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排序方式: 共有65条查询结果,搜索用时 46 毫秒
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An experimental study of the thermal decomposition of a β‐hydroxy alkene, 3‐methyl‐3‐buten‐1‐ol, in m‐xylene solution, has been carried out at five different temperatures in the range of 513.15–563.15 K. The temperature dependence of the rate constants for the decomposition of this compound in the corresponding Arrhenius equation is given by ln k (s?1) = (25.65 ± 1.52) ? (17,944 ± 814) (kJ·mol?1)·T?1. A computational study has been carried out at the M05–2X/6–31+G(d,p) level of theory to calculate the rate constants and the activation parameters by the classical transition state theory. There is a good agreement between the experimental and calculated rate constants and activation Gibbs energies. The bonding characteristics of reactant, transition state, and products have been investigated by the natural bond orbital analysis, which provides the natural atomic charges and the Wiberg bond indices. Based on the results obtained, the mechanism proposed is a one‐step process proceeding through a six‐membered cyclic transition state, being a concerted and slightly asynchronous process. The results have been compared with those obtained previously by us (Struct Chem 2013, 24, 1811–1816) for the thermal decomposition of 3‐buten‐1‐ol, in m‐xylene solution. We can conclude that in the compound studied in this work, 3‐methyl‐3‐buten‐1‐ol, the effect of substitution at position 3 by a weakly activating CH3 group is the stabilization of the transition state formed in the reaction and therefore a small increase in the rate of thermal decomposition. 相似文献
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Abstract Due to the wide use of polymers in medicine, researchers are required to solve a very important problem–to understand the interaction between materials of nonphysiological origin and the surrounding biological liquids, and tissues, particularly blood. 相似文献
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Song Y Challa SR Medforth CJ Qiu Y Watt RK Pena D Miller JE van Swol F Shelnutt JA 《Chemical communications (Cambridge, England)》2004,(9):1044-1045
Nanotubes prepared by the self-assembly of D-Phe-D-Phe molecules are investigated by electron microscopy and Monte Carlo simulations; the nanotubes appear to be porous and are capable of forming novel peptide-nanotube platinum-nanoparticle composites. 相似文献
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Medforth CJ Haddad RE Muzzi CM Dooley NR Jaquinod L Shyr DC Nurco DJ Olmstead MM Smith KM Ma JG Shelnutt JA 《Inorganic chemistry》2003,42(7):2227-2241
Previous studies of 5,10,15,20-tetraarylporphyrins have shown that the barrier for meso aryl-porphyrin rotation (DeltaG++(ROT)) varies as a function of the core substituent M and is lower for a small metal (M = Ni) compared to a large metal (M = Zn) and for a dication (M = 4H(2+)) versus a free base porphyrin (M = 2H). This has been attributed to changes in the nonplanar distortion of the porphyrin ring and the deformability of the macrocycle caused by the core substituent. In the present work, X-ray crystallography, molecular mechanics (MM) calculations, and variable temperature (VT) (1)H NMR spectroscopy are used to examine the relationship between the aryl-porphyrin rotational barrier and the core substituent M in some novel 2,3,5,7,8,10,12,13,15,17,18,20-dodecaarylporphyrins (DArPs), and specifically in some 5,10,15,20-tetraaryl-2,3,7,8,12,13,17,18-octaphenylporphyrins (TArOPPs), where steric crowding of the peripheral groups always results in a very nonplanar macrocycle. X-ray structures of DArPs indicate differences in the nonplanar conformation of the macrocycle as a function of M, with saddle conformations being observed for M = Zn, 2H or M = 4H(2+) and saddle and/or ruffle conformations for M = Ni. VT NMR studies show that the effect of protonation in the TArOPPs is to increase DeltaG++(ROT), which is the opposite of the effect seen for the TArPs, and MM calculations also predict a strikingly high barrier for the TArOPPs when M = 4H(2+). These and other findings suggest that the aryl-porphyrin rotational barriers in the DArPs are closely linked to the deformability of the macrocycle along a nonplanar distortion mode which moves the substituent being rotated out of the porphyrin plane. 相似文献
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Z.M. Babar Wan Mohd Azizi Solachuddin JA Ichwan Qamar Uddin Ahmed Abul Kalam Azad Imranul Mawa 《Natural product research》2019,33(15):2266-2270
The current study provides a way of extraction for both active NSO and WSE from Nigella sativa seeds using 98% methanol. About 1?kg of ground seeds was macerated by 1:2.5 w/v (g/mL) for 72?hours. After rotary evaporation and 7 days of continuous drying and chilling at 50 and 4?°C, NSO and WSE were obtained at the same instant. Solubility tests of 24 solvents and 11 thin layer chromatographic analyses while 2, 2-diphenyl-1-picrylhydrazyl free radical scavenging assay of NSO (73.66) , WSE (33.32) and NSO?+?WSE (78.22) against ascorbic acid (IC50?=?4.28?mg/mL) was performed. WSE was found to be highly soluble in water and 5% NaOH exhibiting the same Rf value of 0.95 for EtOH:DMSO (9:1) against the honey. WSE has revealed more than twofold higher anti-oxidant activity than others. Formulation of WSE with Tualang honey may provide better targeted hydrophilic drug delivery systems. 相似文献
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F van Swol SR Challa JA Shelnutt 《Physical chemistry chemical physics : PCCP》2012,14(38):13309-13318
Producing nanostructures with high surface area that are stable is important to accomplish sustained use of catalytic materials in practical settings. Avoiding the processes of ripening and sintering that typically hinder stability has long been recognized as a significant challenge and much research is focused on addressing these issues. In this article, we investigate a Pt nanostructure-a holey nanosheet-that exhibits high surface area and stability. The findings from lattice gas simulations produce a stability diagram that relates a critical hole diameter to sheet thickness. The stability is now addressed from a thermodynamic point of view, and, in particular, the crucial role of curvature is considered. We find that the stability of certain sized holes is due to the near zero mean curvature of the surface of the holes and of the surrounding flat sheet. Molecular dynamics simulations of Pt (using an embedded atom potential) are reported for small nanoclusters and model holes in sheets to illustrate the strong effects of curvature on thermodynamic properties such as the lowering of melting and surface melting temperatures. 相似文献
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