Decoupler offset

SWi, which in turn is related to the homonuclear or heteronuclear coupling constants. In homonuclear 2D spectra, the transmitter offset frequency is kept at the center of (i.e., at = 0) and F domains. In heteronuclear-shift-correlated spectra, the decoupler offset frequency is kept at the center (Fi = 0) of thei i domain, with the domain corresponding to the invisible or decoupled nucleus. 

In single-frequency off-resonance decoupled (SFORD) spectra the magnitude of the coupling interaction between 13C and H is reduced so that normally only one bond C-H coupling patterns are observed and an A X situation is assumed. In such a decoupling the residual coupling JK is less than /(C-H) and depends upon the decoupling power (yB2) and the decoupler offset (Av). For most applications to structural work... 

The results of a homonuclear decoupling experiment on sucrose are shown in Fig. 5.18. The experiment is set up by acquiring a normal H spectrum and determining the exact RF frequency of each peak (each resonance) we wish to test by CW irradiation during the acquisition of the FID. The desired frequencies are actually offsets from a fundamental decoupler frequency for example, an offset (Bruker o2 for channel 2 offset Varian dof for decoupler offset) of 132.6 Hz is added to the fundamental frequency (Bruker BF2... 

Although CP experiments are predominantly between H->X, other combinations of nuclei exist. In particular, can be used as the source of magnetisation (Sebald et al. 1992). can be more difficult to set-up since its chemical shift range is that much greater than in H, so the decoupler offset must be recalibrated. It is also recommended that the F is observed directly to check where the decoupler frequency is set relative to the chemical shift for that compound. Appropriate set-up compounds are necessary for optimisation of the CP condition. Na2Sip6 is a good set-up compound for Si. 

In some respects, the H-decoupling operation can be viewed as a simultaneous H NMR experiment. Just as there is a transmitter offset that positions the X-nucleus observation frequency, so is there a corresponding decoupler offset for the H-decoupling fiequency. Many spectrometers have both transmitter and decoupler power levels. Three parameters, however, are specific to decoupling and have no counterparts among the spectral observation parameters that have been discussed. 

Figure 3.56. Calibration of the homonuclear decoupling field strength via the Bloch-Siegert shift. The decoupler offset from the unperturbed resonance was 47.5 Hz causing a shift of 5.5 Hz, indicating the mean if field to be 23 Hz.

Figure 9.8. Experimental comparison of the decoupling offset profiles of (a) WALTZ-16 and (b) GARP relative to the decoupling rf field strength yB2. The sample was carbon-13 labelled methanoic acid in D2O.

Figure 5. Three series of C spectra from a C-F doublet with different F decoupler modulations schemes. In each series, the decoupler offset was incremented in 5kHz step , (a) WALTZ-16, (b) GARP, and (c) WURST-40.

Fig. 10 (a) Rf trajectory, (b) pulse sequence, (c) offset (upper) and rf inhomogeneity (lower) profile for EXPORT (c subpanels c, e, f) relative to DCP (c subpanels a, d) and optimal control DCP (c subpanel b), along with a 2D 13C-13C correlation spectrum of GB1 obtained using EXPORT with 23.8 kHz spinning and no 1H decoupling (reproduced from [48] with permission)... 

As shown in Fig. 17, the BSFS, by a double adiabatic decoupling, is not only significantly reduced compared with that by a single adiabatic decoupling but it also becomes linear as a function of the offset as predicted by Eq. (102). This linear BSFS is corrected by the application of a dilated evolution time =[l+(/lnnS/A/)2]h. 

Fig. 17. The BSFS versus the offset S for a single (open circles) and a double adiabatic (filled circles) decoupling, respectively. By application of a dilated evolution time t = 1.00223/], the remaining BSFS, which is linearly proportional to the offset, is eliminated (filled diamonds). Reprinted from Ref. 47 with permission from Elsevier.

For off-resonance decoupling of 13C NMR spectra, frequency offsets of about 0.5 1 kHz are used. In order to avoid complete collapsing of multiplets by large, noise-modulated frequency bands, non-modulated decoupling fields are usually applied. The decoupling frequency offset can be adjusted until the multiplets are so narrow that no or only slight overlapping occurs. 

The residual coupling JR of an individual carbon with the carbon-proton coupling constant JCH is a linear function of the decoupling frequency offset A f2 from the protons attached to that carbon, provided the constant decoupling power y BJ2 n is sufficiently... 

Fig. 2.21. Series of stacked proton off-resonance decoupled 13C NMR spectra with varying frequency offset Af2.

The offsets necessary for selective proton decoupling do not have to be measured, provided the decoupling frequency of TMS protons is known and the proton shifts of the compound are available from the literature. Tn this case, the decoupling frequency offsets are calculated from the proton shifts. This is performed for the protons of 6-methoxy-a-tetralone. The result of decoupling experiments with these values is shown in F ig. 2.25 (c). A complete assignment of all protonated carbons is achieved. 

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