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排序方式: 共有1045条查询结果,搜索用时 312 毫秒
1.
Dominik Vítek Prof. Aleš Růžička Lise Vermeersch Dr. Libor Dostál Dr. Jan Turek Prof. Roman Jambor 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(70):e202202639
Reactivity studies of the GeII→B complex L(Cl)Ge⋅BH3 ( 1 ; L=2-Et2NCH2-4,6-tBu2-C6H2) were performed to determine the effect on the GeII→B donation. N-coordinated compounds L(OtBu)Ge⋅BH3 ( 2 ) and [LGe⋅BH3]2 ( 3 ) were prepared. The possible tuning of the GeII→B interaction was proved experimentally, yielding compounds 1-PPh2-8-(LGe)-C10H6 ( 4 ) and L(Cl)Ge⋅GaCl3 ( 5 ) without a GeII→B interaction. In 5 , an unprecedented GeII→Ga coordination was revealed. The experimental results were complemented by a theoretical study focusing on the bonding in 1 − 5 . The different strength of the GeII→E (E=B, Ga) donation was evaluated by using energy decomposition analysis. The basicity of different L(X)Ge groups through proton affinity is also assessed. 相似文献
2.
A path-integral molecular dynamics technique for strongly interacting atoms using ab initio potentials derived from density functional theory is implemented. This allows the efficient inclusion of nuclear quantum dispersion in ab initio simulations at finite temperatures. We present an application to the quantum cluster H
5
+
. 相似文献
3.
Treatment of the phenyl-substituted silane 4 with lithium metal afforded the functionalized lithiosilane rac-2 by selective cleavage of one Si-C bond between silicon and a phenyl group. The resulting lithiosilane rac-2 crystallizes as the dimer (2.THF)2, which represents the first example of a dimeric organyl-substituted lithiosilane in the presence of THF. 相似文献
4.
Dominik Rejman 《Tetrahedron》2007,63(5):1243-1253
The work deals with the synthesis of hydroxypyrrolidine analogs of nucleosides. Starting from the optically pure l- or d-tartaric acid, we improved the synthesis of enantiomeric trans-3,4-dihydroxypyrrolidines and elaborated a procedure for the synthesis of all possible diastereoisomers of 3-hydroxy-4-pyrrolidinyl derivatives of both purine and pyrimidine nucleobases. The prepared compounds were tested for cytostatic and antiviral properties but no significant activity was found. 相似文献
5.
The phase β-K0.25Pb0.75F1.75 previously found in the KF-PbF2 system appears to be metastable at low temperatures relative to a mixture of orthorhombic PbF2 and a new phase suspected to be KPbF3 II. KPbF3 II transforms to KPbF3 I at 298.5°C at atmospheric pressure. The KPbF3 II/I transition line rises with pressure, but the substance appears to reversibly disproportionate above ~360°C, 5 kbar, possibly to a mixture of PbF2 and K4PbF6. Instead of β-K0.25Pb0.75F1.75, a mixture with this composition yielded, in addition to weak heat events due to the KPbF3 II/I transition, strong heat events at 254.5°C and atmospheric pressure (thermal hysteresis ~13°C) which were ascribed to the PbF2 orthorhombic/cubic transition. This transition rises with pressure to 673°C at 37.8 kbar. 相似文献
6.
DeClue MS Baldridge KK Künzler DE Kast P Hilvert D 《Journal of the American Chemical Society》2005,127(43):15002-15003
Isochorismate pyruvate lyase (IPL) catalyzes the cleavage of isochorismate to give salicylate and pyruvate, a key step in bacterial siderophore biosynthesis. We investigated the enzyme from Pseudomonas aeruginosa using isochorismate selectively deuterated at C2 as a substrate. Monitoring the reaction by 2H NMR spectroscopy revealed that the label is quantitatively transferred from C2 to C9, producing stoichiometric amounts of [3-2H]pyruvate as product. Moreover, the deuterium kinetic isotope effect of 2.34 +/- 0.08 on kcat indicates that C-H cleavage is significantly rate limiting. Consistent with these data, hybrid density functional theory (HDFT) calculations at the Becke3LYP/DZ+(2d,p) level of theory predict a concerted but highly asynchronous pericyclic transition structure, in which carbon-oxygen bond cleavage is more advanced than hydrogen atom transfer from C2 to C9; the calculated 2H isotope effect of 2.22 at C2 is in excellent accord with the experimental value. Together, these findings indicate that IPL should be added to the small set of proteins that are known to catalyze pericyclic reactions. They also raise the possibility that enzymes, such as chorismate pyruvate lyase, salicylate synthase, 4-amino-4-deoxychorismate lyase, and anthranilate synthase, which accelerate formally similar reaction steps, may also exploit pericyclic mechanisms. 相似文献
7.
8.
Unprotected thymidine and uridine react with 4-nitrophenylsulfonyl ethene () in a base catalyzed Michael type addition to give O4-(4-nitrophenylsulfonylethyl)thymidine-() and -uridine (), respectively. The 4-nitrophenylsulfonylethyl group is cleaved within 2.5 hours at 50–55°C by concentrated aqueous ammonia, via β-elimination. 相似文献
9.
We describe a spectrophotometric method to determine the sum of Fe(II) plus Fe(III) in HCl and oxalate extracts. The principle of the method is to reduce Fe(III) by ascorbate in near-neutral solution and to sequester the Fe(II) formed as a tri-ferrozine complex, which is then determined photometrically at 562 nm. Because the complex is stable, the reaction is irreversible and complete. Fe(III) in HCl solution reacted very rapidly, whereas oxalate decelerated the overall reaction so that pseudo first-order kinetics with respect to Fe(III) was detected. However, when extractions were conducted at the recommended soil:solution ratio, the absorption reached 98% of its final value within a few minutes. To test the method, four soils differing considerably in texture and carbonate, organic matter, and Fe(III)(hydr)oxide contents were extracted with oxalate in the dark for amorphous (Fe(o)), and with boiling oxalate for total Fe(III)(hydr)oxide (Fe(bo)). This newly developed spectrophotometric method showed excellent correspondence with the conventional atomic absorption spectroscopy (AAS) method. The method presented here can therefore be used as an alternative method to determine the Fe content of oxalate and hydrochloric acid extracts if AAS is not available. Oxalate extracts low in Fe content, which cannot be diluted, are easier to determine by the photometric method than by AAS. 相似文献
10.
For some saline environments (e.g. deeply percolating groundwater, interstitial water in marine sediments, water sample collected after several steps of fractionation) the volume of water sample available is limited. A technique is presented which enables simultaneous determination of major and trace elements after preconcentration of only 60 mL sample on chelating resins. Chelex-100 and Chelamine were used for the preconcentration of trace elements (Cd, Cu, Pb, Zn, Sc) and rare earth elements (La, Ce, Nd, Yb) from saline water before their measurement by inductively coupled plasma mass spectrometry. Retention of the major elements (Na, Ca, Mg) by the Chelamine resin was lower than by Chelex; this enabled their direct measurement in the solution after passage through the resin column. For trace metal recoveries both resins yield the same mass balance. Only Chelex resin enabled the quantitative recovery of rare earth elements. The major elements, trace metals and rare earth elements cannot be measured after passage through one resin only. The protocol proposes the initial use of Chelamine for measurement of trace and major elements and then passage the same sample through the Chelex resin for determination of the rare earth elements. The detection limit ranged from 1 to 12 pg mL(-1). At concentrations of 1 ng mL(-1) of trace metals and REE spiked in coastal water the precision for 10 replicates was in the range of 0.3-3.4% (RSD). The accuracy of the method was demonstrated by analyzing two standard reference waters, SLRS-3 and CASS-3. 相似文献