A COMPARATIVE QUANTUM-MECHANICAL STUDY OF BOND SEPARATION ENERGIES ASA 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 cal culate isodesmic bond separation energies for reactions involving a nu mber of representative five- and six-membered ring organic compounds. The MP2 and density functional approaches yield reasonably good energi es; the density functional method agrees particularly well-with experi ment, exhibiting a root-mean-square error of only 2.5 kcal/mol. Ring g eometries are calculated satisfactorily in all approaches but are give n particularly accurately by the MP2 approach. A comparison of the B-L YP bond separation energies with several other definitions of resonanc e energy shows that these different approaches correlate with each oth er in a reasonable fashion. (C) 1995 by John Wiley & Sons, Inc.