Enhanced Resolution in EPR Spectroscopy: Solid para-Hydrogen Resolves Proton Hyperfine Splitting in the Diphenylphosphinyl Radical

Abstract

The diphenylphosphinyl radical 1 was generated from iododiphenylphosphine by visible-light photolysis and flash-vacuum pyrolysis under cryogenic matrix isolation conditions and characterized by IR, UV/Vis, and EPR spectroscopy. In solid para-hydrogen (p-H₂), photolysis proceeds markedly more efficiently than in argon and affords substantially narrower EPR signals. Simulation of the low-field wing requires an effective Gaussian broadening of 0.83 G in p-H₂, compared with 1.13 G in normal hydrogen and 4.95 G in argon. This enhanced resolution reveals a seven-line proton hyperfine envelope that remained unresolved in previous studies of 1 in frozen solutions. Spectral simulations and quantum-chemical calculations identify 1 as a π-type radical with pronounced spin localization at phosphorus and additional spin density delocalized over the phenyl rings. Compared with the more rigid dibenzophospholyl radical 4, radical 1 is less stabilized, more conformationally flexible, and exhibits photochemical H-atom abstraction from molecular hydrogen. Ground-state and vertical excited-state calculations exclude a simple thermal or adiabatic photochemical pathway and instead suggest that nonadiabatic or secondary radical-pair processes contribute to the reaction. These results establish solid p-H₂ as a powerful medium for resolving otherwise inaccessible hyperfine structure in reactive phosphorus-centered radicals.