Bloch-Siegert frequency shift decoupling

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... 

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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