On the evaluation of molecular dipole moments from multipole refinement of X-ray diffraction data

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Lack of physical constraints in the purely mathematical multipole refinement model can lead to basis set overlap errors in the evaluation of static molecular properties from X-ray diffraction data. For the molecular dipole moment, the error is large for several of the crystals tested in this study: dl-histidine, dl-proline, p-nitroaniline and p-amino- p′-nitrobiphenyl. Two restricted models are tested. In the first, atomic charges are constrained at κ-refinement values, while in the second κ′-values based on multipole refinements of theoretical ab-initio structure factors are used to reduce the flexibility of the model. Both models provide a more localized description of the pseudo atoms compared with an unrestricted refinement, but the κ′-restricted model gives a more consistent representation of the molecular dipole moments and superior agreement with the theoretical deformation density for dl-histidine.