  The application of NMR spectroscopy for studying molecular and reaction
dynamics relies crucially on the measurement of the magnitude of radiofrequency
(RF) fields that are used to nutate or lock the nuclear magnetization. Here, we
report a method for measuring RF field amplitudes that leverages the intrinsic
modulations observed in offset-dependent NMR nutation profiles of small
molecules. Such nutation profiles are exquisitely sensitive to the magnitude of
the RF field, and B1 values ranging from 1-2000 Hz, as well the inhomogeneity
in B1 distributions, can be determined with high accuracy and precision using
this approach. In order to measure B1 fields associated with NMR experiments
carried out on protein or nucleic acids, where these modulations are obscured
by the large transverse relaxation rate constants of the analyte, our approach
can be used in conjunction with a suitable external small molecule standard,
expanding the scope of the method for large biomolecules.
