# Enhanced Resolution in EPR Spectroscopy Using para‐Hydrogen Matrices

## TL;DR

This paper shows that using solid para-hydrogen improves EPR spectroscopy resolution and sensitivity for studying radicals.

## Contribution

The study introduces solid para-hydrogen matrices as a novel matrix for EPR spectroscopy, offering enhanced resolution and sensitivity.

## Key findings

- The linewidth of TEMPO in p-H2 is narrower than in argon.
- The P-centered mono-radical in p-H2 shows a three-fold increase in spectral resolution compared to argon.
- The non-existing cage effect in p-H2 increases sensitivity for radical detection.

## Abstract

We developed a matrix isolation experiment utilizing a closed‐cycle helium cryostat, which operates at 2.5 K and enables electron paramagnetic resonance (EPR) measurements in solid para‐hydrogen (p‐H2) matrices. The EPR spectra of the persistent 2,2,6,6‐tetramethylpiperidinyloxyl (TEMPO) radical and an in situ generated P‐centered mono‐radical were recorded at 2.5 K in p‐H2. The spectrum of TEMPO in p‐H2 shows a narrower linewidth compared to argon, and its matrix‐isolated spectrum and simulation are reported. The P‐centered mono‐radical was generated in p‐H2 via in situ photolysis of the corresponding phosphorus iodide, which affords roughly a three‐fold increase in spectral resolution compared to argon, as well as higher sensitivity due to the non‐existing cage effect in soft p‐H2.

We demonstrate the advantageous properties of solid para‐hydrogen (p‐H2) matrices for electron paramagnetic resonance (EPR) spectroscopy by investigating the 2,2,6,6‐tetramethylpiperidinyloxyl (TEMPO) radical and an in situ generated P‐centered mono‐radical. The spectral resolution of the P‐centered radical in p‐H2 is roughly enhanced by a factor of three compared to argon.