Nickel catalysts have shown unique ligand control of stereoselectivity in the Suzuki–Miyaura cross-coupling of boronates with benzylic pivalates and derivatives involving C(sp3)–O cleavage. The ...SIMes ligand (1,3-dimesityl-4,5-dihydroimidazol-2-ylidene) produces the stereochemically inverted C–C coupling product, while the tricyclohexylphosphine (PCy3) ligand delivers the retained stereochemistry. We have explored the mechanism and origins of the ligand-controlled stereoselectivity with density functional theory (DFT) calculations. The oxidative addition determines the stereoselectivity with two competing transition states, an SN2 back-side attack type transition state that inverts the benzylic stereogenic center and a concerted oxidative addition through a cyclic transition state, which provides stereoretention. The key difference between the two transition states is the substrate–nickel–ligand angle distortion; the ligand controls the selectivity by differentiating the ease of this angle distortion. For the PCy3 ligand, the nickel–ligand interaction involves mainly σ-donation, which does not require a significant energy penalty for the angle distortion. The facile angle distortion with PCy3 ligand allows the favorable cyclic oxidative addition transition state, leading to the stereoretention. For the SIMes ligand, the extra d–p back-donation from nickel to the coordinating carbene increases the rigidity of the nickel–ligand bond, and the corresponding angle distortion is more difficult. This makes the concerted cyclic oxidative addition unfavorable with SIMes ligand, and the back-side SN2-type oxidative addition delivers the stereoinversion.
Stacking-fault energy (SFE) is a critical parameter that influences plastic deformation of Cu alloys. Here, the SFEs of Cu solid solutions at various temperatures and solute concentrations were ...studied by using first-principles calculations. The interaction energies of 18 substitutional atoms and 5 interstitial atoms with stacking fault (SF) in Cu were obtained for the first time. The results show that the interactions can extend to the second layer with respect to the SF. Substitutional atoms Sn, Al, Zn, P, Si, Ge as well as interstitial atoms Be, C, N, O, H have large attraction energies with the SF; whereas, substitutional atoms Ti, Mn, Cr are repelled by the SF, and the rest of the atoms have small interaction energies. Then, the effects of uniform and Fermi-Dirac distribution of solute atoms on the SFE of Cu alloys were further investigated based on these interaction energies. It is found that the SFE of Cu can be drastically reduced by Sn, Al, Zn, P, Si, Ge, C, N and O atoms with relatively high concentration at low temperatures. Our research suggests that it is possible to obtain twins in bulk Cu by adding alloying elements at circumstances, since the SFEs can be significantly lowered.
•The atoms attracted by SFs can significantly reduce the SFEs of Cu solid solutions.•A few substitutional solutes can greatly lower the SFEs of Cu at low temperatures.•At low temperatures, some interstitial solutes may drastically decrease the SFEs.•The possibility of gaining twins in bulk Cu is increased under certain conditions.
This is the second installment for the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) Quasar Survey, which includes quasars observed from 2013 September to 2015 June. There are ...9024 confirmed quasars in DR2 and 10911 in DR3. After cross-match with the Sloan Digital Sky Survey (SDSS) quasar catalogs and NED, 12126 quasars are discovered independently. Among them, 2225 quasars were released by SDSS DR12 QSO catalog in 2014 after we finalized the survey candidates. 1801 sources were identified by SDSS DR14 as QSOs. The remaining 8100 quasars are considered as newly founded, and among them, 6887 quasars can be given reliable emission line measurements and the estimated black hole masses. Quasars found in LAMOST are mostly located at low-to-moderate redshifts, with a mean value of 1.5. The highest redshift observed in DR2 and DR3 is 5. We applied emission line measurements to H , Hβ, Mg ii, and C iv. We deduced the monochromatic continuum luminosities using photometry data, and estimated the virial black hole masses for the newly discovered quasars. Results are compiled into a quasar catalog, which will be available online.
The single-particle and many-body properties of twisted bilayer graphene (TBG) can be dramatically different from those of a single graphene layer, particularly when the two layers are rotated ...relative to each other by a small angle (θ ≈ 1°), owing to the moiré potential induced by the twist. Here we probe the collective excitations of TBG with a spatial resolution of 20 nm, by applying mid-infrared near-field optical microscopy. We find a propagating plasmon mode in charge-neutral TBG for θ = 1.1−1.7°, which is different from the intraband plasmon in single-layer graphene. We interpret it as an interband plasmon associated with the optical transitions between minibands originating from the moiré superlattice. The details of the plasmon dispersion are directly related to the motion of electrons in the moiré superlattice and offer an insight into the physical properties of TBG, such as band nesting between the flat band and remote band, local interlayer coupling, and losses. We find a strongly reduced interlayer coupling in the regions with AA stacking, pointing at screening due to electron–electron interactions. Optical nano-imaging of TBG allows the spatial probing of interaction effects at the nanoscale and potentially elucidates the contribution of collective excitations to many-body ground states.Moiré potentials substantially alter the electronic properties of twisted bilayer graphene at a magic twist angle. A propagating plasmon mode, which can be observed with optical nano-imaging, is associated with transitions between the moiré minibands.
Empirical critical loads for N deposition effects and maps showing areas projected to be in exceedance of the critical load (CL) are given for seven major vegetation types in California. Thirty-five ...percent of the land area for these vegetation types (99,639
km
2) is estimated to be in excess of the N CL. Low CL values (3–8
kg
N
ha
−1
yr
−1) were determined for mixed conifer forests, chaparral and oak woodlands due to highly N-sensitive biota (lichens) and N-poor or low biomass vegetation in the case of coastal sage scrub (CSS), annual grassland, and desert scrub vegetation. At these N deposition critical loads the latter three ecosystem types are at risk of major vegetation type change because N enrichment favors invasion by exotic annual grasses. Fifty-four and forty-four percent of the area for CSS and grasslands are in exceedance of the CL for invasive grasses, while 53 and 41% of the chaparral and oak woodland areas are in exceedance of the CL for impacts on epiphytic lichen communities. Approximately 30% of the desert (based on invasive grasses and increased fire risk) and mixed conifer forest (based on lichen community changes) areas are in exceedance of the CL. These ecosystems are generally located further from emissions sources than many grasslands or CSS areas. By comparison, only 3–15% of the forested and chaparral land areas are estimated to be in exceedance of the NO
3
− leaching CL. The CL for incipient N saturation in mixed conifer forest catchments was 17
kg
N
ha
−1
yr
−1. In 10% of the CL exceedance areas for all seven vegetation types combined, the CL is exceeded by at least 10
kg
N
ha
−1
yr
−1, and in 27% of the exceedance areas the CL is exceeded by at least 5
kg
N
ha
−1
yr
−1. Management strategies for mitigating the effects of excess N are based on reducing N emissions and reducing site N capital through approaches such as biomass removal and prescribed fire or control of invasive grasses by mowing, selective herbicides, weeding or domestic animal grazing. Ultimately, decreases in N deposition are needed for long-term ecosystem protection and sustainability, and this is the only strategy that will protect epiphytic lichen communities.
Atmospheric humidity in the urban canopy layer is relevant to various fields, such as bioclimatology and public health. However, considerably less research has been conducted about urban effects on ...humidity than on temperature. This study aims to elucidate the spatiotemporal characteristics of humidity at the neighborhood scale. The humidity differences of 8 local climate zones (LCZs) in Nanjing, China, were analyzed based on three-year hourly observations. LCZ D (low plants) was used as the reference site. Differences in temperature (ΔTX−D), relative humidity (ΔRHX−D), and humidity ratio (ΔWX−D) were investigated. The relationships of ΔW with condensation (dew and frost) and precipitation events, meteorological parameters, and ΔT were examined. The results show that variations in ΔRHX−D exhibited a close inverse relationship with variations in ΔTX−D. Greater negative ΔRHX−D occurred during the nighttime when strong heat islands were present. Meanwhile, diurnal and seasonal patterns of ΔWX−D were clearly observed. Greater and more frequent negative ΔWX−D (moisture deficit) were observed during the daytime than during the nighttime and in warm months than in cold months, whereas the opposite trend was observed for positive ΔWX−D (moisture excess). The frequency of urban moisture excess increased during the nighttime with heavy condensation. Precipitation events significantly increased the frequency of urban moisture excess, and this effect could last until 24 h after precipitation. ΔW was influenced by meteorological parameters, but the trends were not clear. No relation was observed between ΔW and ΔT. The findings can improve understanding of urban humidity and provide valuable information for humidity-related studies.
•Hourly temperature and humidity data were collected from 8 LCZ sites for 3 years.•Differences in relative humidity and humidity ratio among the LCZ sites were analyzed.•Impact of condensation and precipitation events on humidity difference was examined.•ΔRH among LCZ classes is significant at night but insignificant during daytime.•Urban moisture deficit is more frequent and greater during daytime and warm months.
To determine the imaging characteristics of SAPHO (synovitis, acne, pustulosis, hyperostosis, and osteitis) syndrome and seronegative spondyloarthropathies (SpAs) on whole-spine magnetic resonance ...imaging (WS-MRI) and evaluate the role of WS-MRI in the diagnosis and differentiation of the two diseases.
Twenty-eight patients with SAPHO and 44 with SpAs were included. All patients were symptomatic and clinically diagnosed with SAPHO or SpAs, and all underwent WS-MRI for comparison of imaging characteristics.
The mean age of the SAPHO patients was 48.7 ± 12.7 years, while that of the SpA patients was 34.7 ± 12.3 years (p<0.001). WS-MRI showed that the frequency of cervical, thoracic, and lumbar spine involvement was 53.6% versus 52.3%, 75% versus 88.6%, and 60.7% versus 63.6%, respectively (p=0.70, 0.13, and 0.80). The frequency of sacroiliac joint involvement was 7.1% and 100% (p<0.001). Continuous spinal involvement accounted for 50% versus 43.2%, 60.7% versus 84.1%, and 39.3% versus 40.9% in the cervical, thoracic, and lumbar vertebrae, respectively (p=0.03). WS-MRI showed that bone marrow oedema of spinal anterior corner was observed in 50% versus 75% (p=0.03). Vertebral body and posterior attachment involvement accounted for 85.7% versus 93.2% and 14.3% versus 34.1% (p=0.3, 0.06). The frequency of bone erosion in mobile spine was 75% and 36.4%, respectively (p=0.02). The frequency of intervertebral disc, endplate, anterior thoracic wall, and paraspinal soft-tissue swelling was 42.9% versus 18.2%, 53.6% versus 22.7%, 85.7% versus 42.2%, and 50% versus 11.4% (p=0.02, 0.00).
Factors differentiating the two groups at WS-MRI were bone marrow oedema of the spinal anterior corner, bone erosion, and swelling of the intervertebral disc, endplate, anterior thoracic wall, and paraspinal soft-tissue.
•WS-MRI can comprehensively assess spinal involvement in SAPHO and SpA.•WS-MRI can provide related information in distinguishing SAPHO and SpA.•WS-MRI can also provide information about lesion activity.
A 173-point genetic linkage map of cucumber (Cucumis sativus L.), consisting of 116 SRAPs, 33 RAPDs, 11 SSRs, 9 SCARs, 3 ISSRs, and 1 STS, was constructed using 130 F₂ progeny derived from a narrow ...cross between line S94 (Northern China open-field type) and line S06 (greenhouse European type). The seven linkage groups spanned 1016 cM with a mean marker interval of 5.9 cM. Using the F₂ population and its F₃ derived families, a total of 38 QTLs were detected on five linkage groups with an LOD threshold of 3.0 for nine fruit-related traits: fruit weight, length, and diameter, fruit flesh thickness, seed-cavity diameter, fruit-stalk length, fruit pedicel length, length/diameter and length/stalk ratio. Of the identified QTLs, fsl4.3 for fruit-stalk length explained the largest portion of phenotypic variation (r² = ~30%). Several QTLs were detected in the same linkage region in different generations and different seasons. Additionally, several QTLs for various fruit traits were mapped to the same or neighbouring marker intervals, suggesting they are possible character associations for controlling cucumber fruit development.
A compact online proton spectrometer based on a CMOS plate detector was proposed for the diagnosis of picosecond intense-laser accelerated protons. Inside the CMOS plate detector, a composite ...structure that composed of a scintillator screen, a fiber optic plate (FOP), and a CMOS sensor was designed to detect the protons safely and efficiently. The scintillator screen converted the incident protons to visible light, which, in turn, was collected by using the FOP and detected by using the CMOS sensor. An additional aluminum housing was designed to decrease the strong electromagnetic pulse (EMP) impact on the electronic readout system from picosecond laser–plasma interaction. This online spectrometer is compact and cost-effective than the system based on the micro-channel plate or scintillator screen in combination with an electron multiplying CCD camera and has better resistance to radiation damage than the system based on a single CMOS sensor.
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