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Reconciling Structural and Spectroscopic Fingerprints of the Oxygen Evolving Complex of Photosystem II: A Computational Study of the S <sub/>2</sub> State
摘要： The catalytic cycle of photosynthetic water oxidation occurs at the Mn4CaO5 oxygen-evolving complex (OEC) of Photosystem II (PSII). Extensive spectroscopic data have been collected on the intermediates, especially the S2 (Kok) state, although the proton and electron inventories (Mn oxidation states) are still uncertain. The “high-oxidation” paradigm, assigns S2 Mn oxidation level (III, IV, IV, IV) or (IV, IV, IV, III), whereas a “low-oxidation” paradigm posits two additional electrons. Here we investigate the geometric (XRD, EXAFS) and spectroscopic (EPR, ENDOR) properties of the S2 state using quantum chemical DFT calculations, focusing on the neglected low paradigm. Two interconvertible electronic spin configurations are predicted as ground states, producing multiline (S = 1/2) and broad (S = 5/2) EPR signals in the low paradigm oxidation state (III, IV, III, III) and with W2 as OH– and O5 as OH–. They have “open” (S = 5/2) and “closed” (S = 1/2) Mn3CaO4-cubane geometries. Other energetically accessible isomers with ground spin states 1/2, 7/2, 9/2, or 11/2 can be obtained through perturbations of hydrogen-bonding networks (e.g. H+ from His337 to O3 or W2), consistent with experimental observations. Conformers with the low oxidation state configuration (III, IV, IV, II) also become energetically accessible when the protonation state is O5 (OH–), W2 (H2O) and neutral His337. The configuration with (III, IV, III, III) agrees well with earlier low temperature EPR and ENDOR interpretations, while the MnII-containing configuration agrees partially with recent ENDOR data. However, the low-oxidation paradigm does not yield isotropic ligand hyperfine interactions in good agreement with observed values. We conclude that the low Mn oxidation state proposal for the OEC can closely fit most of the available structural and electronic data for S2 at accessible energies.
关键词： EPR,low-oxidation paradigm,oxygen-evolving complex,ENDOR,Photosystem II,DFT calculations,S2 state
更新于2025-09-10 09:29:36
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Silver polymer complexes as precursors of nanocomposites based on polymers of 1-vinyl-1,2,4-triazole
摘要： Bioactive silver-containing (Ag content of about 6.8-8.2 wt %) polymer complexes based on the synthetic homopolymer of 1-vinyl-1,2,4-triazole and its copolymers have been synthesized and characterized. Structural features of macromolecular complexes were studied by magnetic resonance techniques (EPR, NMR) and other analytical methods (FTIR, SEM, UV-vis, atomic absorption analysis, etc.). The spatial structure of the polymer chain fragment and silver ion/1-vinyl-1,2,4-triazole complexes were investigated by quantum-chemical calculations. The formation of coordinatively saturated polymer complexes of Ag(I) and highly oxidized forms of Ag(II) silver was analyzed. The sites of ions localization and reduced metallic silver in the structure of the polymer matrix were determined.
关键词： Complex,EPR,Polymer,Silver,1-Vinyl-1,2,4-triazole,NMR
更新于2025-09-09 09:28:46
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Nuclear and Electron Magnetic Resonance Spectroscopies of Atomically Precise Gold Nanoclusters
摘要： Atomically precise gold nanoclusters display properties that are unseen in larger nanoparticles. When the number of gold atoms is sufficiently small, the clusters exhibit molecular properties. Their study requires extensive use of classic molecular physical chemistry and, thus, methods such as theory and vibrational spectroscopies, electrochemistry, density functional molecular dynamics calculations, and of course nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectroscopies. NMR and EPR studies have been mostly carried out on the benchmark, stable molecules Au25(SR)18, Au38(SR)24, Au102(SR)44, and Au144(SR)60 (where SR = thiolate). In this Account, we showcase examples primarily taken from our previous and ongoing NMR and EPR studies, which we hope will trigger further interest in the use of these sensitive, though often underutilized, techniques. Indeed, 1D and 2D NMR spectra of pure, atomically precise clusters can be very detailed and informative. Molecular clusters are molecules and, thus, have discrete energy levels and undergo stepwise oxidation or reduction. The effect of the charge state on the chemical shifts and line shapes is a function of the ligand type (ligands differ due to specific bonds with different Au atom types) and the position of the chemical group along the ligand backbone: for groups near the Au core, they can be very dramatic. Ligand-protected gold clusters are hard?soft molecules where a hard metal core is surrounded by a dynamic molecular layer. The latter provides a nanoenvironment that interfaces the cluster core with the surrounding environment and can be permeated by molecules and ions. NMR spectroscopy is especially useful to assess its structure. For example, the data show that whereas long alkanethiolates form bundles, shorter chains exhibit more conformational freedom and are quite folded. NMR spectroscopy allows studying diastereotopic effects and provides information on possible hydrogen bonds of ligands with sulfur or surface gold atoms. EPR spectroscopy is a very precise technique to check and characterize the magnetic state of gold clusters or clusters doped with foreign-metal atoms. Electron nuclear double resonance (ENDOR) provides a powerful tool to assess the interaction of an unpaired electron with nuclei, as we showed for 197Au and 1H. It can be used as a sensitive probe of the spin-density distribution in nanoclusters: for example, it showed that the singly occupied molecular orbital may span outside the Au core by nearly 6 ?. Solid-state EPR spectroscopy has provided compelling evidence that the specific ligands and the crystallinity degree are very important factors in determining the interactions between clusters in the solid state. Depending on the condition, paramagnetic, superparamagnetic, ferromagnetic, or antiferromagnetic behavior can be observed. Time-resolved EPR was successfully tested to determine the efficiency of singlet-oxygen generation via sensitization of Au25 clusters. This Account thus demonstrates some of the remarkable insights that can be gained into the properties of atomically precise clusters through detailed NMR and EPR studies.
关键词： EPR,gold nanoclusters,molecular properties,atomically precise,NMR
更新于2025-09-09 09:28:46
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Cr3+ Doped Al2O3 Obtained by Non-Hydrolytic Sol-Gel Methodology
摘要： This paper reports the synthesis and characterization of Cr3+-doped alumina by the sol-gel non-hydrolytic methodology. The resulting sample was treated at different temperatures. X-ray diffraction revealed that the ruby phase emerged in the sample treated at 1100 oC, which was later confirmed by absorption bands correspondent to Cr3+ ions allowed transitions at 4A2 → 4T1, 4T2, and forbidden at 4A2 → 2T1, 2E, observed by diffuse reflectance UV-Vis. The luminescence spectroscopy showed the intensity band at 694 nm in red region, characterized of the Cr3+ ion. The peaks at 702 and 705 nm correspond to N1 and N2 lines, respectively, which arose from the second and fourth nearest-neighbor exchange-coupled pairs of chromium(III) ion, respectively, ascribed to high chromium(III) concentration. The Cr3+ cluster formation was observed in electron paramagnetic resonance signal as discussed in this work. Nuclear magnetic resonance evidenced that the 27Al symmetry changed in the samples treated between 900 and 1100 oC.
关键词： EPR,NMR,sol-gel,luminescence,ruby
更新于2025-09-09 09:28:46
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The effects of 150?kGy dose γ rays on Nd3+ doped lead fluoroborate glasses
摘要： Authors aim to make a study on gamma (γ) rays effects over inorganic glassy materials. The Nd3+ content as well as high energetic induced radiation in the glass compositions resulted in the B-O-B bonds depolymerization creating NBOs (Non-Bridging Oxygens) in the matrices, interpreted through FTIR (Fourier Transform Infrared) spectra. The decrease in the Tg, Hr and H values obtained from Differential Scanning Calorimetry (DSC) thermograms and Vickers Microhardness Tester respectively, proved the changes in the glass structure after irradiation. The drastic decrease in the intensities of UV-VIS absorption and Photoluminescence (PL) emission peaks after irradiation, prompted the creation of defect points in the samples. The JO (Judd-Ofelt) theory was implemented to understand the asymmetric and covalent nature of the matrix and also to predict the radiative properties. The EPR (Electron Paramagnetic Resonance) spectra proved the presence of defects in the samples both pre and post irradiation. These changes were correlated with the Nd (Neodymium) and O (Oxygen) concentration ratios, studied through EDS (Energy-Dispersive X-ray spectroscopy), which showed decrease in the elemental composition post irradiation. The UV-VIS differential absorption spectra of the samples after γ irradiation depicts defects in the glass network with feasibility in valency change from Nd3+ to Nd2+ and decrease in Nd3+ ions number density after irradiation. Dipole-Dipole interactions within the ions were analyzed through lifetime decay measurement. 1.0 mol% of Nd3+ ions showed better radiation resistant nature and quality optical properties when compared to other samples both before and after γ rays interactions.
关键词： Gamma rays,EPR,Nd3+,PL,JO
更新于2025-09-04 15:30:14
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Shoreline analysis using Landsat-8 satellite image
摘要： The shoreline is a boundary between wet and dry part of the beach, and it is dynamic in nature. Natural and human factors are always influencing shoreline configuration. One of the important natural events which are responsible for the shoreline configuration along the Karnataka coast is southwest monsoon, and hence there is a change in shoreline position between pre- and post-monsoon. For the present research work, Karwar beach with two beaches, Rabindranath Tagore beach and Devabagh beach along Karnataka coast, West coast of India were selected as study area. Landsat-8 satellite images for the years 2013–2017 were used in the present study and processed for May and October of every year, using ERDAS imagine 2014 and ArcGIS 10.3 tools to generate shoreline configuration maps. Finally, the comparison was made between 2013 and 2017 years, and the results indicate that the Devbagh beach during pre-monsoon season has an average shoreline change rate of ?7.54 m/yr (EPR) and ?5.57 m/yr (LRR) and during post-monsoon season it is 0.34 m/yr (EPR) and ?0.46 m/yr (LRR). Similarly, Rabindranath Tagore beach during pre-monsoon seasons has an average shoreline change rate of 0.004 m/yr (EPR) and 1.67 m/yr (LRR), and in post-monsoon season, it is ?5.77 m/yr (EPR) and ?6.55 m/yr (LRR) respectively. The total uncertainty error was estimated and found to be (cid:1) 5.00 m/yr.
关键词： shoreline change,ArcGIS,EPR and LRR,ERDAS imagine,Landsat-8
更新于2025-09-04 15:30:14
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AIP Conference Proceedings [Author(s) SolarPACES 2017: International Conference on Concentrating Solar Power and Chemical Energy Systems - Santiago, Chile (26–29 September 2017)] - Investigation of molecular and radical structures of ethylene glycol
摘要： In this study, the conformational variety of ethylene glycol molecule was theoretically searched via energy minimization calculations carried out by using the Merck Molecular Force Field (MMFF) and eight stable conformers were determined for the free molecule. For each of the found conformers, a geometry optimization calculation was performed by using the DFT/B3LYP method and 6-311++G(d,p) basis set. From these calculations some structural data of the conformers, such as optimized geometry, relative energy and dipol moment, were obtained and then they were used in evaluation of the stabilities of the investigated conformers. In the next step of the study, theoretically possible forteen radicals were derived from the most stable conformer of free ethylene glycol and geometry optimization was also performed for each of them. Afterwards, the “Electron Paramagnetic Resonance (EPR)” parameters of these radicals were calculated using DFT/B3LYP method and TZVP basis set and then the obtained values were compared to the corresponding experimental data reported in the literature. The results have demonstrated that the experimentally observed EPR parameters match the theoretical EPR values obtained for the model radical called here “Rad 5” (?OHCH2OH). The model “Rad 5” is an anion radical formed by abstraction of an hydrogen atom from the neutral free molecule.
关键词： DFT,EPR parameters,radical structures,ethylene glycol,molecular structure
更新于2025-09-04 15:30:14