Temperature Dependence of the Proton Overhauser DNP Enhancements on Aqueous Solutions of Fremy's Salt Measured in a Magnetic Field of 9.2 T

The temperature dependence of the water-proton dynamic nuclear polarization (DNP) enhancement from Fremy's salt nitroxide radicals was measured in a magnetic field of 9.2 T (corresponding to 260 GHz microwave (MW) and 392 MHz NMR frequencies) in the temperature range of 15-65 °C. The temperature could be determined directly from the proton NMR line shift of the sample. Very high DNP enhancements of -38 (signal integral) or -81 (peak intensity) could be achieved with a high-power gyrotron MW source. The experimental findings are compared with classical Overhauser theory for liquids, which is based on the translational and rotational motion of the molecules and with molecular dynamics calculations of the coupling factor. © 2012 Springer-Verlag

A DFT, X- and W-band EPR and ENDOR study of nitrogen-centered species in (Nano)hydroxyapatite

© Springer-Verlag Wien 2014. Incorporation of the nitrogen-containing impurities in hydroxyapatite (HAp) powders with the sizes of the crystallites of (20–50) nm was studied using first-principles modeling combined with the multi-frequency (9 and 94 GHz) electron paramagnetic resonance (EPR) methods. It is shown that the observed EPR spectra are undoubtedly due to the presence of the bulk radiation-induced NO3 2- radicals. This conclusion is based on spin-polarized density functional theory calculations of spectroscopic parameters within gauge-including projector augmented wave framework followed by the exact comparison of the simulated EPR and electron–nuclear double resonance spectra with the experimental findings. In addition, a comprehensive analysis of the simulated properties allows us to suggest that the paramagnetic centers preferably occupy PO4 3- sites in the HAp structure

Phonon Spectrum in Hydroxyapatite: Calculations and EPR Study at Low Temperatures

© 2015 Springer Science+Business Media New York Density functional theory-based calculations within the framework of the plane-wave pseudopotential approach are carried out to define the phonon spectrum of hydroxyapatite (Formula presented.) (HAp). It allows to describe the temperature dependence of the electronic spin-lattice relaxation time (Formula presented.) of the radiation-induced stable radical (Formula presented.) in HAp, which was measured in X-band (9 GHz, magnetic field strength of 0.34 T) in the temperature range T = (10–300) K. It is shown that the temperature behavior of (Formula presented.) at (Formula presented.) 20 K can be fitted via two-phonon Raman type processes with the Debye temperature (Formula presented.) evaluated from the phonon spectrum

Phonon Spectrum in Hydroxyapatite: Calculations and EPR Study at Low Temperatures

© 2015, Springer Science+Business Media New York.Density functional theory-based calculations within the framework of the plane-wave pseudopotential approach are carried out to define the phonon spectrum of hydroxyapatite Ca10(PO4)6(OH)2 (HAp). It allows to describe the temperature dependence of the electronic spin-lattice relaxation time T 1e of the radiation-induced stable radical NO32- in HAp, which was measured in X-band (9 GHz, magnetic field strength of 0.34 T) in the temperature range T = (10–300) K. It is shown that the temperature behavior of T1e at T> 20 K can be fitted via two-phonon Raman type processes with the Debye temperature ΘD≈280K evaluated from the phonon spectrum

Combination of EPR measurements and DFT calculations to study nitrate impurities in the carbonated nanohydroxyapatite

We demonstrate the application of the combined experimental-computational approach for studying the anionic impurities in hydroxyapatite (HAp). Influence of the carbonation level (x) on the concentration of the NO3 2- radicals in the HAp nanocrystals of Ca10-xNa x(PO4)6-x(CO3)x(OH) 2 with x in the range 0 < x < 2 and average sizes of 30 nm is investigated by different analytical methods including electron paramagnetic resonance (EPR). Stable NO3 2- radicals are formed under X-ray irradiation of nano-HAp samples from NO3 - ions incorporated in trace amounts during the wet synthesis process. Density functional theory (DFT) based calculations show energetic preference for the PO4 group substitution by NO3 - ions. Comparison of the calculated and experimental spectroscopic parameters (g and hyperfine tensors) reveals that EPR detects the NO3 2- radicals located in the positions of the PO4 group only. It is shown that with the increase in x, the carbonate ions substitute the NO3 2-/NO3 - ions. DFT calculations confirm that carbonate incorporation in HAp structure is energetically more favorable than the formation of the nitrate defect. © 2014 American Chemical Society

In Situ Identification of Various Structural Features of Vanadyl Porphyrins in Crude Oil by High-Field (3.4 T) Electron-Nuclear Double Resonance Spectroscopy Combined with Density Functional Theory Calculations

© 2017 American Chemical Society.Structural characterization of metalloporphyrins in complex systems, such as native hydrocarbons, has been the focus of scientific and industrial interests for many years. We describe electron-nuclear double resonance (ENDOR) of crude oil from the well without any additional sample treatment (i.e., in the native environment) in the magnetic field of about 3.4 T and temperature of 50 K by applying microwave pulses at 94 GHz (W band) and radio frequency pulses at near the proton Larmor frequencies of 144 MHz to probe the paramagnetic vanadyls. By means of density functional theory calculations, ENDOR features are explained and ascribed to certain vanadyl porhyrin structural forms known to be present in crude oil

Electron paramagnetic resonance studies of asphaltenes complexes in heavy oils and bitumen

The present paper is focused on the application of some of the multifrequency (9.5 and 94 GHz) continuous wave and pulsed EPR techniques to study the dynamics and structure of asphaltenes and vanadyl complexes from the crude oils and bitumen at near room temperature. The features of the observation of EPR in these systems at high frequencies are pointed out. Longitudinal and transverse relaxation times of asphaltenes and vanadyl complexes are measured. Usage of the data obtained for DNP of protons of crude oils and bitumen is discussed

Study of Organic Self-Assembled Nanosystems by Means of High-Frequency ESR/ENDOR: The Case of Oil Asphaltenes

© 2018, Pleiades Publishing, Ltd. An approach to investigate self-assembly of oil disperse systems based on high-field electron nuclear double resonance has been suggested. As exemplified by asphaltenes, the oil components most prone to self-assembly, the formation of planar rather than multilayer structures has been directly evidenced experimentally. The suggested method can be applied to elucidate the self-assembly mechanism in wide range of organic objects

The Interplay of manganese and nitrate in hydroxyapatite nanoparticles as revealed by pulsed EPR and DFT

© the Owner Societies 2015. The interplay of oppositely charged substitutions in the structure of hydroxyapatite (HAp) nanopowders is investigated on the atomic level by pulsed electron paramagnetic resonance (EPR) technique and ab initio density functional theory calculations. Benefits of EPR to determine Mn2+ ions in nano-HAp samples are demonstrated. A simple approach based on the measurements of electron spin relaxation times allowed observing the strong influence of fast-relaxing Mn2+ ions on the relaxation characteristics of the nitrate ions (NO3-/NO32-) incorporated in trace amounts. Based on the results of ab initio calculations, we show the propensity of Mn2+ and NO3-/NO32- to associate within the HAp crystal lattice. This could have a direct impact on the functional properties of the material especially to resorption and ion exchange. Furthermore, such an effect can increase a propensity of undesired impurities to incorporate into the doped nanocrystals