A combined EPR and DFT study of the overcrowded aromatic radical cations from Friedel–Crafts alkylation reactions

Electron paramagnetic resonance and electron–nuclear double resonance methods were used to study the polycyclic aromatic radical cations produced in a Friedel–Crafts alkylating system, and the following radical cations were indentified: 3,6,11,14-tetramethyl dibenzo (a, c) triphylene and 2,6-dimethyl-9,10-di(p-methylbenzyl) anthracene radical cations. The results indicate that the observed electron paramagnetic resonance spectra are due to overcrowded polycyclic aromatic radical cations formed from the parent hydrocarbons. It is suggested that benzyl halides produced in the Friedel–Crafts alkylation reactions undergo Scholl condensation to give polycyclic aromatic hydrocarbons, which are converted into corresponding polycyclic aromatic radical cations in the presence of AlCl3. We carried out the theoretical calculation of the isotropic 1H hyperfine coupling constants for studied both PAHs radical cations. The results indicate that the IEFPCM–DFT calculation at B3LYP level with 6-31++G(d,p), EPRII and EPRIII basis sets could well support the experimental hfcc assignment of the observed radicals. Optimized geometry indicates that the aromatic rings in both PAHs radical cations twisted significantly out of co-planarity.

► The polycyclic aromatic radical cations were investigated by EPR and ENDOR.
► The aromatic rings in both PAHs radical cations twist out of co-planarity.
► The theoretical hfcc support the experimental assignment of the observed radicals.