1002/chiCIF/PLATON31–33 to ensure the overall validity of the structure determination. For absolute-configuration deter￾mination one pays particular attention to any indications of pseudosymmetry, incorrect space group, and insufficient number of intensity measurements of Friedel opposites. Moreover, the noncrystallographic chemical evidence con￾cerning the enantiopurity both of the bulk and of the indi￾vidual single crystal used for the diffraction studies needs to be reviewed. x(u) 5 0.05(2), space group P21/c: This report is com￾plete rubbish as a crystal structure with this space group is centrosymmetric implying that both absolute structure and the Flack parameter are meaningless. If there are chi￾ral molecules in this structure, they are present as the rac￾emate. x(u) 5 0.05(2), space group Pna21: Absolute-configura￾tion determination is not possible as this space group con￾tains rotoinversion operations in the form of the glide reflections n and a. The crystal structure is noncentrosym￾metric and achiral. If there are chiral molecules in this structure, they are present as the racemate. Absolute￾structure determination, but not absolute-configuration determination, may have been possible in this case. x(u) 5 0.05(2), space group C2, good Friedel coverage, enantiomerically-pure bulk compound: The standard uncertainty on the Flack parameter is less than 0.10, and the Flack parameter itself is within three standard uncer￾tainties of zero. Absolute-structure determination has been achieved. Moreover, as the space group contains only pure rotations and screw rotations, the crystal structure is noncentrosymmetric and chiral. The enantiopurity of the bulk compound ensures the enantiopurity of the single crystal used for the diffraction studies and consequently absolute-configuration determination has been achieved. Characterization of the bulk compound by OR, CD or enantioselective chromatography is necessary to make the absolute-configuration determination complete. x(u) 5 0.05(2), space group C2, good Friedel coverage, racemic bulk compound: Similar to the case immediately above but the compound has crystallized by spontaneous resolution. For measurements on a series of crystals all refined using the same structure model, about 50% of the crystals would give x(u) 0.05(2) and the other 50% would give x(u) 0.95(2). Absolute configuration is achieved if it is possible to characterize by either CD or enantioselective chromatography the single crystal used for the diffraction study. x(u) 5 20.1(2): The value 0.2 of the standard uncer￾tainty is larger than either of the limiting values 0.04 or 0.1, and consequently absolute-structure and absolute-con- figuration determination are not possible from the experi￾mental intensity measurements. The negative value of the Flack parameter is within three standard uncertainties of zero and entirely compatible with the statistical fluctua￾tions inherent in the experimental measurements. Possi￾ble remedies to achieve a smaller standard uncertainty on the Flack parameter are more accurate intensity measure￾ments, measurement at a lower temperature, accurate measurement of a selected set of Bragg reflections having the largest model Friedel intensity differences, use of a radiation of different wavelength, synthesis of a derivative containing resonant-scattering atoms, and synthesis of a cocrystal or solvate containing resonant scatterers. x(u) 5 0.00: No standard uncertainty on the Flack pa￾rameter has been reported and the Flack parameter may not even have been refined. As it is hence impossible to assess the accuracy of the Flack parameter, absolute-struc￾ture and absolute-configuration determination have not been achieved. x(u) 5 0.0(3): This is a situation similar to that of x(u) 5 20.1(2) but with an even greater uncertainty. x(u) 5 0.81(12): For values of the Flack parameter greater than 0.5, the structure model should be inverted and the refinement restarted. x(u) 5 0.19(12): When using SHELXL28 there is the danger that positive values of the Flack parameter notice￾ably different from zero may not have converged. One must use the TWIN/BASF commands13,29 and report the value of BASF1 for the Flack parameter. If this has been done in the present case and there is good Friedel cover￾age, the crystal is probably twinned by inversion contain￾ing 81% of the model and 19% of the inverted model. See the section Absolute-configuration determination from a bulk racemate by combined CD and XRD. However x(u) 5 0.19(12) is also rather typical of structure solutions suffer￾ing from a partial-polar ambiguity.34 x(u) 5 0.49(2), good Friedel coverage, enantiomerically pure bulk compound: The chemical and the crystallo￾graphic evidence are contradictory. One needs to examine both very critically. A similar situation pertains when a centrosymmetric crystal structure is obtained for an enan￾tiomerically pure bulk compound. The chemists’ curse is the opposite enantiomer as impurity in a binary system for which the melting temperature of the racemic compound is much higher than that of the enantiomerically pure com￾pounds. The crystallographers’ curse is bad software im￾plementing inappropriate averaging algorithms or absorp￾tion corrections. No absolute-configuration determination is possible. Watch out as well for disordered racemates which may be enantiomerically-pure crystals produced by spontaneous resolution. x(u) 5 0.042(8), poor Friedel coverage, indications from checkCIF/PLATON that a center of symmetry needs to be added to the space group, no chemical data indicating that the compound is enantiomerically pure: Due to insufficient intensity data, the refinement of the Flack parameter has probably stuck at its starting value of zero13,14 for this cen￾trosymmetric structure refined as noncentrosymmetric. The analyst should undertake refinement in the appropri￾ate centrosymmetric space group. No absolute-configura￾tion determination is possible. x(u) 5 0.042(8), good Friedel coverage, indications from checkCIF/PLATON that a center of symmetry needs to be added to the space group, strong chemical evidence that the compound is enantiomerically pure: This is a very nice absolute-configuration determination. x(u) 5 0.49(2), good Friedel coverage, bulk compound is either a racemate or achiral: If there are indications from checkCIF/PLATON that a center of symmetry needs to be added to the space group, this is most likely the USE OF X-RAY CRYSTALLOGRAPHY 687 Chirality DOI 10.1002/chir