Bloch-Siegert shifts

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... 

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)... 

It should be noted that in both the TRAPDOR and REAPDOR experiments, the rf field on the dephasing quadrupolar nucleus remains on for a considerable time, which can result in a Bloch-Siegert shift (Bloch and Siegert 1940) appearing as a misset in the zero-order phasing of the reduced echo compared to the full echo spectrum. 

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.

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 4.7. The Bloch-Siegert shift causes resonances near to an applied if to move away its point of application, (a) is the conventional spectrum and (b) is that acquired with a decoupling field applied at the position of the arrow.

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