Nuclear magnetic resonance site-molecule interactions

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. 

Multinuclear solid state nuclear magnetic resonance (NMR) has been applied to study the interaction of pyrrole with extra framework compensating cations in zeolites LiNaY and LiNaX. Upon adsorption over zeolite LiNaY, Na and Li cations migrate towards accessible positions in the supercage to interact with one molecule of pyrrole. The adsorption over zeolite LiNaX decreases the mobility of SIIT Na cations, while pyrrole molecules do not interact with Li" cations. At lower loading, pyrrole adsorbs over more basic sites, which are associated with Na cations in zeolite LiNaY. 

Lunsford [3b] and Hoffman and Nelson [23] first reported the ESR spectra for adsorbed NO molecules. Then, Kasai [4b] revealed that ESR spectra of NO probe molecules are very sensitive to the interaction with metal ions and Lewis acid sites in zeolites. The earlier ESR studies of the NO/zeolite system have been summarized in several review papers [3a, 4a, 8]. A number of ESR studies have been also carried out for NO adsorbed on metal oxides such as MgO and ZnO as reviewed by Che and Giamello [5]. Modern ESR techniques such as pulsed ESR [25-27], ENDOR (Electron Nuclear Double Resonance) [26], and multi-frequency (X-, Q-, and W-band) ESR [28] are especially useful for an unambiguous identification of the ESR magnetic parameters (g, hyperfine A, and quadrupole tensors, etc.) and, consequently, for a detailed characterization of structural changes and motional dynamics involved. Some recent advancements in ESR studies on NO adsorbed on zeolites are presented in this section. 

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