New aspects of electrophylic aromatic substitution mechanism: Computational model of nitration reaction
The SM2.1 calculations give many conclusions similar to those of gas phase. But there are certain differences, one example being that the process of solvation/desolvation of reagents results in classic profile of reaction with distinct activation barrier. Appearance of the barrier is due to positive-charge delocalization in TS caused by desolvation of
NO2+ cation. The SM2.1 calculations predict no prereaction complex, meaning that there is no special affinity between
NO2+ and aromatics, just solvation. The σ-complex becomes endothermic, unstable structure, in good agreement with chemical intuition. Its decomposition with
H+-elimination is an exothermic process with hardly noticeable TS.
In comparison with results of gas-phase calculations, σ-complex in liquid phase is characterized by more deformed and polarized
C-H bond. It is lengthened by 15%, charge of H-atom is 0.43, and frequency of valent
C-H vibration changes to 2200 sm−1 (from the usual 3100 sm−1). Reactions of low reactive substrates with late TS have no σ-complex at all: proton leaves synchronously with addition of
Transition states and reaction coordinate
In the area of the first TS the reaction coordinate is again the length of
C-N distance. The second stage of reaction, proton elimination, has more complex movement, including both valent and deformation vibrations of
C-H bond. This second stage has practically no barrier, 1.5 kcal/mol for reaction of benzene (see Table IV). Traveling from TS along gradient vector, one can trace changes of energy or charges occurring during reaction (Figs. 4 and 5). Some parameters of stationary points on the pathway are shown in Table IV.
Figure 4. Dependence of heat of formation (kcal/mol) of supermolecule on
C-N distance (Å) at the first stage (σ-complex formation) for AM1/SM2.1.
Figure 5. Dependence of charge of
C-N distance (Å) at first stage for AM1/SM2.1.