Pulse technique, sensitivity enhancement polarization-transfer

The NMR signals of insensitive nuclear spins can be enhanced by transferring polarization from a more sensitive species to which they are coupled. The well-known pulse sequences as the polarization transfer techniques are insensitive nuclei enhanced by polarization transfer (INEPT), distortionless enhancement by polarization transfer (DEPT), and reverse insensitive nuclei enhanced by polarization transfer (RINEPT) The INEPT sequence is an alternative to the nuclear Overhauser effect. The INEPT experiment does not require any particular relaxation mechanism and therefore a better enhancement factor can be obtained. Furthermore it is demonstrated that INEPT sequence can be used to determine the multiplicity of each signal in a NMR spectrum. More recently, the INEPT and DEPT experiments were used for the coherence transfer via heteronuclear J-coupling between spin-1/2 and quadrupolar nuclei in the solids. " Fyfe et showed that coherence transfer via the scalar coupling between spin-1/2 and quadrupolar nuclei can be obtained in the solid state by using INEPT experiment. 

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-... 

The most economical and efficient method of sensitivity enhancement in 13C NMR of organic molecules is the pulse Fourier transform technique (PFT) in combination with decoupling methods such as proton broad band decoupling and polarization transfer. These methods will be described in the following sections. 

An alternative pulse sequence that provides the same multiplicities as INEPT but with intensity ratios that follow the binomial theorem is DEPT (distortionless enhancement by polarization transfer).The pulse sequence, depicted in Fig. 12.3a, can be used, like refocused INEPT, for sensitivity enhancement but is usually employed as an editing technique. The three evolution periods T are chosen to approximate 1/2/, but the length of the pulse labeled 0 can be varied. As we show in the following, CH has maximum intensity at 0 = 90° CH2 has zero... 

If the approximate magnitude of coupling constants /( Sn, H) is known, polarization transfer pulse sequences such as INEPT or DEPT ° can be successfully applied in order to enhance the sensitivity of the NMR experiments. These techniques, if applied correctly with optimized setting of the spectrometer, normally give Sn NMR signals for which the line widths are mainly governed by natural transverse relaxation processes (see Figs 1 and 2). 

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

In an article with a limited length and depth of coverage like this, dealing with 10 elements and 14 NMR-active nuclei, it is impossible to discuss all the important NMR parameters. Therefore, most attention has been paid to NMR chemical shifts while spin-spin coupling constants and relaxation times are only mentioned in the contexts where they should be taken into account to obtain useful NMR results. However, as well as direct NMR detection, one should remember, for example, the usefulness of the (inverse) two-dimensional pulse sequences and the MAS technique to utilize homo- and heteronu-clear spin-spin coupling constants for polarization transfer between nuclei to enhance the sensitivity of the measurement. These approaches have increased... 

Isotopes in low abundance have long spin-lattice relaxation times which give rise to poor signal-to-noise ratios. Sensitivity can be improved by using a technique known as cross polarization where a complex pulse sequence transfers polarization from an abundant nucleus to the dilute spin thereby enhancing the intensity of its signal. 

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