Electron density analysis of nonlinear optical materials: an ab initio study of different conformations of benzene derivatives

Ab initio Hartree-Fock calculations using polarized basis sets have been performed on different conformations of p-nitroaniline, nitrobenzene, aniline, and benzene. The polarizability and first hyperpolarizability have been calculated with the finite field
(CHF) and sum over states (UCHF) methods. It has been found that the first hyperpolarizability is strongly influenced by rotating the nitro and/or amino group out of the plane of the benzene ring, while this has only a limited effect on the polarizability and dipole moment. An analysis of the charge density has been performed to investigate which atoms and molecular orbitals contribute most to the (hyper)polarizability in the different conformations of these organic molecules. The atomic charges of almost all atoms contribute to the polarizability, but only the oxygen atoms and the nitrogen atom of the amino group contribute significantly to the first hyperpolarizability of p-nitroaniline.