ENDOR-Induced EPR of Disordered Systems: Application to X‑Irradiated Alanine

The electron paramagnetic resonance (EPR) spectra of radiation-induced radicals in organic solids are generally composed of multiple components that largely overlap due to their similar weak <i>g</i> anisotropy and a large number of hyperfine (HF) interactions. Such properties make these systems difficult to study using standard cw EPR spectroscopy even in single crystals. Electron–nuclear double-resonance (ENDOR) spectroscopy is a powerful and widely used complementary technique. In particular, ENDOR-induced EPR (EIE) experiments are useful for separating the overlapping contributions. In the present work, these techniques were employed to study the EPR spectrum of stable radicals in X-irradiated alanine, which is widely used in dosimetric applications. The principal values of all major proton HF interactions of the dominant radicals were determined by analyzing the magnetic field dependence of the ENDOR spectrum at 50 K, where the rotation of methyl groups is frozen. Accurate simulations of the EPR spectrum were performed after the major components were separated using an EIE analysis. As a result, new evidence in favor of the model of the second dominant radical was obtained

Dominant Stable Radicals in Irradiated Sucrose: <b>g</b> Tensors and Contribution to the Powder Electron Paramagnetic Resonance Spectrum

Ionizing radiation induces a composite, multiline electron paramagnetic resonance (EPR) spectrum in sucrose, that is stable at room temperature and whose intensity is indicative of the radiation dose. Recently, the three radicals which dominate this spectrum were identified and their proton hyperfine tensors were accurately determined. Understanding the powder EPR spectrum of irradiated sucrose, however, also requires an accurate knowledge of the <b>g</b> tensors of these radicals. We extracted these tensors from angular dependent electron nuclear double resonance-induced EPR measurements at 110 K and 34 GHz. Powder spectrum simulations using this completed set of spin Hamiltonian parameters are in good agreement with experimentally recorded spectra in a wide temperature and frequency range. However, as-yet nonidentified radicals also contribute to the EPR spectra of irradiated sucrose in a non-negligible way

<i>In Situ</i> Electron Paramagnetic Resonance and X‑ray Diffraction Monitoring of Temperature-Induced Breathing and Related Structural Transformations in Activated V‑Doped MIL-53(Al)

The metal–organic framework MIL-53­(Al) is characterized by a distinct reversible structural transition between a narrow pore (NP) and a large pore (LP) state, resulting in expansion or contraction of this three-dimensional porous framework also called breathing. This transition is studied for vanadium-doped MIL-53­(Al), induced by temperature (<i>T</i>) using <i>in situ</i> electron paramagnetic resonance (EPR) and X-ray diffraction (XRD) in air and in vacuum. The EPR active V^IVO molecular ions are used as local probes to detect the NP to LP transitions. The EPR spectra of V^IVO embedded in the NP and LP MIL-53­(Al) states are clearly distinguishable. The temperature-dependent EPR and XRD data can consistently be interpreted in terms of <i>T</i>-ranges in the experiments where one of the states is predominantly present and a narrow <i>T</i>-range in which the two states coexist. In addition the XRD data indicate that the NP state undergoes a transition to a metastable state characterized by different lattice parameters than the NP state at room temperature, before the transition to the LP state occurs. The EPR spectra, however, show that only in the LP state the V^IVO ions can exhibit an interaction with paramagnetic O₂ molecules from air