A DFT study of the conformational behavior of para-substituted acetophenones in vacuum and in various solvents

The barriers to internal rotation around the acetyl-phenyl ring in a series of para-substituted acetophenones X-C6H4COMe, with X = H, F, Cl, Br, I, Me, CF3, CN, NO2, OMe, NH2, NMe2, have been calculated in the gas phase using RHF- and DFT-based methods at different levels of theory. All the computational methods led to only one stable conformer, planar with respect to the heavy atoms, but overestimated activation Gibbs energies of rotation. The influence of the environment has been discussed with respect to the SCRF solvent effect computations using Onsager and PCM solvation models at B3LYP/6-311++G(2d,2p) level of theory for all atoms and B3LYP/aug-cc-pVQZ-PP for iodine. Onsager model seemed to work quite well in predicting the rotational barriers and gave values substantially nearer to experimental data than those given by PCM model. Theoretical and experimental values of rotational barriers, dipole moments and vibrational frequencies of the carbonyl group were correlated.