Two-dimensional electron paramagnetic resonance spectroscopy of nitroxides: Elucidation of restricted molecular motions in glassy solids

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Abstract
The combination of concepts of two‐dimensional (2D) spectroscopy with the well‐known field step electron–electron double resonance (ELDOR) method offers a practical route to recording 2D ELDOR spectra covering the full spectral range needed for electron paramagnetic resonance (EPR) of nitroxide spin labels in the solid state. The 2D ELDOR pattern provides information about molecular reorientation measured in real time, the anisotropies of electron phase, and electron spin‐lattice relaxation as well as nuclear spin‐lattice relaxation all of which are connected with the detailed geometry of the molecular reorientation. Thus, in 2D ELDOR the same electron spin probes the motional behavior over a wide range of correlation times from 10−4 to 10−12 s. An efficient algorithm for simulating 2D ELDOR spectra is derived, based on analytical solutions of the spin relaxation behavior for small‐angle fluctuations and offers a means of quantitatively analyzing experimental data. As an example, the motion of nitroxide spin labels in a liquid‐crystalline side‐group polymer well below its glass transition is determined as a β‐relaxation process with a mean angular amplitude of 5° and a distribution of correlation times with a mean correlation time of 0.9×10−10 s and a width of 2.5 decades.