AB-INITIO INVESTIGATION OF THE CONFORMATIONAL ENERGIES, ROTATIONAL BARRIERS, MOLECULAR-STRUCTURES, VIBRATIONAL FREQUENCIES, AND DIPOLE-MOMENTS OF ALDEHYDES AND KETONES
Rj. Berry et al., AB-INITIO INVESTIGATION OF THE CONFORMATIONAL ENERGIES, ROTATIONAL BARRIERS, MOLECULAR-STRUCTURES, VIBRATIONAL FREQUENCIES, AND DIPOLE-MOMENTS OF ALDEHYDES AND KETONES, Journal of physical chemistry, 99(26), 1995, pp. 10511-10520
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.