Теоретичне дослідження делокалізації заряду у четвертинних амонієвих катіонах на прикладі цетилпіридинію

Abstract

Cetylpyridinium chloride or bromide are detergents which are widely used in industrial and household cleaners and in pharmaceuticals as cationic surfactants and disinfectants. Moreover, cetylpyridinim salts are popular catalysts in organic synthesis.In this work, we have considered the delocalization of charge in the cation of cetylpyridinium. The starting geometry of cetylpyridinim cation was modeled in the Avogadro program. Further geometry optimization was carried out by using AM1 Hamiltonian in the MOPAC216 package. Preparation of incoming mopac files was carried out through the Gabedit program interface. The values of the dielectric permittivity of different mediums were varied from 1 to 120. It was established that the positive charge is not localized on the nitrogen atom, as it is commonly shown, but delocalized over the pyridine ring and the cetyl chain. The pyridine ring is partially positively charged, however, the group charge does not correspond to the unit while lies within the range from +0.65e to +0.71e. The rest residual positive charge is delocalized along the cetyl chain. Moreover, in polar solvents, the first four methylene groups of cetyl substituent carry the main part in delocalization of the positive charge. As for modeling in the gas phase, the charge is even more delocalized, and the seventh methylene group has a partial group charge is about +0.005e. Such an effect of the polar medium can be explained by the localization of charge due to electrostatic interaction with the polar solvents. The methyl-group and the neighboring methylene group adjacent to it, have rather high values of partial charges, which explains some of the features in the spectral (NMR) properties of the terminal CH3 group. To simulate large systems based on the CP cation, it is advisable to reduce the length of the chain to six carbon atoms (of course, if not the effects caused by the chain's length itself is planned to be investigated). The choice of a chain containing six carbon atoms is due to the need for at least four carbon atoms to adequately simulate delocalization of a charge in a similar way as in the CP and two more carbons to reproduce the effects of the terminal methyl group. Keywords: cetylpyridinium; AM1; charge; delocalization; semiempirical.