PERSISTENT SILYLATED PHOSPHORANYL RADICALS - APPLICATION TO DYNAMIC NUCLEAR-POLARIZATION

New phosphoranyl radicals [(R3SiO)(3)POSiR'(3)](.) (R = R' = Et, 1, R = iPr, R' = Et, 2) were generated and studied by EPR, in both liquid a nd frozen n-hexane. 1 and 2 were shown to be very persistent (for 1, t au(1/2)similar to 50 min at 293 K), and their EPR isotropic spectrum i s composed of a large doublet resulting from the phosphorus hyperfine coupling (Ap similar to 2800 MHz). The modulation of the anisotropic p art of the hyperfine phosphorus coupling was shown to be the main rela xation mechanism contributing to the EPR line width. Its contribution was close to 98% at 230 K and decreased to 75% at 330 K. The contribut ion of the exchange coupling between two radicals was negligible at 23 0 K but reached 25% at 330 K. Radicals 1 and 2, strongly influenced th e 1 of the solvent hydrogens, and in n-hexane at 273 K the efficiency factor of 2 was 0.55. Using this value and a model previously describe d for stable nitroxides, the maximum value of the low-field dynamic nu clear polarization factor of an n-hexane solution of 2 was found to be close to 23 x 10(3). This value is approximately 10 times higher than the value measured and calculated for the most efficient nitroxides c urrently used in low-field DNP experiments.