Polarization Transfer Methods

WALTZ-16 is based on an element R = 90J180° x-270°. WALTZ-16 is more effective than MLEV-16, primarily because it includes only 180° phase shifts, not the 90° phase shift inherent in MLEV. To simplify the notation, these pulse cycles are usually abbreviated in terms of multiples of 90° pulses, with a phase inversion denoted by a bar, as in the R,R notation. In these terms the basic WALTZ element R becomes 123 (hence the acronym WALTZ). Permutations of R and R lead to WALTZ-16. Further computer-optimized improvements have been devised and are not restricted to integral multiples of 90° pulses. 

FIGURE 9.9 Energy levels, populations, and, 3C spectra in the SPT experiment. Top Energy levels and excess populations in (a) the normal Boltzmann distribution and (b) the distribution after a selective pulse on line vx has inverted the populations of the aa and /3a levels. Bottom , 3C spectra (coupled and decoupled) obtained with a) Boltzmann distribution (b) altered population distribution (c) altered distribution with phase inversion of line v3. 

The sequence can easily be modified to allow the two separate components to combine—refocused INEPT. After the final 90° pulses but before data acquisition, a period 2A is added, with A = 1/4J, during which the two vectors combine. H and 13C 180° pulses are placed in the middle of this precession period to refocus chemical shifts. If decoupling is now applied during data acquisition, a single line of intensity 8 is obtained, that is, four times as large as would be obtained 

FIGURE 9.10 Summary of the INEPT experiment as applied to a coupled H-13C system. Top Pulse sequence. Bottom Depiction of the behavior of and 13C magnetizations, as described in the text. In practice, the 90° l3C pulse may be applied coincident with the final H pulse but is shown slightly displaced to illustrate in (g) and (h) the behavior of the 13C magnetization after the population inversion has been established in (f). 

As in the case of a spin echo with coupled nuclei, our explanation of INEPT grafted the quantum concept of energy levels onto the classical picture of precess- 

Those familiar with the routine acquisition of NMR spectra are aware of the consequences of the nuclear Overhauser effect (NOE). Saturation of protons has the effect of increasing the net mag- 

Since quantitation is crucial in many polymer analyses, it is important to obtain data with T] and NOE in mind. Highly flexible polymers undergoing rapid segmental motion typically give narrow lines. Often these carbons can have s of several 

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 (1 ) 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 ( ). This technique uses strong non-selective pulses and gives general sensitivity enhancement. 

One drawback is the effect of short T2 s on the sensitivity improvement. Since the sequence requires 

The combination of these two spectra then allows direct identification of XH,XH2 and XH3 signals in even the most complex molecules in a similar manner as in the J modulated spin echo experiments described above. 

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In the following, we will discuss heteronuclear polarization-transfer techniques in four different contexts. They can be used as a polarization-transfer method to increase the sensitivity of a nucleus and to shorten the recycle delay of an experiment as it is widely used in 1H-13C or 1H-15N cross polarization. Heteronuclear polarization-transfer methods can also be used as the correlation mechanism in a multi-dimensional NMR experiment where, for example, the chemical shifts of two different spins are correlated. The third application is in measuring dipolar coupling constants in order to obtain distance information between selected nuclei as is often done in the REDOR experiment. Finally, heteronuclear polarization transfer also plays a role in measuring dihedral angles by generating heteronuclear double-quantum coherences. 

Satellites are not restricted to 13C, but may be seen with other magnetic nuclei that are present at low abundance when the principal isotope has 1=0. Among the best known are 29Si (8.5%), nlCd and l13Cd (each about 9%), 199Hg (7.6%), and 207Pb (8.9%). Other nuclides, such as 15N, have a natural abundance so low that satellite signals are rarely observed in normal one-dimensional NMR spectra, but with polarization transfer methods described in Chapters 9 and 10, the existence of these weak satellites often permits observation of the less sensitive, low abundance nuclide by indirect detection. 

Three pulse schemes which were proposed for indirect Ti measurements and produce the same results as the inversion-recovery sequence are shown in Fig. 12. The first method is an inversion-recovery experiment on the insensitive "Y spin, with a subsequent DEPT transfer to the observed nucleus "X. Although successfully applied in model studies, the experiment suffers from the necessity of long relaxation delays and was considered to be too insensitive for application to and insensitive metal nuclei. Higher sensitivity can be achieved with double polarization transfer methods which start with "X magnetization. TWo sequences were employed on the basis of double DEPT (Fig. 12(b)) or INEPT transfer (Fig. 12(c)). In order to remove effects of... 

For leading references to the spin polarization transfer method, see (a) Dahlquist FW, Longmur KJ, Du Vernet RB (1975) J Magn Reson 17 406 (b) Frim R, Zilber G,... 

The favorable properties of 180° refocussing pulses have been exploited in two main efforts obtaining more spectral information by causing spectra to depend on J and the number of coupled nuclei, and secondly, discussed later, in polarization transfer methods. 

Two new polarization transfer techniques have recently been reported INEPT (2) and DEPT (3,4). These pulse sequences lack the limitations of previous polarization transfer methods, and allow the routine collection of 29Si-NMR data. The principal virtues of both the INEPT and DEPT pulse sequences are that the polarization transfer enhancements are substantial (five- to ninefold) (12) and relatively nonselective and that they can easily be used by chemists familiar with normal FT-NMR spectroscopy on available commercial multinuclear FT-NMR instruments. 

Encyclopedia of Nuclear Magnetic Resonance, Vol. 9, ed. D.M. Grant and R.K. Harris, John Wiley Sons Ltd., Chichester, UK., 2002 R 182 M. Ernst and B.H. Meier, Adiabatic Polarization-Transfer Methods in MAS Spectroscopy , p. 23... 

The chemical shifts of C-atoms in lichen substances are in the range of 10-220 ppm. The multiplicity of the signals in the coupled spectrum gives information about the number of protons at the carbon atom =C= singlet, =C-H doublet, -CH2- triplet, and -CH3 quartet. The DEPT (Distortionless Enhancement by Polarization Transfer) method is a modern technique to differentiate between CH, CH2 or CH3 signals. C-NMR shifts of lichen substances are summarized in Table 10. 

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