X-ray charge density study of p-amino-p′-nitrobiphenyl at 20 K using a CCD area detector and synchrotron radiation: a very large dipole moment enhancement in the solid state

Accurate 20 K intensity data were collected on a crystal of p-amino-p′-nitrobiphenyl [a = 24.348(1), b = 5.802 (1), c = 7.158 (1) Å, α = β = γ = 90°, space group Pca2₁] at the SUNY X3A1 beamline at the National Synchrotron Light Source, using a CCD area detector and a DISPLEX cryostat. The sharp vertical profile of the synchrotron beam, combined with a slight instability of the cryostat, necessitates a φ-dependent correction, made possible by a large redundancy in the data set, the average multiplicity of the measurements being 8.02. In the multipole refinement, net atomic charges were constrained to those from a κ-refinement, corresponding to a more local definition of the pseudoatoms in the crystal. Both localized and unrestricted refinements show a very large (more than threefold) enhancement of the dipole moment in the solid state compared with that of the isolated molecule.