Enhancement by polarization transfer

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. 

Techniques developed in recent years make it possible to obtain large amounts of information from l3C NMR spectra. For example, DEPT-NMR, for distortionless enhancement by polarization transfer, allows us to determine the number of hydrogens attached to each carbon in a molecule. 

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. 

The most widely used method for determining multiplicities of carbon atoms is DEPT (Distortionless Enhancement by Polarization Transfer). This has generally replaced the classical method of recording off-resonance C spectra with reduced CH couplings from which the multiplicity could be read directly. 

DEPT (distortionless enhancement by polarization transfer) A onedimensional C-NMR experiment commonly used for spectral editing that allows us to distinguish between CH, CH2, CH, and quaternary carbons. Detectable magnetization The magnetization processing in the x y -plane induces a signal in the receiver coil that is detected. Only single-quantum coherence is directly detectable. 

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. 

Multidimensional spectraas well as techniques including DEPT (distortionless enhancement by polarization transfer), COSY (correlated spectroscopy), and ROESY (rotating-frame overhauser enhancement spectroscopy) have been increasingly used. 

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. 

You may be told that your NMR laboratory does not routinely use APT spectra but provides DEPT spectra (Distortionless Enhancement by Polarization Transfer) instead. This is no problem, as DEPT spectra also provide you with the information you need just go back and read what we have said about the relative merits of APT and DEPT. 

The first of these tools is the distortionless enhancement by polarization transfer (DEPT) pulse sequence. There are a number of versions of this experiment which can be very useful for distinguishing the different types of carbons within a molecule. Of these, we have found the DEPT 135 sequence to be the most useful. In this experiment, the quaternary carbons are edited out of the spectrum altogether. 

DEPT Distortionless enhancement by polarization transfer. A useful one-dimensional technique which differentiates methyl and methine carbons from methylene and quaternary carbons. 

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). 

The most useful 1-D pulse sequence applied to 13C nuclei is known as distortionless enhancement by polarization transfer (DEPT). A decoupled... 

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. 

The 13C NMR spectrum of 64, an amide of 63, showed sixty-two carbon signals of which partial assignments, shown in Table 16, were made based upon distortionless enhancement by polarization transfer(DEPT), H-13C correlation experiments and literature data describing 13C NMR analysis of polyene macrolides. 

INEPT insensitive nuclei enhanced by polarization transfer... 

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-... 

Deoxyepinephrine, chemiluminescence, 647 Deoxygenation, hydroperoxides, 153 DEPT (distortionless enhancement by polarization transfer), 725 -6 6,9-Desdimethylartemisinin, synthesis, 288 Desferrioxamine, PfATP6 enzyme inhibition, 1313... 

Distortionless enhancement by polarization transfer (DEPT), 725-6 Disubstituted alkenes, regioselectivity, 842-4 o-Ditoluidine, glucose determination, 632, 634 o,o -Dityrosine, low-density lipoprotein, 610 DMD see Dimethyldioxirane DMDO see Dimethyldioxirane DNA... 

Substituted pyridines Insensitive nuclei enhanced by polarization transfer procedure (INEPT) for 1SN NMR with natural isotope abundance 2J, 3J and 4/(15N, H) coupling constants obtained in (CD3)2CO and DMSO-d6 81OMR(16)170... 

The imaging of conversion within the fixed bed was achieved by using a distortionless enhancement by polarization transfer (DEPT) spectroscopy pulse sequence integrated into an imaging sequence, as shown in Fig. 44. In theory, a signal enhancement of up to a factor of 4 (/hZ/c 7i is the gyromagnetic ratio of nucleus i) can be achieved with DEPT. In this dual resonance experiment, initial excitation is on the H channel. Consequently, the repetition time for the DEPT experiment is constrained by Tih (< T lc) where Tn is the Ty relaxation time of... 

B This pulse sequence, the ID DEFT (Distorsionless Enhancement by Polarization Transfer) experiment, was developed to measure carbon chemical shifts with enhanced sensitivity and to determine at the same time their multiplicities, to differentiate between CH, CH, CH and C. It is a heteronuclear multiple pulse experiment with pulses applied to perturb both carbon and proton spins. Il consists of a preparation, a mixing (used to transfer proton polarization to the directly bound carbons) and a detection period. 

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