Al quadrupole interaction in zeolites loaded with probe molecules—a quantum-chemical study of trends in electric field gradients and chemical bonds in clusters

The electric field gradient (EFG) has been calculated in zeolite clusters at the aluminium site surrounded by four SiO₄ tetrahedra. Density functional theory (DFT) with the 6-31G^{* *} basis set has been employed. Formation of a Brønsted acid site by protonation of one oxygen atom of the A1O₄ tetrahedron perturbs the coordination of aluminium, i.e., the corresponding Al-O bond is considerably weaker than in the unprotonated case. This leads to a large EFG, and the calculated quadrupole coupling constant (QCC) for ²⁷Al is 18.2 MHz. Different probe molecules were adsorbed on the Brønsted site. The hydrogen bond formed between the acid proton and the probe molecule weakens the zeolitic O---H bond. For conservation of the overall bond order of the oxygen atom, its bonds to the neighboring tetrahedral atoms (Si, Al) become stronger. As a consequence, the perturbation of the AlO₄ tetrahedron and the EFG at the aluminium position decrease depending on the strength of the hydrogen bond. Perturbation of an oxygen atom of the AlO₄ tetrahedron by accepting a hydrogen bond from the base molecule also affects the corresponding AlO---O bond order. A linear correlation is found between the calculated QCC constants for ²⁷Al and the AlO---O bond orders of the oxygen atoms which are perturbed by protonation or by hydrogen bonds. A geometrical shear strain parameter and a simple electrostatic point charge model are less successful at predicting the trends in EFG which clearly shows the importance of the chemical bonds. Published by Elsevier Science B.V.     

New method for determining the concentration of ionic impurities in solvent polymeric membranes

A new method is proposed for ascertaining the amount of ionic impurities in solvent polymeric membranes of ion-selective electrodes. The method is based on determining the selectivity coefficient for ions of different valences as a function of the concentration of a lipophilized tetraphenylborate salt added to the membrane phase. Thus, the concentration of anionic impurities in commercially available poly(vinyl chloride) and of cationic ones in Tecoflex® (a polyurethane) was obtained as 0.063 ± 0.016 and 0.044 ± 0.006 mmol/kg, respectively.     

H/D exchange between CD4 and the OH-groups of H-zeolites and γ-Al2O3

The H/D exchange between CD₄ and OH-groups was studied over zeolite H-ZSM-5, H-mordenite, H-beta and over -Al₂O₃ using the pulsed microcatalytic method. The apparent activation energy of the exchange was found to be independent of the Brönsted acidity of the solid. Results suggested that methane was activated for the reaction by dissociative adsorption over Lewis acid-Lewis base pair sites.     

Two-Dimensional Two-Component Plasma with Adsorbing Impurities

We study the behavior of the two-dimensional two-component plasma in the presence of some adsorbing impurities. Using a solvable model, we find analytic expressions for the thermodynamic properties of the plasma such as the n-body densities, the grand potential, and the pressure. We specialize in the case where there are one or two adsorbing point impurities in the plasma, and in the case where there are one or two parallel adsorbing lines. In the former case we study the effective interaction between the impurities, due to the charge redistribution around them. The latter case is a model for electrodes with adsorbing sticky sites on their surface.     

Fine speciation of active sites in zeolites by a CO probe: Dynamics and IR frequencies

The influence of system dynamics onto infrared spectra calculated and measured for probe molecules weakly interacting with active sites is discussed. OH stretching frequencies for Brønsted sites in Y zeolite and their shift on CO adsorption (measured by IR spectroscopy at ca. 170 K) are simulated from AIMD trajectory, based on BO DFT calculations. We present here very good agreement between experimental and computed OH frequency red‐shift on CO adsorption in dealuminated zeolite Y. It is shown that extracting nonharmonic OH frequencies from ab initio molecular dynamics trajectory provides better estimate of experimental IR results than standard vibrational analyses due to realistic computational model and better correspondence with experimental conditions.     

Conformers,properties, and docking mechanism of the anticancer drug docetaxel: DFT and molecular dynamics studies

The conformational structures and properties of the anticancer drug docetaxel (DTX) are studied theoretically. A total of 3888 trial structures were initially generated by all combinations of internal single‐bond rotamers and screened with the B3LYP/3‐21G* method. A total of 31 unique conformers were further optimized at the B3LYP/6‐311G* method. Their relative energies, dipole moments, rotational constants, and harmonic vibrational frequencies were predicted. Single‐point relative energies were then determined at the M06‐L/6‐311G(2df,p) level. The UV spectrum of the lowest‐lying DTX conformer in methanol was investigated with the TD‐CAM‐B3LYP/6‐311 + G(2df,p) method. The 31 unique DTX structures are mainly docked at three different sites within β‐tubulin. Based on the results of molecular docking and double‐float MD simulations, the lowest‐lying DTX conformer consistently exhibits good docking performance with β‐tubulin. We identified the residues LYS299, ARG215, GLN294, LEU275, THR216, GLU290, PRO274, and THR276 on β‐tubulin as active sites forming a binding pocket responsible for locking DTX within β‐tubulin to make the combination more stable. The RMSD values show that the predicted complexes are favorable, and the SASA analysis shows that the hydrophilic properties of DTX are better than paclitaxel. © 2018 Wiley Periodicals, Inc.     

Coupling Magnetic Single‐Crystal Co2Mo3O8 with Ultrathin Nitrogen‐Rich Carbon Layer for Oxygen Evolution Reaction

Transition‐metal oxides as electrocatalysts for the oxygen evolution reaction (OER) provide a promising route to face the energy and environmental crisis issues. Although palmeirite oxide A₂Mo₃O₈ as OER catalyst has been explored, the correlation between its active sites (tetrahedral or octahedral) and OER performance has been elusive. Now, magnetic Co₂Mo₃O₈@NC‐800 composed of highly crystallized Co₂Mo₃O₈ nanosheets and ultrathin N‐rich carbon layer is shown to be an efficient OER catalyst. The catalyst exhibits favorable performance with an overpotential of 331 mV@10 mA cm^?2 and 422 mV@40 mA cm^?2, and a full water‐splitting electrolyzer with it as anode catalyst shows a cell voltage of 1.67 V@10 mA cm^?2 in alkaline. Combined HAADFSTEM, magnetic, and computational results show that factors influencing the OER performance can be attributed to the tetrahedral Co sites (high spin, t₂³e⁴), which improve the OER kinetics of rate‐determining step to form *OOH.     

Computational prediction of protein ubiquitination sites mapping on Arabidopsis thaliana

Among the protein post-translational modifications (PTMs), ubiquitination is considered as one of the most significant processes which can regulate the cellular functions and various diseases. Identification of ubiquitination sites becomes important for understanding the mechanisms of ubiquitination-related biological processes. Both experimental and computational approaches are available for identifying ubiquitination sites based on protein sequences of different species. The experimental approaches are time-consuming, laborious and costly. In silico prediction is an alternative time saving, easier and cost-effective approach for identifying ubiquitination sites. Moreover, the sequence patterns in the different species around the ubiquitination sites are not similar which demands species-specific predictors. Therefore, in this study, we have proposed a novel computational method for identifying ubiquitination sites based on protein sequences of A. thaliana species which will be robust against outlying observations also. Through the comparative study of two encoding schemes and three classifiers, the random forest (RF) based predictor was selected as the best predictor under the CKSAAP encoding scheme with 1:1 ratio of positive and negative samples (i.e. ubiquitinated and non-ubiquitinated) in training dataset. The proposed predictor produced the area under the ROC curve (AUC score) as 0.91 and 0.86 for 5-fold cross-validation test with the training dataset and the independent test dataset of A. thaliana respectively. The proposed RF based predictor also performed much better than the other existing ubiquitination sites predictors for A. thaliana.     

Coupling Magnetic Single-Crystal Co2Mo3O8 with Ultrathin Nitrogen-Rich Carbon Layer for Oxygen Evolution Reaction

Transition-metal oxides as electrocatalysts for the oxygen evolution reaction (OER) provide a promising route to face the energy and environmental crisis issues. Although palmeirite oxide A₂Mo₃O₈ as OER catalyst has been explored, the correlation between its active sites (tetrahedral or octahedral) and OER performance has been elusive. Now, magnetic Co₂Mo₃O₈@NC-800 composed of highly crystallized Co₂Mo₃O₈ nanosheets and ultrathin N-rich carbon layer is shown to be an efficient OER catalyst. The catalyst exhibits favorable performance with an overpotential of 331 mV@10 mA cm⁻² and 422 mV@40 mA cm⁻², and a full water-splitting electrolyzer with it as anode catalyst shows a cell voltage of 1.67 V@10 mA cm⁻² in alkaline. Combined HAADFSTEM, magnetic, and computational results show that factors influencing the OER performance can be attributed to the tetrahedral Co sites (high spin, t₂³e⁴), which improve the OER kinetics of rate-determining step to form *OOH.     

Revealing Isolated M−N3C1 Active Sites for Efficient Collaborative Oxygen Reduction Catalysis

Single atom catalysts (SACs) are of great importance for oxygen reduction, a critical process in renewable energy technologies. The catalytic performance of SACs largely depends on the structure of their active sites, but explorations of highly active structures for SAC active sites are still limited. Herein, we demonstrate a combined experimental and theoretical study of oxygen reduction catalysis on SACs, which incorporate M−N₃C₁ site structure, composed of atomically dispersed transition metals (e.g., Fe, Co, and Cu) in nitrogenated carbon nanosheets. The resulting SACs with M−N₃C₁ sites exhibited prominent oxygen reduction catalytic activities in both acidic and alkaline media, following the trend Fe−N₃C₁ > Co−N₃C₁ > Cu−N₃C₁. Theoretical calculations suggest the C atoms in these structures behave as collaborative adsorption sites to M atoms, thanks to interactions between the d/p orbitals of the M/C atoms in the M−N₃C₁ sites, enabling dual site oxygen reduction.