VIBRATIONAL ANALYSES OF TRANS,TRANS-1,3,5,7-OCTATETRAENE AND ALL-TRANS-1 ,3,5,7,9-DECAPENTAENE BASED ON AB-INITIO MOLECULAR-ORBITAL CALCULATIONS AND OBSERVED INFRARED AND RAMAN-SPECTRA

Normal coordinate analyses are performed for trans, trans-1,3,5,7-octa tetaene and all-trans-1,3,5,7,9-decapentaene on the basis of ab initio molecular orbital (MO) calculations and observed infrared and Raman s pectra. The infrared and Raman spectra of all-trans-1,3,5,7,9-decapent aene are measured in this study. Ab initio MO calculations are carried out at the second-order Moller-Plesset perturbation (MP2) level with the 6-31G basis set. The calculated force fields are scaled by using the scale factors determined to obtain good fits between the calculate d and observed frequencies of trans-1,3,5-hexatriene. The calculated f requencies are in satisfactory agreement with the observed for all the modes of both octatetraene and decapentaene including the in-phase C= C stretches, for which the calculated frequencies at the Hartree-Fock (HF) level deviate considerably from the observed. It is concluded tha t good force fields of all-trans oligoenes can be obtained from ab ini tio MP2 calculations by use of the scale factors derived in this study . From the comparison of the force fields at the MP2/6-31G and HF/6-3 1G levels, it is clear that electron correlation has large effects no t only on the absolute values of diagonal and off-diagonal force const ants of the C=C and C-C stretches but also on the chain-length depende nce of these force constants. Important features of the vibrational mo des are also discussed. (C) 1995 American Institute of Physics.