scholarly journals Radiation-Induced Stable Radicals in Calcium Phosphates: Results of Multifrequency EPR, EDNMR, ESEEM, and ENDOR Studies

2021 ◽  
Vol 11 (16) ◽  
pp. 7727 ◽  
Author(s):  
Fadis F. Murzakhanov ◽  
Peter O. Grishin ◽  
Margarita A. Goldberg ◽  
Boris V. Yavkin ◽  
Georgy V. Mamin ◽  
...  
Keyword(s):  
Calcium Phosphates ◽  
Spin Echo ◽  
Double Resonance ◽  
Electron Spin Echo ◽  
Octacalcium Phosphate ◽  
Powder Materials ◽  
Electron Nuclear Double Resonance ◽  
Paramagnetic Resonance ◽  
Radiation Induced ◽  
Synthetic Calcium Phosphate

This article presents the results of a study of radiation-induced defects in various synthetic calcium phosphate (CP) powder materials (hydroxyapatite—HA and octacalcium phosphate—OCP) by electron paramagnetic resonance (EPR) spectroscopy at the X, Q, and W-bands (9, 34, 95 GHz for the microwave frequencies, respectively). Currently, CP materials are widely used in orthopedics and dentistry owing to their high biocompatibility and physico-chemical similarity with human hard tissue. It is shown that in addition to the classical EPR techniques, other experimental approaches such as ELDOR-detected NMR (EDNMR), electron spin echo envelope modulation (ESEEM), and electron-nuclear double resonance (ENDOR) can be used to analyze the electron–nuclear interactions of CP powders. We demonstrated that the value and angular dependence of the quadrupole interaction for 14N nuclei of a nitrate radical can be determined by the EDNMR method at room temperature. The ESEEM technique has allowed for a rapid analysis of the nuclear environment and estimation of the structural positions of radiation-induced centers in various crystal matrices. ENDOR spectra can provide information about the distribution of the nitrate radicals in the OCP structure.

scholarly journals Simulations of the 1 H electron spin echo–electron nuclear double resonance and 2 H electron spin echo envelope modulation spectra of exchangeable hydrogen nuclei coupled to the S 2 –state photosystem II manganese cluster

2002 ◽  
Vol 357 (1426) ◽  
pp. 1359-1366 ◽  
Author(s):  
Constantino P. Aznar ◽  
R. David Britt
Keyword(s):  
Photosystem Ii ◽  
Electron Spin ◽  
Spin Echo ◽  
Double Resonance ◽  
Electron Spin Echo ◽  
Oxygen Evolving Complex ◽  
Electron Nuclear Double Resonance ◽  
Paramagnetic Resonance ◽  
Pulsed Epr ◽  
Envelope Modulation

The pulsed EPR methods of electron spin echo envelope modulation (ESEEM) and electron spin echo–electron nuclear double resonance (ESE–ENDOR) are used to investigate the proximity of exchangeable hydrogens around the paramagnetic S 2 –state Mn cluster of the photosystem II oxygen–evolving complex. Although ESEEM and ESE–ENDOR are both pulsed electron paramagnetic resonance techniques, the specific mechanisms by which nuclear spin transitions are observed are quite different. We are able to generate good simulations of both 1 H ESE–ENDOR and 2 H ESEEM signatures of exchangeable hydrogens at the S 2 –state cluster. The convergence of simulation parameters for both methods provides a high degree of confidence in the simulations. Several exchangeable protons–deuterons with strong dipolar couplings are observed. In the simulations, two of the close (≈2.5 Å) hydrogen nuclei exhibit strong isotropic couplings and are therefore most probably associated with direct substrate ligation to paramagnetic Mn. Another two of the close (≈2.7 Å) hydrogen nuclei show no isotropic couplings and are therefore most probably not contained in Mn ligands. We suggest that these proximal hydrogens may be associated with a Ca 2+ –bound substrate, as indicated in recent mechanistic proposals for O 2 formation.

EPR, ESE and Pulsed ENDOR Study of Nitrogen Related Centers in 4H-SiC Wafers Grown by Different Technologies

2007 ◽  
Vol 556-557 ◽  
pp. 355-358 ◽  
Author(s):  
Ekaterina N. Kalabukhova ◽  
S.N. Lukin ◽  
D.V. Savchenko ◽  
W.C. Mitchel ◽  
Siegmund Greulich-Weber ◽  
...  
Keyword(s):  
Lattice Site ◽  
Spin Echo ◽  
Double Resonance ◽  
Hexagonal Lattice ◽  
Electron Spin Echo ◽  
Epr Spectrum ◽  
Electron Nuclear Double Resonance ◽  
Paramagnetic Resonance ◽  
Hall Effect Measurements ◽  
Electron Paramagnetic

D-band electron paramagnetic resonance (EPR) measurements as well as X and Q-band field-swept Electron Spin Echo (ESE) and pulsed Electron Nuclear Double Resonance (ENDOR) studies were performed on a series of n-type 4H-SiC wafers grown by different techniques including sublimation sandwich method (SSM), physical vapor transport (PVT) and modified Lely method. Depending on the C/Si ratio and the growth temperature the n-type 4H-SiC wafers revealed, besides a triplet due to nitrogen residing on the cubic site (Nc), two nitrogen (N) related EPR spectra with g||=2.0055, g⊥=2.0010 and g||=2.0063, g⊥=2.0005 with different intensities. In the samples with low C/Si ratio the EPR spectrum with g|| =2.0055, g⊥=2.0010 consists of a triplet with low intensity which is tentatively explained as a N-related complex, while in the samples with high C/Si ratio the triplet is transformed into one structureless line of high intensity, which is explained as being due to an exchange interaction between N donors. In the samples grown at low temperature with enhanced carbon concentration the EPR line with g||=2.0063, g⊥=2.0005 and a small hyperfine (hf) interaction dominates the EPR spectrum. It is attributed to N on the hexagonal lattice site. The interpretation of the EPR data is supported by activation energies and donor concentrations obtained from Hall effect measurements for three donor levels in this series of 4H-SiC samples.

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