Origin of Enantioselection in Chiral Alcohol Oxidation Catalyzed by Pd[(-)-sparteine]Cl2

A kinetic investigation into the origin of enantioselectivity for the Pd[(-)-sparteine]Cl2-catalyzed aerobic oxidative kinetic resolution (OKR) is reported. A mechanism to account for a newly discovered chloride dissociation from Pd[(-)-sparteine]Cl2 prior to alcohol binding is proposed. The mechanism includes (1) chloride dissociation from Pd[(-)-sparteine]Cl2 to form cationic Pd(-)-sparteine]Cl, (2) alcohol binding, (3) deprotonation of Pd-bound alcohol to form a Pd-alkoxide, and (4) â-hydride elimination of Pd-alkoxide to form ketone product and a Pd-hydride. Utilizing the addition of (-)-sparteine HCl to control the [Cl-] and [H+] and the resulting derived rate law, the key microscopic kinetic and thermodynamic constants were extracted for each enantiomer of sec-phenethyl alcohol. These constants allow for the successful simulation of the oxidation rate in the presence of exogenous (-)-sparteine HCl. A rate law for oxidation of the racemic alcohol was derived that allows for the successful prediction of the experimentally measured krel values when using the extracted constants. Besides a factor of 10 difference between the relative rates of â-hydride elimination for the enantiomers, the main enhancement in enantiodetermination results from a concentration effect of (-)-sparteine HCl and the relative rates of reprotonation of the diastereomeric Pd-alkoxides

The auxiliary use of LANDSAT data in estimating crop acreages: Results of the 1975 Illinois crop-acreage experiment

The author has identified the following significant results. It was found that classifier performance was influenced by a number of temporal, methodological, and geographical factors. Best results were obtained when corn was tasselled and near the dough stage of development. Dates earlier or later in the growing season produced poor results. Atmospheric effects on results cannot be independently measured or completely separated from the effects due to the maturity stage of the crops. Poor classifier performance was observed in areas where considerable spectral confusion was present

Realization of an Asymmetric Non‐Aqueous Redox Flow Battery through Molecular Design to Minimize Active Species Crossover and Decomposition

This communication presents a mechanism‐based approach to identify organic electrolytes for non‐aqueous redox flow batteries (RFBs). Symmetrical flow cell cycling of a pyridinium anolyte and a cyclopropenium catholyte resulted in extensive capacity fade due to competing decomposition of the pyridinium species. Characterization of this decomposition pathway enabled the rational design of next‐generation anolyte/catholyte pairs with dramatically enhanced cycling performance. Three factors were identified as critical for slowing capacity fade: (1) separating the anolyte–catholyte in an asymmetric flow cell using an anion exchange membrane (AEM); (2) moving from monomeric to oligomeric electrolytes to limit crossover through the AEM; and (3) removing the basic carbonyl moiety from the anolyte to slow the protonation‐induced decomposition pathway. Ultimately, these modifications led to a novel anolyte–catholyte pair that can be cycled in an AEM‐separated asymmetric RFB for 96 h with >95 % capacity retention at an open circuit voltage of 1.57 V.Applied molecular design! This study presents a mechanism‐based approach to the molecular design of electrolytes for implementation in an asymmetric non‐aqueous redox flow battery.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/154972/1/chem202000749-sup-0001-misc_information.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154972/2/chem202000749.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154972/3/chem202000749_am.pd

Nitrogen and oxygen isotope constraints on the origin of atmospheric nitrate in coastal Antarctica

Throughout the year 2001, aerosol samples were collected continuously for 10 to 15 days at the French Antarctic Station Dumont d'Urville (DDU) (66°40' S, l40°0' E, 40 m above mean sea level). The nitrogen and oxygen isotopic ratios of particulate nitrate at DDU exhibit seasonal variations that are among the most extreme observed for nitrate on Earth. In association with concentration measurements, the isotope ratios delineate four distinct periods, broadly consistent with previous studies on Antarctic coastal areas. During austral autumn and early winter (March to mid-July), nitrate concentrations attain a minimum between 10 and 30 ng m^−3 (referred to as Period 2). Two local maxima in August (55 ng m^−3) and November/December (165 ng m^−3) are used to assign Period 3 (mid-July to September) and Period 4 (October to December). Period 1 (January to March) is a transition period between the maximum concentration of Period 4 and the background concentration of Period 2. These seasonal changes are reflected in changes of the nitrogen and oxygen isotope ratios. During Period 2, which is characterized by background concentrations, the isotope ratios are in the range of previous measurements at mid-latitudes: δ¹⁸O_vsmow=(77.2±8.6)‰; Δ¹⁷O=(29.8±4.4)‰; δ¹⁵N_air=(−4.4±5.4)‰ (mean ± one standard deviation). Period 3 is accompanied by a significant increase of the oxygen isotope ratios and a small increase of the nitrogen isotope ratio to δ¹⁸O_vsmow=(98.8±13.9)‰; Δ¹⁷O=(38.8±4.7)‰ and δ¹⁵N_air=(4.3±8.20‰). Period 4 is characterized by a minimum ¹⁵N/¹⁴N ratio, only matched by one prior study of Antarctic aerosols, and oxygen isotope ratios similar to Period 2: δ¹⁸O_vsmow=(77.2±7.7)‰; Δ¹⁷O=(31.1±3.2)‰; δ¹⁵N_air=(−32.7±8.4)‰. Finally, during Period 1, isotope ratios reach minimum values for oxygen and intermediate values for nitrogen: δ¹⁸O_vsmow=63.2±2.5‰; Δ¹⁷O=24.0±1.1‰; δ¹⁵N_air=−17.9±4.0‰). Based on the measured isotopic composition, known atmospheric transport patterns and the current understanding of kinetics and isotope effects of relevant atmospheric chemical processes, we suggest that elevated tropospheric nitrate levels during Period 3 are most likely the result of nitrate sedimentation from polar stratospheric clouds (PSCs), whereas elevated nitrate levels during Period 4 are likely to result from snow re-emission of nitrogen oxide species. We are unable to attribute the source of the nitrate during periods 1 and 2 to local production or long-range transport, but note that the oxygen isotopic composition is in agreement with day and night time nitrate chemistry driven by the diurnal solar cycle. A precise quantification is difficult, due to our insufficient knowledge of isotope fractionation during the reactions leading to nitrate formation, among other reasons

Avoiding catastrophic failure in correlated networks of networks

Networks in nature do not act in isolation but instead exchange information, and depend on each other to function properly. An incipient theory of Networks of Networks have shown that connected random networks may very easily result in abrupt failures. This theoretical finding bares an intrinsic paradox: If natural systems organize in interconnected networks, how can they be so stable? Here we provide a solution to this conundrum, showing that the stability of a system of networks relies on the relation between the internal structure of a network and its pattern of connections to other networks. Specifically, we demonstrate that if network inter-connections are provided by hubs of the network and if there is a moderate degree of convergence of inter-network connection the systems of network are stable and robust to failure. We test this theoretical prediction in two independent experiments of functional brain networks (in task- and resting states) which show that brain networks are connected with a topology that maximizes stability according to the theory.Comment: 40 pages, 7 figure

Two Modes of Change in Southern Ocean Productivity Over the Past Million Years

Export of organic carbon from surface waters of the Antarctic Zone of the Southern Ocean decreased during the last ice age, coinciding with declining atmospheric carbon dioxide (CO2) concentrations, signaling reduced exchange of CO2 between the ocean interior and the atmosphere. In contrast, in the Subantarctic Zone, export production increased into ice ages coinciding with rising dust fluxes, thus suggesting iron fertilization of subantarctic phytoplankton. Here, a new high-resolution productivity record from the Antarctic Zone is compiled with parallel subantarctic data over the past million years. Together, they fit the view that the combination of these two modes of Southern Ocean change determines the temporal structure of the glacial-interglacial atmospheric CO2 record, including during the interval of “lukewarm” interglacials between 450 and 800 thousand years ago

The Early Sociocognitive Battery: a clinical tool for early identification of children at risk for social communication difficulties and ASD?

BACKGROUND: A substantial proportion of preschool children referred to speech and language therapy (SLT) services have social communication difficulties and/or autistic spectrum disorders (SC&/ASD) that are not identified until late childhood. These 'late' diagnosed children miss opportunities to benefit from earlier targeted interventions. Prior evidence from a follow-up clinical sample showed that preschool performance on the Early Sociocognitive Battery (ESB) was a good predictor of children with social communication difficulties 7-8 years later. AIMS: The aims were three-fold: (1) to determine the impact of child/demographic factors on ESB performance in a community sample of young children; (2) to assess the ESB's concurrent validity and test-retest reliability; and (3) to use cut-offs for 'low' ESB performance derived from the community sample data to evaluate in a clinical sample the predictiveness of the ESB at 2-4 years for outcomes at 9-11 years, including parent-reported SC&/ASD diagnosis. METHODS & PROCEDURES: A community sample of 205 children aged 2-4 years was assessed on the ESB and a receptive vocabulary test. A subsample (n = 20) was retested on the ESB within 2 weeks. Parents completed a questionnaire providing background child/demographic information. The clinical sample from our previous study comprised 93 children assessed on the ESB at 2;6 to < 4;0 whose parents completed the Social Responsiveness Scale (SRS), our measure of social communication, when the children were 9-11 years. Cut-offs for 'low' ESB performance derived from the community sample were used to determine the predictive validity of 'low' ESB scores for social communication outcomes and parent-reported SC&/ASD diagnosis according to age of ESB assessment. OUTCOME & RESULTS: Findings from the community sample confirmed the ESB as psychometrically robust, sensitive to age and language delay, and, in contrast to the receptive vocabulary measure, unaffected by bilingualism. While overall associations between ESB performance and later social communication difficulties in the clinical sample were particularly strong for the youngest age group (2;6 to < 3;0; r = .71, p < .001), 'low' ESB performance was equally predictive across age groups and overall identified 89% of children with 'late' SC&/ASD diagnoses (sensitivity), and 75% of those without (specificity). CONCLUSIONS & IMPLICATIONS: Results indicate that the ESB is a valid preschool assessment suitable for use with children from diverse language backgrounds. It identifies deficits in key sociocognitive skills and is predictive of social communication difficulties in school-age children that had not been identified in preschool clinical assessment, supporting earlier targeted interventions for these children

Foraminiferal isotope evidence of reduced nitrogen fixation in the ice age Atlantic Ocean

Fixed nitrogen (N) is a limiting algal nutrient in the low latitude ocean, and the oceanic N inventory has been suggested to increase during ice ages so as to lower atmospheric CO_2. In organic matter within planktonic foraminifera shells in Caribbean Sea sediments, the ^(15)N/^(14)N from the last ice age is higher than that from the current interglacial, indicating higher nitrate ^(15)N/^(14)N in the Caribbean thermocline. This and species-specific differences are best explained by less N fixation in the Atlantic during the last ice age. The fixation decrease was most likely a response to a known ice age reduction in ocean N loss, and it would have worked to balance the ocean N budget and to curb ice age-to-interglacial change in the N inventory