AN AB-INITIO MOLECULAR-ORBITAL STUDY ON HYPERPOLARIZABILITIES OF AN INTERACTING 2-METHYL-4-NITROANILINE MOLECULAR PAIR - A MOLECULAR STUDY ON THE ORIENTED-GAS APPROXIMATION

The oriented-gas approximation was examined at the molecular level by using an ab initio molecular orbital method in order to investigate it s basic assumption: the additivity of hyperpolarizabilities between mo lecules. The additivity was examined in an interacting 2-methyl-4-nitr oaniline (MNA) pair, the first-neighboring molecular pair in MNA cryst al. The first-order hyperpolarizabilities beta of the MNA pair were es timated by using the supermolecule technique in coupled perturbed Hart ee-Fock calculations, fully taking into account quantum mechanical int ermolecular interactions between MNA molecules. The results indicated that hyperpolarizabilities were not additive between the molecules and the intermolecular interactions considerably influenced the hyperpola rizabilities of the MNA pair. The results were analyzed by using the e lectrical interaction model of Dykstra et al. (J. Mol. Struct.: THEOCH EM 1986, 113, 357), and it was found that electrostatic intermolecular interactions were the predominant influence on the hyperpolarizabilit ies, similar to the case of urea. The importance of the local field co rrection in the oriented-gas approximation was discussed from the mole cular viewpoint, considering its microscopic basis which corrects the effects of electrostatic interactions between a molecule and the cryst al field on hyperpolarizabilities within the framework of mean field t heory.