Equilibria of Association

All discussed Ksem s are related to the separately solvated members of a two step redox system. In solvents of low polarity the charged forms may form ion pairs. Especially prone to this association are anionic redox systems of Type C (RED + OX = 2 SEM ) since the often used gegenions K , Na and Li tend to form ion pairs with the anions. These exhibit special UV/VIS-, NMR- and ESR-spectra as well as g-values Dimeres of the type (SEM M )2 may also be formed, as demonstrated with the anion radicals of pyrazines ° heptafulvalene and tetracyano-quinodimethanes. Corresponding associations are reported for dian- 

In cationic redox systems of type A with SEM and OX ion pairing can be more easily avoided by choosing anions as BF4, CIO4, PF and BfCgHs) of low nucleophilicities and low solvation energies With these precautions 5 10 molar solutions of type A redox systems in acetonitrile or DMF hardly show any anomalies, even in the presence of 2 10 m Et4N BF. In methylenechloride, 

R groups. The unexpected positive AS values for smaller R groups point to a stronger solvent shell which has to be removed on dimerisation. Therefore, a face to face structure of the dimer similar to (76sem)2 has been proposed in accordance with conclusions from the UV-spectra of Paraquat in aequous solution The stretched structure of isEM seems to be important since the radical cation derived from Diquat (2R2sem) does not dimerize  

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Chapman, W.G., Jackson, G., and Gubbins, K.E., 1988. Phase equilibria of associating fluids Chain molecules with multiple bonding sites. Mol. Phys., 65 1057. 

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