Multiplicities from INEPT

The INEPT (Insensitive Nuclei Enhanced by Polarization Transfer) experiment [6, 7] was the first broadband pulsed experiment for polarization transfer between heteronuclei, and has been extensively used for sensitivity enhancement and for spectral editing. For spectral editing purposes in carbon-13 NMR, more recent experiments such as DEPT, SEMUT [8] and their various enhancements [9] are usually preferable, but because of its brevity and simplicity INEPT remains the method of choice for many applications in sensitivity enhancement, and as a building block in complex pulse sequences with multiple polarization transfer steps. The potential utility of INEPT in inverse mode experiments, in which polarization is transferred from a low magnetogyric ratio nucleus to protons, was recognized quite early [10]. The principal advantage of polarization transfer over methods such as heteronuclear spin echo difference spectroscopy is the scope it offers for presaturation of the unwanted proton signals, which allows clean spec-... 

Pulse sequences for non-selective polarization transfer, not only useful for signal enhancement but also for multiplicity selection, are referred to as INEPT [54], abbreviated from Insensitive Nuclei Enhanced by Polarization Transfer . An improved method denoted as Distortionless Enhancement by Polarization Transfer or DEPT" [55] permits the cleanest multiplicity selection known so far, with full enhancement and low sensitivity to individual CH coupling constants. In addition, fully enhanced and undistorted coupled spectra can be recorded. Finally, subspectra for CH, CH2 and CH3 groups can be generated. 

FIGURE 45. 29Si spectra of HN(SiMe3)2 (at 71.55 MHz, 80% in CgDf, 10 mm sample tube, 2 K data points zero-filled to 8 K) acquired (a) with INEPT (16 transients) (b) the same as (a) after Gaussian multiplication (LB = —1 Hz, GB = 0.7) (c) with DEPT (16 transients) — note the effect of the longer sequence (d) with HEED-INEPT (64 transients, x = 35.7 ms, A = 19.06 ms, T = 0.3 s). Reproduced by permission of Academic Press from Reference 307... 

FIGURE 12.16 Pulse sequence for the triple resonance 3D NMR experiment HNCO. H and N denote H and 15N, C denotes 13C=0, and K denotes 13C . Pulses at times 1, 2, and 3 constitute an INEPT sequence that transfers coherence from H to. V, where it precesses during q. Pulses at times 6, 7, and 8 represent an HMQC sequence that creates multiple quantum coherence in C (where it precesses during and transfers coherence back to N. Pulses 10 and 11 are an inverse INEPT sequence that transfers coherence back to H for detection during f3.The other 180° pulses refocus heteronuclear spin couplings. Note that coherence is not transferred to spin K. 

In practice, HNCO is now carried out by a somewhat more complex pulse sequence than that given in Fig. 12.16 in order to improve its efficiency. Pulsed field gradients are added to aid coherence pathway selection an INEPT transfer from N to K replaces the multiple quantum coherence step and the N evolution is carried out with a constant time experiment. 

The DEPT experiment [33] (Distortionless Enhancement by Polarisation Transfer) is the most widely used polarisation transfer editing experiment in carbon-13 spectroscopy, although its application is certainly not limited to the proton-carbon combination. It enables the complete determination of all carbon multiplicities, as does the refocused INEPT discussed above, but has a number of distinct advantages. One of these is that it directly produces multiplet patterns in proton-coupled carbon spectra that match those obtained from direct observation, meaning methylene carbons display the familiar 1 2 1 and methyl carbons the 1 3 3 1 intensity patterns this is the origin of the term distortionless . However, for most applications proton decoupling is applied during acquisition and multiplet structure is of no consequence, so the benefits of DEPT must lie elsewhere. 

The classical approach to generating multiplicity-edited proton spectra was to use the INEPT or DEPT sequences in reverse to transfer initial carbon magnetisation onto the proton for detection. These suffered from low sensitivity and poor suppression of resonances, so were never popular. 

In practice, the appearance of decoupled INEPT and DEPT 29Si-NMR spectra are usually the same. However, coupled INEPT and DEPT spectra differ dramatically. Coupled DEPT spectra essentially appear as greatly enhanced standard acquisition spectra the multiplicity, phase, and relative intensities of multiplets using DEPT are the same as those obtained from normal FT-NMR techniques. In contrast, coupled INEPT spectra contain several distinctive distortions (1) the outer lines of multiplets in INEPT spectra are much enhanced compared to relative multiplet intensities obtained using standard acquisition or DEPT-NMR techniques (2) the central line of odd line multiplets in INEPT has zero intensity and (3) the two halves of a multiplet in INEPT are 180° out of phase. Thus, a triplet and a quartet in INEPT would appear as 1 0 —1 and 1 1 —1 patterns, respectively, instead of the normal 1 2 1 and 1 3 3 1 patterns seen with DEPT (see Section IV,A). 

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. 

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