116 research outputs found

    Amine-Directed Hydroboration: Scope and Limitations

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    Iodine activation induces intramolecular hydroboration of homoallylic and bis-homoallylic amine boranes with good to excellent control of regiochemistry compared to control experiments using excess THF•BH<sub>3</sub>. Deuterium labeling and other evidence confirm that the iodine-induced hydroboration reaction of homoallylic amine boranes occurs via an intramolecular mechanism equivalent to the classical 4-center process and without competing retro-hydroboration. Longer carbon chain tethers result in lower regioselectivity, whereas the shorter tether in allylic amines results in a switch to dominant intermolecular hydroboration. Regioselectivity in THF•BH<sub>3</sub> control experiments is higher for the allylic amine boranes compared to the iodine activation experiments, whereas the reverse is true for homoallylic amine borane activation

    Evaluation of Gridded Precipitation Data for Driving SWAT Model in Area Upstream of Three Gorges Reservoir

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    <div><p>Gridded precipitation data are becoming an important source for driving hydrologic models to achieve stable and valid simulation results in different regions. Thus, evaluating different sources of precipitation data is important for improving the applicability of gridded data. In this study, we used three gridded rainfall datasets: 1) National Centers for Environmental Prediction - Climate Forecast System Reanalysis (NCEP-CFSR); 2) Asian Precipitation - Highly-Resolved Observational Data Integration Towards Evaluation (APHRODITE); and 3) China trend - surface reanalysis (trend surface) data. These are compared with monitoring precipitation data for driving the Soil and Water Assessment Tool in two basins upstream of Three Gorges Reservoir (TGR) in China. The results of one test basin with significant topographic influence indicates that all the gridded data have poor abilities in reproducing hydrologic processes with the topographic influence on precipitation quantity and distribution. However, in a relatively flat test basin, the APHRODITE and trend surface data can give stable and desirable results. The results of this study suggest that precipitation data for future applications should be considered comprehensively in the TGR area, including the influence of data density and topography.</p></div

    Spatial distribution of annual precipitation value for various precipitation datasets for the Dong River (up) and the Puli River (down) basins, from left to right is Monitoring data, NCEP CFSR data, APHRODITE and trend surface data.

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    <p>Spatial distribution of annual precipitation value for various precipitation datasets for the Dong River (up) and the Puli River (down) basins, from left to right is Monitoring data, NCEP CFSR data, APHRODITE and trend surface data.</p

    Automatic Reaction Pathway Search via Combined Molecular Dynamics and Coordinate Driving Method

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    We proposed and implemented a combined molecular dynamics and coordinate driving (MD/CD) method for automatically searching multistep reaction pathways of chemical reactions. In this approach, the molecular dynamic (MD) method at the molecular mechanics (MM) or semiempirical quantum mechanical (QM) level is employed to explore the conformational space of the minimum structures, and the modified coordinate driving (CD) method is used to build reaction pathways for representative conformers. The MD/CD method is first applied to two model reactions (the Claisen rearrangement and the intermolecular aldol reaction). By comparing the obtained results with those of the existing methods, we found that the MD/CD method has a comparable performance in searching low-energy reaction pathways. Then, the MD/CD method is further applied to investigate two reactions: the electrocyclic reaction of benzocyclobutene-7-carboxaldehyde and the intramolecular Diels–Alder reaction of ketothioester with 11 effectively rotatable single bonds. For the first reaction, our results can correctly account for its torquoselectivity. For the second one, our method predicts eight reaction channels, leading to eight different stereo- and regioselective products. The MD/CD method is expected to become an efficient and cost-effective theoretical tool for automatically searching low-energy reaction pathways for relatively complex chemical reactions

    R<sup>2</sup> and NSE of various precipitation datasets for the Dong River during the validation period.

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    <p>R<sup>2</sup> and NSE of various precipitation datasets for the Dong River during the validation period.</p

    Annual precipitation value of various precipitation datasets for the Dong River (left) and the Puli River (right) basins.

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    <p>Annual precipitation value of various precipitation datasets for the Dong River (left) and the Puli River (right) basins.</p

    Basic geo-information of Dong River and Puli River basins.

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    <p>Note: *Area means the basin area of runoff observation stations,</p><p>**Std means the standard error of the elevation.</p><p>Basic geo-information of Dong River and Puli River basins.</p

    Evaluation results of different gridded precipitation datasets in the Dong River basin.

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    <p>Evaluation results of different gridded precipitation datasets in the Dong River basin.</p
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