Internal rotation of the NN-dimethylamino-group in aromatic and heteroaromatic systems

Abstract

The origin of the rotational barrier of the dimethylamino-group in several aromatic and heteroaromatic systems has been analysed by a comparison of the results of MO calculations and experimental energies. For most of the compounds considered theory predicts that the dimethylamino-group should be coplanar with the attached ring in the ground state, except in a few cases where steric effects are present. For NN-dimethylaniline nitrogen inversion was also studied for several conformations (different angles of rotation θ around the C–N bond) by ab initio MO calculations with a limited basis set. It was thus found that, while the pyramidal structure is stable both for the planar and perpendicular conformations, the ground state of the molecule should correspond to the fully planar conformation averaged over two rapidly equilibrating pyramidal structures. The rotational behaviour for the planar-averaged dimethylamino-group was calculated for several angles of rotation by different MO approaches, namely extended Hückel, CNDO/2, INDO, PCILO, and ab initio STO-3G. The different MO approaches provide qualitatively similar results with the planar conformation corresponding to the ground state, except extended Hückel which indicates the perpendicular conformation to be more stable. The π-bond order given by CNDO/2 shows a linear dependence on the experimental free-energy of activation (ΔG*), and it appears that the resulting equation can be usefully employed for predicting the ΔG* values for different systems. Even the energy difference, ΔE, are roughly proportional to the corresponding ΔG* values, but the correlation is less satisfactory than that obtained with the π-bond order. Provisions of energy barriers with the above equations for a number of protonated and alkylated forms show that in molecules where conjugation is expected the barriers should increase. This has been verified experimentally in two molecules where measurements were made possible.