A
bstract
We study the spectrum, unitarity bound and holographic central charge of spin-2 operators in a warped AdS
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background in the Type IIB theory. We were able to ...identify a class of solutions that is completely independent of the functions that define the background solution. We comment on the relation of our results to the previous ones in the literature.
Pore fluid sulfate, calcium and magnesium concentrations in ocean drilling sites 981–984, 925–929 and 803–807 are modeled to estimate the rates of sulfate reduction and carbonate ...precipitation/dissolution. These sites represent scenarios where the sulfate reduction rate is high, moderate, and low. Results show that the rate of carbonate precipitation decreases with decreasing sulfate reduction rate. When sulfate reduction rate is sufficiently low carbonate minerals dissolve. Sulfate reduction produces H+ and bicarbonate which compete in driving carbonate minerals to dissolve or precipitate. The results suggest that the carbonate reactions are mainly driven by the produced H+ when sulfate reduction rate is low, and by the produced bicarbonate when the sulfate reduction rate is high.
•Sulfate reduction is a major mechanism for carbonate precipitation/dissolution in deep sea sediments.•The rates of suflate reduction and carbonate dissolution/precipitation are estimated from pore fluid geochemical data.•Carbonate minerals precipitate when the rate of sulfate reduction is high, and dissolve when the rate is low.•Dissolution of carbonate minerals is likely because of the decrease of pH caused by sulfate reduction.
Radical C−H bond functionalization provides a versatile approach for elaborating heterocyclic compounds. The synthetic design of this transformation relies heavily on the knowledge of ...regioselectivity, while a quantified and efficient regioselectivity prediction approach is still elusive. Herein, we report the feasibility of using a machine learning model to predict the transition state barrier from the computed properties of isolated reactants. This enables rapid and reliable regioselectivity prediction for radical C−H bond functionalization of heterocycles. The Random Forest model with physical organic features achieved 94.2 % site accuracy and 89.9 % selectivity accuracy in the out‐of‐sample test set. The prediction performance was further validated by comparing the machine learning results with additional substituents, heteroarene scaffolds and experimental observations. This work revealed that the combination of mechanism‐based computational statistics and machine learning model can serve as a useful strategy for selectivity prediction of organic transformations.
Mechanism‐based computational statistics allowed the machine learning prediction of regioselectivity in radical C−H functionalization of heterocycles. The developed random forest model with physical organic features achieved satisfying performance without any experimental input.
Despite the rich body of research on the outward foreign direct investment (OFDI) by Chinese multinationals, little attention has been given to the fact that China’s OFDI is facing a high failure ...rate even in their initial attempt to enter a foreign market. Grounded on institutional theory, this study provides a nuanced view of the expansion dynamic of Chinese multinational firms overseas using a unique dataset that contains both successful and troubled Chinese foreign market entry attempts between 2018 and 2021. We find that at the firm level, state-owned firms are more likely to face difficulties when trying to enter a new market compared to their private counterparts. Firms’ corporate social responsibility (CSR) reporting reduces the chance of failure. These effects are conditioned on the political, economic, and cultural distances between the home and host counties.
Conspectus Transition-metal-catalyzed C–O bond activation provides a useful strategy for utilizing alcohol- and phenol-derived electrophiles in cross-coupling reactions, which has become a research ...field of active and growing interest in organic chemistry. The synergy between computation and experiment elucidated the mechanistic model and controlling factors of selectivities in these transformations, leading to advances in innovative C–O bond activation and functionalization methods. Toward the rational design of C–O bond activation, our collaborations with the Jarvo group bridged the mechanistic models of C(sp2)–O and C(sp3)–O bond activations. We found that the nickel catalyst cleaves the benzylic and allylic C(sp3)–O bonds via two general mechanisms: the stereoinvertive SN2 back-side attack model and the stereoretentive chelation-assisted model. These two models control the stereochemistry in a wide array of stereospecific Ni-catalyzed cross-coupling reactions with benzylic or allylic alcohol derivatives. Because of the catalyst distortion, the ligands can differentiate the competing stereospecific C(sp3)–O bond activations. The PCy3 ligand interacts with nickel mainly through σ-donation, and the Ni(PCy3) catalyst can undergo facile bending of the substrate–nickel–ligand angle, which favors the stereoretentive benzylic C–O bond activation. The N-heterocyclic carbene SIMes ligand has additional d(metal)–p(ligand) back-donation with nickel, which leads to an extra energy penalty for the same angle bending. This results in the preference of stereoinvertive benzylic C–O bond activation under Ni/SIMes catalysis. In addition to ligand control, a Lewis acid can increase the selectivity for stereoinvertive C(sp3)–O activation by stabilizing the SN2 back-side attack transition state. The oxygen leaving group complexes with the MgI2 Lewis acid in the stereoinvertive activation, leading to the exclusive stereoinvertive Kumada coupling of benzylic ethers. We also identified that the competing C(sp3)–O bond activation models have noticeable differences in charge separation. This leads to the solvent polarity control of the stereospecificity in C(sp3)–O activations. Low-polarity solvents favor the neutral stereoretentive C–O bond activation, while high-polarity solvents favor the zwitterionic stereoinvertive cleavage. In sharp contrast to the nickel catalysts, the C(sp2)–O bond activation under palladium catalysis mainly proceeds via the classic three-membered ring oxidative addition mechanism instead of the chelation-assisted mechanism. This is due to the lower oxophilicity of palladium, which disfavors the oxygen coordination in the chelation-assisted-type activation. The three-membered ring activation model selectively cleaves the weak C–O bond, resulting in the exclusive chemoselectivity of acyl C–O bond activation in Pd-catalyzed cross-coupling reactions with aryl carboxylic acid derivatives. This explains the overall acylation in the Pd-catalyzed Suzuki–Miyaura coupling with aryl esters. In collaboration with the Szostak group, we revealed that the three-membered ring model applies in the Pd-catalyzed C–O bond activation of carboxylic acid anhydride, which stimulated the development of a series of Pd-catalyzed decarbonylative functionalizations of aryl carboxylic acids.
•We propose a spinning parallelogram operator (SPO) for depth estimation on EPI.•We apply the SPO to both synthetic and real light field camera images.•The SPO is proved to be insensitive to ...occlusion, noise, spatial aliasing.•The SPO has no requirement for depth range and angular resolution.
Removing the influence of occlusion on the depth estimation for light field images has always been a difficult problem, especially for highly noisy and aliased images captured by plenoptic cameras. In this paper, a spinning parallelogram operator (SPO) is integrated into a depth estimation framework to solve these problems. Utilizing the regions divided by the operator in an Epipolar Plane Image (EPI), the lines that indicate depth information are located by maximizing the distribution distances of the regions. Unlike traditional multi-view stereo matching methods, the distance measure is able to keep the correct depth information even if they are occluded or noisy. We further choose the relative reliable information among the rich structures in the light field to reduce the influences of occlusion and ambiguity. The discrete labeling problem is then solved by a filter-based algorithm to fast approximate the optimal solution. The major advantage of the proposed method is that it is insensitive to occlusion, noise, and aliasing, and has no requirement for depth range and angular resolution. It therefore can be used in various light field images, especially in plenoptic camera images. Experimental results demonstrate that the proposed method outperforms state-of-the-art depth estimation methods on light field images, including both real world images and synthetic images, especially near occlusion boundaries.
Reported is a highly enantioselective copper‐catalyzed Markovnikov protoboration of unactivated terminal alkenes. A variety of simple and abundant feedstock α‐olefins bearing a diverse array of ...functional groups and heterocyclic substituents can be used as substrates, and the reaction proceeds under mild reaction conditions at ambient temperature to provide expedient access to enantioenriched alkylboronic esters in good regioselectivity and with excellent enantiocontrol. Critical to the success of the protocol was the development and application of a novel, sterically hindered N‐heterocyclic carbene, (R,R,R,R)‐ANIPE, as the ligand for copper.
By design: A highly enantioselective copper‐catalyzed Markovnikov protoboration of α‐olefins is described. This mild and general process effectively converts simple and abundant feedstock α‐olefins into a diverse array of chiral alkylboronic esters with excellent enantiocontrol. Key to the success of the protocol was the use of a newly developed, sterically hindered N‐heterocyclic carbene, (R,R,R,R)‐ANIPE, as the ligand.
Fractional-order neural networks play a vital role in modeling the information processing of neuronal interactions. It is still an open and necessary topic for fractional-order neural networks to ...investigate their global stability. This paper proposes some simplified linear matrix inequality (LMI) stability conditions for fractional-order linear and nonlinear systems. Then, the global stability analysis of fractional-order neural networks employs the results from the obtained LMI conditions. In the LMI form, the obtained results include the existence and uniqueness of equilibrium point and its global stability, which simplify and extend some previous work on the stability analysis of the fractional-order neural networks. Moreover, a generalized projective synchronization method between such neural systems is given, along with its corresponding LMI condition. Finally, two numerical examples are provided to illustrate the effectiveness of the established LMI conditions.
The mechanisms and chemo- and regioselectivities of Ru(II)-catalyzed decarboxylative C–H alkenylation of aryl carboxylic acids with alkynes were investigated with density functional theory (DFT) ...calculations. The catalytic cycle involves sequential carboxylate-directed C–H activation, alkyne insertion, decarboxylation and protonation. The facile tether-assisted decarboxylation step directs the intermediate toward the desired decarboxylative alkenylation, instead of typical annulation and double alkenylation pathways. The decarboxylation barrier is very sensitive to the tether length, and only the seven-membered ring intermediate can selectively undergo the designed decarboxylation, suggesting a tether-dependent chemoselectivity. This tether-dependent chemoselectivity also applies to the alkyl tethers. In addition, the polarity of solvent is found to control the chemoselectivity between the decarboxylative alkenylation and 4 + 2 annulation. Solvent with low polarity (toluene) favors the decarboxylation pathway, leading to the decarboxylative alkenylation. Solvent with high polarity (methanol) favors the ionic stepwise C–O reductive elimination pathway, leading to the 4 + 2 annulation. To understand the origins of regioselectivity with asymmetric alkynes, the distortion/interaction analysis was applied to the alkyne insertion transition states, and led to a predictive frontier molecular orbital model. The asymmetric alkynes selectively use the terminal with the larger HOMO orbital coefficient to form the C–C bond in the insertion step.
