Premium
AM1 calculations on the tautomerism of free and hydrated hydroxypyridines: Barriers to proton transfer in 2‐hydroxypyridine–pyrid‐2(1 H )‐one and effect of solvation and self‐association
Author(s) -
Fabian Walter M. F.
Publication year - 1990
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.610030510
Subject(s) - tautomer , chemistry , solvation , ab initio , computational chemistry , hydrogen bond , ab initio quantum chemistry methods , proton , supermolecule , transition state , solvent , molecule , stereochemistry , organic chemistry , catalysis , physics , quantum mechanics
AM1 calculations for the tautomerism of the three isomeric hydroxypyridines are reported and compared with recent ab initio results. Intrinsic stabilities of the various tautomers are predicted by AM1 with an accuracy comparable to or even better than the best available ab initio calculations. Solvation is accounted for by the supermolecule approach and also a continuum model for solvent effects. Except for 3‐hydroxypyridine, AM1 correctly accounts for the observed shift of the tautomeric equilibria due to hydration. Energetics of hydrogen bonding are reasonably described by this semiempirical method. With respect to structures, bifurcated hydrogen bonds are preferred by AM1. In addition, for 2‐hydroxypyridine and its lactam tautomer self‐association in addition to barriers to proton transfer are considered. With respect to tautomerization transition states, AM1 shows serious shortcomings. Compared with both experimental and ab initio results, barriers to proton transfer are considerably overestimated by AM1, especially in the hydrated and associated species. For the dimers AM1 predicts an unsymmetrical transition state which, however, is only slightly lower in energy than the symmetrical structure with two negative eigenvalues of the force constant matrix. Despite several attempts, the transition state for proton transfer in the doubly hydrated species could not be located.