Absolute Configuration Assignment From a Single Reflection Using Resonant X-Ray Diffraction
Abstract
Determination of the absolute configuration of chiral molecules is essential across multiple scientific disciplines, as handedness determines the chiroptical, biological, and pharmaceutical properties. For crystalline materials, single crystal x-ray diffraction is the standard technique for assigning the absolute configuration. However, its application typically requires a large number of reflections and analysis of Friedel pair intensities using the Flack parameter or related approaches, such as those of Hooft or Parsons. Here, we introduce a conceptually distinct approach for determining the absolute configuration of a known crystal structure, based on the x-ray energy dependent intensity of a single Bragg reflection that is sensitive to the absolute configuration. By monitoring the variation in the intensity of a Bragg peak across an absorption edge, this method provides an unambiguous probe of chirality. The proposed approach is demonstrated for the 211 reflection of a 2-(3-bromophenyl)-N,N-diisopropyl-2-oxoacetamide single crystal, showing that the absolute configuration can be reliably determined from a single reflection. To confirm the assignment of chirality and assess potential limitations, the x-ray energy dependence of additional diffraction peaks was also investigated. This method is particularly advantageous for systems with a limited number of accessible reflections, where conventional statistical methods based on Flack parameter analysis are not reliable.
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