The DEPT Experiment

0 = 90° primarily methine carbons, with possible small breakthroughs of some other signals 

0 - 45°, 90°, and 135° full spectral editing (CH and CH3, positively phased CH2, negatively phased) 

Because DEPT is a polarization transfer experiment, the relaxation delay times are a function of the H-, and not the X-nucleus , T s. The following are additional suggested spectral parameters  

number of scans = multiple of 4 (for phase-cycling purposes) 

A very useful pulse sequence in spectroscopy is employed in the experiment called Distortionless Enhancement by Polarization Transfer, better known as DEPT. The DEPT method has become one of the most important techniques available to the NMR spectroscopist for determining the number of hydrogens attached to a given carbon atom. The pulse sequence involves a complex program of pulses and delay times in both the and ehannels. The result of this pulse sequenee is that carbon atoms with one, two, and three attached hydrogens exhibit different phases as they are recorded. The phases of these hydrogens will also depend on the duration of the delays that are programmed into the pulse sequence. In one experiment, called a DEPT-45, only carbon atoms that 

There are several variations on the DEPT experiment. In one form, separate spectra are traced on a single sheet of paper. On one spectrum, only the methyl carbons are shown on the second spectrum, only the methylene carbons are traced on the third spectrum, only the methine carbons appear and on the fourth trace, all carbon atoms that bear hydrogen atoms are shown. In another variation on this experiment, the peaks due to methyl, methylene, and methine carbons are all traced on the same line, with the methyl and methine carbons appearing as positive peaks and the methylene carbons appearing as negative peaks. 

The two methyl carbons (numbered 1) can be seen as the tallest peak (at 22.3 ppm), while the methyl group on the acetyl function (numbered 6) is a shorter peak at 20.8 ppm. The methine carbon (2) is a still smaller peak at 24.9 ppm. The methylene carbons produce the inverted peaks carbon 3 appears at 37.1 ppm, and carbon 4 appears at 63.0 ppm. Carbon 4 is deshielded since it is near the electronegative oxygen atom. The carbonyl carbon (5) does not appear in the DEPT spectrum since it has no attached hydrogen atoms. 

Nuclear Magnetic Resonance Spectroscopy Part Five Advanced NMR Techniques 

Another example that demonstrates some of the power of the DEPT technique is the terpenoid alcohol citronelloL 

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C NMR spectra 14 were recorded of cw-l,2-dimethylcyclohexane at the temperatures given the DEPT experiment at 223 K was also recorded in order to distinguish the CH multiplicities (CH and CHs positive, C// negative). Which assignments of resonances and what thermodynamic data can be deduced from these spectra ... 

In the //broadband decoupled C NMR spectrum, 15 carbon signals can be identified, in agreement with the molecular formula which indicates a sesquiterpene. The DEPT experiments show that the compound contains four quaternary C atoms, three CH units, seven CH units and a CH3... 

The sample prepared is not particularly pure, so instead of the 30 signals expected, 33 signals are observed in the // broadband decoupled C NMR spectrum. Only by pooling information from the DEPT experiment and from the reliable analysis be obtained, as shown in Table 51.1. Here the AB systems of the geminal CH2 protons are assigned. 

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. 

Figure 2.14 (A) Pulse sequence for the DEPT experiment. (B) Effect of the pulse...

In the DEPT experiment, all the signals of the insensitive nuclei are in phase at the start of acquisition, so no refocusing period A (with accompanying loss in sensitivity) is required. Since the multiplets appear in-phase, it is called a distortionless experiment. Moreover, DEPT spectra depend on the angle 0 of the last polarization transfer pulse, and are less dependent on the delay times between the pulses. An error of 20% in the estimation of/values still affords acceptable DEPT... 

The theory behind both of these experiments, and in particular the DEPT experiment, is rather complicated, so that we refer you to NMR textbooks for details. The important feature of both is that the carbon signals appear to have been simply broad-band decoupled, but that according to the multiplicity they appear either in positive (normal) phase or in negative phase, according to their multiplicity. 

Of the multitude of ID 13C NMR experiments that can be performed, the two most common experiments are a simple broadband proton-decoupled 13C reference spectrum, and a distortionless enhancement polarization transfer (DEPT) sequence of experiments [29]. The latter, through addition and subtraction of data subsets, allows the presentation of the data as a series of edited experiments containing only methine, methylene and methyl resonances as separate subspectra. Quaternary carbons are excluded in the DEPT experiment and can only be observed in the 13C reference spectrum or by using another editing sequence such as APT [30]. The individual DEPT subspectra for CH, CH2 and CH3 resonances of santonin (4) are presented in Fig. 10.9. 

Fig. 10.9. Multiplicity edited DEPT traces for the methine, methylene and methyl resonances of santonin (4). Quaternary carbons are excluded in the DEPT experiment and must be observed in the 13C reference spectrum or through the use of another multiplicity editing experiment such as APT.

In more advanced applications, the DEPT experiment can be used to separate the signals arising from carbons in CH3, CH2 and CH groups. This is termed spectral editing and can be used to produce separate i C sub-spectra of just the CH3 carbons, just the CH2 carbons or just the CH carbons. 

Chapter 6 has been almost completely rewritten. There is more emphasis on pulse sequences and on the use of inverse detection (e.g., HMQC and HMBC experiments). Some experiments from the Fifth Edition have been eliminated (e.g., /-Resolved), and others have been added. The chapter has been renamed Correlation NMR Spectrometry to better reflect the emphasis of the chapter. Because of this name change, the DEPT experiment has been moved to Chapter 5 the APT experiment has been eliminated. Gradient field NMR is presented as a recent development. Problems are assigned. 

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... 

The experiment is used for solving simple structural problems and for the evaluation of chemical shifts. This experiment is usually combined with the DEPT experiment (see 3.3.2.2) for additional information and for signal assignments. 

Fig. 2.44. The DEPT experiment for a C- H doublet pulse sequences in the proton and carbon-13 channel (a-g) and the motion of proton and carbon-13 magnetization, controlled by the pulses and by /-modulation (a-h).

In the DEPT experiments, both peaks around 50 and 58 ppm are divided into three components and the levels of the protonation for these six individual resonances are evaluated. The peaks at 57.4, 58.0 and 58.6 ppm are assigned to the polysulfidic crosslinks in the Al, B1 and B2-type structures, respectively. The peaks at 37.2 and 50.7 ppm are due to Al-type polysulfidic crosslinks. There was no apparent structural match for the quaternary peak at 50.2 ppm. 

The DEPT sequence (distortion enhancement by polarization transfer) has developed into the preferred procedure for determining the number of protons directly attached to the individual 13C nucleus. The DEPT experiment can be done in a reasonable time and on small samples in fact it is several times more sensitive than the usual 13C procedure. DEPT is now routine in many laboratories and is widely used in the Student Exercises in this textbook. The novel feature in the DEPT sequence is a variable proton pulse angle 9 (see Figure 4.11) that is set at 90° for one subspectrum, and 135° for the other separate experiment. 

The DEPT experiment is generated from available computer software with the following concomitant pulse sequence... 

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