Bloch-Siegert frequency shift

Phase incremented double adiabatic decoupling for compensation of the Bloch-Siegert frequency shift... 

As with any other homonuclear decoupling, the transverse magnetization of the 13C , for example in the evolution time, is perturbed by the decoupling pulse, resulting in an additional precession virtually around the z axis. Since the adiabatic decoupling is applied during the entire evolution time of the 13C , a non-linear frequency shift rather than a phase shift appears in the spectrum. This is termed the Bloch-Siegert (frequency) shift in the NMR literature in honour of their discovery of the phenomenon. 

The object of this family of experiments is to correlate the chemical shifts of mutually coupled spins. Thus they replace tedious multiple decoupling experiments, and have the additional advantage of high specificity and an absence of Bloch-Siegert frequency distortions. 

The spectrum was obtained with the proton and fluorine decoupling frequencies set on resonance for the respective and nuclei in PE and PTFE. The reasonably well-defined powder patterns imply effective decoupling and suggest the absence of any large Bloch-Siegert shifts, (e.g., see... 

The method of symmetric points was used to determine the center of the interference curve. Extensive calculations showed that the line profile should be symmetric about the center frequency. The line center was then corrected for the second order Doppler shift, The Bloch-Siegert and rf Stark shifts, coupling between the rf plates, the residual F=1 hyperfine component, and distortion due to off axis electric fields. A small residual asymmetry in the average quench curve was attributed to a residual variation of the rf electric field across the line and corrected for on the assumption this was the correct explanation. Table 1 shows the measured interval and the corrections for one of the 8 data sets used to determine the final result. 

In practice one will be dealing with a more complex spin system but it is easy to extend these equations to the AX and AMX cases and hence calculate the frequencies at which the selective proton rf field, B3, must be applied to perturb the Bloch-Siegert shifted transitions. (199)... 

One consequence of high-power proton decoupling during observation of F spectra is that resonance frequencies will be affected by the Bloch-Siegert effect [11]. This has been thoroughly investigated by Vierkotter [12] for some nonpolymeric fluorinated compounds, and has been observed for polymeric systems [13]. The shift (expressed as a fraction) is given by ... 

Fig. 6.6.2. Bloch-Siegert shift on the F PVDF spectrum. The signals are shifted to low frequency (and resolution of the shoulders assigned to crystalline domains is improved) by the decoupling. The decoupler power corresponds to ca. 100 kHz. The sample is of 10 xm film (containing both a and crystalline forms as well as amorphous domains). MAS is used at 14 kHz.

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