S. Marque et al., PERSISTENT SILYLATED PHOSPHORANYL RADICALS - APPLICATION TO DYNAMIC NUCLEAR-POLARIZATION, The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory, 101(31), 1997, pp. 5640-5645
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.