AB-INITIO STUDIES OF THE DIPOLE POLARIZABILITIES OF CONJUGATED MOLECULES .5. THE 5-MEMBERED HETEROCYCLICS C(4)H(4)E (E=BH, ALH, CH2, SIH2, NH, PH, O AND S)

We report high quality ab initio calculations of the geometries, the static dipole polarizability and the first hyperpolarizability of the electronic ground state for a series of five-membered conjugated heterocyclics C(4)H(4)E, E = BH, AlH, CH2, SiH2, NH, PH, O and S (borole, aluminocyclopentadiene, cyclopentadiene, silacyclopentadiene, pyrrole, phosphole, furan and thiophene, respectively). The geometries, fully optimized at HF/6-31G** level of theory, show good agreement with reported experimental and/or other theoretical ones. Changes induced by the heteroatom on the structural and electronic properties of these C4H4 rings are explained in terms of the heteroatom electronegativity. The sensitivity of the polarizability calculations to the variation of basis set was studied. We show that increasing the number of multiple d and p polarization functions in the 6-31 + G(nd, mp) basis set at Hartree-Fock level leads to values of the polarizability tenser components that are in good agreement with available experimental data. Electron correlation effects on the static dipole polarizability are shown to be non-negligible, when calculated at the MP2/6-31+G(d,p) level of theory for borole, cyclopentadiene, pyrrole, furan and thiophene. We find that the calculated average dipole polarizability of these molecules is linearly related to the bond distance C-E of the C(4)H(4)E ring with a correlation coefficient of r(2) = 0.98, by alpha(ave)/10(-40)CV(-1)m(2) = 1.523 + 4.973a/a(0) with phosphole being an exception; this compound shows an enhanced and anomalous polarizability with respect to the rest of the series. In addition, we show that these polarizabilities can be used as an aromaticity scale for explaining the known aromaticity in compounds such thiophene, pyrrole and furan. We also give the first-ever high quality calculation at the HF/6-31+G(3d,3p) level of theory for the first hyperpolarizability tensors of these molecules.