Inversion-Recovery of Nitroxide Spin Labels in Solution and Microheterogeneous Environments

Two modifications of a conventional inversion-recovery experiment which exclude the effect of spectral diffusion on the measured spin−lattice relaxation times of rapidly tumbling nitroxide spin labels are described. In the first approach an almost uniform inversion is achieved by means of specially designed pulse trains with excitation patterns which match the three-line nitroxide ESR spectrum. In the second approach we essentially combine an inversion-recovery for the M I = 0 line with an analog of a pulsed ELDOR technique for M I = ±1 lines in a single experiment. This allows us to reconstruct the recovery of the total magnetization of the ensemble of radicals which is not affected by spectral diffusion. The spin−lattice relaxation times of several nitroxide spin labels in different homogeneous and microheterogeneous environments are measured and compared. The temperature dependence of T 1 times is compared for two solvents, methylcyclohexane and carbon disulfide, which differ in nuclear spin concentration by almost a factor of 1000. The reported experimental evidence suggests that interactions with the nuclear spins of a solvent do not significantly contribute to the spin−lattice relaxation of nitroxide spin labels.