AB-INITIO INVESTIGATION OF THE CONFORMATIONAL ENERGIES, ROTATIONAL BARRIERS, MOLECULAR-STRUCTURES, VIBRATIONAL FREQUENCIES, AND DIPOLE-MOMENTS OF ALDEHYDES AND KETONES

A detailed quantum mechanical study of aldehydes and ketones has been carried out at the HF/6-31G level. Computed relative conformational e nergies, rotational barriers, and geometries were calculated for a wid e variety of molecules and compared with experiment. For the most part , both the computed relative conformational energies and the barriers are in reasonable agreement with experiment. In several cases there we re differences observed between the quantum calculations and experimen t which suggested a reinterpretation of the experimental data. For exa mple, in the case of diisopropyl ketone, it was suggested that two con formers rather than the three assumed by experiment were present in eq uilibrium in the gas phase. For both cyclobutanecarboxaldehyde and met hylcyclobutyl ketone the calculations predicted an additional (axial, gauche) low-energy minimum which has not been observed experimentally but should be possible to detect in the microwave spectrum. Relative t o experiment, the computed C=O bond lengths are similar to 0.025 Angst rom smaller and the computed C=O stretches are similar to 280 cm(-1) h igher. For cycloalkanones the calculations qualitatively reproduce the experimentally observed variation in the C=O bond length and the dram atic decrease in the vibrational frequency with increasing ring size. The dipole moments computed for aldehydes and ketones are similar to 1 0% higher than experiment with the exception of equatorial,trans-cyclo butanecarboxaldehyde, where it is 59% higher.