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X-ray powder diffractometry in a conventional laboratory powder diffractometer was used to discriminate between two chiral diastereoisomeric 7-oxanitronorbornenes that could not be distinguished unambiguously by nuclear magnetic resonance spectroscopy. It was found that the traditional approach consisting of successive steps of molecular modeling, direct-space methods, and crystallographic Rietveld refinement is not efficient for that purpose. This was attributed to the minimization trajectories in the phase spaces becoming trapped in local minima. On the contrary, the use of a final Rietveld refinement incorporating additional energy constraints limited the number of physically-reasonable atomic arrangements, thus allowing in this case unequivocal differentiation between the two nearly identical chiral diasteroisomeric 7-oxanitronorbornenes. This was possible thanks to the use of a multiobjective global optimization procedure (“Pareto optimization”) that searches for the minimum of a cost function defined as the weighted average of the crystallographic and energy residuals.

Discriminating between two chiral diastereoisomeric 7-oxanitronorbornenes by conventional X-ray powder diffractometry