INTERNAL-ROTATION IN ALPHA-CHLORINATED TOLUENES - A THEORETICAL MO AB-INITIO STUDY OF THE ROTATIONAL BARRIERS AND GROUND-STATE CONFORMATIONS

Potential energy profiles for internal rotation in benzyl chloride (1), benzal chloride (2), benzotrichloride (3), and in the corresponding derivatives ortho substituted with a chlorine atom (4-6) were derived from total molecular energies calculated with MO ab initio methods at the 6-31G*//6-31G* level. Energy values for critical points were recalculated at the 6-311G**//6-31G* level using MP2 perturbation theory. The geometrical structure of the minima and transition states is discussed and the origin of the barriers to internal rotation analysed. From a natural bond orbital (NBO) model based on localized orbitals and within a donor-acceptor viewpoint, the electronic interactions governing the energy barriers in these molecules are rationalized in terms of effects familiar to the chemical nomenclature. Thus, in compounds 1-3 an electronic interaction between the pi system and the chlorine atom is found to be operative, with prevailing donor character from the pi system, but interactions involving other orbitals also contribute to the rotational barrier. These effects operate to the same extent in the o-Cl substituted derivatives 4-6 and do not justify the higher rotational barrier calculated for these compounds. An empirical approach was used to estimate non-bonded atom-atom potentials. These interactions, which can be included within the so-called ''steric effects'', were found to be the most important ones contributing to the energy barriers in compounds 4-6.