ESR studies of ion pairing

In slow exchange under controlled condi tions, separated features for A and the ion-pair (A M ) are detectable. As the temperature is increased and reaction rates increase, the features broaden and accurate reaction rates can be estimated from the width increments. In fast exchange, the Na splitting is averaged and lost, and the spectra become similar to those of the free anions. 

These results came at a time of intense controversy amongst physical chemists. Conductivity, ion mobility and related studies clearly indicated that ionic association occurred, the extent increasing as the dielectric constant of the solvent decreased. However, the structures of the ion pairs and clusters were unknown. Some considered that a pair of oppositely charged ions, within 

In these ways, conductivity results could be fully explained in terms of ESR structural and kinetic data [13], and some of the extensive controversies were settled. 

A similar change from 9 to 5 lines results sometimes when a simple salt with a common cation is added. In this case reaction (3.2) occurs, so that the lifetimes of the asymmetric units are reduced, but different sodium ions (with varying nuclear spins) are involved, so that the cation hyperfine splitting is lost at the same rate. As usual, at intermediate rates, lines become broad and from the width increments, rates and activation energies and entropies can be obtained. 

Activation energies and entropies are quite large. The barriers involved are mainly shape changes and solvation changes. Thus, the electron will not move from a site favoured by the correct shape and solvent orientation and will wait until these are equivalent within the reaction intermediate. This is a rare event, so the reactions are not as fast as might be expected. 

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Although the ESR studies of ion pairs give the most detailed information about such systems and provide valuable data about the thermodynamics, as well as about kinetics of the interconversion, other methods of investigations are also important. An interesting spectrophotometric technique was developed in our laboratory by Smid and Hogen-Esch (21). Their work led to the understanding of various factors which affect the ability of ethereal solvents to interact with alkali ions of fluorenyl salts. 

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