Theoretical Study of the Barriers to Internal Rotation in Formic Acid

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Abstract
The nonempirical self‐consistent‐field molecular‐orbital method with a basis set of groups of accurate Gaussian atomic orbitals has been used to study formic acid, HCOOH, for three different geometries. The conformer with the hydroxyl hydrogen cis to the carbonyl oxygen is correctly computed to be the most stable. This geometry is 13.0 kcal lower in energy than the geometry with the hydroxyl hydrogen at a 90° dihedral angle with the carbonyl oxygen (experimental estimate: 13.4 kcal) and 8.1 kcal lower than the trans form (experimental estimate: ≥ 4 kcal). The population analysis suggests a screening of the carbonyl oxygen in the high nuclear repulsion cis form and a partial breaking of the hydroxyl oxygen–carbon bond from cis to 90°. The electronic contribution to the calculated dipole moment shows shifts of electronic charge toward the carbonyl end of the molecule in cis and trans forms compared to the 90° form. Molecular properties related to the nuclear–nuclear and nuclear–electron potentials at the various nuclei exhibit increased attraction of electronic charge for a given nucleus when that nucleus is in an energetically unfavorable nuclear potential field, illustrating nuclear screening as an important feature of the barrier.