Ab initio Hartree—Fock and post-Hartree—Fock studies on molecular structures and vibrational spectra of formamide and formamidic acid

Abstract

The molecular parameters (geometries, rotational constants and dipole moments) and vibrational IR spectra (wavenumbers and absolute intensities) of formamide and its tautomer formamidic acid have been computed at the ab initio SCF/6-31G** and MP2/6-31G** levels. The molecular structure of planar formamide has also been predicted at the MP2/6-311G(3df,2p) level. The relative stabilities of the title species are discussed by comparing their relative internal energies at 0 K; the SCF/6-311G** energies, electron correlation MP4(SDTQ) contributions and zero-point vibrational energies are included. The calculated relative internal energies of the studied tautomers suggest that the formamide molecule is more stable than the acidic form by 50 kJ mol −1, in good agreement with the experimental prediction. The MP2/6-31G** calculations suggest a small deviation from planarity for formamide, but the nonplanar structure is predicted to be only 0.003 kJ mol −1 more stable than the planar structure. The predicted MP2/6-31G** and MP2/6-311G(3df,2p) molecular parameters are in better agreement with the experimental data than those calculated at the SCF/6-31G** level. The calculated vibrational spectra of formamide at both SCF/6-31G** and electron correlation MP2/6-31G** levels compare favourably with the available experimental data, and the observed shifts of the IR wavenumbers upon deuteration of the molecule are correctly predicted by the calculations.