[Paper Review] High-Resolution Electron Paramagnetic Resonance

Colin J. Stephen, Anton Tcholakov+8 more|arXiv (Cornell University)|Jan 19, 2026

Fullerene Chemistry and Applications0 citations

TL;DR

The paper demonstrates a 14 T EPR spectrometer using 396 GHz excitation to achieve ultra-narrow EPR lines (210 ppb FWHM) and extremely precise g-factor measurements (±16 ppb), with in-situ NMR referencing to 3He for accurate calibration; it reports g = 2.00209909(3) for N@C60 in deuterated toluene.

ABSTRACT

Electron paramagnetic resonance (EPR) is a valuable tool for physics, chemistry, biology and medicine, providing complementary spectroscopic information to NMR. It has long been known that EPR at high magnetic fields offers greater spectral resolution, but limitations in the THz instrumentation have prevented the full realization of these opportunities. Here we describe an EPR spectrometer at the high magnetic field of 14 T using 396 GHz excitation, which adapts techniques from liquid-state NMR to obtain sharp EPR resonances with a width of 210 ppb (full-width half-maximum). We use this to measure resonance positions, and hence g-factors, with a precision that reaches $\pm$16 ppb. Our use of in-situ liquid-state NMR of our solvent within the same sample improves the accuracy of these measurements: it allows us to reference our EPR measurement back to dilute gas 3He NMR for which quantum calculations are accurate. We measure the g-factor of N@C60 in deuterated toluene as g = 2.002 099 09 (3), where the 3 in brackets means that the uncertainty on the last digit is $\pm$3.

Motivation & Objective

• Motivate high-resolution EPR at high magnetic fields to overcome spectral broadening limitations.
• Adapt liquid-state NMR techniques to EPR to achieve sharp resonances and accurate g-factor determination.
• Enable precise g-factor measurements by referencing EPR data to a well-calibrated NMR standard (dilute gas 3He).

Proposed method

• Operate EPR at 14 T with 396 GHz excitation.
• Adopt liquid-state NMR techniques within the sample to sharpen resonances.
• Reference EPR data to in-situ NMR of solvent and 3He NMR for calibration.
• Measure resonance positions to extract g-factors with sub-ppb precision.
• Report specific g-factor for N@C60 in deuterated toluene as g = 2.00209909(3).

Experimental results

Research questions

• RQ1 Can high-field EPR at 14 T with 396 GHz excitation achieve substantially narrower linewidths than conventional EPR?
• RQ2 What is the achievable precision in g-factor determination using in-situ NMR referencing?
• RQ3 How effectively can solvent NMR references improve the accuracy of EPR measurements in a single sample?
• RQ4 What is the measured g-factor for N@C60 under the described conditions?

Key findings

• EPR resonances with width of 210 ppb (FWHM) at 14 T and 396 GHz excitation.
• g-factor precision reaching ±16 ppb.
• g-factor for N@C60 in deuterated toluene determined as 2.00209909(3).
• In-situ liquid-state NMR of the solvent enables referencing back to 3He NMR for accurate quantum-calculation-backed calibration.
• The approach combines high-field EPR with NMR-based referencing to improve measurement accuracy.