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EPR Line Shifts and Line Shape Changes Due to Spin Exchange of Nitroxide Free Radicals in Liquids 2. Extension to High Spin Exchange Frequencies and Inhomogeneously Broadened Spectra
The work in part 1 of this series (J. Phys. Chem. B. 1997, 101, 8707) is extended experimentally and theoretically to include inhomogeneously broadened nitroxide spectra and high spin exchange frequencies. The nitroxide spin probe 16-doxylstearic acid methyl ester, which is severely inhomogeneously broadened by unresolved hyperfine structure in the absence of spin exchange, is studied under conditions in which the spin probe undergoes spin exchange varying from slow exchange to high exchange leading to collapse of the three 14N hyperfine lines into a single line. Over the entire spin exchange frequency range, theoretically predicted spectra are described essentially perfectly by the sum of three absorption and two “dispersion” lines of Lorentzian shape. The dispersion components are of opposite signs for the outer lines and zero for the inner. The intensities and the line widths of the outer absorption lines are different from the center line. Theoretically, adding unresolved hyperfine structure which inhomogeneously broadens the lines, has no effect on the line widths or intensities of the absorption lines, the intensity of the dispersion lines, or the line shifts. Comparing theory with experiment shows that the line widths, intensities, and line shapes are in excellent accord, whereas the line shifts are not; experimental lines move toward one another faster than predicted by theory.