Cross-polarization efficiency

Figure 5.12 shows the CPMAS/DD NMR spectra of semi-crystalline a-PBT [poly(butylene terephthalate)] recorded [22] below (a) and well above (b) its 7g of 55°C. At ambient temperatures, both crystalline and amorphous carbons are sufficiently rigid to cross-polarize efficiently, but at 105°C the amorphous carbons are mobile and conformationally averaging. The ambient spectrum is broadened by the rigid and conformationally diverse amorphous carbon nuclei which resonate at a variety of frequencies. At lOS C the amorphous carbons, which are mobile with small 7, are conformationaUy averaging to (Ti, and no longer cross-polarize efficiently. Thus at elevated temperatures, only the crystalline carbons in the sample are contributing to the CPMAS/DD spectrum. 

Fig. 16. The experimental CP/MAS spectrum of solid glassy aPP (top) is compared with simulations for the various groups (rows 2-4). For all groups the shape is satisfactorily matched. Note that for all spectra - experiment and theory - the width of the spectrum is 50 ppm. The absolute position is not correctly reproduced by the quantum chemical simulations due to the small atomic basis employed and has been adjusted for better comparison. The absolute height of the peaks does not convey any information since, experimentally, different cross polarization efficiencies tend to distort the stoicheometric relations. Thus, the simulated spectra were all scaled to the same peak height. The fine line shows the contributions of the various conformations, Eq. (3.15) (slightly broadened for typographical reasons), the hold line is the simulation with the d-function being replaced by a Gaussian function s, Eq. (3.16)...

MAS does have an effect on the cross-polarization efficiency for mobile systems like elastomers [72]. For a static sample, the transfer rate is a function of u>c and has a maximum polarization-transfer rate at the Hartman-Hahn match and a slower polarization transfer with increasing mismatch. However, when the sample is spun with MAS, there is a very slow transfer of polarization at the Hartman-Hahn match, where the transfer is expected to be most rapid. A series of maxima in the transfer rate occurs when co c and co w are mismatched by a multiple of the spinning firequency. Thus, mismatching the cuic and )ih produces high signals at multiples of the MAS firequency. A series of maxima in Tch are observed as a result of an amplitude-modulated pattern due to C-H heteronuclear coupling [73]. 

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.

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]. 

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... 

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. 

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