INEPT experiment

Figure 2.9 Pulse sequence for the INEPT experiment. (B) Effect of pulses on H magnetization. Application of the pulse sequence shown results in population inversion of one of the two proton vectors of the CH doublet and therefore causes an intensification of the corresponding C lines.

In what respect is the INEPT experiment superior to the APT or GASPE experiment ... 

Figure 2.10 (A) Relative intensities in an INEPT spectrum presented as a variation of the Pascal triangle. (B) Signal phase and amplitudes of (a) quaternary, (b) CH (c) CH-2, and (d) CH5 carbons in a normal INEPT experiment.

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. 

Based on the equation K = yn/yc, we would expect about a threefold enhancement in the signal intensities of proton-bearing C nuclei in the broad-band decoupled INEPT experiment. In practice, the intensification is more than threefold. Why ... 

The INEPT experiment can be modified to allow the antiphase magnetization to be precessed for a further time period so that it comes into phase before data acquisition. The pulse sequence for the refocused INEPT experiment (Pegg et al., 1981b) is shown in Fig. 2.13. Another delay, A. is introduced and 180° pulses applied at the center of this delay simultaneously to both the H and the C nuclei. Decoupling during data acquisition allows the carbons to be recorded as singlets. The value of Z), is adjusted to enable the desired type of carbon atoms to be recorded. Thus, with D, set at V4J, the CH carbons are recorded at VsJ, the CH2 carbons are recorded and at VeJ, all protonated carbons are recorded. With D3 at %J, the CH and CH ( carbons appear out of phase from the CH2 carbons. 

What are the characteristic differences between basic INEPT and refocused INEPT experiments ... 

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 nuclear Overhauser effect resulting from the broad-band decoupling during the decoupled INEPT experiment also contributes to the signal enhancement of the C lines. 

The basic INEPT spectrum cannot be recorded with broad-band proton decoupling, since the components of multiplets have antiphase disposition. With an appropriate increase in delay time, the antiphase components of the multiplets appear in phase. In the refocussed INEPT experiment, a suitable refocusing delay is therefore introduced that allows the C spin multiplet components to get back into phase. The pulse sequences and the resulting spectra of podophyllotoxin (Problem 2.21) from the two experiments are given below ... 

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. 

The selective INEPT experiment also requires a separated proton resonance for excitation and also uses rectangular soft pulses (Bax and Freeman, 1982). Magnetization is selectively transferred by the soft proton pulses to the corresponding carbons to which the protons are coupled. 

For the new C-hydroxybenzyl glycoflavones from Citnillus colocynthis," the complete C NMR data were assigned and additionally confirmed by 2D-COSY, HETCOR, and selective INEPT experiments. 

During the structural elucidation of the first flavone-xanthone dimer (swertifranche-side) through a series of ID or 2D NMR techniques including COSY, phase-sensitive ROESY, reversed-detected HMQC, HMBC, and selective INEPT experiments, H and C signals appeared as two peaks indicating two conformers and classically, the coalescence of the split signal was observed as the temperature was increased. 

DEPT to enhance the signal of some other experiment in the sections devoted to those experiments (e.g. heteronuclear correlations, INADEQUATE, DQF COSY, 7-resolved spectra). Similarly, an INEPT experiment can be made selective in a number of ways some are discussed in Section VI.C.l on selective experiments. 

The simplest triple resonance experiments to perform are H—29Si INEPT experiments employing decoupling of another nucleus, usually 31P. These experiments are described in Section V.B. 

What does this thought experiment have to do with the INEPT experiment The same spin state that results from the selective H 180° pulse on the downfield component of the doublet (-2I7S7 + S7) can be obtained in a more practical way by a nonselective 90° pulse followed by a delay of 1/(2J) and another nonselective XH 90° pulse ... 

This may seem more complicated, but it is much more practical because all of the pulses are nonselective ( hard ) pulses. If we add the final 13C 90° pulse used in the SPT experiment to bring the magnetization to the x-y plane, we have the INEPT experiment ... 

In the INEPT experiment, the final step of coherence transfer is the simultaneous 90° pulses on 13C and H ... 

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