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

Abstract

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 Mo/ller–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.