Nonempirical Molecular-Orbital Calculations for Protonated Formaldehyde, Acetaldehyde, and Formic Acid

Abstract

Nonempirical molecular‐orbital calculations based on the Roothaan scheme are reported for protonated species of formaldehyde, acetaldehyde, and formic acid. In these calculations a small Gaussian basis is used, which is obtained from optimized atomic orbitals. The results are interpreted with the help of a Mulliken‐type population analysis. A number of structural parameters was obtained by minimization of the total energy. Values found for the C–O distances are: 1.23 Å in formaldehyde, 1.27 Å in protonated formaldehyde, 1.31 Å in protonated formic acid. In all protonated molecules the C–O–H angle was close to 120°. For protonated acetaldehyde the stability of the configuration [Complex Equation] was found to be greater than that of [Complex Equation] by 1.4 kcal/mole. For protonated formic acid the stablest system was found to be [Complex Equation] followed by [Complex Equation] which is 1.5 kcal/mole higher in energy. The other possible structure [Complex Equation] was found to be another 6.0 kcal/mole higher in energy. The mechanism of the intramolecular interconversion that transforms one configuration of the protonated system into another depends on the strength of the C– bond. In protonated aldehydes a rotation around the C–O bond is rather difficult and has an energy barrier of 25–30 kcal/mole. A motion in the plane, however, requires only 17–18 kcal/mole, so that it is likely that in aldehyde systems this motion causes the interconversion. In protonated formic acid the C–O bond is not as strong as in aldehydes, and a rotation around the C–O bond requires only 15 kcal/mole, while the motion in the plane still requires 17 kcal/mole.