INEPT polarization transfer

In both the one-dimensional carbon- or proton-detected INEPT polarization transfer experiment and the two-dimensional proton-detected experiment, one needs to optimize the polarization transfer delay with an estimate of the coupling constant. In addition, the refocusing delay. A, must be set separately for an IS and IS2 spin system. For an IS spin system, the optimum delay is A = (l/4)Jis, whereas for an IS2 system the delay should be set to A = (l/8)Jis to get maximum polarization transfer. If one is interested in, for instance, the determination of relaxation parameters for carbons carrying two protons as well as for CH carbons, this requires separate experiments. 

The sensitivity of these experiments can be increased by employing INEPT polarization transfer to nucleus Y in HETCOR or to nucleus X in HMQC or HSQC in place of the first 90° pulse (as in a number of 3D experiments). The INEPT-HMQC combination ... 

To improve the sensitivity the selective J-resolved pulse sequence may be combined with a refocused INEPT polarization transfer experiment. In contrast to the original heteronuclear J-resolved experiment the first excitation pulse is executed on the F2 channel with the spin-echo sequence sandwiched between two incremental delay. Coupling evolves during the second incremental delay before the refocused INEPT unit creates in-phase coherence for the nuclei which are coupled to the selected proton nucleus allowing decoupling on the F2 channel during data acquisition. 

The second method of creating 3D-NMR experiments, concatenation of INEPT polarization transfer sequences, is... 

With improvements in Instrument sensitivity and the use of techniques such as enhancement by polarization transfer (INEPT), it can be expected that natural abundance N NMR spectra will become increasingly Important in heterocyclic chemistry. The chemical shifts given in Table 10 illustrate the large dispersion available in N NMR, without the line broadening associated with N NMR spectra. 

Population transfer experiments may be selective or nonselective. Selective population transfer experiments have found only limited use for signal multiplicity assignments (SSrensen et al, 1974) or for determining signs of coupling constants (Chalmers et al., 1974 Pachler and Wessels, 1973), since this is better done by employing distortionless enhancement by polarization transfer (DEPT) or Correlated Spectroscopy (COSY) experiments. However, nonselective population transfer experiments, such as INEPT or DEPT (presented later) have found wide application. 

In polarization transfer experiments such as INEPT, the recorded multiplets do not have the standard binomial intensity distributions of... 

The sensitivity enhancement in the INEPT experiment is particularly marked when nuclei of low magnetogyric ratios are being detected. A comparison of the signal intensities obtained by polarization transfer against those obtained by full NOE for various nuclei is presented in Table 2.1. 

The DEPT experiment (Doddrell elal, 1982) involves a similar polarization transfer as the INEPT experiment, except it has the advantage that all the C signals are in phase at the start of acquisition so there is no need for an extra refocusing delay as in the refocused INEPT experiment. Coupled DEPT spectra, if recorded, would therefore retain the familiar phasing and multiplet structures (1 1 for doublets, 1 2 1 for triplets, etc.). Moreover, DEPT experiments do not require as accurate a setting of delays between pulses as do INEPT experiments. 

The inverse INEPT (Bodenhausen and Ruben, 1980) and inverse DEPT (Brooks et ai, 1984) experiments utilize such an approach. In the inverse INEPT experiment, successive 90° pulses are applied to the C nucleus, followed by a H read pulse. Protons not coupled to the C nucleus are suppressed by presaturation of the entire H-NMR spectrum before the polarization transfer, so only those signals will be detected that are generated by polarization transfer from the C nucleus. 

The APT pulse sequence provides limited information about the number of hydrogens bonded to the carbons in a molecule, since it does not readily allow us to distinguish between the CH, and CH carbons or between CH and quaternary carbons. The INEPT spectrum not only can yield information about the multiplicity of all the carbons, but also affords sensitivity-enhanced C signals due to polarization transfer. 

Both experiments are based on polarization transfer from sensitive nuclei to insensitive nuclei, and therefore the mjyor portions of their pulse sequences are common. The INEPT experiment, without refocusing and decoupling, however, yields spectra with distorted" multiplets. For instance, the two lines of a doublet appear in antiphase with respect one another. Similarly, the central line of a triplet may be too small to be visible, while the outer two lines of the triplet will be antiphase to one another. Introducing a variable refocusing delay A and broadband decoupling in the INEPT sequence can convert this experiment into a more useful one. 

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-... 

INEPT (insensitive nuclei enhanced by polarization transfer) Polarization transfer pulse sequence used to record the NMR spectra of insensitive nuclei, e.g., C, with sensitivity enhancement may be used for spectral editing. 

There is another member of this family of experiments known as INEPT (Insensitive Nuclei Enhancement by Polarization Transfer), which was the forerunner of DEPT. INEPT still has its uses for obtaining spectra of really insensitive nuclei such as silicon-29 or nitrogen-15. 

NMR spectra can be recorded using various techniques. INEPT (see Section 1.2.5) is useful, because the polarization transfer from protons to silicon leads to a considerable increase in signal intensity. 

J splittings cannot be directly resolved. In addition to the obvious advantage of providing a map of chemical bonds between the spins, /-based transfers do not require spin-locking and are not disturbed by molecular motions. The major drawback of polarization transfer through J coupling is that the delays involved in the pulse sequences, such as insensitive nuclei enhanced by polarization transfer (INEPT) [233] or heteronuclear multiple-quantum coherence (HMQC)... 

Insensitive Nuclei Enhanced by Polarization Transfer (INEPT) method (see Table 3.13). 

In the case of the 7-hydroxy-substituted compounds 44 (Scheme 2), 54 different derivatives were investigated by 13C NMR spectroscopy and, in some cases, also by 1SN NMR spectroscopy <1995JST(335)273>. With the help of proton-coupled 13C NMR spectra, semi-selective INEPT (insensitive nuclei enhanced by polarization transfer) experiments, and heteronuclear multiple bond correlation (HMBC) two-dimensional 2D-NMR spectra, all shifts could be unequivocally assigned. While the C-7 shifts did not allow the existing tautomeric situation to be determined, a clear decision could be made by H NMR spectroscopy in this respect. The 1SN NMR spectra revealed an equilibrium between the N(4)H and N(3)H tautomeric forms, which is fast on the NMR timescale. 

Fig. 2. Different polarization transfer elements for the 1Hn-15N spin pair, INEPT (a), CRIPT (b), and CRINEPT (c) elements. Narrow and wide bars correspond to 90° and 180° flip angles, respectively, applied with phase x unless otherwise stated.

INEPT insensitive nuclei enhanced by polarization transfer... 

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