Twinning in γ-o-nitroaniline: Improved Structure via Dual Lattice Processing of Raw Data

What is it about?

This article presents a detailed crystallographic study of the γ-form of o-nitroaniline, a polymorphic compound known to exhibit complex twinning behavior that complicates unit-cell determination. The scope includes collection of new diffraction data using an in-house APEXII diffractometer with Mo Kα radiation and analysis of these data by both single-lattice and twin-lattice processing methods. Methodologically, the study employs EVAL software for data integration, SADABS/TWINABS for scaling, and structure refinement with SHELXL, with particular attention to the effects of twinning and the application of a twin matrix. The results demonstrate that processing raw data with explicit consideration of both twin lattices yields significantly improved refinement statistics (R1[I > 2σ(I)] = 0.0314 and wR2(all refl.) = 0.0860) compared to attempts to de-twin already processed structure factors. The structure is shown to have P2₁/a symmetry with two independent molecules forming hydrogen-bonded layers, and the twinning involves a rotation about the c-axis with a small obliquity of 0.743°. The study further discusses the implications of pseudo-orthorhombic twinning and stacking faults in the context of OD theory, as well as the importance of using raw diffraction data for resolving complex twinning phenomena. Overall, the findings underscore the superiority of raw data reprocessing with twin models over post hoc de-twinning approaches for accurate crystal structure determination in twinned systems.

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Why is it important?

This research addresses the crystallographic challenges associated with the γ-form of o-nitroaniline, a compound known for its polymorphism and complex twinning behavior. By systematically re-examining unit cell determination and data processing techniques, the study provides a clear pathway for overcoming difficulties that have previously hindered accurate structure solution in twinned crystals. The findings have broader implications for crystallographic studies of polymorphic materials, particularly those exhibiting non-merohedral twinning. Key Takeaways: 1. The study demonstrates that proper processing of raw diffraction data using two twin lattices significantly improves crystallographic refinement statistics for γ-form o-nitroaniline, as opposed to de-twinning post-processed structure factors. 2. Findings reveal that the γ-form exhibits pseudo-orthorhombic twinning with a non-zero twin obliquity (0.743°), resulting in split reflections and non-merohedral twinning that complicates traditional space-group assignment and structure solution. 3. The research highlights that the availability and analysis of raw diffraction data enable accurate identification of twin relationships and stacking faults, leading to a more reliable determination of crystal structure and hydrogen-bonded layer arrangements in o-nitroaniline.