A charge density study of the effect of irradiation on the α-form of p-nitrophenol

Careful investigation of the structure and charge density of the α polymorph of p-nitrophenol has been carried out following prolonged irradiation by visible light. Though the change in colour from yellow to red is remarkable, the molecular geometry and the intermolecular hydrogen contacts exhibit only minor changes. However, a charge density analysis in comparison with the pristine α and β polymorphs has brought out significant differences in the charge distribution in the intra- and intermolecular bonding regions. The deformation density map of the a form reveals many differences in the bonding regions of the molecule before and after irradiation. Charge migration from the nitro and the hydroxyl groups to the phenyl ring of the molecule occurs on irradiation, resembling more the charge distribution in the light stable β form. The molecular dipole moment in the irradiated crystal is significantly lower compared to that before irradiation. Relief maps of the negative Laplacian in the hydrogen bond region also show less polarization of the oxygen lone pairs after irradiation