Tautomeric equilibria of heterocyclic molecules. A test of the semiempirical AM1 and MNDO‐PM3 methods

Abstract

AbstractTautomeric equilibria (mainly of the lactam‐lactim type) for a rather large number of six‐membered heterocyclic molecules are calculated by the semiempirical AM1, MNDO‐PM3, and MNDO methods. Except for compounds with adjacent pyridine‐like lone pairs both AM1 as well as MNDO‐PM3 give rather reliable predictions for relative stabilities of the various tautomeric species–comparable to quite high level ab initio calculations. The known errors associated with MNDO in the treatment of heterocyclic tautomerism are thus largely corrected in AM1 as well as MNDO‐PM3. For 2‐hydroxypyridine‐pyrid‐2(1H)‐one the effect of self‐association is less satisfactorily described by MNDO‐PM3 than by AM1. MNDO‐PM3 calculated relative stabilities of methylated derivatives are, however, in considerably closer agreement with experimental values than those obtained by AM1. Ionization potentials, especially those for lone‐pair orbitals, are overestimated by all three semiempirical methods. MNDO‐PM3 results for nitrogen lone‐pair orbital energies are slightly better than those obtained by the AM1 or MNDO method.