Bloch-Siegert effect

Fig. 15. Adiabatic decoupling of 13CO from 13C with a compensating pulse applied on the other side of the peaks. The compensating and decoupling pulses have the same shape but opposite frequency sweep. Due to the Bloch-Siegert effects, both the left and the right peaks are pushed towards the center while the centre peak is balanced and remains in its position. Reprinted from Ref. 47 with permission from Elsevier.

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 ... 

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... 

It is known that, whenever excitation bands of the detection and pump pulses are close, a reduction of echo amplitude due to a Bloch-Siegert mechanism takes place. This effect is particularly important for Gd(iii)-nitroxide pairs as the pump pulse is set up to flip low-spin nitroxide species, it has a higher turning angle ( 3-4ti, instead of ti) for the Gd(iii) species. Therefore, for the same frequency offset the effect of echo reduction is stronger for Gd(iii)-nitroxide pairs as compared to pairs of identical spin-centres, like nitroxide-nitroxide or Gd(iii)-Gd(iii). 

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