The pi-electron delocalization is destabilizing in N-(phenylethenylidene) benzeneamine; a method of separating out the pi and sigma systems in its ketenimine fragment

To understand the nature of pi-electron delocalization, we have developed a new method of energy partition. As it is seen from our previous contributi ons devoted to stilbene-like species (J. Phys. Chem. A, 102 (1998) 2016-202 8), whether the pi-electron delocalization is stabilization or not depends upon the response of the a framework to the delocalization, In the case of stilbene-like species, the quantum mechanical resonance energy is generally destabilizing. A method for separating out its pi and a systems in ketenim ine fragment -CH=C=N- is a three-step procedure: (i) the dissection of the bridge -CH=C=N- into two fragments -CH=(P) and =C=N- (Q); (ii) two FMO basi s sets psi'(pj) and psi'(qi) obtained from the full UHF computation for eac h of two fragments P and Q followed by the Kost's localization based on the ir respective fragment molecules H-CH=O and O=C=N-F; (iii) the superpositio n of psi'(pj) and psi'(qi) followed by two consecutive conditional UHF comp utations for fragment -CH=C=N- and a conditional RHF computation for fragme nt molecule H-CH=C=N-H at the STO-3G level. Based on our procedure, the Mor okuma energy partitioning provides the various pi and a interaction energie s. In each of the four hypothetical electronic states of the theta = 0 degr ees geometry, the pi-electron delocalization is found to be destabilization . The DPI state is most destabilized. In the meantime, the delocalized pi s ystem is always destabilized, and it is most destabilized in the FUD state. The pi-sigma orbital interaction is also destabilizing. The a-electron del ocalization, on the other hand, strongly stabilizes the DSI state. The incr ease in the stability of the DSI states with the rotation of fragment C is discussed in detail, and it appears to confirm the concept: "stability" mea ns "smaller destabilization" rather than "greater stabilization''. (C) 1999 Elsevier Science B.V. All rights reserved.