CONFORMATIONS OF STILBENE-LIKE SPECIES AND NEW METHOD OF ENERGY PARTITION

To understand the nature of pi-electron delocalization, while question ing the abnormally large twist angle of N-benzylideneaniline, we prepa red four stilbene-like species, (4-X-Ph)-CH=N-Ar (Ar = 2-pyridyl, X= - Cl, -NO2, -N(Me)(2): Ar = 2-pyrimidyl, X = -NO2), and four ketenimine derivatives, (4-X-Ph)(2)C=C=N-(Ph-Y-4) (Y=-H, X=-H; Y=-NO2, X=-H; Y=-N O2, X=-OMe; Y=-N(Me)(2), X=-H), and determined their crystal structure s using X-ray difl%action. Our new procedure for constructing a comple te fragment molecular orbital (FMO) basis set is described in detail. Based on our procedure, the Morokuma's energy partitioning provides, i n the framework of ab initio SCF-MO computation at the STO-3G level, t he various pi and sigma energies associated with the inter-and intrafr agment interactions. The pi-electron delocalization in the DPI state o f stilbene-like species is found to be destabilization. The DPI state is most destabilized at the coplanar geometry, and its electronic ener gy is the highest of four hypothetical electronic states. The characte ristics of the quantum mechanical resonance energy (QMRE), including i ts role with regard to chemical reactivities toward electrophile attac k, depend upon the response of the a framework to the pi-electron delo calization. In the case of stilbene-like species, the QMRE is destabil izing. Conversely, the QMRE of benzene is stabilizing. However, it is the cr framework of benzene, rather than the sr system itself, which i s strongly stabilized by the QMRE, revealing that benzene is a aromati c. The driving forces for the out-of-plane twist of stilbene-like spec ies arise from the QMRE and the sigma orbital interaction. The electro n-withdrawing (-I) groups and the ring-nitrogen atoms seem to have an obvious influence upon the twist angle.