REEXAMINATION OF THE VIBRATIONAL FORCE-FIELDS OF TRANS-1,3,5-HEXATRIENE AND CIS-1,3,5-HEXATRIENE - EFFECT OF ELECTRON CORRELATION, TRANSFERABILITY OF SCALE FACTORS AND CORRESPONDENCE OF NORMAL-MODES BETWEEN THE 2 ISOMERS

Ab initio molecular orbital calculations of the structures and vibrati onal force fields of trans- and cis-1,3,5-hexatriene are performed at the Hartree-Fock (HF) and the second-order Moller-Plesset perturbation (MP2) levels with the 6-31G basis set. Scale factors for force const ants are optimized with reference to the observed vibrational frequenc ies of the trans isomer, and the transferability of the scale factors between the trans and cis isomers is examined. The scale factors for t he in-plane force constants obtained at the MP2/6-31G level are trans ferable between the trans and cis isomers. However, the scale factors for the out-of-plane force constants obtained at the HF/6-31G level a re better in transferability than those obtained at the MP2/6-31G lev el. Correspondence between the normal modes of the trans and cis isome rs is examined by calculating the pseudo-Duschinsky rotation matrix. M ode-to-mode correspondence is easily understood by inspecting the elem ents of the pseudo-Duschinsky rotation matrix.