ESR investigation of chemical exchange in geminally diphosphorylated linear nitroxide radicals

For geminally diphosphorylated linear nitroxide radicals, a marked line width alternation (LWA) can be observed. The LWA appears irrespectively whether the investigated radicals R"N(O-.)C(P(O)(OEt)(2))(2)R' contain any chiral group or not. If R" = tert-butyl and R' = H, the small P-hydrogen coupling indicates a completely blocked rotation around the N-C sigma-bond and the LWA can be assigned to a chemical exchange between conformations in which the phosphoryl groups have a symmetric and a nonsymmetric geometry. The conformational change is accompanied by the deformation of the CP2 bonding angle and the rate of exchange is slowed in pentane for which the solvent molecules can be trapped by the chelating phosphoryl groups. If R" = benzyl, both the proton hyperfine lines of the CH2 group and the phosphorus lines of the CP2 group produce a LWA; two coalescences can be observed. In the case of a chiral R" group (R" = secondary butyl, R' = methyl) a tentative four-site model can explain the highly complex LWA