Spin guest-host effect

A variety of spin probe methods have also been used to study the morphological features of the nano-channels present within MCM 41, as well as dynamical aspects connected to molecular diffusion in the inner pores,186-188 EPR has been used to investigate the adsorption and interactions of nitroxide-labelled de-ndrimers within porous silica.181 This method allows one to investigate the effective porosity of a solid surface (as a host) which is determined by the accessibility of the host surface to an adsorbed guest molecule. Information on the adsorption and interaction of dendrimers with the porous surface arises from computer-aided analysis of the EPR spectra based on of the well-established procedure proposed by Schneider and Freed.189... 

More quantitative information about host-guest proximity may be obtained by application of the nuclear overhauser effect (NOE). Very simply, the NOE involves the saturation of the spin of one nucleus by continuous irradiation, and monitoring the resulting intensity enhancements of the NMR resonances of adjacent atoms. The idea is that the irradiation of one nucleus causes it to become excited to a nonequilibrium distribution of spin states. Relaxation of this excited state situation occurs by dipole-dipole spin-lattice transfer of the excess energy, resulting in enhancement of the intensity of signals for nuclei physically close to the irradiated nucleus, irrespective of whether they are actually bonded. NOE intensity enhancements may be anywhere... 

During the excited triplet state lifetime, hyperhne interactions between polarized electrons and neighboring (guest and host" ) nuclei permit polarization transfer via (a) mixing of states, (b) cross-relaxation or (c) applied electromagnetic helds (nearly) resonant with the electron-spin transitions, i.e., the the solid effect . The relative importance of these mechanisms depends on the system and the experimental conditions the distinctions can be a bit subtle, and merit some further discussion. 

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