Solvent-Free Synthesis of ZnO Nanoparticles by a Simple Thermal Decomposition Method

This paper reports on a novel processing route for producing ZnO nanoparticles by solid-state thermal decomposition of zinc(II) acetate nanostructures obtained by the sublimation of zinc(II) acetate powder. The sublimation process of the Zn(OAc)₂ powder was carried out in the temperature 150 °C for 2 h. In addition, nanoparticles of ZnO were obtained by solid-state thermal decomposition of the synthesized Zn(OAc)₂ nanostructures. The synthesized products were characterized by X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy, photoluminescence spectroscopy, and energy dispersive X-ray spectroscopy. The sublimation process of the Zn(OAc)₂ powder was carried out within the range of 150–180 °C. The XRD studies indicated the production of pure hexagonal ZnO nanoparticles after thermal decomposition.     

Monitoring the morphology development of polymer‐monolithic stationary phases by thermal analysis

Thermal analysis and SEM were employed to gain insights in the different stages of morphology development and the thermal properties of polymer‐monolithic stationary phases. The studied system was a thermally initiated free‐radical copolymerization reaction at 70°C of styrene and divinylbenzene in the presence of tetrahydrofuran and 1‐decanol. The key events in the early stages of morphology development are initiation, chain growth, branching, and cyclization, leading to microgel particles. Interparticle reactions through pendant vinyl groups lead to the formation of microgel clusters. The rapid increase in molecular weight and cross‐link density of the microgel clusters causes a reaction‐induced phase separation, and the formation of a macroscopic network of interconnected globules was observed (macrogelation) at around 45 min. After 3 h or 65% conversion, a space‐filling macroporous monolithic network was observed. Afterwards, mainly growth of existing globules takes place, reducing the macropore size. The porogen ratio affects the timing of the reaction‐induced phase separation, strongly influencing the morphology of the polymer material. The use of a mixture of divinylbenzene isomers yielded a monolithic material that is less cross‐linked at the surface compared to the central part of the polymer backbone due to copolymerization‐composition drift. The less cross‐linked outer layer starts devitrifying at 100°C.     

Metabolism of Halogenated Alkanes by Cytochrome P450 enzymes. Aerobic Oxidation versus Anaerobic Reduction

The cytochromes P450 are a large class of heme‐containing enzymes that catalyze a broad range of chemical reactions in biosystems, mainly through oxygen‐atom transfer to substrates. A relatively unknown reaction catalyzed by the P450s, but very important for human health, is the activation of halogenated substrates, which may lead to toxicity problems. However, its catalytic mechanism is currently unknown and, therefore, we performed a detailed computational study. To gain insight into the metabolism of halogenated compounds by P450 enzymes, we have investigated the oxidative and reductive P450‐mediated activation of tetra‐ and trichloromethane as halogenated models with density functional theory (DFT) methods. We propose an oxidative halosylation mechanism for CCl₄ under aerobic conditions by Compound I of P450, which follows the typical Groves‐type rebound mechanism. By contrast, the metabolism of CHCl₃ occurs preferentially via an initial hydrogen‐atom abstraction rather than halosylation. Kinetic isotope effect studies should, therefore, be able to distinguish the mechanistic pathways of CCl₄ versus CHCl₃. We find a novel mechanism that is different from the well accepted P450 substrate activation mechanisms reported previously. Moreover, the studies highlight the substrate specific activation pathways by P450 enzymes leading to different products. These reactivity differences are rationalized using Marcus theory equations, which reproduce experimental product distributions.     

Oxygen-pressure dependence of the crystallinity of MgO films grown on Si(1 0 0) by PLD

Effects of the oxygen partial pressure on pulsed-laser deposition of MgO buffer layers on silicon substrates were investigated. The overall growth process was monitored in situ by reflection high-energy electron diffraction (RHEED) method. It was found that the crystallinity and surface morphology of the MgO films were strongly affected by oxygen partial pressure in the deposition chamber. The oxygen-pressure dependence could be explained in terms of interactions of oxygen with species in the plume-like plasma. The MgO film obtained at an optimal oxygen-pressure range of 1×10⁻²–1 Pa exhibited an atomic-smooth and defect-free surface (the root-mean-square roughness being as low as 0.82 nm). For the metal–insulator–metal (MIM) structure of Au/MgO (150 nm)/TiN prepared at the optimal growth conditions achieved a very low leak current density of 10⁻⁷ A cm⁻² at an electric field of 8×10⁵ V cm⁻¹ and the permittivity (ε_r) of about 10.6, virtually the same as that of the bulk MgO single crystals.     

Is the bound substrate in nitric oxide synthase protonated or neutral and what is the active oxidant that performs substrate hydroxylation?

We present here results of a series of density functional theory (DFT) studies on enzyme active site models of nitric oxide synthase (NOS) and address the key steps in the catalytic cycle whereby the substrate (L-arginine) is hydroxylated to N(omega)-hydroxo-arginine. It has been proposed that the mechanism follows a cytochrome P450-type catalytic cycle; however, our calculations find an alternative low energy pathway whereby the bound L-arginine substrate has two important functions in the catalytic cycle, namely first as a proton donor and later as the substrate in the reaction mechanism. Thus, the DFT studies show that the oxo-iron active species (compound I) cannot abstract a proton and neither a hydrogen atom from protonated L-arginine due to the strength of the N-H bonds of the substrate. However, the hydroxylation of neutral arginine by compound I and its one electron reduced form (compound II) requires much lower barriers and is highly exothermic. Detailed analysis of proton transfer mechanisms shows that the basicity of the dioxo dianion and the hydroperoxo-iron (compound 0) intermediates in the catalytic cycle are larger than that of arginine, which makes it likely that protonated arginine donates one of the two protons needed during the first catalytic cycle of NOS. Therefore, DFT predicts that in NOS enzymes arginine binds to the active site in its protonated form, but is deprotonated during the oxygen activation process in the catalytic cycle by either the dioxo dianion species or compound 0. As a result of the low ionization potential of neutral arginine, the actual hydroxylation reaction starts with an initial electron transfer from the substrate to compound I to create compound II followed by a concerted hydrogen abstraction/radical rebound from the substrate. These studies indicate that compound II is the actual oxidant in NOS enzymes that performs the hydroxylation reaction of arginine, which is in sharp contrast with the cytochromes P450 where compound II was shown to be a sluggish oxidant. This is the first example of an enzyme where compound II is able to participate in the reaction mechanism. Moreover, arginine hydroxylation by NOS enzymes is catalyzed in a significantly different way from the cytochromes P450 although the active sites of the two enzyme classes are very similar in structure. Detailed studies of environmental effects on the reaction mechanism show that environmental perturbations as appear in the protein have little effect and do not change the energies of the reaction. Finally, a valence bond curve crossing model has been set up to explain the obtained reaction mechanisms for the hydrogen abstraction processes in P450 and NOS enzymes.     

Triterpenoid saponins from the seeds of Pharbitis nil

From the seeds of Pharbitis nil (Convolvulaceae), two new oleanene-type triterpene glycosides, pharbitosides A (1) and B (2), together with beta-sitosterol, beta-sitosterol glucoside (daucosterol), caffeic acid, and methyl caffeate were isolated. The structure of pharbitoside A (1) was elucidated to be queretaroic acid 3-O-alpha-L-rhamnopyranosyl-(1-->2)-O-beta-D-glucopyranosyl-(1-->2)-beta-D-glucopyranoside (1). Pharbitoside B (2) is a 21alpha-hydroxyoleanolic acid saponin carrying the same sugar moiety as that of pharbitoside A (1).     

A crossover lattice fluid equation of state for pure fluids

A lattice fluid model is one of the most versatile, molecular-based engineering equations of state (EOS) but, in common with all analytic equations of state, the lattice fluid (LF) EOS exhibits classical behaviour in the critical region rather than the non-analytical, singular behaviour seen in real fluids. In this research, we use the LF EOS and develop a crossover lattice fluid (xLF) equation of state near to and far from the critical region which incorporates the scaling laws valid asymptotically close to the critical point while reducing to the original classical LF EOS far from the critical point. We show that, over a wide range of states, the xLF EOS yields the saturated vapour pressure data and the density data with much better accuracy than the classical LF EOS.     

Estimation of 2nd-order derivative thermodynamic properties using the crossover lattice equation of state

We apply the crossover lattice equation of state (xLF EOS) [M.S. Shin, Y. Lee, H. Kim, J. Chem. Thermodyn. 40 (2007) 174–179] to the calculations of thermodynamic 2nd-order derivative properties (isochoric heat capacity, isobaric heat capacity, isothermal compressibility, thermal expansion coefficient, Joule–Thompson coefficient, and sound speed). This equation of state is used to calculate the same properties of pure systems (carbon dioxide, normal alkanes from methane to propane). We show that, over a wide range of states, the equation of state yields properties with better accuracy than the lattice equation of state (LF EOS), and near the critical region, represents singular behavior well.     

A (vapor + liquid) equilibria of the {carbon dioxide + isopropoxyethanol (iC3E1)} and the {carbon dioxide + isobutoxyethanol (iC4E1)} systems

Binary (vapor + liquid) equilibrium data were measured for the {carbon dioxide + isopropoxyethanol (iC₃E₁)} and the {carbon dioxide + isobutoxyethanol (iC₄E₁)} systems at temperatures ranging from (313.15 to 333.15) K. These experiments were performed with a circulating-type apparatus with on-line gas chromatography. The experimental data correlated well with the Peng–Robinson equation of state using the Wong–Sandler mixing rules.     

Redox electron-transfer reactions: electrochemically mediated rearrangement, mechanism, and a total synthesis of daucene

The term "housane" refers to molecules possessing a bicyclo[2.1.0]pentane core. One was designed, synthesized, and used as a precursor of daucene, a member of the carotane class of sesquiterpenes. The total synthesis was completed, thereby marking the first time that housane-derived cation radicals have been used as the key intermediate in the synthesis of a natural product. The transformation used in the construction and featured in the text involves an oxidation to generate the cation radical via either a chemically or an electrochemically mediated electron transfer, the latter process using tris(p-bromophenyl)amine as the mediator. The two methods are compared, and guiding principles are formulated to assist in deciding how best to implement each. Both processes afford an unfavorable equilibrium state that is subsequently drained toward the product by two irreversible events, viz., a 1,2 carbon migration to the site that best stabilizes a positive charge and a second electron transfer, this time being a highly exothermic reduction of the rearranged species to generate the neutral product. A mechanistic proposal calling for the use of a catalytic quantity of the electrochemical mediator and the consumption of exceptionally small quantities of current is advanced. Experimental deviations from these predictions are noted, and a rationale to account for them is presented. Finally, significant differences were noted between the cyclic voltammograms of housanes bearing a CH2OR substituent rather than a methyl group at the bridgehead carbon. Those having the inductively withdrawing group displayed broad and ill-defined curves. The differences were investigated quantum mechanically, and a stereoelectronic argument is formulated stating that broadness of the curve for the ROCH2-substituted systems is the result of a time-averaged sampling of the HOMO energies over the distribution of conformers. The possible generality of the stereoelectronic effect is noted.