Db. Chesnut et Km. Davis, RESONANCE REVISITED - A CONSIDERATION OF THE CALCULATION OF CYCLIC CONJUGATION ENERGIES, Journal of computational chemistry, 18(4), 1997, pp. 584-593
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.