Enhanced Resolution in EPR Spectroscopy Using para-Hydrogen Matrices
J. Am. Chem. Soc.: RESOLV members Prof. Wolfram Sander and Dr. André K. Eckhardt report the development of a matrix isolation experiment utilizing a closed-cycle helium cryostat operating at 2.5 K, enabling electron paramagnetic resonance (EPR) measurements in solid para-hydrogen (p-H₂) matrices.
Using this setup, the EPR spectra of the persistent 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) radical as well as an in situ generated phosphorus-centered mono-radical were successfully recorded at 2.5 K in p-H₂. The TEMPO spectrum shows significantly narrower linewidths compared to argon matrices, and its matrix-isolated spectrum was supported by spectral simulations.
Furthermore, the phosphorus-centered mono-radical was generated in solid p-H₂ by in situ photolysis of the corresponding phosphorus iodide precursor. This approach resulted in approximately three-fold higher spectral resolution compared to argon matrices, along with improved sensitivity attributed to the absence of cage effects in the soft p-H₂ environment. This work demonstrates the advantages of para-hydrogen matrices for high-resolution EPR spectroscopy of reactive radical species.
