Fast orientational-motional correlation time measurements from ESR hyperfine spectra of perdeuterated nitroxides with 15N (I=1/2)

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Abstract
A simple, direct approach is presented for determining fast orientational-motional correlation times τ associated with motionally narrowed electron-spin-resonance (ESR) hyperfine spectra of perdeuterated nitroxide radicals enriched with 15N (I=1/2). This approach is based on (1) the experimental determination of net dynamic line broadening by taking the difference in linewidth between the M=+1/2 and M=−1/2 nitroxide hyperfine components, (2) the Kivelson expression for fast tumbling motional ESR hyperfine linewidth, (3) a simple, effective method of treating the problem of experimental nitroxide ESR hyperfine line shapes consisting of Gaussian and Lorentzian components, and (4) the spin-Hamiltonian constants adjusted for fast tumbling nitroxide radicals at high temperature from their rigid limit values at low temperatures. It is shown that the present method yields more reliable and accurate experimental τ values than those determined by conventional methods for fast tumbling nitroxide radicals. Detailed experimental procedures are described for the case of poly(ethylene oxide) polymer containing perdeuterated and 15N-enriched TANOL (TANOLD-15) spin probes.