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21.
Patrick DL Flanagan JF Kohl P Lynden-Bell RM 《Journal of the American Chemical Society》2003,125(22):6762-6773
Chemical force microscopy and related force measurement techniques have emerged as powerful tools for studying fundamental interactions central to understanding adhesion and tribology at the molecular scale. However, detailed interpretation of these interactions requires knowledge of chemical and physical processes occurring in the region of the tip-sample junction that experiments cannot provide, such as atomic-scale motions and distribution of forces. In an effort to address some of these open issues, atomistic molecular dynamics simulations were performed modeling a chemical force microscope stylus covered with a planar C12 alkylthiolate self-assembled monolayer (SAM) interacting with a solid wall. A complete loading-unloading sequence was simulated under conditions of near-constant equilibrium, approximating the case of infinitely slow tip motion. In the absence of the solid wall, the stylus film existed in a fluid state with structural and dynamic properties similar to those of the analogous planar SAM at an elevated temperature. When the wall was brought into contact with the stylus and pressed against it, a series of reversible changes occurred culminating with solidification of the SAM film at the largest compressive force. During loading, the chemical composition of the contact changed, as much of the film's interior was exposed to the wall. At all tip heights, the distribution of forces within the contact zone was uneven and subject to large local fluctuations. Analysis using the Johnson-Kendall-Roberts, Derjaguin-Muller-Toporov, and Hertz contacts mechanics models revealed significant deviations from the simulation results, with the JKR model providing best overall agreement. Some of the discrepancies found would be overlooked in an actual experiment, where, unlike the simulations, contact area is not separately known, possibly producing a misleading or incorrect interpretation of experimental results. These shortcomings may be improved upon by using a model that correctly accounts for the finite thickness of the compliant components and nonlinear elastic effects. 相似文献
22.
Burke RM Pearce JK Boxford WE Bruckmann A Dessent CE 《The journal of physical chemistry. A》2005,109(43):9775-9785
Multiply charged anions (MCAs) represent highly energetic species in the gas phase but can be stabilized through formation of molecular clusters with solvent molecules or counterions. We explore the intramolecular stabilization of excess negative charge in gas-phase MCAs by probing the intrinsic stability of the [adenosine 5'-triphosphate-2H](2-) ([ATP-2H](2-)), [adenosine 5'-diphosphate-2H](2-) ([ADP-2H](2-)), and H(3)P(3)O(10)(2-) dianions and their protonated monoanionic analogues. The relative activation barriers for decay of the dianions via electron detachment or ionic fragmentation are investigated using resonance excitation of ions isolated within a quadrupole trap. All of the dianions decayed via ionic fragmentation demonstrating that the repulsive Coulomb barriers (RCB) for ionic fragmentation lie below the RCBs for electron detachment. Both the electrospray ionization mass spectra (ESI-MS) and total fragmentation energies for [ATP-2H](2-), [ADP-2H](2-), and H(3)P(3)O(10)(2-) indicate that the multiply charged H(3)P(3)O(10)(2-) phosphate moiety is stabilized by the presence of the adenosine group and the stability of the dianions increases in the order H(3)P(3)O(10)(2-) < [ADP-2H](2-) < [ATP-2H](2-). Fully optimized, B3LYP/6-31+G* minimum energy structures illustrate that the excess charges in all of the phosphate anions are stabilized by intramolecular hydrogen bonding either within the phosphate chain or between the phosphate and the adenosine. We develop a model to illustrate that the relative magnitudes of the RCBs and hence the stability of these ions is dominated by the extent of intramolecular hydrogen bonding. 相似文献
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24.
Lindsey?Gillian?MackayEmail author C.?Peter?Taylor Richard?Bruce?Myors Ruth?Hearn Bernard?King 《Accreditation and quality assurance》2003,8(5):191-194
The selection of an appropriate isotope dilution mass spectrometry technique for the practical application of this potentially primary method of analysis is highly important. The NARL approach for the application of the exact matching double isotope dilution mass spectrometry technique developed by Henrion is presented. NARL's approach utilises exact matching to minimise the effect of measurement biases within the method but also includes the thorough examination of all other biasing factors. The approach has been successfully tested in international CCQM intercomparisons with other national metrology institutes. 相似文献
25.
Abstract— The quenching by oxygen of the α- and β-phosphorescence of trypaflavine adsorbed on silica gel has been found to obey the simple Stern-Volmer relationship for oxygen partial pressures up to the point at which, for low temperatures, the β-phosphorescence becomes negligible. At room temperatures above this partial pressure the Stern-Volmer plot is no longer linear. An explanation is proposed, based on the formation of a triplet-oxygen complex, analogous to that proposed by Porter and Wright for paramagnetic quenching. 相似文献
26.
Ruth Freitag Thomas Baltes Martin Eggert 《Journal of polymer science. Part A, Polymer chemistry》1994,32(16):3019-3030
Several batches of poly-N,N-diethylacrylamide were synthesized by anionic and by group transfer polymerization (GTP). A radical poly-N,N-diethylacrylamide prepared from the same monomer was also included in the comparison. According to matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) both types of living polymerization resulted in narrow molecular weight distributions with Mw/Mn values below 1.5. Average molecular weights (Mn) between 888 and 4678 g/mol were calculated in these cases. The radical polymer had an average molecular weight (Mn) of approximately 130,000 g/mol. The dry anionic and GTP polymers were investigated by differential scanning calorimetry (DSC) and x-ray diffraction spectrometry. Evidence for partial crystallinity in the solid state was found. The conformation of all polymers was examined by high resolution (600 MHz) NMR. According to these measurements, 75% of the ? CHR? groups of the anionic poly-N,N-diethylacrylamide were located in an isotactic triade. The remaining 25% had heterotactic structure, while no indication for the presence of syndiotactic protons was found. Poly-N,N-diethylacrylamide prepared by GTP, on the other hand, had mainly syndiotactic structure. The aqueous solutions of the polymers showed phase separation upon heating. Whereas the lower critical solution temperature (LCST) was approximately 30°C in the case of the poly-N,N-diethylacrylamide prepared by GTP and by radical polymerization, uncommonly high LCSTs of more than 40°C were observed for the anionic poly-N,N-diethylacrylamide. © 1994 John Wiley & Sons, Inc. 相似文献
27.
Barro R Ares S Garcia-Jares C Llompart M Cela R 《Analytical and bioanalytical chemistry》2005,381(1):255-260
A combination of sorbent enrichment and ultrasound-assisted solvent extraction has been used to determine polychlorinated biphenyls in air. Analytes were sampled by pumping a known volume of air through a porous polymer (Tenax TA). The enriched adsorbent was transferred into a glass vial, and ultrasound-assisted extraction of the analytes was then performed in n-hexane. Quantification was carried out by using gas chromatography coupled to tandem mass spectrometry. Breakthrough volume of the sampling step was studied, indicating that 10 m3 of air could be processed without losses of the most volatile compounds. Good recoveries (75–96%) were obtained, and limits of detection at the sub ng m–3 were achieved for all the analytes. The proposed method is very simple and fast, avoiding the use of large solvent volumes and time-consuming preconcentration steps. 相似文献
28.
Dong Xiao Brian J. Lavey Anandan Palani Robert G. Aslanian Neng-Yang Shih Gerard P. Randolph Ruth A. Duffy 《Tetrahedron letters》2005,46(44):7653-7656
Unlike the lithiation of N-Boc-2-alkylpiperidines, which occurs at the 6-position, N-Boc-2-phenylpiperidine and N-Boc-2-phenylpyrrolidine can be lithiated exclusively at the 2-position. The tertiary carbanions can be trapped with a variety of electrophiles. This chemistry was used for the synthesis of a potent NK1 ligand (Ki = 0.3 nM). The bioactive configuration at the piperidine quaternary center was determined by X-ray analysis to be (S). 相似文献
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