Can we trust the experiment? Anisotropic displacement parameters in 1‐(halomethyl)‐3‐nitrobenzene (halogen = Cl or Br)

1‐(Chloromethyl)‐3‐nitrobenzene, C7H6NClO2, and 1‐(bromomethyl)‐3‐nitrobenzene, C7H6NBrO2, were chosen as test compounds for benchmarking anisotropic displacement parameters (ADPs) calculated from first principles in the harmonic approximation. Crystals of these compounds are isomorphous, and theory predicted similar ADPs for both. In‐house diffraction experiments with Mo Kα radiation were in apparent contradiction to this theoretical result, with experimentally observed ADPs significantly larger for the bromo derivative. In contrast, the experimental and theoretical ADPs for the lighter congener matched reasonably well. As all usual quality indicators for both sets of experimental data were satisfactory, complementary diffraction experiments were performed at a synchrotron beamline with shorter wavelength. Refinements based on these intensity data gave very similar ADPs for both compounds and were thus in agreement with the earlier in‐house results for the chloro derivative and the predictions of theory. We speculate that strong absorption by the heavy halogen may be the reason for the observed discrepancy. Anisotropic displacement parameters for the isomorphous compounds 1‐(halomethyl)‐3‐nitrobenzene (halo = chloro and bromo) were calculated from first principles and determined by X‐ray diffraction experiments. Unexpectedly, the experiment for the bromo compound proved more challenging than theory.