Decoupling, off-resonance

When a high-power decoupling field is applied to one of the two nuclei, it results in an equalization of the populations of levels 1 and 3, as well as of levels 2 and 4. What this means, in effect, is that half the excess d) of nuclei in levels 3 and 4 are promoted to levels 1 and 2. The populations of the levels at equilibrium and immediately after application of the decoupling field are shown in Table 3.3. 

The difference in population between levels 1 and 4 was 2d at equilibrium but after application of the saturation pulse it is only d. This difference is restored to some extent by the relaxation pathway involving direct relaxation from level 1 to level 4, primarily a dipole-dipole interaction between the coupled nuclei. If the number of nuclei relaxed by this path from level 1 to level 4 is x, then the intensity of the lines (which is dependent on the difference in populations between levels 1 and 2, or between levels 3 and 4) will be increased by an amount x due to this additional relaxation, and a corresponding effect will be observed. 

In an A- X experiment (i.e., irradiate X, observe A), the NOE is dependent on the gyromagnetic ratio of A and X nuclei, provided that the intramolecular dipolar relaxation mechanism is the predominant pathway through which nuclei A are relaxing. Thus the maximum NOE factor /a(X) for the A signal in an A- X experiment is given by 

Since the increase in signal-to-noise ratio is 1 plus the NOE factor, the signals will be increased by 1 + 1.988 = 2.988 (i.e., up to a threefold increase will be observed at maximum NOE). 

In the corresponding experiment in which are being irradiated and protons are being observed, the NOE for would be 

In many cases it would be helpful to have the information about the attached hydrogens that a proton-coupled spectrum provides, but frequently the spectrum becomes too complex, with overlapping multiplets that are difficult to resolve or assign correctly. A compromise technique called off-resonance decoupling can often provide multiplet information while keeping the spectrum relatively simple in appearance. 

In this technique, the frequency of a second radiofrequency transmitter (the decoupler) is set either upheld or downheld from the usual sweep width of a normal proton spectrum (i.e., off resonance). In contrast, the frequency of the decoupler is set to coincide exactly with the range of proton resonances in a true decoupling experiment. Furthermore, in off-resonance decoupling, the power of the decoupling oscillator is held low to avoid complete decouphng. 

Every silver lining has a cloud Although an H-decoupled 13C spectrum has the advantages of simplicity and improved 

One way to do this involves a technique called off-resonance decoupling. As the term implies, we do not set v2 at 

In like manner, 2Jr = Vr = 1 Hz, which is negligibly small. So, only one-bond C-H couplings are preserved in an off-resonance decoupled l3C spectrum.  

As useful as off-resonance decoupling once was, there is an even better way to get the same type of information, as we will see later in this chapter and in Chapter 13. 

only one-bond C-H couplings are preserved in an off-resonance decoupled C spectrum.  

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Decoupling (Section 13.17) In NMR spectroscopy, any process that destroys the coupling of nuclear spins between two nuclei. Two types of decoupling are employed in C NMR spectroscopy. Broadband decoupling removes all the couplings off-resonance decoupling removes all H- C couplings except those between directly bonded atoms. 

How can spectral editing in INEPT be used to generate separate spectra for CH, CH2, and CH3 groups, and how is it superior to the traditional off-resonance decoupled spectra ... 

In the study of 13C-NMR spectra of 8 and 6, the presence of the hydroxyl group at C-14 is also evident since a downfield shift of 43 ppm from 6 (<521.2) to 8 (<563.87) is observed. The previously ambiguous assignments of C-15, C-16, C-5, and C-20 are also clarified through irradiation of H-16, H-5, and H-20 with low-power single-frequency off-resonance decoupling (SFORD) experiments. 

Singh et al [54] used a 13C NMR spectrometric method and reported the chemical shifts of primaquine and chloroquine. The signals are assigned on the basis of substituent effects on benzene shifts, intensities, multiplicities in single-frequency off-resonance decoupled and the comparison with structurally related compounds. 

Proton-noise decoupled and single-frequency off-resonance decoupled carbon-13 NMR spectra were determined for the CTC Working Standard (Figure 13). 

From the off-resonance decoupled spectrum there are 2 CH resonances in the aromatic/olefinic chemical shift range, one CH2 and one CH3 carbon in the aliphatic chemical shift range. 

Hydrochlorinated 1,4-po1ydimethylbutadiene. Figure 8 shows the 22.6 MHz C NMR spectrum of the hydrochlorinated 1,4-polydimethyl butadiene sample. Identification of methyl, methylene, methine, and quaternary carbon resonances was made by off-resonance decoupling experiments. It can be seen that all resonances but the methyl resonance at 15.1 ppm, and the methine resonance at... 

There are two new singlets (no attached protons as verified by off-resonance decoupling) in the carbonyl region at 178.66 and 175.75 ppm. The slight downfleld shift and the appearance of two lines are expected for the carbonyls in the 150 °C product (carbons f" and g" in 3) since they are not equivalent. Comparison of the intensity of these two lines to the carbonyl line at 169.56 ppm, which comes from carbon a of compound 1., in the 5 hr spectrum allows one to conclude that the rate of reaction approximately corresponds to 30X consumption of reactants in 5 hrs at 150 °C. 

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