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

摘要

Accurate 20K intensity data were collected on a crystal of p-amino-p'-nitrobiphenyl [a=24.348(1), b=5.802(1), c=7.158(1)A, #alpha#=#beta#=#gamma#=90deg, space group Pca2_1] 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 #phi#-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 k-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.