Selective polarization inversion

The physics behind SPI (Selective Polarization Inversion)77 or SPT (Selective Polarization Transfer)78,79 experiments is described and explained in every current NMR textbook, since it provides a nice introduction to understanding some of the more common current experiments (e.g. INEPT or 2D homo- and heteronuclear correlation experiments). The two names, SPI and SPT, are used indiscriminately for the same experiment, although in general SPI might be considered a special case of an SPT experiment with maximum polarization transfer achieved by inversion78. 

Bromek et al. have presented a polychromatic selective polarization inversion (PC-SPI) as an alternative for the application of NMR to large biological molecules. Theory indicate that PC-SPI has the potential for more efficient polarization transfer under conditions of rapid transverse relaxation compared to J coupling- and cross-correlated relaxation-based transfers. They demonstrate these improvements by measuring chemical shift correlation... 

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A fundamentally different approach to signal excitation is present in polarization transfer methods. These rely on the existence of a resolvable J coupling between two nuclei, one of which (normally the proton) serves as a polarization source for the other. The earliest of these type of experiments were the SPI (Selective Population Inversion) type (19>) in which low-power selective pulses are applied to a specific X-satellite in the proton spectrum for an X-H system. The resultant population inversion produces an enhanced multiplet in the X spectrum if detection follows the inversion. A basic improvement which removes the need for selective positioning of the proton frequency was the introduction of the INEPT (Insensitive Nucleus Excitation by Polarization Transfer) technique by Morris and Freeman (20). This technique uses strong non-selective pulses and gives general sensitivity enhancement. 

The most popular and generally successful surface-selective polarization strategies to date employ H —> X cross-polarization (CP) 111], where X is a nucieus present at the surface (and presumably also within the bulk) 112,13]. These strategies, which assume that essentially all the protons in the system are present at the surface, are based on the dependence of cross-polarization upon a static component of the H-X dipolar interaction. The dipolar coupling varies as the inverse cube of the H-X intemuclear distance, and the cross-polarization rate... 

A number of studies have recently been devoted to membrane applications [8, 100-102], Yoshikawa and co-workers developed an imprinting technique by casting membranes from a mixture of a Merrifield resin containing a grafted tetrapeptide and of linear co-polymers of acrylonitrile and styrene in the presence of amino acid derivatives as templates [103], The membranes were cast from a tetrahydrofuran (THF) solution and the template, usually N-protected d- or 1-tryptophan, removed by washing in more polar nonsolvents for the polymer (Fig. 6-17). Membrane applications using free amino acids revealed that only the imprinted membranes showed detectable permeation. Enantioselective electrodialysis with a maximum selectivity factor of ca. 7 could be reached, although this factor depended inversely on the flux rate [7]. Also, the transport mechanism in imprinted membranes is still poorly understood. 

Irrespective of the nature of the retention that is due to adsorption, solubility, chemical binding, polarity or molecular filtration, the column does retain some components longer than others. When in the gas phase the components are moved toward the column outlet, they are selectively retarded by the stationary phase. Consequently, all components pass through the column at varying speeds and emerge in the inverse order of their retention by the column materials. The aforesaid process may be outlined schematically as shown in Figure 29.1. 

It can be seen that these are very powerful selection rules indeed. On the other hand, we might have assumed that the symmetry of the environment of the metal ion could have been adequately approximated by considering only the six coordinated oxygen atoms. In this case, the symmetry would be Dm, in which there is a center of inversion and the transitions would be governed by vibronic selection rules. When these are worked out, it is found that all of the transitions are vibronically permitted. Thus, experimental study of the polarizations should provide clear-cut evidence as to the correct effective symmetry and selection rules. Such a study has been reported and shows conclusively that the selection rules followed are those given above for pure electronic transitions in Dy symmetry. 

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