Structural studies and vibrational analyses of stable and less stable conformers of 1,3,5-hexatriene based on ab initio MO calculations

For 3-trans and 3-cis isomers of 1,3,5-hexatriene, structure parameters of not only the stable (trans about the CC single bonds) but also less stable conformers have been optimized by restricted Hartree-Fock calculations at the 6-31G level. The less stable conformers are found to have torsional angles about the CC single bonds in the range between 32° and 48° from the planar s-cis conformation. Force constants in the Cartesian-coordinate space have been calculated and transformed into the group-coordinate force constants, which have then been adjusted to obtain good agreement between the calculated and observed vibrational frequencies. Vibrational assignments for the stable conformers are reexamined in view of the latest experimental data. Infrared bands of less stable conformers previously reported by two of the present authors are reasonably assigned on the basis of the results of normal-coordinate calculations. Characteristics features of some normal modes of individual conformers are discussed in detail.