Methods of Polarization Transfer

A number of methods can be used to enhance the signals of nuclei with low gyromagnetic ratios through polarization transfer. These methods exploit the nuclear spin polarization of protons which are characterized by much larger Boltzmann population differences than nitrogen. (See also atoms (B 71)). 

The sensitivity limitations inherent in the observation of proton decoupled N signals become more significant if undecoupled spectra are desired. The method of selective population inversion (M 31i), which is capable of noticecibly enhancing the line intensities of coupled spectra, therefore seems very attractive. In this method a tt pulse is applied selectively to a given component (or an appropriate sub-multiplet) of the satellite proton spectrum pertaining to the molecules which contain the N isotope. Just after this selective inversion, a non selective 

A Basic sequence for an SPI experiment (M 31i). At is the acquisition time. D, the delay for magnetization recovery, is conditioned by the proton relaxation times. 

C Diagram of a J-cross polarization experiment (B 57), t is the cross polarization time. 

Due to population inversion of the energy levels involved in the pertur- 

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If a particular NMR technique does not specifically favor detection of nuclei at the surface, then the resulting spectrum will be dominated by peaks due to nuclei from the bulk of a particle. There are simply many more nuclei in the main framework than at structural sites on the surface, unless the surface area is very large, say, greater than 100 m-/g. Exceptions may occur either if observed nuclei are located largely at the surface or if the method of polarization transfer discriminates strongly in favor of surface nuclei. The former situation often... 

The INEPT (Insensitive Nuclei Enhanced by Polarization Transfer) experiment [6, 7] was the first broadband pulsed experiment for polarization transfer between heteronuclei, and has been extensively used for sensitivity enhancement and for spectral editing. For spectral editing purposes in carbon-13 NMR, more recent experiments such as DEPT, SEMUT [8] and their various enhancements [9] are usually preferable, but because of its brevity and simplicity INEPT remains the method of choice for many applications in sensitivity enhancement, and as a building block in complex pulse sequences with multiple polarization transfer steps. The potential utility of INEPT in inverse mode experiments, in which polarization is transferred from a low magnetogyric ratio nucleus to protons, was recognized quite early [10]. The principal advantage of polarization transfer over methods such as heteronuclear spin echo difference spectroscopy is the scope it offers for presaturation of the unwanted proton signals, which allows clean spec-... 

Isotropic mixing [29] known also as Hartmann-Hahn polarization transfer [30,31] is a unique and very efficient method of coherence transfer between spins. Non-selective isotropic mixing is widely used in different types of correlation experiments. Selective Hartmann-Hahn transfer has been introduced quite recently [32-37] and provides the means for multiplet-selective [33-37] or band-selective [32] correlation experiments. 

In 1975, Schaefer and Stejskal [26] combined MAS with a method of magnetization transfer known as cross-polarization (CP), developed by Pines et al. [27]. The combined method, known as CP/M AS, can be apphed to organic substances and permits high-resolution spectra of at its natural low abundance... 

Nuclear Magnetic Resonance Spectroscopy. Like IR spectroscopy, NMR spectroscopy requires little sample preparation, and provides extremely detailed information on the composition of many resins. The only limitation is that the sample must be soluble in a deuterated solvent (e.g., deuterated chloroform, tetrahydro-furan, dimethylformamide). Commercial pulse Fourier transform NMR spectrometers with superconducting magnets (field strength 4-14 Tesla) allow routine measurement of high-resolution H- and C-NMR spectra. Two-dimensional NMR techniques and other multipulse techniques (e.g., distortionless enhancement of polarization transfer, DEPT) can also be used [10.16]. These methods are employed to analyze complicated structures. C-NMR spectroscopy is particularly suitable for the qualitative analysis of individual resins in binders, quantiative evaluations are more readily obtained by H-NMR spectroscopy. Comprehensive information on NMR measurements and the assignment of the resonance lines are given in the literature, e.g., for branched polyesters [10.17], alkyd resins [10.18], polyacrylates [10.19], polyurethane elastomers [10.20], fatty acids [10.21], cycloaliphatic diisocyanates [10.22], and epoxy resins [10.23]. 

Lopez-Ortiz F and Carbajo RJ (1998) Applications of polarization transfer and indirect detection NMR spectroscopic methods based on phosphorus-31 in organic and... 

In such instances new methods for C assignment are more suitable. Compared with the 7-modulated spin-echo experiment, the DEPT (Distortionless Enhancement by Polarization Transfer) pulse sequence is insensitive to variations in VcH- The method is based on the use of polarization transfer, whereby magnetization is transferred from protons to C nuclei, and as a result quaternary carbon atoms are not detected in such experiments. This experiment involves pulsing both the H and C channels. The final proton pulse is along the v-axis and by performing two... 

There are several types of heteronuclear 2D J-resolved experiments (i) the gated decoupler method, (ii) the spin-flip method, (iii) the selective spin-flip method, (iv) the semi-selective spin-flip method, and (v) the use of polarization transfer, e.g., INEPT. In all these experiments, the pulse sequence results in the generation of spin echoes which are modulated during the evolution period by coupling frequencies. 

Two-dimensional indirectly detected through-space and through-bond solid-state NMR experiments utilizing fast MAS and homonuclear multipulse decoupling have been evaluated. It has been shown that remarkable efficiency of polarization transfer can be achieved at MAS rate of 40 kHz by both CP and INEPT, which makes these methods applicable for routine characterizations of natural abundance solids. The first measurement of 2D HETCOR spectrum of natural abundance... 

Another method of polarizing nuclear spins is dynamic nuclear polarization (DNP), whereby the comparably large electron spin polarization (see Fig. 1) is transferred to nuclear spins by saturating the electron resonance. DNP is almost as old as NMR spectroscopy, building on the aforementioned theoretical work by Overhauser [21] who predicted what is today known as the Overhauser effect (OE). DNP was soon after demonstrated experimentally by Carver and Slichter [22, 23]. The enhancement, s, that can be obtained by DNP is determined by the gamma ratio 7e/7n> which is 660 for protons and 2,625 for... 

Muns ENDOR mvolves observation of the stimulated echo intensity as a fimction of the frequency of an RE Ti-pulse applied between tlie second and third MW pulse. In contrast to the Davies ENDOR experiment, the Mims-ENDOR sequence does not require selective MW pulses. For a detailed description of the polarization transfer in a Mims-type experiment the reader is referred to the literature [43]. Just as with three-pulse ESEEM, blind spots can occur in ENDOR spectra measured using Muns method. To avoid the possibility of missing lines it is therefore essential to repeat the experiment with different values of the pulse spacing Detection of the echo intensity as a fimction of the RE frequency and x yields a real two-dimensional experiment. An FT of the x-domain will yield cross-peaks in the 2D-FT-ENDOR spectrum which correlate different ENDOR transitions belonging to the same nucleus. One advantage of Mims ENDOR over Davies ENDOR is its larger echo intensity because more spins due to the nonselective excitation are involved in the fomiation of the echo. 

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