RESONANCE REVISITED - A CONSIDERATION OF THE CALCULATION OF CYCLIC CONJUGATION ENERGIES

A homomolecular differential bond separation reaction may be defined a s the difference between the conventional bond separation reactions in volving the unsaturated system and its saturated counterpart. Such a r eaction is homomolecular in that the basic molecular structures involv ed are the same on both sides of the reaction. The type of homodesmoti c reaction that also conserves structure in this way may be termed a h omomolecular homodesmotic reaction. Both types of homomolecular reacti ons are readily related to hydrogenation reactions and more importantl y, to each other. Delta B(n), the energy of the homomolecular differen tial bond separation reaction involving a system with n double bonds, and H(n), the corresponding homomolecular homodesmotic reaction, are r elated by:Delta B(n) - H(n) = n . (h(1) - h(e)) where h(1) and h(e) ar e the hydrogenation energies of the system's monoene and of ethylene, respectively. Both types of reactions yield measures of cyclic conjuga tion energies that for certain classifications of molecules are simply related to each other. Consideration of extra conjugation in the mono enes allows a ready interpretation of those cases in which a simple cl assification is not obtained. Ab initio calculations illustrating thes e effects have been carried out on a variety of molecules including ma ny five- and six-membered ring systems using second order Moller-Pless et and density functional approaches. (C) 1997 by John Wiley & Sons, I nc.