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

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 ‐H ₂ ), 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 ‐H ₂ , 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 ‐H ₂ as a powerful medium for resolving otherwise inaccessible hyperfine structure in reactive phosphorus‐centered radicals.