Ab initio post-Hartree—Fock calculations of the 2-hydroxypyridine/2(1 H)-pyridinone system: molecular structures, vibrational IR spectra and tautomeric stability

Abstract

The results of ab initio post-Hartree-Fock (MP2(full)/6-31G(d), MP2(fc)/6-31G(d,p)) and Hartree-Fock (HF/6-311G(d,p)) calculations are reported for molecular parameters (geometries, rotational constants and dipole moments) and vibrational IR spectra (harmonic wavenumbers and absolute intensities) of the oxo and hydroxy tautomers of 2-oxopyridine. The results are compared with the recent gas phase microwave and matrix isolation IR spectroscopic data. The molecular parameters (rotational constants and dipole moments) predicted at the MP2 level agree much better with the recorded experimental data than do the parameters predicted at the HF level. However, the calculated MP2 vibrational IR spectra do not show considerable improvement over the spectra predicted at the HF level. The relative internal energies at 0 K of the tautomeric species were calculated at different levels of theory (with electron correlation contributions computed up to fourth-order perturbation theory (MP4) using the 6-31G(d,p) and 6-311G(d,p) basis sets). The temperature-dependent contributions to the free energies of tautomerization are also computed within the rigid rotor-harmonic oscillator-ideal gas approximation. The calculated total free energies of tautomerization are compared with recent ab initio calculations and with the available experimental data.