The EPR spectrum of the novel radical Mes*(CH₃)P—PMes* (Mes*=2,4,6-(tBu)₃C₆H₂) was measured in the temperature range 100-300 K, and was found to be drastically temperature dependent as a result of the large anisotropy of the ³¹P hyperfine tensors. Below 180 K, a spectrum of the liquid solution is accurately simulated by calculating the spectral modifications due to slow tumbling of the radical. To achieve this simulation, an algorithm was developed by extending the well-known nitroxide slow-motion simulation technique for the coupling of one electron spin to two nuclear spins. An additional dynamic process responsible for the observed line broadening was found to occur between 180 K and room temperature; this broadening is consistent with an exchange between two conformations. The differences between the isotropic ³¹P couplings associated with the two conformers are shown to be probably due to an internal rotation about the P—P bond.

Stable for at least one week below -30°C: crystals of 1, the first highly persistent diphosphanyl radical, have been isolated and characterized. This phosphorus-centered radical exhibits hyperfine coupling whose anisotropy is considerably larger than that for well-established nitrogen radicals (hydrazyls, nitroxides). This feature is of potential interest for studies of fast molecular movements. Mes*=2,4,6-tBu₃C₆H₂.