EPR Spectroscopy in para-Hydrogen Matrices

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 a higher spectral resolution and intensity compared to argon, neon and deuterium matrices.