Theoretical Study of Hyperpolarizabilities in Crystalline m-Nitroaniline

A variational−perturbation method in terms of molecular orbitals (MOs) has been developed to study the effect of a permanent crystal field on hyperpolarizability (β) in van der Waals organic crystals. The method has the advantage of giving a numerical value of the dipole moment for each fragment molecule in a weakly interacting system with an accuracy comparable to the supermolecule method. Our method has been applied to a m-nitroaniline molecule in different model crystal environments to analyze the crystal field effect on the β-values. The results indicate that the crystal field induces the intramolecular charge transfer (CT) of the π-electrons from an NH2 group to a NO2 group in the highest occupied MO while the lowest unoccupied MO almost remains unchanged. From a net atomic charge analysis, the CT will be derived from the reversed polarization of the σ-electrons due to the polarized π-electrons and it will then contribute to the enhancement of the β x -value additively (βadd). The effect of the different crystal environments on the β xxx -values has been also discussed with a focus on the degree of the ground state polarization.