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 s tatic dipole polarizability and the first hyperpolarizability of the e lectronic ground state for a series of five-membered conjugated hetero cyclics C(4)H(4)E, E = BH, AlH, CH2, SiH2, NH, PH, O and S (borole, al uminocyclopentadiene, cyclopentadiene, silacyclopentadiene, pyrrole, p hosphole, furan and thiophene, respectively). The geometries, fully op timized at HF/6-31G* level of theory, show good agreement with report ed experimental and/or other theoretical ones. Changes induced by the heteroatom on the structural and electronic properties of these C4H4 r ings are explained in terms of the heteroatom electronegativity. The s ensitivity of the polarizability calculations to the variation of basi s set was studied. We show that increasing the number of multiple d an d p polarization functions in the 6-31 + G(nd, mp) basis set at Hartre e-Fock level leads to values of the polarizability tenser components t hat are in good agreement with available experimental data. Electron c orrelation effects on the static dipole polarizability are shown to be non-negligible, when calculated at the MP2/6-31+G(d,p) level of theor y for borole, cyclopentadiene, pyrrole, furan and thiophene. We find t hat 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 comp ound shows an enhanced and anomalous polarizability with respect to th e rest of the series. In addition, we show that these polarizabilities can be used as an aromaticity scale for explaining the known aromatic ity in compounds such thiophene, pyrrole and furan. We also give the f irst-ever high quality calculation at the HF/6-31+G(3d,3p) level of th eory for the first hyperpolarizability tensors of these molecules.