Scandium dimetallofullerene with a single-electron metal-metal bond as a spectroscopic ruler for EPR measurements.

Measurements of electron paramagnetic resonance (EPR) close to the saturation region of iron-core electromagnets ( 1 Tesla) require precise magnetic field calibration due to nonlinear effects. Endohedral fullerenes Sc@C(CHPh) have unique spectral features due to the large hyperfine interaction (509 MHz) of the delocalized electron with two scandium nuclei ( = 7/2), resulting in a maximum spectral width of 0.25 Tesla. The spectrum consists of 64 well resolved lines divided into 15 groups, depending on the projection of the total spin of the scandium nuclei. Each group has a reference line with the largest possible total nuclear spin, located in the right edge of the group. These reference lines are shown to be equidistant in the low-field and high-field regions of the spectrum, and can therefore serve as a benchmark for magnetic field calibration. The calibration procedure is explained by theoretical calculations and verified on endofullerene spectra in Q-band EPR. To compare the spectral properties, measurements in the X- and W-bands were also performed. Absolute values of magnetic field strength were taken from standard Hall sensors of the spectrometer (X- and Q-bands) or on the measurements of the applied current (W-band); additional control of these values is carried out using a Gaussmeter at the sample location. The results show the high accuracy of the correction procedure for linear and nonlinear magnetic field offsets.