New aspects of electrophylic aromatic substitution mechanism: Computational model of nitration reaction


Calculations were performed by means of Austin model 1 (AM1) method [1] in approximations of gas and liquid phase AM1/SM2.1* [2]) implemented in AMSOL v. 5.4 package. Mostly restricted Hartree–Fock (RHF) approximation was used because calculations of reaction path with configuration interaction (CI) demonstrated that one-determinant solution was valid (contribution of main state over 98%) for intermolecular distances up to 2.2 Å, that is, in the area of transition states (TSs). Geometries of all TSs were found by means of standard routine and checked for the presence of one negative-force constant. When CI was used, the variant was 3 × 2.

Although the SM2.1 method was proposed for simulation of aqueous media, we considered that with certain stipulations it enabled us to obtain general regulations of influence of liquid phase on investigated reaction. Thus, one must shift his attention from absolute values of enthalpies presented below to relative reactivity of considered compounds.

* Solvation model for AM1 method. Coulomb component of energy is calculated from the assumption that dielectric continuum is situated outside of molecule’s cavity (constructed by intersection of atomic spheres), so this requires numerical integration of its potential at the time of self-consistent field (SCF) and geometry optimization procedures. Additionally, the cavitation energy is taken into consideration.

The method was parameterized for calculation of enthalpies of solvation of C, N, O, P, and Hal containing compounds in water (including ions). Accuracy for neutral molecules was approximately 1 kcal/mol, ions 4 kcal/mol [2].