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排序方式: 共有166条查询结果,搜索用时 31 毫秒
91.
A study of the asymptotic decay of the pair radial correlations in the bare quantum hard-sphere (QHS) fluid and in the quantum hard-sphere Yukawa (QHSY) fluid is presented. The conditions explored are far from quantum exchange and are contained within the region (0.1相似文献
92.
Six stabilised phosphonium ylides bearing ortho-benzylaminophenyl and cinnamoyl (or a heterocyclic analogue) groups have been prepared and upon flash vacuum pyrolysis at 800 °C were found to undergo cascade cyclization processes to give mainly 3-styrylquinolines but also in some cases ring-fused carbazoles and other fused-ring heterocyclic products. By starting with an appropriate ring-methylated precursor the natural product Eustifoline D was obtained in 19% yield in the pyrolysis in addition to the 3-(2-furylethenyl)quinoline (46%). 相似文献
93.
A practical Buchwald-Hartwig amination of 2-bromopyridines with volatile amines is developed in sealed tubes. The method provides an expedient entry to a variety of secondary and tertiary aminopyridines that are otherwise not readily synthesized. 相似文献
94.
Sounart TL Liu J Voigt JA Huo M Spoerke ED McKenzie B 《Journal of the American Chemical Society》2007,129(51):15786-15793
Recently we discovered that under certain conditions new crystal growth (branch) can be induced on specific crystalline planes of the same material. This is a new phenomenon and is in sharp contrast to typical nucleation and growth in which a crystal will simply grow larger in preferred directions depending on the surface energy of the specific crystalline planes. Based on our observation, we developed a sequential nucleation and growth technique offering the power to assemble complex hierarchical crystals step-by-step. However, the key questions of when and how the secondary nucleation takes place have not been answered. Here we systematically study secondary ZnO crystal growth using organic diamine additives with a range of chain lengths and concentration. We found that ZnO branches form for a narrow diamine concentration range with a critical lower and upper critical nucleation concentration limit, which increases by about a factor of 5 for each additional carbon in the diaminoalkane chain. Our results suggest that the narrow window for secondary growth is dictated by the solubility of the ZnO crystals, where the low critical nucleation concentration is determined by slight etching of the surface to produce new nucleation sites, and the upper critical concentration is determined by the supersaturation concentration. Kinetic measurements show that the induction time and growth rate increase with increasing diamine concentration and follow classical nucleation and growth theory. Observations of branch morphological evolution reveal the mechanisms guiding the tunable crystal size and morphology. 相似文献
95.
Armstrong DR Clegg W Dale SH Graham DV Hevia E Hogg LM Honeyman GW Kennedy AR Mulvey RE 《Chemical communications (Cambridge, England)》2007,(6):598-600
Benzene can be easily 1,4-dideprotonated stoichiometrically on reaction with two equivalents of a synergic mixture of tBu2Zn, NaTMP and TMEDA to give a unique 1,4-dizincated benzene product which has been characterised by X-ray crystallography and NMR spectroscopy as well as modelled theoretically by DFT computational studies; a related synergic dimagnesiation of benzene is also reported. 相似文献
96.
Stimson L Dong L Karttunen M Wisniewska A Dutka M Róg T 《The journal of physical chemistry. B》2007,111(43):12447-12453
Spin-labeled stearic acid species are commonly used for electron paramagnetic resonance (EPR) studies of cell membranes to investigate phase transitions, fluidity, and other physical properties. In this paper, we use large-scale molecular dynamics simulations to investigate the position and behavior of nitroxide spin labels attached to stearic acid molecules in dipalmitoylphosphatidylcholine (DPPC) bilayers. The results of these studies are potentially very important for the interpretation of EPR spectra, which rely on assumptions about the position of the label in the membrane. Additionally, we investigate the effect of chirality and ionization of the carboxyl group of the label. For a non-ionized species, we observe that spin-label molecules are even able to make flip-flop transitions between the leaflets of the bilayer. Such transitions have been previously observed only in very rare cases in molecular simulations. 相似文献
97.
Dr. Mihoko Yamada Tomoya Sawazaki Mae Fujita Fumio Asanoma Yoshiko Nishikawa Prof. Tsuyoshi Kawai 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(49):e202201286
We have designed and synthesized photochromic tetrathienyl corannulene compounds, 1,2,7,8-tetrakis(2-methyl-5-phenylthiophen-3-yl)corannulene ( 1 ) and 1,2,7,8-tetrakis(2,4-dimethyl-5-phenylthiophen-3-yl)corannulene ( 2 ), by fusing two units of photochromic terarylene with a curved aromatic corannulene with a promising antenna effect. Compound 1 exhibited highly sensitive photoreactivity, with a large molar absorption coefficient of 8.2×104 M−1 cm−1 and practically photon-quantitative photocyclization. On the other hand, a terarylene derivative with a planar aromatic phenanthrene, 9,10-bis(2,4-dimethyl-5-phenylthiophen-3-yl)phenanthrene ( 4 ) showed no photoreactivity. The reason for such a difference was attributed to the predominance of the photoreactive atropisomers amplified by energy migration, and the shortened distance between reactive carbons induced by the curved structure. 相似文献
98.
Hall Z Politis A Bush MF Smith LJ Robinson CV 《Journal of the American Chemical Society》2012,134(7):3429-3438
Collapse to compact states in the gas phase, with smaller collision cross sections than calculated for their native-like structure, has been reported previously for some protein complexes although not rationalized. Here we combine experimental and theoretical studies to investigate the gas-phase structures of four multimeric protein complexes during collisional activation. Importantly, using ion mobility-mass spectrometry (IM-MS), we find that all four macromolecular complexes retain their native-like topologies at low energy. Upon increasing the collision energy, two of the four complexes adopt a more compact state. This collapse was most noticeable for pentameric serum amyloid P (SAP) which contains a large central cavity. The extent of collapse was found to be highly correlated with charge state, with the surprising observation that the lowest charge states were those which experience the greatest degree of compaction. We compared these experimental results with in vacuo molecular dynamics (MD) simulations of SAP, during which the temperature was increased. Simulations showed that low charge states of SAP exhibited compact states, corresponding to collapse of the ring, while intermediate and high charge states unfolded to more extended structures, maintaining their ring-like topology, as observed experimentally. To simulate the collision-induced dissociation (CID) of different charge states of SAP, we used MS to measure the charge state of the ejected monomer and assigned this charge to one subunit, distributing the residual charges evenly among the remaining four subunits. Under these conditions, MD simulations captured the unfolding and ejection of a single subunit for intermediate charge states of SAP. The highest charge states recapitulated the ejection of compact monomers and dimers, which we observed in CID experiments of high charge states of SAP, accessed by supercharging. This strong correlation between theory and experiment has implications for further studies as well as for understanding the process of CID and for applications to gas-phase structural biology more generally. 相似文献
99.
Murray PR Crawford S Dawson A Delf A Findlay C Jack L McInnes EJ Al-Musharafi S Nichol GS Oswald I Yellowlees LJ 《Dalton transactions (Cambridge, England : 2003)》2012,41(1):201-207
We report the preparation and electrochemical studies of a systematic series of mono- and di-nitro-substituted 2,2'-bipyridine (bipy) compounds [x-NO(2)-bipy (x = 3,4) and x,x'-(NO(2))(2)-bipy (x,x' = 3, 4, 5)] and their complexes with platinum(II), [Pt(x-NO(2)-bipy)Cl(2)] and [Pt(x,x'-(NO(2))(2)-bipy)Cl(2)]. The effect of the number and substitution pattern of the nitro groups on the low-lying acceptor molecular orbitals (involved in charge transfer transitions) is probed by in situ UV/Vis/NIR and EPR spectroelectrochemical methods, supported by DFT calculations. The LUMOs of x-NO(2)-bipy (x = 3-5) are largely localised on the NO(2)-pyridyl moiety; this is also true of their {PtCl(2)} complexes but with a small but significant shift of electron density from the nitro groups. The LUMOs of x,x'-(NO(2))(2)-bipy with x = 3 and 5 are delocalised over both NO(2)-pyridyl rings, but for 4,4'-(NO(2))(2)-bipy is localised on a single NO(2)-pyridyl ring. In all cases the LUMO of the [Pt(x,x'-(NO(2))(2)-bipy)Cl(2)] complexes is delocalised over both nitro-pyridyl rings. For all complexes, the 4(4') derivatives allows greatest overlap with metal valence orbitals in the LUMO. 相似文献
100.
Thomas JB Duewer DL Mugenya IO Phinney KW Sander LC Sharpless KE Sniegoski LT Tai SS Welch MJ Yen JH 《Analytical and bioanalytical chemistry》2012,402(2):749-762
Standard Reference Material 968e Fat-Soluble Vitamins, Carotenoids, and Cholesterol in Human Serum provides certified values
for total retinol, γ- and α-tocopherol, total lutein, total zeaxanthin, total β-cryptoxanthin, total β-carotene, 25-hydroxyvitamin
D3, and cholesterol. Reference and information values are also reported for nine additional compounds including total α-cryptoxanthin,
trans- and total lycopene, total α-carotene, trans-β-carotene, and coenzyme Q10. The certified values for the fat-soluble vitamins and carotenoids in SRM 968e were based on the agreement of results from
the means of two liquid chromatographic methods used at the National Institute of Standards and Technology (NIST) and from
the median of results of an interlaboratory comparison exercise among institutions that participate in the NIST Micronutrients
Measurement Quality Assurance Program. The assigned values for cholesterol and 25-hydroxyvitamin D3 in the SRM are the means of results obtained using the NIST reference method based upon gas chromatography-isotope dilution
mass spectrometry and liquid chromatography-isotope dilution tandem mass spectrometry, respectively. SRM 968e is currently
one of two available health-related NIST reference materials with concentration values assigned for selected fat-soluble vitamins,
carotenoids, and cholesterol in human serum matrix. This SRM is used extensively by laboratories worldwide primarily to validate
methods for determining these analytes in human serum and plasma and for assigning values to in-house control materials. The
value assignment of the analytes in this SRM will help support measurement accuracy and traceability for laboratories performing
health-related measurements in the clinical and nutritional communities. 相似文献