SOLVENT EFFECTS .3. TAUTOMERIC EQUILIBRIA OF FORMAMIDE AND 2-PYRIDONE IN THE GAS-PHASE AND SOLUTION - AN ABINITIO SCRF STUDY

High level ab initio molecular orbital studies, using basis sets up to 6-31 + G**, with electron correlation included at the second-order Moller-Plesset perturbation (MP2) and quadratic configuration interaction with singles and doubles (QCISD) levels, are reported for the tautomeric equilibria of formamide/formamidic acid and 2-pyridone/2-hydroxypyridine in the gas phase and solution. The solvent effects on the tautomeric equilibria were investigated by self-consistent reaction field (SCRF) theory. The calculated tautomerism free energy changes for 2-pyridone in the gas phase, cyclohexane, and acetonitrile are -0.64,0.36, and 2.32 kcal mol-1, in very good agreement with the experimental values (-0.81, 0.33, and 2.96 kcal mol-1, respectively). The introduction of a dielectric medium has little effect on the electronic structure of the enol forms. On the other hand, significant effects on the molecular geometry, charge distributions, and vibrational frequencies are found for the more polar keto tautomers. The calculated changes are readily understood in terms of the increasing weight of the dipolar resonance structure.