Investigation of bonding in the solid state using experimental charge density

Abstract

A method of investigating chemical bonding in crystaline solids using experimental charge density is presented. A refinement procedure based on multiple formalism for X-ray diffraction data has been described along with topographical analysis of charge distribution. The method has been applied to 2 study of alpha and beta polymorphs of p-nitrophenol and their photochemical activity. Besides revealing many structural differences, the study has brought out signifcant differences in intra- and intermolecular regions of the two forms as seen from deformation density maps. Rellef maps of the negative Laplacians to the plane of intemolecular hydrogen boods show polarization of the oxygen lone-pair electrons towards hydrogen. Charge appears to migrate from the benzene ring region of the molecule to nitro and hydroxyl groups as the structure changes from alpha to beta form. On prolonged irradiation of the a form, charge migrates in the opposite way resembling more like charge distribution in light stable alpha forn. Molecular dipole moments in solid stale are found to be considerably larger than the value in the free molecule.