Applications of the Bloch-Siegert shift in solid-state proton-dipolar-decoupled F-19 MAS NMR

In solid-state proton-dipolar-decoupled F-19 MAS NMR spectroscopy, F-19 chemical-shift data need to be corrected for the Bloch-Siegert shift. Assigning the single sharp F-19 resonance of 2-fluoroadamantane to its proton-coupled F-19 shift of -174.4 ppm results in chemical-shift referencing that is independent of the amplitude of the proton-decoupling field, The Bloch-Siegert shift is also a useful tool to characterize the amplitude and homogeneity of the proton-decoupling field, H-1H, and to monitor probe performance. Considerable inhomogeneity in H-1H along the long axis of the right-cylinder sample rotor was detected, In our commercial 7 mm H-F MAS probe, the proton field strength, <(gamma)over bar>H-H(1H), decreases to 25% of the maximum value across the usable sample volume. Measurement of the Bloch-Siegert shift revealed that the proton-decoupling field strength decreases during the first few scans of an acquisition. Reductions in the proton field strengths can exceed 10%, and they are explained by the heating of the RF coil circuitry which is caused by high-power proton decoupling. The extent of reduction in field amplitude is a function of the decoupling duty cycle. Losses in <(gamma)over bar>H-H(1H) can be avoided by tuning the probe proton RF circuitry at the operating temperature of the probe, using the Bloch-Siegert shift as an optimization parameter. (C) 1996 Academic Press, Inc.