Ab initio studies of the dipole polarizabilities of conjugated molecules: Part 2. Monocyclic azines

Abstract

We report high quality ab initio calculations of the dipole polarizability and first hyperpolarizability of the following single monoeyclic azines: pyridine, pyridazine, pyrimidine, pyrazine, s-triazine and s-tetrazine. A study of the sensitivity of such calculated properties to the variation of basis set shows that increasing the number of multiple d and p diffuse functions in the 6-31 + G( nd, mp) basis set at Hartree-Fock level gives values of the polarizability tensor components that are comparable with the experimental ones, and consistent values of hyperpolarizability tensor components. We have also found that correlation effects are negligible, provided that such calculations are performed at the SCF level with very extended basis sets. We show that standard sp basis sets need to be extended in the sp space, to have extra diffuse functions added and also to include suitable d and p gaussian primitives in order to be useful for the calculation of response functions. We find that the average dipole polarizability (α ave) is related to the number of nitrogen atoms N in the ring by α ave/10 −40(CV −1 m 2) = 10.90–0.90 N We also find that the calculated average and anisotropy polarizability components are both linearly related to the sum of the HOMO and LUMO orbital energies. We also give the first ever high-quality calculation of the hyperpolarizability tensors for these molecules. Electric dipole and quadrupole moments agree well with experiment, as do the asymmetry parameter and the nitrogen quadrupole coupling constants.