Carbocations arenium ions

The carbocation formed m this step is a cyclohexadienyl cation Other commonly used terms include arenium ion and a complex It is an allylic carbocation and is stabilized by electron delocalization which can be represented by resonance... 

Arenium ion (Section 12 2) The carbocation intermediate formed by attack of an electrophile on an aromatic substrate in electrophilic aromatic substitution See cyclohexadienyl cation... 

General agreement on what to call these ions has not yet been reached. The term s complex is a holdover from the time when much less was known about the structure of carbocations and it was thought they might be complexes of the type discussed in Chapter 3. Other names have also been used. We will call them arenium ions, following the suggestion of Olah, G.A. J. Am. Chem. Soc., 1971, 94, 808. 

The results suggest that chinoid type structures are the predominant resonance contributors for 88. The IGLO/DZ//3-21G calculated 13C NMR chemical shifts of benzylic monocations 88 correlate reasonably well with the experimentally obtained data. The 13C NMR chemical shifts of the carbocation centers (CH2 carbon) are calculated 10.6-12.5 ppm too deshielded. Similar results were obtained for benzylic dications 89. NMR chemical shifts of arenium ions derived from various classes of polycyclic aromatic hydrocarbons have been calculated using GIAO-DFT methods.103... 

The NMR studies are complemented with theoretical work. Given the size of the systems, AMl minimizations and single-point calculations were performed in earlier studies. Relative arenium ion energies are typically computed for all possible carbocations derived from a given PAH, for comparison with experiment (this method proved to work quite well). 

Since charge delocalization into the 1-Py-, 4-Py-, and 9-Phe-substituents was quite effective, the presence of a-Cp3 greatly increased Ti-participation and the arenium ion character. On the other hand, diminished charge delocalization into a 2-Py-substituent, coupled with destabilization by a-Cp3, provoked ring protonation. Representative NMR data for these carbocations are shown in Pig. 5. 

Energies for various possible arenium ions and regioisomeric benzylic cations were computed at the DFT B3LYP/6-31G(d) level or by AMI for comparison with the experimental results. These findings provided further evidence in support of the stability sequence 1-pyrenyl > 4-pyrenyl > 2-pyrenyl in a-pyrene-substituted carbocations as models for the intermediates arising from BaP-epoxide ring opening (Fig. 10). 

A systematic carbocation concentration dependency study on NMR chemical shifts was performed for the C-l-protonated 477-cyclopenta[fi e/ phenanthrenium cation 7H+ and the C-l-protonated pyrenium cation 2H+ (Fig. 11). Shielding of the PAH arenium ion protons and carbons was observed with decreasing FSO3H PAH ratios without noticeable line-broadening. This was attributed to cation-anion interactions in the low FSO3H PAH domain and possible formation of contact ion... 

The mono- and diprotonated carbocations and the two-electron oxidation dications derived from parent pyrene and its nonalternant isomers azupyrene (dicyclo-penta[e/, /]heptalene) (DCPH) 72 and dicyclohepta[eJ,g /z]pentalene (DCHP) 73 were studied at the B3LYP/6-31G(d) level (Fig. 29). The most likely site(s) for mono-and diprotonation were determined based on relative arenium ion energies and the structures of the energetically most favored carbocations were determined by... 

Laali et al. have characterized carbocations generated from substituted polycyclic aromatic compounds. The related cation 157 is a true aryl-methyl-type ion, whereas cations 158 have arenium ion character because the strongly electron-withdrawing a-CF3 group enhances charge delocalization into the pyrenyl and phenyl groups. 

This carbocation is called an arenium ion. Although it has considerable resonance stabilization, it has lost the aromatic resonance energy of benzene because the cycle of orbitals does not extend entirely around the ring. Note that the resonance structures have the positive charge located on the carbons ortho and para to the carbon that is bonded to the electrophile. 

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