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

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.

Molecular dynamics in polystyrene from electron spin resonance (ESR) measurements: comparison between spinprobes and ‐labels attached to the chain ends

“…g-tensor: g, = 2.0093, g, = 2.0058, g, = 2.0023 hyperfine tensor: A , = 7.2, A , = 7.2, A , = 47.7 (all values in G) The value A,, = 47.0 G from ref. 13) had to be corrected slightly in order to give the same extrema separation in the rigid limit (100 K) for the simulated and recorded spectra.…”

Librational dynamics of nitroxide molecules in a molecular glass studied by echo-detected EPR

“…Therefore it may be attributed to high-frequency librations. Magnetization transfer experiments on the basis of the rapid stepping of the magnetic field also indicate molecular librations [11][12][13]. This kind of experiments has shown that large-amplitude orientational motion in glasses and supercooled liquids is slow, with a characteristic time larger than 10-5 s [14,15].…”

CW EPR, echo-detected EPR, and field-step ELDOR study of molecular motions of nitroxides ino-terphenyl glass: Dynamical transition, dynamical heterogeneity and β-relaxation

“…Experiments with phenyl-imidasoline embedded into a crystalline o-terphenyl state gave the same kinetics (data not shown). MT normally is related with molecular motion [15][16][17][18][19][20]. As molecular motion is expected to depend substantially on the physical phase state of the matrix, we conclude that the time kinetics seen in Fig.…”

“…Different approaches in EPR were employed: continuous-wave (CW) EPR [1][2][3][4], including high-field CW EPR [5,6], echo-detected (ED) EPR [7][8][9][10][11][12], stimulated echo decay [13,14], field-step electron-electron double resonance (FS ELDOR) [15][16][17][18][19][20], pulsed two-dimensional EPR [21], and some others. EPR allows addressing the important issues of molecular dynamics 638 s.A. Dzuba et al in glasses, such as the mechanism of motion of individual guest molecules, the cooperative nature of motion, temperature dependence of motion, the relation to glass transition, the distribution of correlation times, ~tynamical heterogeneity and others.…”