A COMPARATIVE QUANTUM-MECHANICAL STUDY OF BOND SEPARATION ENERGIES AS A MEASURE OF CYCLIC CONJUGATION

Restricted Hartree-Fock (RHF), second-order Moller-Plesset (MP2), and density functional calculations [using the Becke/Lee-Yang-Parr (B-LYP) exchange/correlation gradient-corrected functionals] employing the 6-311G(d,p) and 6-311 + + G(d,p) basis sets have been carried out to calculate isodesmic bond separation energies for reactions involving a number of representative five- and six-membered ring organic compounds. The MP2 and density functional approaches yield reasonably good energies; the density functional method agrees particularly well-with experiment, exhibiting a root-mean-square error of only 2.5 kcal/mol. Ring geometries are calculated satisfactorily in all approaches but are given particularly accurately by the MP2 approach. A comparison of the B-LYP bond separation energies with several other definitions of resonance energy shows that these different approaches correlate with each other in a reasonable fashion. (C) 1995 by John Wiley & Sons, Inc.