The authors describe the first enantioselective α-arylation of ketones with aryl triflates to provide access to enantioenriched quaternary stereocenters through use of chiral biarylphosphine-palladium and -nickel complexes. The combination of electron-rich aryl triflates and a palladium catalyst, and that of electron-poor aryl triflates and a nickel catalyst led to a series of α-arylations that occur with much higher enantioselectivity than the previously reported reactions of aryl bromides with other catalysts.
The aim of this study was to investigate the influence of Si/Al ratio on the locations of exchangeable cations in H-MFI and on the monomolecular cracking and dehydrogenation reactions of n-butane. On the basis of UV-visible spectroscopic analysis of Co(II) exchanged into MFI, it was inferred that the fraction of Co(II) (and, by extension, Brønsted protons) located at channel intersections relative to straight and sinusoidal channels increases with increasing Al content. Concurrently, turnover frequencies for all monomolecular reactions, and the selectivities to dehydrogenation versus cracking and to terminal cracking versus central cracking, generally increased. The changes in selectivity with Al content are consistent with the finding that the transition-state geometry for dehydrogenation is bulky and resembles a product state, and should therefore exhibit a stronger preference to occur at channel intersections relative to cracking. Increases in turnover frequencies are attributed partly to increases in intrinsic activation entropies that compensate for concurrent increases in intrinsic activation energies, most strongly for dehydrogenation and terminal cracking, resulting in increased selectivity to these reactions at higher Al content. This interpretation contrasts with the view that intrinsic activation barriers are constant. It is also observed that isobutene inhibits the rate of n-butane dehydrogenation. Theoretical calculations indicate that this effect originates from adsorption of isobutene at the channel intersections. Because cracking reaction rates are not affected by the presence of isobutene, this result suggests that the preference of dehydrogenation to occur at channel intersections is much stronger than the preference for cracking to occur at these locations.
An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTLaser Raman spectroscopy of supported vanadium oxide catalystsGregory T. Went, S. Ted. Oyama, and Alexis T. BellCite this: J. Phys. Chem. 1990, 94, 10, 4240–4246Publication Date (Print):May 1, 1990Publication History Published online1 May 2002Published inissue 1 May 1990https://pubs.acs.org/doi/10.1021/j100373a067https://doi.org/10.1021/j100373a067research-articleACS PublicationsRequest reuse permissionsArticle Views2334Altmetric-Citations349LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access options Get e-Alerts
In this work, a computational study of the dielectric absorption and Q-factor at microwave frequencies in rocksalt oxides is presented. This work is performed within a theory for the two-phonon anharmonic absorption process in terms of third-order force constant matrices, phonon eigendata, and Born effective charges. The complex dielectric permittivity is expressed by means of Green’s functions. This theory is used on model systems that are described using empirical Buckingham potentials with shells. The Q-factor and its temperature dependence in AO (A=Mg, Ca, Ba, Sr) oxides with the rocksalt structure is calculated. For MgO, the calculated Q-factor at room temperature agrees relatively well with experiments, and the temperature dependence is in qualitative agreement. The Q-factor in a model “AO” system is also calculated to determine the effects of cation mass and size on the Q-factor. Increased cation size is found to lower the Q-factor, whereas increasing cation mass is found to increase the Q-factor when the cation mass is higher than that of Ca.
Gamma-TiAl based alloys have recently received attention for potential elevated temperature applications in gas-turbine engines. However, although expected critical crack sizes for some targeted applications (e.g. gas-turbine engine blades) may be less than ∼500 μm, most fatigue-crack growth studies to date have focused on the behavior of large (on the order of a few millimeters) through-thickness cracks. Since successful implementation of damage-tolerant life-prediction methodologies requires that the fatigue properties be understood for crack sizes representative of those seen in service conditions, the present work is focused on characterizing the initiation and growth behavior of small (a∼25–300 μm) fatigue cracks in a γ-TiAl based alloy, of composition Ti–47Al–2Nb–2Cr–0.2B (at.%), with both duplex (average grain size of ∼17 μm) and refined lamellar (average colony size of ∼145 μm) microstructures. Results are compared to the behavior of large (a>5mm), through-thickness cracks from a previous study. Superior crack initiation resistance is observed in the duplex microstructure, with no cracks nucleating after up to 500 000 cycles at maximum stress levels (R=0.1) in excess of the monotonic yield stress, σ y. Comparatively, in the lamellar microstructure cracks nucleated readily at applied maximum stresses below the yield stress (85% σ y) after as few as 500 cycles. In terms of crack growth, measurements for small fatigue cracks in the duplex and lamellar microstructures showed that both microstructures have comparable intrinsic fatigue-crack growth resistance in the presence of small flaws. This observation contrasts previous comparisons of large-crack data, where the lamellar structure showed far superior fatigue-crack growth resistance than the duplex structure. Such “small-crack effects” are examined both in terms of similitude (i.e. crack tip shielding) and continuum (i.e. biased microstructural sampling) limitations of traditional linear elastic fracture mechanics.
Mechanisms of initiation and unstable propagation of transgranular cleavage cracks are compared for brittle fracture ahead of sharp cracks and rounded notches, e.g. for fatigue pre-cracks and Charpy V-notches, respectively, in standard toughness specimens. The comparison is made over a range of temperatures, from the lower shelf into the ductile/brittle transition region, for a single phase material containing a known distribution of particles where weakest link statistics can be used to model the onset of catastrophic failure. Using linear and nonlinear elastic solutions for the stress distribution ahead of a sharp crack, and slip-line field solutions, modified for a power hardening material, for the rounded notch, statistical modelling is employed to define the critical dimensions ahead of the crack or notch tip where initial cracking events are most probable. The analysis provides an interpretation of the role of stress gradient in governing microscopic fracture behavior. Predictions are evaluated by comparison with experimental results on the low temperature flow, Charpy V-notch and plane strain fracture toughness behavior of a low carbon mild steel with simple ferrite/grain boundary carbide microstructures.
This chapter covers the notion of hyperelasticity—the concept that stress is derived from a strain—energy function–by invoking an analogy between elastic materials and springs. Alternatively, it can be derived by invoking a work inequality; the notion that work is required to effect a cyclic motion of the material.
