•Review of environmental redox classifications.•Calibration of commonly used elemental proxies to redox facies.•Analysis of proxies from modern Black Sea, Saanich Inlet, and California Margin.•Use of ...compound covariation of redox proxies to identify key redox thresholds.•No universal proxy values: calibration necessary for each depositional system under study.
Existing redox classifications and the calibrations of elemental proxies to modern environmental redox scales are in need of re-evaluation. Here, we review environmental redox classifications, commonly used elemental redox proxies, and their intercalibration, and we propose a novel approach to improve the calibration of such proxies, using datasets from the modern Black Sea, Saanich Inlet, and California Margin as examples. Our approach is based on recognition of compound covariation patterns among pairs of elemental redox proxies within a redox framework based on three key thresholds: (1) the Re4+/Re3− couple near the suboxidized/subreduced boundary of the suboxic zone, (2) the U6+/U4+ couple in the middle of the subreduced zone, and (3) the SO42−/H2S couple at the suboxic/euxinic boundary. Within this framework, it is possible to determine the relative timing of onset and the degree of enrichment of other elemental redox proxies. Our analysis demonstrates that, even though some elements exhibit limited enrichment within the suboxic zone, the bulk of authigenic enrichment of the redox-sensitive elements considered in this study occurs within the euxinic zone. One important finding of our study is that the threshold value associated with a given elemental proxy can vary considerably between depositional systems. For this reason, it is inadvisable to transfer published threshold values (i.e., from earlier paleoredox studies) to completely different formations, and redox proxies must be internally calibrated for each individual paleodepositional system under investigation.
We are entering an era that emphasizes greenness and sustainability. Based on such a philosophy, it is critical to uncover novel and original sustainable reaction modes for future green chemical ...syntheses. The cross-dehydrogenative coupling (CDC) reaction has thus been widely developed as one of the most sustainable and efficient synthesis strategies for constructing C-C bonds. This review summarizes the development of this field over the past 20 years, with a discussion on future trends and directions: from the original reaction model at the beginning and its development in the first decade, to extensive research in the second decade. The latest development sees the emergence of alternative forms of energy inputs (photoredox, mechano, microwave, electrochemical, continuous-flow and solar quantum dots) to facilitate CDC reactions, gradually replacing the classical form of thermal energy, which will inspire broader applications and innovations in the future.
We provide a review of the progress of cross-dehydrogenative coupling reactions in constructing a wide variety of C-C bonds. Sustainable cross-dehydrogenative coupling reactions can be combined with multiple forms of energy output.
In this work, a theoretical method for the prediction of both the relaxation time of quantum tunneling of magnetization (
τ
QTM
) and the effective barrier of magnetic reversal (
U
eff
) is proposed ...for single-ion magnet (SIM) systems of Kramers type. The reliability of theoretical
τ
QTM
is tested within a large series of 18 lanthanide SIMs. Compared to the experimental results, the deviations of theoretical
τ
QTM
are within one order of magnitude for 11 tested SIMs and the largest order-of-magnitude deviation is only 1.86. In the aspect of
U
eff
, for 5 typical high-performance Dy-SIMs of the local coordination mode of a pentagonal bipyramid, the relative deviations of theoretical values lie within the range of 1.4-7.2%. Thus this method possesses good reliability, at least in the aspect of the order of magnitude. Besides an empirical estimate of the local magnetic field experienced by the central ion, for a given SIM, one
ab initio
calculation, providing accurate
g
-factors of both ground and excited Kramers doublets (KDs), is the only computational cost. Therefore this method has a high degree of both reliability and efficiency. Based on the temperature dependence of theoretically predicted
U
eff
and its contributions from various KDs, some mechanistic information on the magnetic relaxation could be given by this method too. Therefore it is reasonable to expect the bright prospect of this method in the aspects of both the interpretation of the existing experimental results and rational design of future high-performance SIMs.
A method to predict the relaxation time of quantum tunneling of magnetization and the magnetic reversal barrier with efficiency and reliability.
Following the severe acute respiratory syndrome coronavirus (SARS‐CoV) and Middle East respiratory syndrome coronavirus (MERS‐CoV), another highly pathogenic coronavirus named SARS‐CoV‐2 (previously ...known as 2019‐nCoV) emerged in December 2019 in Wuhan, China, and rapidly spreads around the world. This virus shares highly homological sequence with SARS‐CoV, and causes acute, highly lethal pneumonia coronavirus disease 2019 (COVID‐19) with clinical symptoms similar to those reported for SARS‐CoV and MERS‐CoV. The most characteristic symptom of patients with COVID‐19 is respiratory distress, and most of the patients admitted to the intensive care could not breathe spontaneously. Additionally, some patients with COVID‐19 also showed neurologic signs, such as headache, nausea, and vomiting. Increasing evidence shows that coronaviruses are not always confined to the respiratory tract and that they may also invade the central nervous system inducing neurological diseases. The infection of SARS‐CoV has been reported in the brains from both patients and experimental animals, where the brainstem was heavily infected. Furthermore, some coronaviruses have been demonstrated able to spread via a synapse‐connected route to the medullary cardiorespiratory center from the mechanoreceptors and chemoreceptors in the lung and lower respiratory airways. Considering the high similarity between SARS‐CoV and SARS‐CoV2, it remains to make clear whether the potential invasion of SARS‐CoV2 is partially responsible for the acute respiratory failure of patients with COVID‐19. Awareness of this may have a guiding significance for the prevention and treatment of the SARS‐CoV‐2‐induced respiratory failure.
Research Highlights
SARS‐CoV2 causes epidemic pneumonia characterized by acute respiratory distress.
This novel coronavirus is similar to SARS‐CoV in sequence, pathogenesis, and cellular entry.
Some coronaviruses can invade brainstem via a synapse‐connected route from the lung and airways.
The potential invasion of SARS‐CoV2 may be one reason for the acute respiratory failure.
Awareness of this will have guiding significance for the prevention and treatment.
Conventional approaches for Pd‐catalyzed ring‐opening cross‐couplings of gem‐difluorocyclopropanes with nucleophiles predominantly deliver the β‐fluoroalkene scaffolds (linear selectivity). Herein, ...we report a cooperative strategy that can completely switch the reaction selectivity to give the alkylated α‐fluoroalkene skeletons (branched selectivity). The unique reactivity of hydrazones that enables analogous inner‐sphere 3,3′‐reductive elimination driven by denitrogenation, as well as the assistance of steric‐embedded N‐heterocyclic carbene ligand, are the key to switch the regioselectivity. A wide range of hydrazones derived from naturally abundant aryl and alkyl aldehydes are well applicable, and various gem‐difluorocyclopropanes, including modified pharmaceutical and biological molecules, can be efficiently functionalized with high value alkylated α‐fluorinated alkene motifs under mild conditions.
A highly effective Pd‐catalyzed defluorinative alkylation of gem‐difluorocyclopropane with branched selectivity was achieved by using a cooperative strategy that integrated the unique trifunctional character of hydrazones with Pd/NHC catalysis.
Employing phenols and phenol derivatives as electrophiles for cross-coupling reactions has numerous advantages over commonly used aryl halides in terms of environmental-friendliness and ...sustainability. In the early stage of discovering such transformations, most efforts have been devoted to utilizing highly activated sulfonate types of phenol derivatives (e.g., OTf, OTs, etc.), which have similar reactivities to the corresponding aryl halides. However, a continuing scientific challenge is how to achieve the direct C–O functionalizations of relatively less-activated phenol derivatives more efficiently. In this review, we will focus on the recent updates on the C–O functionalizations of less-activated phenol derivatives, from aryl carboxylates (e.g., pivalates, acetates, etc.), aryl carbamates and carbonates, to aryl ethers (anisoles, diaryl ethers, aryl pyridyl ethers, aryl silyl ethers), to phenolate salts, and ultimately to simply unprotected phenols, sorted by the types of bond formations. Both transition-metal-catalyzed and transition-metal-free protocols will be covered and discussed in detail. Instead, the C–O functionalizations of aryl sulfonates will not be covered extensively unless they are closely related, due to their high reactivity. Since aryl ethers and phenols represent the main linkages or units in lignin biomass, the successes of such transformations will potentially make major contributions to the direct lignin biomass upgrading and depolymerization.
Synthetic chemists aspire both to develop novel chemical reactions and to improve reaction conditions to maximize resource efficiency, energy efficiency, product selectivity, operational simplicity, ...and environmental health and safety. Carbon−carbon bond formation is a central part of many chemical syntheses, and innovations in these types of reactions will profoundly improve overall synthetic efficiency. This Account describes our work over the past several years to form carbon−carbon bonds directly from two different C−H bonds under oxidative conditions, cross-dehydrogenative coupling (CDC). We have focused most of our efforts on carbon−carbon bonds formed via the functionalization of sp3 C−H bonds with other C−H bonds. In the presence of simple and cheap catalysts such as copper and iron salts and oxidants such as hydrogen peroxide, dioxygen, tert-butylhydroperoxide, and 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ), we can directly functionalize various sp3 C−H bonds by other C−H bonds without requiring preactivation. We demonstrate (1) reaction of α-C−H bonds of nitrogen in amines, (2) reaction of α-C−H bonds of oxygen in ethers, (3) reaction of allylic and benzylic C−H bonds, and (4) reaction of alkane C−H bonds. These CDC reactions can tolerate a variety of functional groups, and some can occur under aqueous conditions. Depending on the specific transformation, we propose the in situ generation of different intermediates. These methods provide an alternative to the separate steps of prefunctionalization and defunctionalization that have traditionally been part of synthetic design. As a result, these methods will increase synthetic efficiencies at the most fundamental level. On an intellectual level, the development of C−C bond formations based on the reaction of only C−H bonds (possibly in water) challenges us to rethink some of the most fundamental concepts and theories regarding chemical reactivities. A successful reaction requires the conventionally and theoretically less reactive C−H bonds to react selectively in the presence of a variety of functional groups. With further investigation, we expect that C−C bond formations based on cross-dehydrogenative coupling will have a positive economic and ecological impact on the next generation of chemical syntheses.
Quantum computational advantage using photons Zhong, Han-Sen; Wang, Hui; Deng, Yu-Hao ...
Science (American Association for the Advancement of Science),
12/2020, Letnik:
370, Številka:
6523
Journal Article
Recenzirano
Odprti dostop
Quantum computers promise to perform certain tasks that are believed to be intractable to classical computers. Boson sampling is such a task and is considered a strong candidate to demonstrate the ...quantum computational advantage. We performed Gaussian boson sampling by sending 50 indistinguishable single-mode squeezed states into a 100-mode ultralow-loss interferometer with full connectivity and random matrix-the whole optical setup is phase-locked-and sampling the output using 100 high-efficiency single-photon detectors. The obtained samples were validated against plausible hypotheses exploiting thermal states, distinguishable photons, and uniform distribution. The photonic quantum computer,
, generates up to 76 output photon clicks, which yields an output state-space dimension of 10
and a sampling rate that is faster than using the state-of-the-art simulation strategy and supercomputers by a factor of ~10
.
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