Do Diradicals Behave Like Radicals? Stuyver, Thijs; Chen, Bo; Zeng, Tao ...
Chemical reviews,
11/2019, Volume:
119, Issue:
21
Journal Article
Peer reviewed
Open access
This review sets out to understand the reactivity of diradicals and how that may differ from monoradicals. In the first part of the review, we delineate the electronic structure of a diradical with ...its two degenerate or nearly degenerate molecular orbitals, occupied by two electrons. A classification of diradicals based on whether or not the two SOMOs can be located on different sites of the molecule is useful in determining the ground state spin. Important is a delocalized to localized orbital transformation that interchanges “closed-shell” to “open-shell” descriptions. The resulting duality is useful in understanding the dual reactivity of singlet diradicals. In the second part of the review, we examine, with a consistent level of theory, activation energies of prototypical radical reactions (dimerization, hydrogen abstraction, and addition to ethylene) for representative organic diradicals and diradicaloids in their two lowest spin states. Differences and similarities in reactivity of diradicals vs monoradicals, based on either a localized or delocalized view, whichever is suitable, are then discussed. The last part of this review begins with an extensive, comparative, and critical survey of available measures of diradical character and ends with an analysis of the consequences of diradical character for selected diradicaloids.
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IJS, KILJ, NUK, PNG, UL, UM
•The control mechanism of low-temperature PODEn/gasoline combustion was revealed.•The collaborative organization of fuel reactivity and concentration dominates PPC.•Reliable control can effectively ...be achieved at the PR of 14.9% for R-RCCI.•R-RCCI can achieve 26% reduction in NOx emissions compared with PPC.•Lower Tin is required for R-RCCI to achieve stable combustion than PPC.
This study aims to investigate the control mechanism of fuel properties and intake temperature (Tin) on the low-temperature polyoxymethylene dimethyl ethers (PODEn)/gasoline combustion to maximize the advantages of PODEn in enhancing engine performance. To achieve this goal, two representative combustion modes of partially premixed combustion (PPC) and reactivity-controlled compression ignition with reverse reactivity stratification (R-RCCI) with different fuel regulating methods were investigated. In PPC, the fuel was regulated by blending PODEn and gasoline outside the cylinder, while for R-RCCI, the in-cylinder fuel was tuned by delivering the two fuels into the cylinder with two different fuel supply systems. The main factors dominating the combustion process and pollutant emissions were identified for these two combustion modes. The results show that the combustion process of PPC is dominated by the collaborative organization of fuel reactivity and concentration in the cylinder, and 50% burn point (CA50) plays only a secondary role, but CA50 determines the nitrogen oxides (NOx) emission level. For PPC operated with the start of injection later than −40 °CA ATDC, increasing local fuel concentration is more effective in improving the combustion efficiency and indicated thermal efficiency than increasing Tin. For R-RCCI, the lower local temperature caused by the heat of vaporization of the directly injected gasoline remarkably influences the combustion process, but this cooling effect is not significant in PPC. The comparison of PPC and R-RCCI shows that the in-cylinder local fuel reactivity impacts the ignition more significantly than the overall fuel reactivity. Compared with PPC, R-RCCI can effectively reduce combustion instability and combustion rate, and simultaneously 26% reduction in NOx emissions was accomplished.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPUK, ZAGLJ, ZRSKP
•The potential of R-RCCI was demonstrated by comparing with PPC and RCCI.•R-RCCI has the highest ITE compared with PPC and RCCI.•R-RCCI has very low soot emissions at both low and medium loads.•The ...reasons for the great advantages of R-RCCI were explained in detail.
To solve the challenges of the high heat release rate of partially premixed combustion (PPC) and low combustion efficiency of reactivity-controlled compression ignition (RCCI), RCCI with reverse reactivity stratification (R-RCCI) fueled with gasoline and polyoxymethylene dimethyl ethers (PODEn) was investigated at low and medium loads in a light-duty compression-ignition engine in this study. The results indicated that R-RCCI has moderate combustion rate and combustion efficiency between PPC and RCCI. At both low and medium loads, R-RCCI has the highest indicated thermal efficiency (ITE) due to its relatively lower heat transfer losses compared with PPC. Moreover, the nitrogen oxides (NOx) and soot emissions of R-RCCI are significantly lower than that of PPC. Although RCCI has the lowest heat transfer losses, its considerably low combustion rate leads to substantial exhaust losses and thereby yields a low ITE. Compared with RCCI, R-RCCI has a better balance between the degree of premixed combustion and combustion temperature, which is crucial for soot oxidation, so it has very low soot emissions at both low and medium loads.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPUK, ZAGLJ, ZRSKP
Iron-hydride and alkene constitute a promising pair of reactants to access alkyl carbon radicals. However, the fundamental mechanistic scenario remains unclear. High-level coupled cluster ...calculations reported in this work demonstrate that for iron(III)-hydrides, hydrogen atom transfer (HAT) is favored over hydrometalation. The oxidation state of iron makes a remarkable difference in the reactivity of the iron-hydride such that ferric FeIII–H is much more reactive in HAT than ferrous FeII–H. Akin to iron(IV)-oxo, exchange-enhanced reactivity (EER) dictates the intrinsic spin HAT reactivity of iron(III)-hydride. However, it is not two-state reactivity (TSR) but single-state reactivity (SSR) that operates in HAT with iron(III)-hydride. The reactivity insights gained herein for alkyl liberation from iron-alkyls may have profound mechanistic implications on the iron-dependent bioorganometallic radical chemistry in radical SAM enzymes.
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Realizing the extraordinary potential of unactivated sp³ C-H bond oxidation in organic synthesis requires the discovery of catalysts that are both highly reactive and predictably selective. We report ...an iron (Fe)-based small molecule catalyst that uses hydrogen peroxide (H₂O₂) to oxidize a broad range of substrates. Predictable selectivity is achieved solely on the basis of the electronic and steric properties of the C-H bonds, without the need for directing groups. Additionally, carboxylate directing groups may be used to furnish five-membered ring lactone products. We demonstrate that these three modes of selectivity enable the predictable oxidation of complex natural products and their derivatives at specific C-H bonds with preparatively useful yields. This type of general and predictable reactivity stands to enable aliphatic C-H oxidation as a method for streamlining complex molecule synthesis.
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BFBNIB, NMLJ, NUK, PNG, SAZU, UL, UM, UPUK
Motivated by the recent synthesis of a new polymorph of germanium selenide (γ-GeSe) with a honeycomb lattice and an astonishingly high conductivity (even higher than graphite), here we conduct a ...study on the hydrogen evolution reaction (HER) electrocatalytic performance and electronic properties of γ-GeSe with respect to the thickness effect from monolayer (1L) to trilayer (3L), along with the defect effect. The band structure of γ-GeSe exhibits a camel’s back-like structure near the Γ point, existing for all the layers and in the presence of dilute Se vacancy and surface adsorbate like the Pt atom, and a narrow bandgap ranging from 0.544 eV for 1L to 0.252 eV for 3L. We find that pristine γ-GeSe is electrocatalytically inert for all the layers with an endothermic uptake of hydrogen, as indicated by the calculated Gibbs free energy (ΔG H). However, upon introduction of Se vacancies and surface Pt adsorbates, the HER performance is enhanced, with the HER activity of 1L γ-GeSe surpassing those of bilayer (2L) and 3L γ-GeSe. Moreover, an increase in defect concentration and thickness leads to a decrease in bandgap, developing semimetallic characteristics. With the absence of a transition element and solely s and p orbitals, semimetallic γ-GeSe is unique and holds great promise as a support for fabricating a single atomic catalyst for HER, and our work offers valuable insights into the rational design of 2D electrocatalysts.
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TiO2/BiOCl heterojunction photocatalysts were synthesized from different exposed facets (BiOCl(010) and BiOCl(001)). Both heterojunctions had higher photocatalytic activity than their respective ...components. The favorable energy band shifts constructed an efficient Z-scheme TiO2/BiOCl heterostructure photocatalytic system, which significantly reduced recombination and accelerated the separation rate of photogenerated carriers. Due to the different electrostatic fields inside the crystal planes of BiOCl, electrons migrate from BiOCl to the TiO2 surface through different transport paths, resulting in the charge transport distance of photogenerated electrons in BiOCl(010) being shorter than that in BiOCl(001), and the electron loss was lower. Therefore, TiO2/BiOCl(010) had a higher photocatalytic activity. Besides, the photocatalytic oxidization mechanism was proposed, TiO2/BiOCl(010) oxidizing NOF mainly via photogenerated h+, while TiO2/BiOCl(001) oxidizing NOF through h+ and ·OH. This study contributes to an in-depth understanding of the charge transfer behavior in the TiO2/BiOCl heterojunction structure and provides a reference for the design of efficient heterojunction photocatalysts.
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Understanding humoral responses to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is critical for improving diagnostics, therapeutics, and vaccines. Deep serological profiling of 232 ...coronavirus disease 2019 (COVID-19) patients and 190 pre-COVID-19 era controls using VirScan revealed more than 800 epitopes in the SARS-CoV-2 proteome, including 10 epitopes likely recognized by neutralizing antibodies. Preexisting antibodies in controls recognized SARS-CoV-2 ORF1, whereas only COVID-19 patient antibodies primarily recognized spike protein and nucleoprotein. A machine learning model trained on VirScan data predicted SARS-CoV-2 exposure history with 99% sensitivity and 98% specificity; a rapid Luminex-based diagnostic was developed from the most discriminatory SARS-CoV-2 peptides. Individuals with more severe COVID-19 exhibited stronger and broader SARS-CoV-2 responses, weaker antibody responses to prior infections, and higher incidence of cytomegalovirus and herpes simplex virus 1, possibly influenced by demographic covariates. Among hospitalized patients, males produce stronger SARS-CoV-2 antibody responses than females.
There exist numerous challenges in hydrogen generation, storage, and transportation. Ammonia has been attractive as an efficient hydrogen carrier by virtue of its high specific density. Designing ...outstanding catalysts for converting NH3 through the ammonia oxidation reaction (AOR) is significant. Because of the synergistic effects resulting from the interactions between metal and support, it is believed that the support is significant for improving the catalyst’s activity. Owing to this, the activity of small M n (M = Pt, Ir; n = 3, 4) clusters supported by CeO2(111) surfaces was theoretically explored. The M n /CeO2(111) surfaces kinetically exhibit the G–M reaction mechanism rather than the N + N mechanism. In addition, we determined the range of operating onset potentials over M n /CeO2(111) surfaces for effective AOR. Using the Sabatier analysis, we also predicted that Pt3/CeO2(111) is the most active catalyst for AOR.
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