2-DIMENSIONAL ELECTRON-PARAMAGNETIC-RESONANCE SPECTROSCOPY OF NITROXIDES - ELUCIDATION OF RESTRICTED MOLECULAR MOTIONS IN GLASSY SOLIDS

The combination of concepts of two-dimensional (2D) spectroscopy with the well-known held step electron-electron double resonance (ELDOR) me thod offers a practical route to recording 2D ELDOR spectra covering t he full spectral range needed for electron paramagnetic resonance (EPR ) of nitroxide spin labels in the solid state. The 2D ELDOR pattern pr ovides information about molecular reorientation measured in real time , the anisotropies of electron phase, and electron spin-lattice relaxa tion as well as nuclear spin-lattice relaxation all of which are conne cted 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 efficien t algorithm for simulating 2D ELDOR spectra is derived, based on analy tical solutions of;the spin relaxation behavior for small-angle fluctu ations and offers-a means of quantitatively analyzing experimental dat a. As an example, the motion of nitroxide spin labels in a liquid-crys talline side-group polymer well below its glass transition is determin ed as a beta-relaxation process with a mean;angular amplitude of 5 deg rees and a distribution of correlation times with a mean correlation t ime of 0.9 X 10(-10) s and a width of 2.5 decades.