Prereactive complexes in chlorination of benzene, triazine, and tetrazine: a quantum chemical study.

In order to perform a complete search for prereactive complexes between arenes and chlorine, the stochastic search method was employed. Stationary points are optimized at B3LYP, M05-2X, and MP2 levels, while improved energetics are calculated using the B2PLYP-D method, which includes corrections important for accurate description of dispersion forces. New intermediates were located and their mechanistic relevance has been discussed. It has been suggested that, at least in the gas-phase, the T-shaped complex precedes the formation of classical benzene/chlorine π-complex. No σ-complex is found on the energy surface, unless explicit counterions are included in calculations. Neither π- nor σ-complexes were located on the reactant side of chlorination of triazine, but only linear and T-shaped complexes were identified as stable minima. These structures represent important prereactive complexes for chlorination of triazine. In the case of tetrazine, which is unlikely to undergo direct chlorination, only two complexes (resting and T-shaped) were located.