Efficiency of cross-polarization

Figure 4 Expected SRI plots for 13C CPMAS (top) and DDMAS (bottom) NMR peak intensities (solid lines) against fluctuation frequency (Hz). The fluctuation frequencies were divided into the following three regions, static (/a or /b), slow (//a or //b), and high frequency (///a or ///b) regions. The maximum intensities are given by S. In the presence of slow fluctuation motions, the peak intensities can be modified as the dotted lines (//a or //b ). In the nearly static region, the peak intensities could be changed into the dotted lines /a or /b, depending upon efficiency of cross-polarization or Tn values. From Ref. 29.

A solid-state C NMR study of the intermolecular hydrogen bonding formation in a blend of phenolic resin and poly(hydroxyl ether) of bisphenol A was reported by Wu et at) " The presence of a single glass transition temperature for all of the blend samples with different composition as disclosed by the DSC measurements demonstrates that the blends are thermodynamically miscible. The solid state NMR parameters, including chemical shift, efficiency of cross-polarization and Ti, confirm the presence of more free OH groups when one of the polymers is the minor component. 

N CP MAS NMR spectra of adsorbed pyridine have been studied by Maciel et al. [103] (pyridine on silica-alumina), Ripmeester [104] (pyridine on y-alumina, mordenite), and by Majors and ElHs [159] (pyridine on y-alumina). Values for the resonance shift relative to solid pyridine are collected in Table 6. The major drawbacks of these experiments are (i) an absolute determination of concentrations is connected with large errors due to the strong dependence of the hne intensities on parameters controlling the efficiency of cross polarization [107] (see Sect. 2.4), and (ii) it is not certain that the exchange rates of the molecules among the various adsorption sites are... 

In addition to providing motional information there is another benefit to be derived from CP studies which will be exemplified further in Section 8 and which depends on the fact that the effectiveness of the CP phenomenon is proportional to r 3, where r is the distance between the / and S nuclei. In other words, the efficiency of transfer of magnetization falls off extremely rapidly as the distance between / and S increases. Bulk Si nuclei in Si02, for example, will be relatively distinct from any proton species, whereas Si nuclei at the surface will be relatively close to surface OH groups (or any chemically bound organic species). The use of cross-polarization Si n.m.r. will therefore enable one to examine Si nuclei at the surface relative to those in the bulk material. 

Both for the SDS/CA/waler and the CMEA/CA/water systems, the all-trans signal disappears above the upper phase transition temperature. The signal of disordered chains is usually of lower intensity since cross-polarization, which requires a dipolar coupling between H and C nuclei, is less efficient for mobile chains. Cross-polarization breaks down completely for liquid-like chains with isotropic mobility since the dipolar coupling is averaged to zero. Highly mobile chains can be observed better by using direct carbon excitation instead of cross-polarization. 

By contrast, in the high field of a superconducting magnet, polarization transfer to heteronuciei is less efficient, because now the difference in resonance frequency of XH and 13C is significant and exceeds the magnitude of the coupling constants between the carbons and the protons. In this latter case, polarization transfer can be achieved most effectively by appropriate pulse sequences (e.g., via cross-polarization). 

Hartmann-Hahn cross polarization between two low-y nuclei has been successfully used to record chemical-shift correlation spectra between 13C and 15 N nuclei. Cross polarization between two low-y nuclei suffers from a high sensitivity to the exact matching condition at one of the side bands of the Hartmann-Hahn condition [101]. Adiabatic methods (APHH-CP) can eliminate most of this sensitivity and lead to high transfer efficiencies [34, 62, 90]. 

In a recent study, Singer221 showed that triethylamine quenches fluorenone fluorescence with a greater efficiency in more polar solvents. According to Eq. (28), this could be caused by an increase in kes or by a decrease in kz. Since the fluorescence yield of fluorenone was also found to increase dramatically in more polar solvents, it is clear from Eq. (27) that ks must decrease. This was rationalized with the idea that increasing solvent polarity reorders the electronic states so that intersystem crossing changes from (7r 1,7r) —> (tt 3,m) to ( 7r 1,7r) -> (ir 3,ir). According to EI-Sayed,222 the former can be as much as 103 faster than the latter which would raise the lifetime and fluorescence quantum yield in polar solvents. 

These intermolecular correlation peaks mean that intermolecular cross polarization (CP) occurs between the carbon and proton of intermolecular -interacting polypeptides in blend. There may be two pathways for the observed intermolecular CP. One is that a direct transfer from proton to carbon exists, and another is that a change in the magnetization by spin diffusion (homonuclear Hartmann Hahn transfer) exists. It is thought that the former is much more efficient than the latter because the former comes from only one magnetization transfer process, but the latter comes from two... 

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