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51.
Interpolation methods such as the nudged elastic band and string methods are widely used for calculating minimum energy pathways and transition states for chemical reactions. Both methods require an initial guess for the reaction pathway. A poorly chosen initial guess can cause slow convergence, convergence to an incorrect pathway, or even failed electronic structure force calculations along the guessed pathway. This paper presents a growing string method that can find minimum energy pathways and transition states without the requirement of an initial guess for the pathway. The growing string begins as two string fragments, one associated with the reactants and the other with the products. Each string fragment is grown separately until the fragments converge. Once the two fragments join, the full string moves toward the minimum energy pathway according to the algorithm for the string method. This paper compares the growing string method to the string method and to the nudged elastic band method using the alanine dipeptide rearrangement as an example. In this example, for which the linearly interpolated guess is far from the minimum energy pathway, the growing string method finds the saddle point with significantly fewer electronic structure force calculations than the string method or the nudged elastic band method.  相似文献   
52.
Vapor-phase adsorption and separation of the C8 alkylaromatic components p-xylene, m-xylene, o-xylene, and ethylbenzene on the metal-organic framework MIL-47 have been studied. Low coverage Henry adsorption constants and adsorption enthalpies were determined using the pulse chromatographic technique at temperatures between 230 and 290 degrees C. The four C8 alkylaromatic components have comparable Henry constants and adsorption enthalpies. Adsorption isotherms of the pure components were determined using the gravimetric technique at 70, 110, and 150 degrees C. The adsorption capacity and steepness of the isotherms differs among the components and are strongly temperature dependent. Breakthrough experiments with several binary mixtures were performed at 70-150 degrees C and varying total hydrocarbon pressure from 0.0004 to 0.05 bar. Separation of the different isomers could be achieved. In general, it was found that the adsorption selectivity increases with increasing partial pressure or degree of pore filling. The separation at a high degree of pore filling in the vapor phase is a result of differences in packing modes of the C8 alkylaromatic components in the pores of MIL-47.  相似文献   
53.
This study applies aimless shooting and likelihood maximization to determine the molecular mechanism in the solid state polymorph transformation in terephthalic acid from over 500 candidate order parameters. The crystals examined here extend the range of crystal sizes considered in our previous work (J. Amer. Chem. Soc. 2007, 129, 4714) and reveal a change in the mechanism with increasing system size. As the crystal size increases beyond that studied in our previous work, the polymorph transformation mechanism changes from a global distortion of the crystal to a local corner nucleation mechanism. In the corner nucleation mechanism, the interfacial area between the two polymorphs is minimized for a given nucleus size. However, this mechanism differs from classical nucleation theory in that the molecular level details are essential to describe the nucleation process, which involves nonspherical domains at the corner of the crystal. These new findings suggest that there is a range of sizes for which corner nucleation is the dominant mechanism of polymorph transitions, thus implying that different mechanistic regimes exist for nucleation based on crystal size. From a computational standpoint, this study demonstrates the utility of aimless shooting and likelihood maximization to identify nonintuitive reaction coordinates in complex systems.  相似文献   
54.
This paper presents fuzzy acoustic emission identification in high precision hard turning process based on extended subtractive cluster analysis combined with the least-square estimation method. The fuzzy identification method provides a simple way to arrive at a definite conclusion based upon the information obtained with the difficulty in understanding the exact physics of the machining process. The experimental results prove that the proposed method is efficient and feasible.  相似文献   
55.
Long-wavelength onset of the fundamental branches is described for a free anisotropic plate with arbitrary through-plate variation of material properties. Main attention is given to the flexural branch. Closed-form expressions for the leading-order dispersion coefficient of the velocity and displacement are derived for a generic case and exemplified for the various types of either continuous, or discrete, or periodic inhomogeneity and for the monoclinic symmetry. The relevance of the static averaging is examined in detail. The bounds for the slope of the flexural velocity branch are established. The upper fundamental branches are considered for the case when these are uncoupled inplane and shear horizontal ones.  相似文献   
56.
Chronic skin exposure to ultraviolet light stimulates the production of cytokines known to be involved in the initiation of skin cancer. Recent studies in mouse models suggested a role for macrophage migration inhibitory factor (MIF) in the UVB‐induced pathogenesis of nonmelanoma skin cancer (NMSC). Our studies aimed at defining the pathophysiological function of MIF in cutaneous inflammatory reactions and in the development and progression of NMSC. Immunohistochemical analysis revealed a moderate expression of MIF in normal human skin samples but an enhanced expression of this cytokine in lesional skin of patients with actinic keratosis or cutaneous SCC. Enzyme‐linked immunosorbent assay studies showed a time‐dependent increase in MIF secretion after a moderate single‐dose UVB irradiation in NHEKs and SCC tumor cells. MIF is known to interact with CXCR2, CXCR4 and CD74. These receptors are not constitutively expressed in keratinocytes and HaCaT cells and their expression is not induced by UVB irradiation either. However, stimulation with IFNγ upregulated CD74 surface expression in these cells. Affymetrix® Gene Chip analysis revealed that only keratinocytes prestimulated with IFNγ are responsive to MIF. These findings indicate that MIF may be an important factor in the pathogenesis of NMSC tumorigenesis and progression in an inflammatory environment.  相似文献   
57.
Experimentally-determined permeation transients do not support the view that the behaviour of water in PDMS is significantly influenced by statistical-mechanical clustering; rather, they suggest that water behaves in a straightforward way. Simple calculations appear to confirm that the incidence of the statistical clustering of water in the polymer is negligible. A diffusion coefficient derived to include the influence of hydrophilic sites within the polymer is partially successful in mathematically reproducing measured quantities. An entropy calculation appears to suggest that the amount of mobile water in PDMS is solely thermally determined; hence the reduction of supposed hydrophilic impurities would probably not lead to a reduction in water permeation. The apparently large difference between the water solubility in PDMS, and that in siloxane liquids, a point of some interest in separation processes, remains unexplained in this paper.  相似文献   
58.
59.
Single crystal structure studies at room temperature have been made for the first stage Mn Cl2 intercalated graphite. The nominal composition was C5·6 Mn Cl2·4, as deduced from chemical analyses and X-ray diffraction intensities. The data are in agreement with the island model, already proposed for Ni Cl2 intercalation. From the decrease in C-C bond length and comparison with As F5 compound, it is shown that the charge transfer is determined by chlorine in excess with respect to free metal halide.  相似文献   
60.
An electrically contacted glucose dehydrogenase (GDH) enzyme electrode is fabricated by the reconstitution of the apo-GDH on pyrroloquinoline quinone (PQQ)-functionalized Au nanoparticles (Au-NPs), 1.4 nm, associated with a Au electrode. The Au-NPs functionalized with a single amine group were attached to the Au surface by 1,4-benzenedithiol bridges, and PQQ was covalently linked to the Au-NPs. The apo-GDH was then reconstituted on the PQQ cofactor sites. The surface coverage of GDH corresponded to 1.4 x 10(-12) mol cm(-2). The reconstituted enzyme revealed direct electrical contact with the electrode surface, and the bioelectrocatalytic oxidation of glucose occurred with a turnover number of 11,800 s(-1). In contrast, a system that included the covalent attachment of GDH to the PQQ-Au-NPs monolayer in a random, nonaligned, configuration revealed lack of electrical communication between the enzyme and the electrode, albeit the enzyme existed in a bioactive structure. The bioelectrocatalytic function of the later system was, however, activated by the diffusional electron mediator 2,6-dichlorophenol-indophenol. The results imply that the alignment of GDH on a Au-NP through the reconstitution process leads to an electrically contacted enzyme-electrode, where the Au-NP acts as a charge-transfer mediator.  相似文献   
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