DEPT distortionless enhancement with

Several other techniques now allow for the easy determination of the number of hydrogens attached to a carbon. One of the most convenient techniques goes by the acronym DEPT (distortionless enhancement with polarization transfer). In this technique, several separate NMR spectra are determined for a molecule under conditions that allow for the appearance of only methine (CH), methylene (CH2), or methyl (CH3) carbons. Quaternary carbons can be determined by difference Any signal in the full spectrum that does not appear in the separate spectra for CH, CH2, and CH3 carbons must belong to a quaternary carbon. 

DEPT (distortionless enhancement with polarization transfer)... 

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. 

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... 

One-dimensional subspectra also may be obtained by combining selective excitation and broadband homonuclear Hartmann mixing with het-eronuclear polarization-transfer steps like INEPT, DEPT (distortionless enhancement by polarization transfer), or heteronuclear Hartmann-Hahn transfer (Doss, 1992 Gardner and Coleman, 1994 Willker et al., 1994). Related experiments with multiple-step selective Hartmann-Hahn mixing in combination with heteronuclear coherence transfer were used by Kupce and Freeman (1993a). 

A number of papers have looked at the development of relationships between base stock composition as measured by NMR and either physi-cal/chemical properties or their performance.22 27 Most of this work has been focused on group II and III base stocks, with less or little attention paid to solvent extracted ones. These have all relied on various techniques to simplify the spectra and the assignments of peaks and make peak integration more reliable. These have many acronyms,23 for example, GASPE (gates spin echo), PCSE (proton coupled spin echo), INEPT (insensitive nuclei enhancement by polarization transfer), DEPT (distortionless enhancement by polarization), QUAT (quaternary-only carbon spectra), 2D COSY (two-dimensional homo-nuclear spectroscopy), and HETCOR (heteronuclear shift correlated spectroscopy)]. Table 4.10 provides an example of some of the chemical shift data generated26 and employed in this type of work, and Adhvaryu et al.25 were able to develop the correlations between base stock properties and carbon types in Table 4.11, whose main features correspond to intuition (e.g., the values of API and aniline points are both decreased by aromatic carbon and increased by the... 

In such instances new methods for C assignment are more suitable. Compared with the 7-modulated spin-echo experiment, the DEPT (Distortionless Enhancement by Polarization Transfer) pulse sequence is insensitive to variations in VcH- The method is based on the use of polarization transfer, whereby magnetization is transferred from protons to C nuclei, and as a result quaternary carbon atoms are not detected in such experiments. This experiment involves pulsing both the H and C channels. The final proton pulse is along the v-axis and by performing two... 

The other commonly seen spectral editing techniques used in carbon NMR involve the transfer of polarization from protons to carbons in particular ways. A good example of this type of experiment is the so-called DEPT (distortionless enhancement by polarization transfer) sequence (Figure 5). By careful choice of experimental parameters, one can obtain spectra that represent only the quaternary, only the methines, only the methylenes and only the methyl groups in the sample. This is done in this case by use of the sequence shown in Figure 5. Four spectra are obtained with <5 =7t/4, jtH, and 3 7r/4 in the sequence... 

The zero sum of intensities in a multiplet in these differential polarization transfer experiments means that any decoupling that causes all the lines to coalesce results in exact cancellation of intensities. Several methods have been devised to avoid this problem when decoupling is required. One is pulse-interrupted precession,which results in a net polarization transfer, as does the insertion of a properly chosen delay period in the INEPT sequence (as in Figure 12) so that all members of a multiplet appear positively enhanced, i.e., INEPT with refocusing (INEPTR). The DEPT (Distortionless Enhancement Polarization Transfer) sequence gives undistorted relative intensities of the multiplet components. 

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 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... 

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. 

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