Applied Magnetic Resonance最新文献

To formulate the relationship between the dextrin hydrate concentration and T1 relaxation rate and create a magnetic resonance imaging (MRI) phantom with an arbitrary T1 value. Dextrin solution with nine different concentrations was prepared by dissolving 0–20 g (2.5 g increments) of dextrin in 25 g of purified water. The T1 values of the phantoms were measured using a 1.5 T MR scanner, and the relationship between the R1 value and dextrin concentration was regressed using linear and quadratic equations. Phantoms with concentrations adjusted to T1 values of 500, 1000, and 1500 ms were created from each regression equation, and the errors between the measured T1 and target values were evaluated. In addition, the temporal changes in the T1 and T2 values of the phantoms were also evaluated. The T1 and T2 values ranged from 367.4 ± 14.1 to 2577.6 ± 76.5 ms and 20.0 ± 0.9 to 1805.3 ± 8.3 ms, respectively. The linear and quadratic regression equations were (y=2.9631x+0.2043) and (y=1.8295{x}^{2}+1.4995x+0.37), with coefficients of determination of 0.9763 and 0.9954, respectively. The maximum errors were 12.3% and 2.1% for the linear and quadratic equations, respectively. The T1 value was maintained at a fluctuation rate of approximately 10% during the first 4 weeks. The T2 value decreased by approximately 20% after 4 weeks. MRI phantoms with arbitrary T1 values in the range of 500–1500 ms with an error within 2.1% can be created using dextrin, which can be used as human tissue-equivalent MRI phantoms for T1 of the grey or white matter of the brain, liver, pancreas, spleen, and prostate.

The dynamics of the photogenerated triplet electron spin polarization (ESP) of two perylenebisimide (PBI) derivatives, PBI-2Br and PBI-Br-NH, is studied by the time-resolved electron paramagnetic resonance (TREPR). The ESP inversion in the triplet TREPR spectra of two compounds is detected at delays after the laser pulse (DAF) of ~ 500–900 ns. A fairly efficient algorithm for the exact numerical solution of the equation of motion of the spin density matrix of an ensemble of photoexcited triplets taking into account the anisotropy of the decay rates of sublevels of the triplet state, spin–lattice and spin–spin relaxations is elaborated in order to analyze the ESP inversion of TREPR spectra. The calculated time evolution of the TREPR spectra and its fitting experimental ones shows that even a slight difference in decay rates for the triplet sublevels of PBI-2Br results in the ESP inversion in the TREPR spectra already at DAF of ~ 500 ns. At the same time, decay rates ( τ_X⁻¹ = 4 µs⁻¹; τ_Y⁻¹ = 4.4 µs⁻¹; τ_Z⁻¹ = 3.3 µs⁻¹) are higher than those for the available examples of the ESP inversion of the triplet TREPR spectra. The replacement of a Br atom by NH leads to noticeable changes in the photoinduced properties of the triplet including decay rates (τ_X⁻¹ = 1.25 µs⁻¹; τ_Y⁻¹ = 0.125 µs⁻¹; τ_Z⁻¹ = 0.125 µs⁻¹). PBI-Br-NH is characterized by an axial decay rate anisotropy with τ_x⁻¹, which is ten times higher than τ_Y⁻¹ and τ_Z⁻¹. The trend in the triplet lifetime change obtained from the TREPR spectra is consistent with the data of the nanosecond transient absorption spectra.

In recent years, the theory of the effect of saturation of EPR spectra of free radicals undergoing spin exchange has been extended to spin exchange frequencies where “peculiar” behavior occurs. In a paper by Salikhov (Appl. Magn. Reson. (2021) 52:1063–1091), analytic expressions were developed that predict the dependence of measurable EPR parameters on the exchange frequencies and the microwave field strength. This work is an experimental test of that theory where, in principle, there are no adjustable parameters.

The relaxation times (T_2^{prime}), characterizing the allowed, and (T_2^{^{primeprime}} ,) characterizing the forbidden and coherence cross-resonances, during free evolutions over the coherent pathways p = ± 1 are estimated from the SECSY (spin echo correlation spectroscopy) signal of an electron–nuclear spin-coupled system in a γ-irradiated malonic acid single crystal. This is accomplished by fitting the intensities of the two main peaks in the Fourier transform of the SECSY signal reported by Lee et al. (J Chem Phys 98:3665, 1993) to (T_2^{prime}) and (T_2^{^{primeprime}}). The values of the fitted relaxation times (T_2^{prime} = 500 {text{ns}}, T_2^{^{primeprime}} = 50,000 {text{ns}}) reveal that the relaxation via the allowed resonance is two orders of magnitude faster than that via the forbidden and coherence cross-resonances. Full details of the calculation exploiting the Liouville–von Neumann equation are presented.

In this study, electrochemical processes in a Li|LiPF₆|LFP cell have been explored applying advanced solid-state NMR technologies. In situ solid-state NMR allows to monitor structural changes in local environments in commercially available cell components during galvanostatic cycling. In collaboration with Dragonfly Energy, ePROBE GmbH and Bruker BioSpin GmbH & Co. KG, we have demonstrated an experimental procedure for routine application of in situ solid-state NMR for battery research. This points out the high potential of this approach for use in the energy storage industry.

Lipid As are the main components of the external leaflet of the outer membrane of Gram-negative bacteria. Their molecular structure has evolved to allow the bacteria survival in specific environments. In the present work, we investigate how and to what extent lipid membranes that include in their composition lipid A molecules of a bacterium of the gut microbiota, Stenotrophomonas maltophilia, differ from those formed by the lipid A of the common Gram-negative bacterium Salmonella enterica, which is not specific to the gut and is here used as a reference. Electron Paramagnetic Resonance (EPR) spectroscopy, using spin-labelled lipids as molecular probes, allows the segmental order of the acyl chain and the polarity across the bilayer to be analyzed in detail. Both considered lipid As cause a stiffening of the outermost segments of the acyl chains. This effect increases with increasing the lipid A content and is stronger for the lipid A extracted from Stenotrophomonas maltophilia than for that extracted from Salmonella enterica. At the same time, the local polarity of the bilayer region just below the interface increases. As the inner core of the bilayer is considered, it is found that the lipid A from Salmonella enterica causes a local disorder and a significant reduction of the local polarity, an effect not found for the lipid A from Stenotrophomonas maltophilia. These results are interpreted in terms of the different lengths and distributions of the acyl tails in the two lipid As. It can be concluded that the symmetrically distributed short tails of the lipid A from Stenotrophomonas maltophilia favors a regular packing within the bilayer.

Silver-modified zeolites have demonstrated promising properties for hydrocarbon feedstock valorization and the oxidation of organic compounds. Within the frame of the scientific interest for selective oxidation of light alkenes on metal-modified zeolites, the transformation of propene on Ag/H-ZSM-5 zeolite in the presence of molecular oxygen has been monitored by ¹³C (CP) MAS NMR spectroscopy at 296–773 K. Silver-allyl intermediate species have been detected by ¹³C CP/MAS NMR, indicating that Ag⁺ sites are involved in propene transformation. It is assumed that oxygen and propene compete for the Ag⁺ adsorption sites, thus prompting propene adsorption on zeolite Brønsted acid sites (BAS) and its further transformation by oligomerization and conjunct polymerization. Among the products of propene oxidation by molecular oxygen, only carbon dioxide has been detected by ¹³C MAS NMR, implying high activity of Ag/H-ZSM-5 in alkene total oxidation. It is thus inferred that silver-modified zeolite is not a promising catalyst for propene selective oxidation to propene oxide and carbonyl group containing organics.

The study presents an analysis of free induction decay (FID) in the ensemble of ring spin clusters using the formalism of collective modes similar to the spin waves. A dispersion relation for the waves is obtained. FID from a spin cluster is estimated and averaged over system parameters to simulate the signal given by a spherical structure or by an ensemble of rings withrandom orientation. There exists a fast relaxation according to the Gaussian law in the initial time interval. After T₂, the signal decays as a power function which matches the direct numerical simulations of the signal from the ring clusters.