X‐Ray Diffraction and Atomic Dipole Moments

When an atom contains one or more lone pairs of electrons in orbitals that are not centrosymmetric, the centroid of the electron cloud does not coincide with the atomic nucleus. An atomic dipole moment arises, which may cause an observable discrepancy between atomic positions in the crystal as revealed by X‐ray and by neutron diffraction. A few crystal structures have been determined by both methods with a positional accuracy of ∼ 0.02 Å; these have not shown the effect. The only positive evidence reported concerns hexamethylene tetramine, for which refined X‐ray and neutron studies have yielded nitrogen positions differing by 0.019 Å, which compares reasonably with the known dipole moment of ammonia. For quantitative determination of the orbital asymmetry, interpretation of X‐ray diffraction data should be based on a model that takes explicit account of such parameters as orbital exponents and hybridization ratios.