Rapid Discrimination of Crystal Handedness by X‐ray Natural Circular Dichroism (XNCD) Mapping. Issue 59 (21st September 2020) - Cortijo, Miguel
Abstract: An original method for determining the handedness of individual non‐centrosymmetric crystals in a mixture using a tightly‐focused, circularly polarized X‐ray beam is presented. The X‐ray natural circular dichroism (XNCD) spectra recorded at the metal K‐edge on selected crystals of [Δ‐M(en)3 ](NO3 )2 and [Λ‐M(en)3 ](NO3 )2 (M=Co II, Ni II ) show extrema at the metal pre‐edge (7712 eV for Co, 8335 eV for Ni). A mapping of a collection of some 220 crystals was performed at the respective energies by using left and right circular polarizations. The difference in absorption for the two polarizations, being either negative or positive, directly yielded the handedness of the crystal volume probed by the beam. By using this technique, it was found that the addition of l ‐ascorbic acid during the synthesis of [Co(en)3 ](NO3 )2 resulted in an enantiomeric enrichment of the Λ‐isomer of 67±13 %, whereas the Ni analogue was similarly, but conversely, enriched in the Δ‐isomer (65±22 %). Abstract : Chiral cartography : Non‐centrosymmetric ( P 63 22) crystals of [Co(en)3 ](NO3 )2 and [Ni(en)3 ](NO3 )2 exhibit X‐ray natural circular dichroism at the metal K‐edge. Measuring the differential absorption of right and left circularly polarized X‐rays at the pre‐edge maxima allowed the rapid mapping of the crystal handedness. This technique unveiled a differential action of the chiral impurity ascorbic acid on the deracemization of the two complexes.