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81.
Gregory B. Kharas Emi Hanawa Benjamin L. Hill Katherine T. Flynn Emily Figueroa Anne M. Roubal 《高分子科学杂志,A辑:纯化学与应用化学》2013,50(6):423-427
Novel copolymers of trisubstituted ethylene monomers, ring-disubstituted 2-phenyl-1,1-dicyanoethylenes, RC6H3CH=C(CN)2 (where R = 3-Br-4-CH3O, 5-Br-2-CH3O, 2-F-5-CH3, 2-F-6-CH3, 3-F-2-CH3, 3-F-4-CH3, 4-F-2-CH3, 4-F-3-CH3) and 4-fluorostyrene were prepared at equimolar monomer feed composition by solution copolymerization in the presence of a radical initiator (ABCN) at 70°C. The composition of the copolymers was calculated from nitrogen analysis, and the structures were analyzed by IR, 1H and 13C-NMR. The order of relative reactivity (1/r 1) for the monomers is 3-F-4-CH3(1.64) > 5-Br-2-CH3O (1.62) > 3-Br-4-CH3O (1.36) > 4-F-2-CH3(1.3) > 4-F-3-CH3(1.26) > 3-F-2-CH3(1.11) > 2-F-5-CH3 (0.98) > 2-F-6-CH3 (0.97). High Tg of the copolymers, in comparison with that of poly(4-fluorostyrene) indicates a substantial decrease in chain mobility of the copolymer due to the high dipolar character of the trisubstituted ethylene monomer unit. Decomposition of the copolymers in nitrogen occurred in two steps, first in the 290–400°C range with residue, which then decomposed in 400–800°C range. 相似文献
82.
Abstract Intrareaction occurs between moieties attached to copolystyrene-2% divinylbenzene resin as used in solid phase synthesis even when only 0.5% of the phenyl residues are functionalized. Evidence for this interaction has been obtained from the dimeric products resulting from Dieckmann cyclization of resin bound sebacates and ω-cyanopelargonyl thiol resin esters, from kinetic and product data on radioactivity scrambling during the Dieckmann cyclization of uniquely singly labeled tertiary alkyl pimeloyl resin esters, and from anhydride formation with carboxymethyl resin. The extent to which site-site interactions can occur as a function of the percentage functionalization has been measured quantitatively by radiotracer studies on intraresin anhydride formation from carboxymethyl substituted resin. The synthesis and characterization of the resin bound re-actants is described, and the significance of these observations is discussed. 相似文献
83.
Adel M. Elsharkawy Harvey W. Yarranton Tahar A. Al‐sahhaf Mohamed A. Fahim 《Journal of Dispersion Science and Technology》2013,34(2):224-229
Asphaltenes and resins separated from emulsion samples collected from Burgan oil field were used with heptane‐toluene mixtures as model oil to study the effect of oil aromaticity, resin content, and pH of the aqueous phase on the stability of water in model emulsions. It was confirmed that, as long as the asphaltenes are completely solubilized, increasing aromaticity leads to less stable emulsions. A consistent correlation between emulsion stability and relative resin mass content (R/(R+A)) was observed for all three of the field samples. There was a sharp decrease in stability when the R/(R+A) value exceeded 0.75. Emulsion stability was enhanced at high pH and possibly at very low pH (<2). 相似文献
84.
85.
Prof. Pierre D. Harvey Dr. Frédérique Brégier Prof. Shawkat M. Aly Prof. Jędrzej Szmytkowski Prof. Matthew F. Paige Prof. Ronald P. Steer 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(13):4352-4368
Two dendrimers consisting of a cofacial free‐base bisporphyrin held by a biphenylene spacer and functionalized with 4‐benzeneoxomethane (5‐(4‐benzene)tri‐10,15,20‐(4‐n‐octylbenzene)zinc(II)porphyrin) using either five or six of the six available meso‐positions, have been synthesized and characterized as models for the antenna effect in Photosystems I and II. The presence of the short linkers, ‐CH2O‐, and long C8H17 soluble side chains substantially reduces the number of conformers (foldamers) compared with classic dendrimers built with longer flexible chains. This simplification assists in their spectroscopic and photophysical analysis, notably with respect to fluorescence resonance energy transfer (FRET). Both steady‐state and time‐resolved spectroscopic measurements indicate that the cofacial free bases and the flanking zinc(II)–porphyrin antennas act as energy acceptor and donor, respectively, following excitation in either the Q or Soret bands of the dendrimers. The rate constants for singlet electronic energy transfer (kEET) extracted from the S1 and S2 fluorescence lifetimes of the donor in the presence and absence of the acceptor are ≤ (0.1–0.3)×109 and ~2×109 s?1 for S1→S1 (range from a bi‐exponential decay model) and about 1.5×1012 s?1 for S2→Sn (n>1). Comparisons of these experimental data with those calculated from Förster theory using orientation factors and donor–acceptor distances extracted from computer modeling suggest that a highly restricted number of the many foldamers facilitate energy transfer. These foldamers have the lowest energy by molecular modeling and consist of one or at most two of the flanking zinc porphyrin antennas folded so they lie near the central artificial special pair core with the remaining antennas located almost parallel to and far from it. 相似文献
86.
87.
Ahmed M. Rozza Marcell Papp Neil R. McFarlane Prof. Jeremy N. Harvey Dr. Julianna Oláh 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(49):e202200930
The binding of small gas molecules such as NO and CO plays a major role in the signaling routes of the human body. The sole NO-receptor in humans is soluble guanylyl cyclase (sGC) – a histidine-ligated heme protein, which, upon NO binding, activates a downstream signaling cascade. Impairment of NO-signaling is linked, among others, to cardiovascular and inflammatory diseases. In the present work, we use a combination of theoretical tools such as MD simulations, high-level quantum chemical calculations and hybrid QM/MM methods to address various aspects of NO binding and to elucidate the most likely reaction paths and the potential intermediates of the reaction. As a model system, the H-NOX protein from Shewanella oneidensis (So H-NOX) homologous to the NO-binding domain of sGC is used. The signaling route is predicted to involve NO binding to form a six-coordinate intermediate heme-NO complex, followed by relatively facile His decoordination yielding a five-coordinate adduct with NO on the distal side with possible isomerization to the proximal side through binding of a second NO and release of the first one. MD simulations show that the His sidechain can quite easily rotate outward into solvent, with this motion being accompanied in our simulations by shifts in helix positions that are consistent with this decoordination leading to significant conformational change in the protein. 相似文献
88.
Fracture experiments of single silicon crystals reveal that after the critical fracture load is reached, the crack speed jumps from zero to approximately 2 km/sec, indicating that crack motion at lower speeds is forbidden. This contradicts classical continuum fracture theories predicting a continuously increasing crack speed with increasing load. Here we show that this threshold crack speed may be due to a localized phase transformation of the silicon lattice from 6-membered rings to a 5-7 double ring at the crack tip. 相似文献
89.
Comstock MJ Levy N Kirakosian A Cho J Lauterwasser F Harvey JH Strubbe DA Fréchet JM Trauner D Louie SG Crommie MF 《Physical review letters》2007,99(3):038301
We have observed reversible light-induced mechanical switching for individual organic molecules bound to a metal surface. Scanning tunneling microscopy (STM) was used to image the features of individual azobenzene molecules on Au(111) before and after reversibly cycling their mechanical structure between trans and cis states using light. Azobenzene molecules were engineered to increase their surface photomechanical activity by attaching varying numbers of tert-butyl (TB) ligands ("legs") to the azobenzene phenyl rings. STM images show that increasing the number of TB legs "lifts" the azobenzene molecules from the substrate, thereby increasing molecular photomechanical activity by decreasing molecule-surface coupling. 相似文献
90.
We show that holographic models of QCD predict the presence of a Chern-Simons coupling between vector and axial-vector mesons at finite baryon density. In the Anti de Sitter/Conformal Field Theory dictionary, the coefficient of this coupling is proportional to the baryon number density and is fixed uniquely in the five-dimensional holographic dual by anomalies in the flavor currents. For the lightest mesons, the coupling mixes transverse rho and a1 polarization states. At sufficiently large baryon number densities, it produces an instability, which causes the rho and a1 mesons to condense in a state breaking both rotational and translational invariance. 相似文献