AB-INITIO STUDIES OF THE DIPOLE POLARIZABILITIES OF CONJUGATED MOLECULES .2. MONOCYCLIC AZINES

We report high quality ab initio calculations of the dipole polarizabi lity and first hyperpolarizability of the following single monocyclic azines: pyridine, pyridazine, pyrimidine, pyrazine, s-triazine and s-t etrazine. A study of the sensitivity of such calculated properties to the variation of basis set shows that increasing the number of multipl e d and p diffuse functions in the 6-31 + G(nd, mp) basis set at Hartr ee-Fock level gives values of the polarizability tensor components tha t are comparable with the experimental ones, and consistent values of hyperpolarizability tensor components. We have also found that correla tion effects are negligible, provided that such calculations are perfo rmed at the SCF level with very extended basis sets. We show that stan dard 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 funct ions. We find that the average dipole polarizability (alpha(ave)) is r elated to the number of nitrogen atoms N in the ring by alpha(ave)/10( -40) (CV-1 m2) = 10.90 - 0.90N. We also find that the calculated avera ge and anisotropy polarizability components are both linearly related to the sum of the HOMO and LUMO orbital energies. We also give the fir st ever high-quality calculation of the hyperpolarizability tensors fo r these molecules. Electric dipole and quadrupole moments agree well w ith experiment, as do the asymmetry parameter and the nitrogen quadrup ole coupling constants.