DEPT experiments

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. 

A DEPT experiment is usually done in three stages, as shown in Figure 13.10 for 6-methyi-5-hepten-2-oi. The first stage is to run an ordinary spectrum (called... 

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...

Figure 2.15 In DEPT experiments, signal intensities of CHj, CH2, and CH carbons depend on the angle 0, of the last polarization pulse. For instance, at 6, = 90°, only CH carbons can be seen, while at 135°, CH, and CH carbons will appear with one phase and CH2 carbons will appear with the opposite phase.

What are the differences between INEPT and DEPT experiments Why is DEPT considered a distortionless experiment ... 

F ure 2.16 Pulse sequence for the inverse (reverse) DEPT experiment. 

The pulse sequence used in the reverse DEPT experiment is shown in Fig. 2.16. Presaturation of the protons removes all H magnetization and pumps up the C population difference due to nOe. Broad-band decoupling of the C nuclei may be carried out. The final spectrum obtained is a one-dimensional H-NMR plot that contains only the H signals to which polarization has been transferred—for instance, from the enriched C nucleus. 

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.

The structure was determined by NMR spectral analysis including a variety of two-dimensional NMR techniques. The 500-MHz XH NMR spectrum of 77 taken in CDCI3 (Figure 26) revealed the presence of 5 aromatic protons, 15 olefinic protons, a methoxy (63.65), an allylic methyl (62.14) and a tertiary methyl group (61.33). The 13C NMR spectrum showed signals due to all 34 carbons, which were assigned to 7 quaternary carbons, 23 methines, 1 methylene and 3 methyls by DEPT experiments. The 13C and XH NMR spectral data are summarized in Table 27. 

IR spectra were taken on an Analect RFX-30 FTIR spectrophotometer neat between NaCI or KBr plates or as KBr disks. 1H NMR spectra were recorded on a Nicolet NT-360 (360 MHz) or on a Varian VXR-200 (200 MHz) spectrometer. All chemical shifts are reported in parts per million (8) downfield from internal tetramethylsilane. Fully decoupled 13C NMR spectra and DEPT experiments were recorded on a Varian VXR-200 (50 MHz) spectrometer. The center peak of CDCI3 (77.0 ppm) was used as the internal reference. 

NMR spectra were recorded on Bruker Digital FT-NMR Avance 400 spectrometer (CDClj solvent) with TMS as internal reference. In the C spectra qnatemaiy, methylene and methyl carbons were identified using DEPT experiments. IR spectra were recorded on Perkin Elmer FT-IR spectrometer (KBr). Reactions were performed under dry nitrogew Melting points were measured on a Gallenkamp melting point apparatus. Sihca gel 60 (Merck) was used for column separations. TLC was conducted on standart conversion aluminium sheets pre-coated with a 0.2 mm layer of sihca gel. 

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. 

The structure of the stable ozonide of a diterpene (300) was elucidated based on knowledge of the original structure assisted by H and NMR spectroscopies. Thus, a NMR DEPT experiment points to the presence of six quaternary C, six CH, three... 

Assignment for C-6 changed due to recent results of 13C-NMR/DEPT experiments. 

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... 

As DEPT relies on the transfer of polarization from a directly bonded H atom to the carbon - resulting in the increased sensitivity of the carbon atoms - only C atoms that are attached to H atoms are detectable by this method, and so no quaternary carbon atoms are seen on DEPT spectra. Depending upon something called the pulse angle (which is expressed as a number after the acronym, but we do not need to know its significance), there are three different DEPT experiments that can be carried out on a particular sample. 

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. 

Recording of the broad-band decoupled 13C NMR spectrum and DEPT experiments allows to obtain the number of carbon resonances (usually, even a non-quantitative spectrum gives good agreement between line intensities and number of carbon atoms) and carbon multiplicities. 

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).

The insensitivity of the DEPT sequence to different JCH coupling constants, as illustrated in Fig. 2.46, makes it useful for editing 13C NMR spectra. To edit a carbon-13 spectrum, three DEPT experiments for the polarization transfer angles... 

Fig. 2.46. 13CNMR spectra of ( — )-menlhol (lOOmg/mL deuteriochloroform 100.6 MHz) (a) proton broadband-decoupled spectrum (b, c) DEPT spectra with 0y = 90" for CH selection with and without proton decoupling (d, e) DEPT experiments with 0y = 135" for positive CH and CH3 but negative CH2 signals with and without proton decoupling (f) gated-decoupled spectrum for reference (a-e) 16 scans (f) 256 scans. 

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