Reaction Mechanism

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SYNTHESIS BY O/W MICROEMULSION METHOD: CHARACTERIZATION AND STUDY OF REACTION MECHANISM

SYNTHESIS BY O/W MICROEMULSION METHOD: CHARACTERIZATION AND STUDY OF REACTION MECHANISM

The concept of using O/W microemulsions as confined reaction media for prepar- ing various types of inorganic nanoparticles has been demonstrated for a variety of nanomaterials. [2, 24, 29, 30, 60, 66] As example in this work we have explored the synthesis of cerium oxide and platinum nanoparticles as well as copper oxide nanostructures. However, understanding the mechanism of nanoparticle formation in the O/W microemulsion media remains challenging. Identifying the nucleation sites in the O/W microemulsion media was the first objective. The hypothesis is that nucleation sites occur at the oil-water (o-w) interface since the precipitating agent (or reducing solution) is added to the aqueous continuous phase, while the organometallic precursor is dissolved in the oil drops (either in the core of the oil droplets, at the interface, or both). Diffusion of OH − ions in the continuous phase towards the oil-water interface where the precursor is thought to be located will lead to a contact of these species and as a consequence the formation of the first nuclei may occur. The next objective was to define more precisely the nucleation site at the o-w interface since it may occur at either the water side or the oil side. Moreover, assuming that the reaction occurs at the o-w interface where the surfac- tant is adsorbed, it should be taken into account that its role may be important in the nucleation and nanoparticle growth. Many parameters have to be taken into account to assess the mechanism of nanoparticle formation.
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102 Lee mas

Computacional studies on the mechanism of the pauson-khand reaction

Computacional studies on the mechanism of the pauson-khand reaction

A reaction mechanism describes how the starting materials of a chemical reaction are transformed into reaction products. This transformation consists in a reorganisation of the connectivity of the molecular system, that is, breaking and formation of chem- ical bonds. Such a reorganisation of the connectivity is accompanied by a change in the system energy, and along the transformation from reactants to products the system passes through transition states (TS) and minima of the energy surface. The minima of the energy surface are identified as intermediates. The transformation from one intermediate over a TS and further to the next intermediate is called an elementary step. The successive elementary steps connecting the reactants with the products constitute a reaction pathway. Typically for a molecular system there are several reaction pathways that can connect the reactants to the products. That is, there are several ways that the molecular connectivity can reorganise to get from the reactants to the products. Additionally, within each of these pathways, there are several conformational variations. The most likely pathway is the one with the lowest energy barrier when going from reactants to the products. However, a certain portion of the molecules follows the higher energy pathways as well. The distribu- tion among the different pathways is given by the Boltzmann distribution. The Boltzmann distribution gives the probability, P i , of a molecule to be in state i with
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173 Lee mas

Elucidating reaction mechanisms for metal-catalyzed oxidations

Elucidating reaction mechanisms for metal-catalyzed oxidations

In a chemical reaction one or more species, i.e. reactants, are transformed leading to a different set of compounds, i.e. products. The description of what occurs at each stage of the overall chemical process is known as reaction mechanism . A good knowledge of reaction mechanisms is crucial as it can provide detailed guidance to further improve chemical processes. Hence, by understanding the most relevant features of a given reaction, new strategies to achieve higher rates, yields and/or selectivities can be developed. Unfortunately, the many steps typically involved and the existence of numerous competing pathways lead to reaction mechanisms rather complicated to elucidate, particularly when transition metal atoms are present. In addition, short-lived intermediates are difficult to detect, which makes the full mechanistic characterization of reactions particularly challenging by using purely experimental techniques.
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Two state reactivity of oxygen reduction reaction over platinum catalyst

Two state reactivity of oxygen reduction reaction over platinum catalyst

Two state reactivity (TSR) has been studied for the oxygen reduction reaction (ORR) on platinum clusters using density functional theory (DFT). The calculations are performed on two different systems, a Pt3 and a Pt15 cluster via the direct four electron reduction reaction mechanism. Since the oxygen is a triplet and water a singlet, a spin change must take place at some point in the reaction path. The results indicate that the system remains as a triplet after oxygen adsorption but a conversion to a singlet could happen after the first protonation where the singlet and triplet lie extremely close in energy. For the second protonation, a spin swap from singlet to triplet is shown by the calculations and another change from triplet to singlet takes place after the third protonation. No spin change was observed after the fourth protonation and the product, water, is a singlet as expected. Although this study uses the associative mechanism (AM) where the dissociation is supposed to happen after the formation of the OOH species, activation barriers for AM and direct dissociation mechanism (DM) were calculated. The results suggest that for the DM, the barrier is about 0.4 eV higher than that for the AM, making the AM a more energetically feasible mechanism. In both cases, a spin crossover takes place between the transition state and the dissociated oxygen. Finally, the results demonstrate that the small Pt3 system yields almost the same qualitative results as the bigger Pt15 cluster.
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Análisis de la polimerización radicálica por transferencia de átomo activada por microondas de metracrilato de metilo y estireno usando herramientas de simulación

Análisis de la polimerización radicálica por transferencia de átomo activada por microondas de metracrilato de metilo y estireno usando herramientas de simulación

The polymerization scheme for ATRP used in this study, as implemented in the Predici software of CiT, [36] is shown in Table 1.The reactions that make up the reaction mechanism are called “steps”, as in the Predici literature. [36] Steps 1-3 and 5-15 correspond to a conventional ATRP scheme, such as the one used by Delgadillo-Velazquez and coworkers in their kinetic model for ATRP. [37] Step 4 accounts for generation of free radicals from monomer due to MI (“microwave effect”). [31-34] Steps 16-21 were included to take into account induction periods which can be attributed to the presence of O 2 and impurities in the reaction system
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Thermodynamic and Kinetics Modeling of H2 Production by dry Reforming of Ethanol With CO2 Evolution

Thermodynamic and Kinetics Modeling of H2 Production by dry Reforming of Ethanol With CO2 Evolution

is required to establish the corresponding reaction mechanism. This was selected according to the following criteria: because there are scarce references of studies related to the kinetics of dry reforming of ethanol and because Jankhah [41] has experimentally found that the kinetic behavior of both the dry and the steam reforming of ethanol follow a very similar kinetic mechanism it was decided that in the present kinetic simulation a reaction mechanism for the dry reforming of ethanol combined with the CO 2

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Second-generation gioethanolo production

Second-generation gioethanolo production

These findings suggest that one can consider the fol- lowing reaction mechanism as follows. The gaseous atmosphere surrounding the sample heated to 1800 K by the Xe-beam irradiation has a fairly lower oxygen partial pressure due to the successive generation of the metal zinc vapor. This successive vaporization would be facilitated by the rapid removal of the vaporized metal Zn by the oxidation with O 2 gas in the air that

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Co solvent effects on reaction rate and reaction equilibrium of an enzymatic peptide hydrolysis

Co solvent effects on reaction rate and reaction equilibrium of an enzymatic peptide hydrolysis

Due to high concentrations of co-solvents the stability and activity of the enzyme was measured in preparation of this work. As shown by Blandamer and Engberts 34 the addition of a co-solvent lowers water activity, which in this work is further lowered during the reaction as water is a substrate of the SPNA hydrolysis. To validate the stability of the enzyme under reaction conditions samples containing the enzyme were prepared in at least 1.5 fold excess of the co-solvent molalities studied in the work. The initial reaction rate was then measured after exposing the enzyme in the co-solvent followed by the addition of SPNA. All co-solvents did not show any impact the enzyme activity correlated to the time of exposure.
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Different approaches to radiation back reaction

Different approaches to radiation back reaction

Therefore the purpose of the present work is to study the problem of radiation back reac- tion, its implications in the dynamics of charged particles from an semi classical perspective and the range of validity of the results obtained. It is semi classical because relativistic co- variant cases will be studied too, whereas quantum effects in principle won’t be taken into account although we may rely on some of the results of this theory to perform our studies. In order to do this we will have to study first the cause for radiation trough the Larmor formula, so we will derive a covariant expression for it in order to leave clear that emission of radiation is a covariant process and not just an effect produced by the reference frame. Then we will make some derivations of the LAD equation in covariant and noncovariant form and explained the already mentioned deficiencies that it possesses. Then based on the works of Landau-Lifshitz, Ford and O’Connell, Rohrlich and Spohn we will demonstrate that the treatment which solves more satisfactorily the problem of the self force is the one done many years ago by Landau and Lifshitz which gives rise to an equation of motion for the charged particles to which some authors give their names, that is the Landau Lifshitz equa- tion. Therefore we will derive the Landau Lifshitz equations from different points of view and this will corroborate the fact that this equation is consistent with different principles from different branches of physics. As an addition we will study the effect that radiation reaction has over the motion on the spin of the electron. Finally we will conclude that the Landau Lifshitz equation is the correct equation for the motion of the electron at a classical level and will analyze its domain of applicability.
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Mechanically activated metathesis reaction in NaNH2–MgH2 powder mixtures

Mechanically activated metathesis reaction in NaNH2–MgH2 powder mixtures

The metathetic reaction was monitored by X-ray diffraction (XRD) using a Stoe Stadi P equipped with a Mo Ka k = 0.71093 A ˚ tube radiation in a Debye– Scherrer geometry. Specimens were sealed in a cap- illary chamber specifically used to keep moisture and oxygen sensitive materials uncontaminated. The angular range was accurately selected to include all the relevant XRD reflections of the involved phases, thus enabling a reliable quantitative analysis of the resulting XRD patterns. Structural and microstruc- tural information was obtained by interpolating the XRD patterns using Materials Analysis Using Diffraction (MAUD) software, which allows the quantitative Rietveld analysis [32].
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9 Lee mas

Dehydration of C6-monosaccharides to 5-hydroxymethylfurfural in dimethyl sulfoxide using sulfonic acid heterogeneous catalysts

Dehydration of C6-monosaccharides to 5-hydroxymethylfurfural in dimethyl sulfoxide using sulfonic acid heterogeneous catalysts

Additionally, aiming to evaluate the reusability of Amberlyst-70 catalyst, recycling tests were conducted at the optimized reaction conditions. Fig. 10 displays the reaction results for the reutilization experiments. Here catalyst recovery between consecutive runs was carried out by simply filtering and washing with a mixture of n-hexane and methanol (in order to remove both polar and non-polar reactants and reaction products from the surface of the catalyst). As shown, using this simple procedure, the catalyst can be reused once without appreciable loss of activity but a second reutilization leads to a gradual decrease in activity, both in terms of glucose conversion and yield to HMF. Furthermore, the yield to the undesired levulinic acid begins to grow. This fact is probably due to the formation of deposits of organic matter over the catalytic centres of Amberlyst-70 that apparently cannot be removed by the double
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Efficient conversion of levulinic acid into alkyl levulinates catalyzed by sulfonic mesostructured silicas

Efficient conversion of levulinic acid into alkyl levulinates catalyzed by sulfonic mesostructured silicas

drawbacks limiting its application beyond the bench scale [16]: reduced yields towards levulinates (on the order of 20%); the intermolecular dehydration of alcohols is observed in significant extent; limited recyclability of the catalysts; high required temperatures especially for dealing with polymeric feedstock; and finally works reported in literature have just focused on the use of low molecular weight alcohols (methanol or ethanol), so that their validity is not assessed for bulkier alcohols. In other approach, levulinates can be obtained by alcoholysis of 5- (chloro-methyl) furfural (CMF; route b, Fig. 1). CMF can be obtained from chemical digestion of sugars, cellulose and lignocellulosic feedstock with hydrochloric acid in a biphasic reactor with product yields in the range of 80-95% [17]. Further reaction of isolated CMF in the presence of alcohols gives the corresponding levulinates with yields around 80%. Although a high levulinate yield is obtained using this approach, there are still some concerns about the recycle of HCl and the waste disposal problems. Other route is the production of alkyl levulinates by alcoholysis of furfuryl alcohol obtained from hydrogenation of biomass-derived furfural (route c, Fig. 1). The furfural hydrogenation step is carried out using Cu-based catalysts, whereas the subsequent alcoholysis has been addressed with acid catalysts such as strong homogenous acids, polymeric resins and zeolites [18-19]. The profitability potential of this approach is limited for the high formation of ether by-products during the alcoholysis step [18-19].
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The stille reaction, 38 years later

The stille reaction, 38 years later

offers an open space, fairly free of ligand repulsions, which facilitates the initial side-on approach of the Ar–X bond to Pd (a nucleophilic attack to Pd by the electron pair of that bond); the electron richness of the phosphine provides efficient elec- tron back-donation to the σ* Ar–X orbital, so that the oxida- tive addition is completed even for reagents with very low nucleophilicity of the Ar–X bond. At the transmetalation step the nucleophilic attack of the stannane occurs on a tricoordi- nated Pd center, which is always more electrophilic that an equivalent tetracoordinated center with one additional donor ligand. This facilitates the reaction with less strong tin nucleo- philes. It is worth commenting that tricoordination at
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SATURATION MECHANISM IN REED INSTRUMENTS

SATURATION MECHANISM IN REED INSTRUMENTS

the oscillation stops. Simulations shows that this phenomenon can be observed only if important losses, such as non linear losses on a side hole, are taken into account in the model. The saturation mechanism is also the cause of the bifurcation to non standard regimes such as the “inverted Helmholtz motion”

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presentación

presentación

““ A A catalyst catalyst is is a a substance substance which which affects affects the the rate rate of of a a chemical chemical reaction reaction without without.. being.[r]

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Global control for the reaction wheel pendulum

Global control for the reaction wheel pendulum

CONTROL GLOBAL PARA EL PÉNDULO CON RUEDA DE REACCIÓN Global Control for the Reaction Wheel Pendulum RESUMEN En el presente artículo se desarrolla e implementa un esquema de control, que [r]

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Data alignment and association mechanism for multimedia data fusion in distributed sensor networks

Data alignment and association mechanism for multimedia data fusion in distributed sensor networks

We have simulated the temporal data alignment mechanism for event-streaming using the Castalia simulator [Bou13]. We configured an arrangement of 50 nodes, separated by distances between 10 and 15 meters in a field of 200 x 200 meters, with a multi-hop communication, as shown in the network graph of Figure 5.12. With this configuration, the depth of the network graph is established by the number of hops, thus, for our network scenario, the graph depth is up to 9. In addition to the network deployment, we adopted a special interference model, which allows to fix the delivery retries executed by a node, to ensure the reception of a packet by another node within a determined operational area. In this way, we obtained an average transmission delay of 0.30442 seconds between a pair of nodes.
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117 Lee mas

Cp*Ir(NHC) complexes as highly versatile and efficient catalysts for the cross coupling of alcohols and amines

Cp*Ir(NHC) complexes as highly versatile and efficient catalysts for the cross coupling of alcohols and amines

the catalytic performances of the most active catalysts recently described for the same processes. This feature confirms the extraordinary versatility and potential synthetic applications of this unique compound. Only Shvo’s catalyst shows a similar activity for the case of the N-alkylation of anilines with aliphatic amines. The catalytic reactions that we studied were carried out in the absence of base, phosphine or any other additive (for the use of the triflate adducts there is no need to add any extra amount of AgOTf), which in fact is not only simplifying the reaction work-ups (the products are more easily separated from the reaction mixtures), but also providing more environmentally benign processes.
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PRIMER AISLAMIENTO E IDENTIFICACION DE UREAPLASMA SPP Y DE MYCOPLASMA LIPOFACIENS DE GALLINAS COMERCIALES EN MEXICO

PRIMER AISLAMIENTO E IDENTIFICACION DE UREAPLASMA SPP Y DE MYCOPLASMA LIPOFACIENS DE GALLINAS COMERCIALES EN MEXICO

The cultures were observed daily for the detection of mycoplasma colonies using a stereo microscope with a zoom of 40X. When colonies were detected, the observations were supplemented with an optic microscope with a zoom of 100X. The colonies were cloned biologically three times in this way: a small agar square containing a single colony was inoculated on Frey liquid medium and then incubated at 37 °C. They were stored at -70 °C with 10% of glycerol to the suspicious colonies that grew in the Frey liquid medium. The colonies were analyzed three times to determine their patterns of biochemical reactions (11,19) . In this case there were used 5 ml of culture medium and 1 ml of inoculum for each culture. The tests included glucose fermentation (glucose at 1% with a pH set at 8.0); arginine hydrolysis (arginine at 0.2% with a pH set at 7.0); phosphatase activity (cultures in Petri plates with diphosphate phenolphthalein at 1%); and urea hydrolysis (urea at 1% with a pH set at 7.0). The positive or negative reaction to the mentioned tests were adjusted to that recommended by Goll (11) and Bradbury (19) . As positive control two mycoplasma species were included: Mycoplasma synoviae strain WVU 1853 and Mycoplasma gallisepticum strain A5969. As negative control tubes were used containing Frey medium without mycoplasmas.
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8 Lee mas

Reaction object constructions in English and Spanish

Reaction object constructions in English and Spanish

Reaction Object Constructions are alternations involving the transitive use of manner of speaking verbs and verbs of signs and gestures (She mumbled her adoration) paraphrased as “express a reaction by V-ing” (Levin 1993). Research on these constructions has been limited to English, often without a thorough discussion of their different elements. A closer look at examples extracted from the Corpus of Contemporary American English shows a wide variety of verbs and objects used in these constructions to denote different types of reported expressive acts. On the other hand, extensive searches of the Corpus de Referencia del Español Actual have revealed the existence of these conflated constructions in Spanish (Ella murmura su incredulidad), where the verbal event denotes the manner by which the second event – the nominalized expressive act – is produced. The data provided in this paper supports recent claims that Romance languages may use some of these “information-packed” constructions the way Germanic languages do.
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