Charge delocalization modes

Generation and NMR studies of the carbocations from various classes of PAHs under stable ion conditions, in combination with computational studies, provide a powerful means to model their biological electrophiles. These approaches allow the determination of their structures, relative stabilities, charge delocalization modes, and substituent effects, as a way to understand structure/reactivity relationships. 

Fig. 19 GIAO-derived charge delocalization modes for hydroxy-substituted carbocations.

Fig. 23 Charge delocalization modes in model carbocations from BaP, BeP, and azaBaP.

Very recently, a series of novel carbocations and carboxonium ions were generated from 77/-benzo[c]fluorene (80), 1 l//-benzo[Z)]fiuorene (81), ll//-benzo[a]fluorene (82), 2-methoxy- (83), 7-methoxy- (84), and 9-methoxy-l l//-benzo[u]fluorene (85), 7//-dibenzo-[c,g]fluorene (86), 137/-dibenzo[u,g]fluorene (87), 2-methoxy-13//-dibenzo [u,g]fluorene (88), and 5,6-dihydro-13//-dibenzo[u,g]fluorene (89) (Fig. 30). Charge delocalization modes in the resulting carbocations were derived based on experimental and/or computed (GIAO-DFT) A8 C NMR values and via the NPA-deiived changes in charges (A ). 

Arenium ion energies (AAH ion - neutral) and changes in carbon charges [Aq = (ion) - q (neutral)] for protonation of 90 and 91 were probed by the AMI method. The singlet oxidation dication of 90 was also calculated. The charge delocalization modes in the PAH arenium ions were discussed and compared. The AMI studies indicated that benzo[<3]coronene cations were less delocalized than benzo[g/z/]perylene ions. Benzannelation (91 92) severely limited the conjugation path in the carboca-tions despite the fact that coronene 92 was still planar. Further benzannelation (92 90) had a minimal effect on the charge delocalization mode. 

Computational data enabled charge delocalization modes, substituent effects, and in relevant cases conformational aspects to be addressed, and comparison with experiments were found to be fairly good. Moreover, the calculated relative stabilities of the carbocations correlated reasonably well with the available biological activities. 

Quantum-mechanical calculations have been successfully applied to the study of the carcinogenic pathways of PAH and aza-PAH derivatives, and very good correlations have been shown with the available experimental reactivities of these compounds (23-28). Furthermore, modeling studies of biological electrophiles from PAHs by density functional theory (DFT) methods have given proper descriptions of the charge delocalization modes and NMR characteristics of their resulting carbocations (29-33). 

The available NMR data for persistent carbocations derived from various classes of PAHs are compiled and their key features are highlighted and compared. The review covers PAH arenium ions, PAH carboxonium ions and a-PAH-substituted carbocations. Charge delocalization mode and substituent effects are evaluated on the basis of the A8,iC values. 

Figure 33 summarizes the NMR data for dihydropyrenium (ethanophenan-threnium) mono- and dications.25 The charge delocalization mode in the symmetrical dications is noteworthy. The pattern differs from that in parent pyrenium cation and its 2,7-di-tert-butyl-substituted derivatives. 

Fig. 21. Charge delocalization mode for alkylated pyrenium ions based on the A513C values.

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