Vibrational polarizability of polyacetylene chains

Within the double harmonic oscillator approximation, ab initio vibrational contributions to the static electric dipole polarizability have been computed for the all-trans polyacetylene chains homologous series. Atomic basis set and electron correlation effects have been studied on ethylene and trans-butadiene as well as the use of semiempirical Hamiltonians. By using the 6-31G* atomic basis set within the Hartree-Fock and the second order Møller-Plesset procedures as well as by using semiempirical Austin model 1 Hamiltonians, the evolution with chain length of the vibrational polarizability per structural unit has been investigated and compared to the electronic contribution. Although smaller than the electronic contributions (α_vibration≈10% α_electronic), the longitudinal component to the polarizability presents a similar exaltation as chain length grows, but a slower saturation to an asymptotic value per unit cell. Inclusion of electron correlation via the second order Møller-Plesset technique turns out to reduce the longitudinal component calculated at the Hartree-Fock level, but to increase the transversal and perpendicular components. Whereas it reproduces correctly the evolution with chain length of the vibrational polarizability tensor components, the Austin model 1 technique underestimates the longitudinal term and overestimates the perpendicular term. The major contribution to the vibrational polarizability results from large charge fluxes associated with asymmetric stretching motions of the carbon backbone and with the torsion motions presenting very low vibrational frequencies.