Ab initio MO study of the internal rotation process in alpha-chlorinated picolines.

Conformational properties and barriers to internal rotation for the three isomeric picolines mono- bi- and trichlorosubstituted at the methyl group were studied at the theoretical level with ab initio MO calculations. Energy profiles for internal rotation of the chlorinated methyl group were obtained at the 6-31G* level with full molecular geometry relaxation, and the energy of critical points was recalculated at the MP2/6-311G** level of theory. The conformational minima and maxima of these derivatives are qualitatively similar to those found for benzene derivatives, yet differences are found in the heights of the barriers to internal rotation and in the presence of two rotational minima or maxima in the molecules with lower rotational symmetry. The origin of the barriers to rotation and of the different energy content of conformational minima is discussed in the light of the natural bond orbital (NBO) approach within the donor-acceptor viewpoint of intramolecular interactions, and electrostatic interactions between the rotating group and the ring.