Bloch-Siegert Shifts

Bloch Siegert shifts are frequency differences between NMR signals observed in the presence and absence an rf field applied during the acquisition time. The shifts arise because the applied rf field changes the effective magnetic field experienced by nearby resonances. The resonances are always displaced away from the frequency of the irradiating field. The shift is inversely related to the difference in frequency between the irradiation and the resonance and therefore is generally not observed when heteronuclear decoupling is applied. When homonuclear decoupling is employed these shifts can become significant and are typically used to calibrate the strength of the homonuclear decoupling field. One must be aware of these effects when reporting chemical shifts in homonuclear decoupling experiments. The figure below shows the effect of applying homonuclear decoupling fields of varying strength in the 300 MHz ¹H NMR spectrum of dimethyl acetamide. One can see that the displacement of the resonances is away from the decoupling frequency and that the magnitude of the shift is inversely related to the frequency difference between the resonance and the irradiation frequency.