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191.
George SJ Tomović Ž Schenning AP Meijer EW 《Chemical communications (Cambridge, England)》2011,47(12):3451-3453
Preferred handedness in the supramolecular chirality of self-assembled achiral oligo(p-phenylenevinylene) (OPV) derivatives is induced by chiral solvents and spectroscopic probing provides insight into the mechanistic aspects of this chiral induction through chiral solvation. 相似文献
192.
Thuo MM Reus WF Nijhuis CA Barber JR Kim C Schulz MD Whitesides GM 《Journal of the American Chemical Society》2011,133(9):2962-2975
This paper compares charge transport across self-assembled monolayers (SAMs) of n-alkanethiols containing odd and even numbers of methylenes. Ultraflat template-stripped silver (Ag(TS)) surfaces support the SAMs, while top electrodes of eutectic gallium-indium (EGaIn) contact the SAMs to form metal/SAM//oxide/EGaIn junctions. The EGaIn spontaneously reacts with ambient oxygen to form a thin (~1 nm) oxide layer. This oxide layer enables EGaIn to maintain a stable, conical shape (convenient for forming microcontacts to SAMs) while retaining the ability to deform and flow upon contacting a hard surface. Conical electrodes of EGaIn conform (at least partially) to SAMs and generate high yields of working junctions. Ga(2)O(3)/EGaIn top electrodes enable the collection of statistically significant numbers of data in convenient periods of time. The observed difference in charge transport between n-alkanethiols with odd and even numbers of methylenes--the "odd-even effect"--is statistically discernible using these junctions and demonstrates that this technique is sensitive to small differences in the structure and properties of the SAM. Alkanethiols with an even number of methylenes exhibit the expected exponential decrease in current density, J, with increasing chain length, as do alkanethiols with an odd number of methylenes. This trend disappears, however, when the two data sets are analyzed together: alkanethiols with an even number of methylenes typically show higher J than homologous alkanethiols with an odd number of methylenes. The precision of the present measurements and the statistical power of the present analysis are only sufficient to identify, with statistical confidence, the difference between an odd and even number of methylenes with respect to J, but not with respect to the tunneling decay constant, β, or the pre-exponential factor, J(0). This paper includes a discussion of the possible origins of the odd-even effect but does not endorse a single explanation. 相似文献
193.
The fluorides KF, RbF and CsF have been known to serve as catalysts for the reaction CF2O + F2→ CF3OF. The list of catalysts for this process has now been enlarged to include NaF, MgF2, CaF2, SrF2, BaF2 and LaF3. Lithium fluoride and thorium fluoride also give CF3OF but are less active. Perhaps the substances CsF·HF, KAgF4 and NiF2 should be included in this list. Silver fluoride, usually as a mixture of AgF2 with AgF, has been known to catalyze the reaction of CF2O with F2 to give both CF3OF and CF3OOCF3. The proportion of the latter in the mixture of products increases with decreasing temperature. At 25°, the reaction is slow and the yield of CF3OOCF3 is very high. It has now been shown that TIF3 behaves like silver fluoride. It has also been shown that many other fluorides of metals give higher yields of CF3OOCF3 than of CF3OF but require higher temperatures than AgF2 (100-ca. 150°) to be effective. Various possible mechanisms for these catalytic processes are discussed. 相似文献
194.
Treatment of lithium dicyclohexyl(trans-1-alkenyl)(1-alkynyl)borates with either boron trifluoride etherate or tri-n-butyltin chloride results in the preferential migration of the alkenyl group from boron to the adjacent alkynyl carbon atom. Protonolysis of the resultant organoboron intermediates with acetic acid affords the corresponding 1,4-disubstituted (E,Z)-1,3-dienes in good yields, provided that the (Z)-alkenyl moiety of the diene does not contain a tertiary alkyl group. Demonstration that this novel procedure is applicable for the preparation of (E,Z)-1,3-dienes containing functional groups has been shown by the synthesis in 66% yield of methyl (10E, 12Z)-hexadecadienoate, a precursor via lithium aluminium hydride reduction of the sex pheromone of the female silk moth, Bombyx mori. 相似文献
195.
J. A. Waters George A. Mortimer 《Journal of polymer science. Part A, Polymer chemistry》1972,10(6):1827-1837
Soluble ethylene polymerization catalysts derived from (π-C5H5)2Ti(R)Cl and R ′AlCl2, where R = methyl or phenyl and R ′ = methyl or ethyl, were studied both by polymerization kinetics at 0°C and by diagnostic experiments. It was found that the first insertion of ethylene into the Ti? R bond is difficult when R = methyl or phenyl, and for this reason these catalysts show a different overall behavior than when R = ethyl or higher alkyl. 相似文献
196.
Mary E. Carter James L. Nash Joseph W. Drueke James W. Schwietert George B. Butler 《Journal of polymer science. Part A, Polymer chemistry》1978,16(5):937-959
β-Nitrostyrene and many of its derivatives have been shown to yield high polymers via anionic initiation with alkoxide ions in protic solvents, e.g., alcohols. A study was conducted to determine the effect of certain substituents on the polymerization characteristics of representative monomers of this series and the properties of their polymers. A kinetic study was conducted and the relative rates of propagation were unexpectedly found to correlate well with brown's σ+ values. The rates of initiation of two representative monomers, β-nitrostyrene and p-methoxy-β-nitrostyrene, with sodium ethoxide were found to be 3.51 and 2.86 liter/mole sec, respectively. The rate of chain transfer in ethanol was studied qualitatively by using gel-permeation chromatography (GPC) to obtain molecular weight distribution (MWD) curves. The low values of the M w/M n ratios indicated little chain transfer in the protic solvent. 相似文献
197.
A classical model coupling a charged adspecies to a laser-induced surface plasmon is presented. Such coupling can enhance the rate and specify the direction of surface migration. For the particular case of an atomic oxygen ion of charge ?1 adsorbed on aluminum which is exposed to CO2 laser radiation of intensity 1 W/cm2, the velocity of migration (61.3 μm/s) is five orders of magnitude greater than the usual thermal velocities observed at room temperature. 相似文献
198.
George C. Schatz 《Chemical physics letters》1984,108(6):532-534
In this paper we present the results of accurate quantum coupled states calculations for the H + D2 → HD + D reaction at 0.55 eV translational energy (relative to υ = 0, j = 0) using the accurate LSTH potential surface. 相似文献
199.
A brief discussion is given of some factors that merit consideration in the analysis of peptides by mass spectrometry, with emphasis on sequence determination. The first systematic study of simple peptides by field ionization (FI) is described, and comparison made with the corresponding electron-impact (EI) ionization spectra, both at low resolution (approx. 1500). The substances examined were either benzyloxycarbonyl or t-butyloxycarbonyl derivatives of di-through pentapeptide methyl esters, containing the amino acids glycine, alanine, leucine, serine, threonine, proline and tyrosine. The incidences of sequence-characteristic cleavage peaks and rearrangement ion peaks were both slightly lower in the FI than the EI spectra, although the peak relative intensities generally were higher under FI conditions. 相似文献
200.
Draper ND Bakhoum SF Haddrell AE Agnes GR 《Journal of the American Chemical Society》2007,129(37):11364-11377
We have investigated the nucleation and growth of sodium chloride in both single quiescent charged droplets and charged droplet populations that were levitated in an electrodynamic levitation trap (EDLT). In both cases, the magnitude of a droplet's net excess charge (ions(DNEC)) influenced NaCl nucleation and growth, albeit in different capacities. We have termed the phenomenon ion-induced nucleation in solution. For single quiescent levitated droplets, an increase in ions(DNEC) resulted in a significant promotion of NaCl nucleation, as determined by the number of crystals observed. For levitated droplet populations, a change in NaCl crystal habit, from regular cubic shapes to dome-shaped dendrites, was observed once a surface charge density threshold of -9 x 10(-4) e.nm(-2) was surpassed. Although promotion of NaCl nucleation was observed for droplet population experiments, this can be attributed in part to the increased rate of solvent evaporation observed for levitated droplet populations having a high net charge. Promotion of nucleation was also observed for two organic acids, 2,4,6-trihydroxyacetophenone monohydrate (THAP) and alpha-cyano-4-hydroxycinnamic acid (CHCA). These results are of direct relevance to processes that occur in both soft-ionization techniques for mass spectrometry and to a variety of industrial processes. To this end, we have demonstrated the use of ion-induced nucleation in solution to form ammonium nitrate particles from levitated droplets to be used in in vitro toxicology studies of ambient particle types. 相似文献