Recent knowledge of the kinetics and intercalation mechanisms are summarized and accompanied by examples of intercalation reactions of water and ethanol into anhydrous vanadyl phosphate and redox intercalation of alkali metal cations into vanadyl phosphate dihydrate. Three possible mechanisms of intercalation are presented which are based on: (i) a concept of exfoliation of layers; (ii) the formation of stages and randomly stacked layers; (iii) co-existence of intercalated and non-intercalated parts of crystals of the host separated by an advancing phase boundary. The corresponding kinetic curves are ascribed to mechanisms (ii) and (iii). 相似文献
Stress relaxation measurements on ionically cross-linked alginate gels in an unconfined uniaxial compression are reported. Data are obtained as a function of ionic (divalent calcium) concentration and for various sample radii. Analysis of the results shows that such gels, at all investigated concentrations, relax the stress by two different concurring mechanisms: (a) an internal relaxation, most probably due to detachment and re-bonding of the cations along the alginate chains, and (b) the diffusion of water out of the gel. 相似文献
The diffusion mechanisms within zeolite catalysts, such as resistance to diffusion at pore mouths, configurational diffusion, adsorption-controlled diffusion, influence of co-existing molecules and pore blocking, were overviewed. Two kinds of diffusivities, the intracrystalline diffusivity and the effective diffusivity, were discussed separately to clarify the diffusion mechanism. 相似文献
Single-atom nanozymes (SAzymes) with specific response to the unique tumor microenvironment (TME) feature providing 100 % metal atoms utilization for high-efficient enzyme-catalyzed therapy and accurate template for the study of therapeutic mechanisms. In this review, we first introduce the various synthetic strategies of SAzymes, and the TME-responsive SAzymes activities. Next, the TME-responsive enhanced antitumor therapeutic approaches based on the enzymatic activities of SAzymes are summarized, and the corresponding therapy mechanisms are elaborated. Subsequently, a concise but concentrated summary, and the challenges and opportunities for the future design and engineering of SAzyme are outlined. As a new discipline, SAzymes have vast space for development in enhanced antitumor therapy. This timely review provides guidance and constructive suggestions for the future of SAzymes. 相似文献
Based on the surface enhanced Raman intensities of ethylene thiourea on the silver electrode, the temporal bond polarizabilities are elucidated to offer important information of the surface enhancement mechanisms. The results demonstrated that both the electromagnetic and charge transfer mechanisms are responsible for the large enhancement. The relaxation rates of the various bond polarizabilities are displayed. All obey a single exponentially decaying function, independent of potentials and mechanisms. The characteristic time for the relaxation due to charge transfer mechanism is larger than that via the electromagnetic mechanism due to that its relaxation requires longer time for the charges to re-distribute. 相似文献
Platinum is added to thermal barrier coatings (TBCs) as it is observed empirically to extend their lifetime, but the mechanism by which Pt acts is unknown. Since Pt has been proposed to alter diffusivities in NiAl, a key component of TBCs, we use first‐principles quantum mechanics calculations to investigate atomic level diffusion mechanisms. Here, we examine the effect of Pt on five previously proposed mechanisms for Ni diffusion in NiAl: next‐nearest‐neighbor jumps, the triple defect mechanism, and three variants of the six jump cycle. We predict that Pt increases the rate of Ni diffusion by stabilizing point defects and defect clusters that are diffusion intermediates. Previously, we predicted the triple defect mechanism to be a dominant Ni diffusion mechanism; it simultaneously results in long‐range Al diffusion in the opposite direction. Since Pt increases the rate of Ni diffusion, it also increases Al diffusion in NiAl, which may be key to extending the coating lifetime. 相似文献
In this research, two-electron reduction potentials are calculated for a set of eight quinones using Density Functional Theory (DFT) at B1B95/6-31G** and B1B95/6-311++G** levels in aqueous solution. Two different mechanisms, direct and indirect, which have been presented before, are employed for these calculations. DPCM and CPCM models of solvation are carried out to include solution phase contribution. The results show that CPCM is properly matched with DFT method at the B1B95 level in both direct and indirect mechanisms. It is found that direct mechanism gives more accurate two-electron reduction potentials in comparison to indirect mechanism. Mean Absolute Deviation (MAD) obtained through indirect mechanism and CPCM model of solvation are about 0.041 and 0.022 V for 6-31G** and 6-311++G**, respectively. The MAD values of direct mechanism are about 0.024 and 0.018 V for 6-31G** and 6-311++G** basis sets, respectively. The calculated MAD for both direct and indirect mechanisms is comparable with MAD previously reported at MP3 level for this set of molecules. 相似文献
We consider the behaviour of single molecules on surfaces and, more generally, in confined environments. These are loosely split into three sections: single molecules in biology, the physics of single molecules on surfaces and controlled (directed) diffusion. With recent advances in single molecule detection techniques, the importance and mechanisms of single molecule processes such as localised enzyme production and intracellular diffusion across membranes has been highlighted, emphasising the extra information that cannot be obtained with techniques that present average behaviour. Progress has also been made in producing artificial systems that can control the rate and direction of diffusion, and because these are still in their infancy (especially in comparison to complex biological systems), we discuss the new physics revealed by these phenomena.
The family of all possible reaction mechanisms on a potential surface has an algebraic structure with potential applications in quantum chemical molecular design and synthesis planning.Transformation properties and equivalence relations of reaction paths on potential energy hypersurfaces lead to a topological definition of reaction mechanisms. The family of all fundamental reaction mechanisms on the hypersurface has a group structure,the fundamental group of an appropriately defined topological space. Isomorphism and homomorphism relations between fundamental groups of reaction mechanisms are used to characterize the chemically important topological properties of various subsets of a hypersurface, or those of different excited state hypersurfaces. 相似文献
One of the most challenging questions in the Lewis base organocatalyst field is how to predict the most electrophilic carbon for the complexation of N‐heterocyclic carbene (NHC) and reactant. This study provides a valuable case for this issue. Multiple mechanisms (A, B, C, D, and E) for the intramolecular cyclization of aldimine catalyzed by NHC were investigated by using density functional theory (DFT). The computed results reveal that the NHC energetically prefers attacking the iminyl carbon (AIC mode, which is associated with mechanisms A and C) rather than attacking the olefin carbon (AOC mode, which is associated with mechanisms B and D) or attacking the carbonyl carbon (ACC mode, which is associated with mechanism E) of aldimine. The calculated results based on the different reaction models indicate that mechanism A (AIC mode), which is associated with the formation of the aza‐Breslow intermediate, is the most favorable pathway. For mechanism A, there are five steps: (1) nucleophilic addition of NHC to the iminyl carbon of aldimine; (2) [1,2]‐proton transfer to form an aza‐Breslow intermediate; (3) intramolecular cyclization; (4) the other [1,2]‐proton transfer; and (5) regeneration of NHC. The analyses of reactivity indexes have been applied to explain the chemoselectivity, and the general principles regarding the possible mechanisms would be useful for the rational design of NHC‐catalyzed chemoselective reactions. 相似文献
The kinetics of a proton transfer into dilute acid solutions containing natural zeolitic tuff was studied by following the
pH evolution of the liquid phase. Four different solutions with tuff contents of 9, 3, 1 and 0.5 (% wt) and three different
particle size fractions (≤ 2000 μm) were studied. The proton concentration of the solution was decreased by increasing the
zeolite amount and decreasing the particle size fraction. The proton transfer reaction was analyzed with chemical reactions
and diffusion model equations. Analysis shows that the adsorption and/or ion exchange are possible mechanisms and are expressed
by a second order reaction model. 相似文献
A detailed mechanism covering 545 species and 3105 reactions for high-temperature combustion of n-propylcyclohexane(n-PCH), generated via a mechanism generation program(ReaxGen) developed by our research group, was validated in this study. A semi-detailed mechanism involved with 195 species and 573 reactions and a skeletal mechanism concerned with 108 species and 393 reactions were obtained by means of rate-of-production analysis and path flux analysis(PFA), respectively. In order to validate the reliability of these mechanisms, ignition delay time, laminar flame speed and concentration profiles of important species were simulated with the help of CHEMKIN software. Numerically predicted results of our mechanisms are in very good agreement with available experimental data. Finally, major reaction pathways of n-PCH combustion and important reactions during the combustion process were investigated by reaction pathway analysis and sensitivity analysis, respectively. The results indicate that these mechanisms are reliable for describing the auto-ignition characteristics of n-PCH. These mechanisms would also be helpful to computational fluid dynamics(CFD) for engine design. Moreover, this systematic approach used in our study, which combines mechanism construction, simplification, validation and analysis for n-PCH, may also be employed to construct mechanisms for the high-temperature combustion of other cycloalkanes with one ring. 相似文献