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- octatetraene and all-trans-1,3,5,7,9-decapentaene on the basis of ab initio molecular orbital (MO) calculations and observed infrared and Raman spectra. The infrared and Raman spectra of all-trans-1,3,5,7,9-decapentaene are measured in this study. Ab initio MO calculations are carried out at the second-order Møller-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 calculated and observed frequencies of trans-1,3,5-hexatriene. The calculated frequencies 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 that good force fields of all-trans oligoenes can be obtained from ab initio 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/6-31G^* levels, it is clear that electron correlation has large effects not only on the absolute values of diagonal and off-diagonal force constants of the C=C and C-C stretches but also on the chain-length dependence of these force constants. Important features of the vibrational modes are also discussed.