Vibrational Analysis of Acetone, Acetaldehyde, and Formaldehyde

Abstract

Normal-coordinate calculations have been carried out for the molecules acetone, acetone-d6, acetaldehyde, acetaldehyde-d4, acetaldehyde-d1, formaldehyde, and formaldehyde-d2. Infrared and Raman spectra for acetone, acetone-d6, and acetaldehyde-d4 were measured and assigned. Together with experimental data and assignments taken from the literature for the other molecules, these yield an almost complete set of fundamental frequencies. These frequencies were used to derive approximate force fields for these molecules and to investigate the transferability of the force constants. Calculations were carried out with both the Urey—Bradley force field (UBFF) and the valence-force field (VFF) with selected interaction constants. The Urey—Bradley force field is not satisfactory unless it is augmented with some valence-force interaction constants. In the valence-force approximation it is not possible to make an unambiguous choice of the nondiagonal constants. However, a most probable force field is indicated, which on the one hand reproduces the experimental frequencies and on the other hand shows reasonable values for diagonal and nondiagonal force constant. The CO stretching constant, KCO, and the force constants around the CO group appear to be nontransferable among these molecules. The most probable force field indicates a decrease in the value of KCO in going from formaldehyde to acetaldehyde, to acetone. It is noted that the choice of interaction constants has a great influence on the values of the diagonal force constants and on the normal coordinates. Anharmonicity effects are also found to influence the values of the force constants and may affect the resulting conclusions.