Carbon DEPT spectra

The C NMR spectrum does not show the three resonances expected for monomeric cyclopenta-diene. Instead, ten distinct signals appear, of which the DEPT spectrum identifies four CH carbon... 

FIGURE 13.26 C NMR spectra of 1-phenyl-1-pentanone. (a) Normal spectrum, (b) DEPT spectrum recorded using a pulse sequence in which CH3 and CH carbons appear as positive peaks, CH2 carbons as negative peaks, and carbons without any attached hydrogens are nulled. 

The broad-band decoupled C-NMR spectrum of ethyl acrylate shows five carbon resonances the DEPT (6 = 135°) spectrum displays only four signals i.e., only the protonated carbons appear, since the quaternary carbonyl carbon signal does not appear in the DEPT spectrum. The CH and CH3 carbons appear with positive amplitudes, and the CHj carbons appear with negative amplitudes. The DEPT (6 = 90°) spectrum displays only the methine carbons. It is therefore possible to distinguish between CH3 carbons from CH carbons. Since the broadband decoupled C spectrum contains all carbons (including quaternary carbons), whereas the DEPT spectra do not show the quaternary carbons, it is possible to differentiate between quaternary carbons from CH, CHj, and CH3 carbons by examining the additional peaks in the broad-band spectrum versus DEPT spectra. The chemical shifts assigned to the various carbons are presented around the structure. 

The broad-band decoupled C-NMR spectrum of malabarolide-Ai shows signals for all 18 carbon atoms. The DEPT spectrum (0 = 135°) exhibits 14 signals for protonated carbons. It is therefore possible to identify four signals of quaternary carbons, i.e., 8 39.2 (C-9), 74.7 (C-8), 126.0 (C-13), and 171.8 (C-18 carbonyl). The DEPT (0 = 90°)... 

Figure 19 shows the normal (broad-band decoupled), APT and DEPT-135 spectra of model compound 1. Note that in the APT spectrum the solvent (CDC13) is visible, but not in the DEPT spectrum, where the two low-field quaternary aromatic carbons are also absent. 

Carbon-13 spectrum with DEPT or APT for multiplicity information... 

The proton-carbon correlation spectrum, which tells you directly which signals in the proton spectrum correspond with which signals in the broad-band decoupled carbon spectrum. This information, together with the integration values and the multiplicities obtained from APT (or DEPT), is invaluable in putting together the molecular fragments. 

The order in which various NMR data are acquired is largely one of user preference. Acquisition of the proton reference spectrum will invariably be undertaken first. Whether a user next seeks to establish homo- or heteronuclear shift correlations is where individual preferences come into play. Many spectro-scopists proceed from the proton reference spectrum to either a COSY or a TOCS Y spectrum next, while others may prefer to establish direct proton-carbon chemical shift correlations. This author s preference is for the latter approach. From a multiplicity-edited HSQC spectrum you obtain not only the carbon chemical shifts, which give an indication of the location of heteroatoms, the degree of unsaturation and the like, but also the number of directly attached protons, which eliminates the need for the acquisition of a DEPT spectrum [51, 52]. The statement in the prior sentence presupposes, of course, that there the sensitivity losses associated with the acquisition of multiplicity-edited HSQC data are tolerable. 

The carbon and DEPT (distortionless enhanced polarization transfer) spectra are shown in Figure 10. The HETCOR (heteronuclear two-dimensional proton-carbon correlation) spectrum is shown in Figure 11. The carbon assignments are listed in Table 5. Long-range HETCOR experiments were used to make the assignments for the thiophene carbons. 

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. 2.60. Deriving the carbon skeleton of an organic compound by DEPT and 2D-IN-ADEQUATE with COSY-like square correlations sample 3-(isopinocampheoxy)-2-methyl-l,3-butadiene, 400 mg in 0.4 mL of hexadeuterioacetone, 100.6 MHz (a) proton-broadband decoupled spectrum (1 scan) (b) DEPT subspectrum of CH carbon nuclei (8 scans) (c) DEPT spectrum with CH, CH3 (positive), and CH2 (negative. 8 scans) (d) 2 D-INADEQUATE experiment of aliphatic carbon nuclei, 256 experiments, 64 scans per experiment. The bicyclic partial structure of the molecule can be derived from the square correlations (e.g.. .. —44.2 — 81.1 —35.6—. ..). A stacked plot of the carbon-proton shift correlation of this sample is displayed on the cover.

The DEPT spectrum of ipsenol is shown in Figure 4.12. It consists of the main spectrum (a), which is a standard -decoupled 13C spectrum the middle spectrum (b) is a DEPT 135 where the CH3 s and CH s are phased up, whereas the CH2 s are phased down. The top spectrum (c) is a DEPT 90, where only CH carbons are detected. Quaternary 13C are not detected in the DEPT subspectra. We can now interpret the 13C peaks in the main spectrum as CH3, CH2, CH, or C by examining the two subspectra peaks along with the main spectrum. The easiest way to approach the interpretation of the 13C/DEPT spectra... 

Yes, interpretation of a DEPT spectrum takes a bit of practice, but the results are most instructive. Not only do we have the number of carbon and hydrogen atoms, but now we have the frequency distribution of carbon atoms with the number of hydrogen atoms attached to each carbon. However, there is a discrepancy between the proton count (see Figure 3.55 for H spectrum for confirmation) in the proton spectrum and in the DEPT spectrum, since the OH is not recorded in the DEPT spectrum nor are protons that are attached to such atoms as l5N, 33S, 29Si, and 31P. It is not difficult to correlate the DEPT spectrum with the H spectrum. In fact, it is striking to observe how the olefinic and alkyl systems are widely separated in both spectra. 

The COSY spectrum for caryophyllene oxide can be understood more clearly when interpreted in conjunction with the information from an HMQC spectrum (Figure 5.17). From the DEPT spectrum (see Figure 5.15), we already know that caryophyllene oxide has three methyl carbon resonances (16.4, 22.6, and 29.3 ppm), six methylene carbon resonances (26.6,29.2, 29.5, 38.4, 39.1, and 112.0 ppm), three methine carbon resonances (48.1, 50.1, and 63.0 ppm) and three quaternary carbon resonances (33.3,59.1, and 151.0 ppm)... 

The proton spectrum consists of classical first-order multiplets. From left to right, the multiplicities and integrations are triplet (2), singlet (1), doublets of triplet (2), triplet (1), which yields six hydrogen atoms. The 13C/DEPT spectra provide four carbon atoms that read from left to right C, CH, CH2, CH2. This discrepancy implies that one of the protons is bonded to a heteroatom. The OH proton at 2.68 ppm in the H spectrum accounts for the difference in proton count between the H spectrum and the l3C/DEPT spectrum. 

Much emphasis is placed on the DEPT spectrum, in fact, it is used in all of the Student Exercises in place of the obsolete decoupled 13C spectrum. The DEPT spectrum provides the distribution of carbon atoms with the number of hydrogen atoms attached to each carbon. 

Carbons with no H s appear only in the normal spectrum, but not in either DEPT spectrum. 

Thus, the 2D INADEQUATE spectrum of 2-chlorobutane confirms that the carbon of signal 1 is connected to the carbon of signal 3,3 is connected to 4, and 4 is connected to 2. Since the DEPT spectrum (or HET2DJ) of 2-chlorobutane already identified signals 1 and 2 as methyls, 3 as a methylene, and 4... 

COSY and a C.H-HSC. With only an edited DEPT spectrum and a 2D INADEQUATE spectrum the atomic sequence (structure without stereochemistry) of most molecules can be determined. However, there is a down side the time and effort required to generate the 2D INADEQUATE spectrum. Because we are looking at very weak sidebands of weak signals, it can take days of pulse sequence repetitions to generate the desired information. Moreover, the recycle delay between pulses must be carefully set to exceed (by 1.5- to 3-fold) the longest carbon T, value in the molecule. Also, the experiment... 

For unknown compounds, the number of attached protons per carbon is best determined by the DEPT experiment (Section 7-2b). The DEPT spectrum of the 50-mg sample of T-2 toxin is shown in Figure 8-3. It reveals the presence of six methyl, four methylene, and seven... 

The NMR spectrum does not show the three resonances expected for monomeric cyclopentadiene. Instead, ten distinct signals appear, of which the DEPT spectrum identifies four CU carbon atoms in each of the shift ranges appropriate for alkanes and alkenes and in the alkane range an additional two C/Zj cfirbon atoms. This fits the [4 + 2]-adduct 2 of cyclopentadiene /. 

At this point, we are finished with the DEPT spectrum and can continue by identifying an appropriate starting point for specific assignments. There is inevitably at least one proton or carbon peak, and usually several, that one can assign immediately. This may be based on chemical shift. For example, a carbonyl should appear as a quaternary significantly downfield in the carbon spectrum, while aliphatic quaternaries appear significantly upfield. Carbon... 

The DEPT spectrum (Figure 4.15) immediately confirms the seven-saccharide repeat, since there are seven anomeric resonances such a confirmation of the results of methylation analysis by NMR is important, because incomplete methylation can give the appearance of additional branch points. Moreover, the hydroxymethylene carbons at high field show inverted peaks, which identifies them as the C6 carbons of the various residues. 

The final step in the assignment of all resonances is correlating the C peaks with the proton peaks. This can often be done from chemical shift alone for the anomeric resonances, but not for other ring atoms. The most important pulse sequence here is HSQC (Heteronuclear Single Quantum Correlation), which involves six pulses to protons and four to C. The spectrum is now a non-symmetrical map with a peak at each carbon attached to a proton the projection on the C axis is the C DEPT spectrum and on the proton axis the ordinary proton spectrum (Figure 4.18). 

As interesting variant of the standard DEPT experiment is the DEPTQ experiment [5.67] which enables the detection of groups such as quaternary carbons atoms which are not directly bonded to a sensitive NMR nucleus. These types of group are not enhanced by the polarization transfer step and are usually missing from the standard DEPT spectrum. Consequently if quaternary carbon are part of the molecule skeleton in a structural determination it is necessary to record both the broadband IR decoupled spectrum and multiplicity edited DEPT135-l C IR spectrum. Rowever the DEPTQ... 

The presentation of subspectra can be condensed to two lines as shown in Figure 5.6 and in Chapter 6. One line (B subspectrum) shows peaks CH3 and CH up, and CH2 down. The other line (A subspectrum) shows the CH peaks up. Quaternary carbon atoms are not recorded in a subspectrum since there is no attached proton, but of course the main (conventional 13C) spectrum does show these peaks. In many laboratories, a DEPT spectrum is considered part of a routine 13C spectrum. 

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