Aromatic hydrocarbons atomic charges

On the basis of these results we embarked on a systematic study on the synthesis of vinyl cations by intramolecular addition of transient silylium ions to C=C-triple bonds using alkynyl substituted disila alkanes 6 as precursors.(35-37) In a hydride transfer reaction with trityl cation the alkynes 6 are transformed into the reactive silylium ions 7. Under essentially nonHnucleophilic reaction conditions, i.e. in the presence of only weakly coordinating anions and using aromatic hydrocarbons as solvents, the preferred reaction channel for cations 7 is the intramolecular addition of the positively charged silicon atom to the C=C triple bond which results in the formation of vinyl cations 8-10 (Scheme 1). 

Transferability of atomic densities was tested by Brock et al. (1991), who applied atomic charge density parameters from an accurate low-temperature study of perylene (I) to data collected at five and six different temperatures on naphthalene (II) and anthracene (III), respectively. The molecules are all aromatic hydrocarbons. To reduce the number of variables, all H atoms were assigned... 

Reactions (30) and (31) may give the same products. In (31) the polarization energy decreases the energy demand for temporal charge separation and it can be exothermic when B has a considerable electron affinity. For aromatic hydrocarbon quenchers (e.g., anthracene) such mechanism leads to dissipation of the excitation energy on the vibrational levels. When the quencher molecules contain Cl or Br atom in the intermediate step, Cl or Br elimination is expected, e.g., with benzyl chloride additive ... 

Representative couplings of aromatic hydrocarbons are summarized in Table 1. Alkyl-substituted aromatic hydrocarbons can be coupled to diphenyls and/or diphenylmethanes depending on their substitution pattern (Table 1, numbers 1-6). The initially formed radical cation I [Eq. (3)] reacts with the starting compound to the diphenyl (II) (Eq. (3), path a] or loses a proton to form a benzyl radical [Eq. (3), path b], which after oxidation to the cation undergoes an electrophilic aromatic substitution at the starting compound to form the diphenylmethane (III). A low charge density on an unsubstituted carbon atom of I favors path a, whereas a low charge density on a substituted carbon atom favors path b[4]. 

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