Aggregate formation nuclear magnetic resonance

Nuclear magnetic resonance (NMR) has proven to be a very powerful technique for probing the structures of carbanions in the condensed phase. In particular, much work has been completed on the ion-pairing behavior of carbanions with lithium cations as well as the formation of aggregates of these lithium salts. A full discussion of this topic, particularly the methodology, is beyond the scope of this chapter, but a brief overview is appropriate. 

On the other hand, the spectroscopic techniques probe individual ionic species which build up the ionic aggregates. These techniques permit the investigation of the immediate chemical environments, the mobility of cations and water-ions Interactions. Metal nuclear magnetic resonance and Mossbauer spectroscopy are sensitive probes of counter cations and provide valuable information on the cations and their environment. Infrared spectroscopy is complementary to the above methods and addresses itself to the bound SO3" anions or water and the interaction of water molecules with the various species with which it is in contact. A common conclusion that is reached in the above mentioned studies is that four or five water molecules are needed to complete the hydration process. Reducing the level of moisture content (which surrounds the ionic species) below four water molecules per unit SOj site enhances the Coulombic interaction between the ionic species. This eventually leads to the formation of ion pairs in the dry membranes. These ion pairs do not necessarily disperse homogeneously in the fluorocarbon matrix but tend to form aggregates, phase separated from the matrix materials as demonstrated in the scattering studies. 

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