Broad-band decoupling

Based on the equation K = yn/yc, we would expect about a threefold enhancement in the signal intensities of proton-bearing C nuclei in the broad-band decoupled INEPT experiment. In practice, the intensification is more than threefold. Why ... 

DEPT (9 = 135° and 90°) and broad-band decoupled C-NMR spectra of ethyl acrylate are shown. Assign the signals to various carbons of the molecule. 

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. 

The DEPT and broad-band decoupled C-NMR spectra of vasicinone, C11H10N2O2, isolated from a plant Adhatoda vasica, are shown. Spec-... 

The nuclear Overhauser effect resulting from the broad-band decoupling during the decoupled INEPT experiment also contributes to the signal enhancement of the C lines. 

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°)... 

The nOe experiment is one of the most powerful and widely exploited methods for structure determination. nOe difference (NOED) or the two-dimensional experiment, NOESY, is used extensively for stereochemical assignments. It provides an indirect way to extract information about internuclear distances. The other use of nOe is in signal intensification in certain NMR experiments, such as the broad-band decoupled C-NMR experiment. 

Figure 5.1 (a) Broad-band decoupled C-NMR spectrum of the indole alkaloid... 

Figure 5.5 shows the heteronuclear 2Dy-resolved spectrum of camphor. The broad-band decoupled C-NMR spectrum is plotted alongside it. This allows the multiplicity of each carbon to be read without difficulty, the F dimension containing only the coupling information and the dimension only the chemical shift information. If, however, proton broad-band decoupling is applied in the evolution period tx, then the 2D spectrum obtained again contains only the coupling information in the F domain, but the F domain now contains both the chemical shift and the coupling information (Fig. 5.6). Projection of the peaks onto the Fx axis therefore gives the Id-decoupled C spectrum projection onto the F axis produces the fully proton-coupled C spectrum.

Figure 5.6 If proton broad-band decoupling is applied in the evolution period, t, then the resulting 2D spectrum contains only chemical shift information in the F, domain, while both chemical shift and coupling information is present in the F domain. Projection onto the /-j-axis therefore gives the H-decoupled C spectrum, whereas projection along F. gives the fully coupled C spectrum.

In homonuclear 2D /-resolved spectra, couplings are present during <2 in heteronuclear 2D /-resolved spectra, they are removed by broad-band decoupling. This has the multiplets in homonuclear 2D /-resolved spectra appearing on the diagonal, and not parallel with F. If the spectra are plotted with the same Hz/cm scale in both dimensions, then the multiplets will be tilted by 45° (Fig. 5.20). So if the data are presented in the absolute-value mode and projected on the chemical shift (F2) axis, the normal, fully coupled ID spectrum will be obtained. To make the spectra more readable, a tilt correction is carried out with the computer (Fig. 5.21) so that Fi contains only /information and F contains only 8 information. Projection... 

Kessler, H., Griesinger, C., Zarbock, J., Loosli, H. R. Assignment of carbonyl carbons and sequence analysis in peptides by heteronudear shift correlation via small coupling constants with broad-band decoupling in ft (COLOC)./. Magn. Reson. 1984, 57, 331-336. 

Thus when it became possible to record carbon-13 spectra routinely it was decided that the logical thing to do would be to decouple ALL of the protons from the carbons simultaneously (a technique known as broad-band decoupling) in order to obtain a carbon-13 spectrum consisting only of singlets. 

So basically there is no point in integrating a broad-band decoupled carbon spectrum. This is not so much of a drawback as it sounds, because the signals are distributed over a range of more than 200 ppm, so that line overlap is very unusual. 

Fig. 15a,b Carbon-13 spectra of compound 1. a Protons broad-band decoupled b carbon-proton coupling present (gated decoupling)... 

Figure 15 shows the normal broad-band decoupled and gated decoupled spectra of compound 1 in the latter we can see the multiplets arising from C-H coupling (across one or more bonds) and C-P coupling. The rules for the number of lines in a multiplet and their intensities are the same as for protons, since 13C and 31P are both spin-Vi nuclei. 

Because of the NOE and differences in relaxation rates, the intensity differences for carbon signals in a broad-band decoupled spectrum are extremely large, so that quantitative information is not available. 

Fig. 18a-c Carbon-13 signals for the methyl carbon in 1. a Complete carbon-proton coupling present b selective decoupling of methylene protons c broad-band decoupled... 

For completeness, the upper trace in Fig. 18 shows the broad-band-decoupled signal, which is of course just a doublet due to the P-C coupling. 

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. 

Fig. 19a-c Carbon-13 spectra of compound 1. a Standard spectrum (broad band decoupling) b APT spectrum c DEPT-135 spectrum... 

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. 

The carbon spectrum, both in the broad-band decoupled form and as an APT spectrum. 

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. 

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