Formation of Closed Contour for Unpaired Electron Delocalization

2 Formation of Closed Contour for Unpaired Electron Delocalization 

In the case of alkali metals, ion pairing can be visualized as HFCs from paramagnetic nuclei of the metal cations associated with organic anion-radicals in ethereal solvents. In this respect, an alkali metal cation associated with the anion-radical of o-dimesitoylbenzene in DME or THF serves as a paradigm (Herold et al. 1965). 

All of the examples considered in this section up to now, have carbonyl (chelating) groups. Alkali salts of anion-radical without chelating groups should also be considered, to complete the picture. 

Low-lying vacant orbitals of alkali metal cations can, consequently, accept an unpaired electron density even if it is delocalized over an extended n system of carbon chains. The anion-radical of 1,4-diphenylbutadiene can exist in i-trans and in -cis forms. The relative amounts of these geometrical isomers appear to depend highly on the counterion/solvent system. Li and K+ were studied as counterions THF, 2-MeTHF, and DME were employed as solvents (Schenk et al. 1991). Interaction between the anion-radical and the cation contributes to a stabilization of 

When a tetraalkylammonium cation is used as a counterion in solvents of high polarity, such as AN or DME, the alkyl groups of the cation hinder the mutual approach of species with different charges. Ion pairs with the potassium cation are stable. This follows from a comparison of the polarographic behavior of the three isomeric dinitrobenzenes in the same solvent (DMF) using tetraethylammonium or potassium perchlorate as the carrier electrolyte (Todres 1970). The halfwave potentials corresponding to the conversion of p- and m-dinitrobenzenes into anion-radicals are independent of whether tetraethylammonium or potassium counterions are employed. The anion-radical is formed from o-dinitrobenzene at a potential that is less negative by almost 100 mV when 

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