VIBRATIONAL ANALYSES OF TRANSPOLYACETYLENE BASED ON AB-INITIO 2ND-ORDER MOLLER-PLESSET PERTURBATION CALCULATIONS OF TRANS-OLIGOENES

Structures and vibrational force fields of all-trans-oligoenes of vari ous chain lengths are studied by ab initio molecular orbital (MO) calc ulations at the second-order Moller-Plesset perturbation (MP?) level w ith the 3-21G and 6-31G basis sets. Dependencies of the structure par ameters and the force constants of trans-oligoenes on the chain length and the position in the chain are analyzed quantitatively. The struct ure and the force field of trans-polyacetylene are extrapolated from t hose of trans-oligoenes. Normal coordinate calculations are performed for trans-polyacetylene on this basis. The following results show the reliability of the structure and the vibrational force field of trans- polyacetylene obtained in the present study. (1) The calculated C=C an d C-C bond lengths of trans-polyacetylene are in satisfactory agreemen t with the observed. (2) The calculated delta=0 (null phase difference ) frequencies are in good agreement with the observed infrared and Ram an frequencies. (3) The calculated phonon dispersion curves are consis tent with some criteria obtained experimentally. (4) Most of the obser ved infrared bands which do not correspond to the delta=0 frequencies of trans-polyacetylene are assignable to peaks in the profile of the c alculated density of vibrational states. (5) The calculated hydrogen-a mplitude-weighted density of states is in reasonable agreement with th e observed inelastic neutron scattering spectrum. (C) 1995 American In stitute of Physics.