To understand the nature of pi-electron delocalization, while questioning the abnormally large twist angle of N-benzylideneaniline, we prepared 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=-NO2, X=-OMe; Y=-N(Me)(2), X=-H), and determined their crystal structures using X-ray difl%action. Our new procedure for constructing a complete fragment molecular orbital (FMO) basis set is described in detail. Based on our procedure, the Morokuma＇s energy partitioning provides, in the framework of ab initio SCF-MO computation at the STO-3G level, the various pi and sigma energies associated with the inter-and intrafragment interactions. The pi-electron delocalization in the DPI state of stilbene-like species is found to be destabilization. The DPI state is most destabilized at the coplanar geometry, and its electronic energy is the highest of four hypothetical electronic states. The characteristics of the quantum mechanical resonance energy (QMRE), including its role with regard to chemical reactivities toward electrophile attack, depend upon the response of the a framework to the pi-electron delocalization. In the case of stilbene-like species, the QMRE is destabilizing. Conversely, the QMRE of benzene is stabilizing. However, it is the cr framework of benzene, rather than the sr system itself, which is strongly stabilized by the QMRE, revealing that benzene is a aromatic. The driving forces for the out-of-plane twist of stilbene-like species arise from the QMRE and the sigma orbital interaction. The electron-withdrawing (-I) groups and the ring-nitrogen atoms seem to have an obvious influence upon the twist angle.