Polarization transfer techniques

Although polarization transfer techniques have been available for over a decade, they have not been widely used to obtain 29Si-NMR spectra. 7-cross polarization (7) (JCP), which evolved from methods used to enhance the solid state spectra of rare spin nuclei, (8) has been applied to 29Si-NMR spectroscopy (9). JCP suffers from several limitations the proton and 29Si pulses must be on resonance, and the Hartmann-Hahn condition (8) (yHHH = VsiHsi) must be established for full enhancement. Neither of these conditions is trivial to obtain, and the difficulty in establishing them has prohibited routine application of JCP methods to 29Si-NMR spectroscopy.3 

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. 

3 It is, however, useful in obtaining solid state 2,Si-NMR spectra (10). 

Several modified INEPT and DEPT pulse sequences have recently been introduced (IS) (see Fig. 1). The new INEPT+, DEPT +, and DEPT + + sequences differ from the original INEPT and DEPT sequences only in that they employ additional refocusing and purging pulses. These serve to reduce or eliminate distortions inherent in the parent pulse sequences. The fundamental polarization transfer mechanism however remains unchanged. 

The INEPT and DEPT pulse sequences shown in Fig. 1 (IS, 14) are all multinuclear pulse sequences in which proton and/or silicon pulses are separated by free precession periods. However, INEPT and DEPT differ in both the number and duration of precession periods. In the INEPT pulse sequences, there are two precession periods of duration t, and one of duration A [a refocusing pulse (15) bisects the A period]. Both t and A are parameters set by the user to optimize enhancements, although t is routinely set to a constant (4J) 1 (where J is the H-29Si coupling constant). The A parameter can be set according to Eq. (5) (Section IV,A) to obtain optimal enhancement, or may be set to selectively invert or suppress specific silicon resonances as shown by Figs. 8-10 (Section III,D). 

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Polarization transfer techniques like INEPT and DEPT have been used to enhance sensitivity in heteronuclear 2D /-resolved spectra. In combination with the semiselective sequence just described, INEPT has been used to suppress long-range Jen couplings and to measure the one-bond couplings (Fig. 5.15) (Rutar, 1984). Driven equilibrium pulses for fast restora-... 

Application of Polarization-Transfer Techniques to Biological Systems 267... 

The techniques discussed in this chapter can be divided into two general classes homonuclear and heteronuclear polarization-transfer techniques. In each of the two classes, further distinctions can be made and are used to structure the following discussion of the different techniques. We do not want to discuss the details of the experiments, which can be found in the original literature or in several reviews about dipolar recoupling techniques [48-50]. 

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. 

The layout of this chapter is as follows. The aspects of relaxation theory of interest for this article are summarized very briefly in Section 2. Section 3 deals with general aspects of relaxation measurements, including polarization transfer techniques for improving the sensitivity. Sections 4, 5 and 6 cover measurements of T, T2 and the nuclear Overhauser enhancement, respectively. 

We see from Table 1 that the only observable nuclide for oxygen, 0, has a very low natural abundance, even in comparison with those of popular nuclides like (1.108%) and N (031%). Moreover, its quadrupole moment prevents any practical utilization of polarization transfer techniques like INEPT or DEPT, now widely used in and N NMR spectroscopies. A range of chemical shifts much wider than those of and N is an important point in favour of utilization of 0. All these properties did not prevent important applications of O NMR spectroscopy in organic chemistry, even from the times of continuous wave NMR spectroscopy. Interesting examples of such pioneering works can be found both at natural abundance as well as with enriched samples . However, also in the case of O NMR spectroscopy, FT NMR proved to be decisive for its development. 

The antiphase relationship of the C13 —C13 doublet signals in the INADEQUATE spectrum can be eliminated by an additional spin-echo sequence (— 1/4Jcc — 180° — 1 /4 Jcc —) before the 90 monitor pulse [58]. The sensitivity of the experiment may be improved by the application of stronger magnetic fields or by using proton polarization transfer techniques [59]. 

Most NMR spectroscopists have on some occasion examined the Periodic Table of NMR-active nuclei with 13C, I70, etc., and contemplated that at first glance essentially the whole Periodic Table is potentially open to study by NMR. Solution-state NMR studies of nuclei with small magnetic moments exist, but often when papers report data concerning such nuclei the information is obtained not by simple direct excitation but indirectly using polarization transfer techniques such as INEPT and DEPT, and/or by using reverse detection methods.1 As many of these indirect methods are not available in the... 

On modem FT spectrometers Sn NMR can be routinely performed by using direct observation of the Sn nucleus or by using polarization transfer techniques like INEPT [22-24]. 

In vivo application of double resonance ( H-19F) spectroscopic techniques [50, 73, 74], including a combination of NOE and proton decoupling, have proved to increase SNR, which can be traded off for improved visibility or improved spatial or temporal resolution of fluorinated compounds. The double resonance decoupling, NOE, and polarization transfer techniques used in vivo are similar to those used in solid state... 

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