EFFECTS OF ELECTRON CORRELATION ON THE VIBRATIONAL FORCE-FIELDS OF CONJUGATED PI-ELECTRON SYSTEMS - COMPARISON OF THE CYCLOBUTADIENE DICATION AND BENZENE AS EXAMPLES OF 4-MEMBERED AND 6-MEMBERED RINGS

Effects of electron correlation on the vibrational force fields were e xamined for four- and six-membered pi-electronic ring systems. The cyc lobutadiene dication and benzene were selected as subject molecules. D etailed vibrational analyses were performed for the cyclobutadiene dic ation and the results were compared with those for benzene. It is show n that the frequency position of the Kekule-type vibration of the cycl obutadiene dication (nu(5)) is strongly affected by electron correlati on in the reverse direction as compared with that of benzene (nu(14)); in the Hartree-Fock approximation, the energy change induced by the K ekule-type vibration is overestimated for the cyclobutadiene dication but is underestimated for benzene. Such results of calculations are ex plained by the difference in the changes of the electronic structures induced by the Kekule-type vibrations. In the case of the cyclobutadie ne dication, the delocalized pi bonding system changes into a pair of radical cations when the molecule is deformed along the Kekule-type vi bration. In contrast, the Kekule-type vibration of benzene leads to tr ansformation of the delocalized pi bonding system into three localized ethylenic pi bonds.