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91.
Qian J Timko MT Allen AJ Russell CJ Winnik B Buckley B Steinfeld JI Tester JW 《Journal of the American Chemical Society》2004,126(17):5465-5474
The rate of the Diels-Alder reaction between N-ethylmaleimide and 9-hydroxymethylanthracene in supercritical carbon dioxide (scCO(2)) was determined by following the disappearance of 9-hydroxymethylanthracene with in situ UV/vis absorption spectroscopy. The reaction conditions were 45-75 degrees C and 90-190 bar, which correspond to fluid densities (based on pure carbon dioxide) ranging between approximately 340 and 730 kg m(-3). The measured reaction rate at low scCO(2) fluid densities was nearly 25x faster than that reported in acetonitrile at the same temperature (45 degrees C). An inverse relationship between reaction rate and fluid density/pressure was observed at all temperatures in scCO(2). The apparent activation volumes were large and positive (350 cm(3) mol(-1)) and only a weak function of reduced temperature. A solvophobic mechanism analogous to those observed in conventional solvents is postulated to describe (a) the rate acceleration observed for this reaction in scCO(2) relative to that in acetonitrile, (b) the observed relationship between reaction rate and pressure/temperature/density, and (c) the large, positive activation volumes. Solubility measurements in scCO(2), rate measurements in conventional solvents, and an empirical correlation are used to support this theory. Our results advance the general understanding of reactivity in supercritical fluids and provide a rationale for selecting reactions which can be accelerated when conducted in scCO(2). 相似文献
92.
Abrams GS Adolphsen CE Averill D Ballam J Barish BC Barklow T Barnett BA Bartelt J Bethke S Blockus D Bonvicini G Boyarski A Brabson B Breakstone A Bulos F Burchat PR Burke DL Cence RJ Chapman J Chmeissani M Cords D Coupal DP Dauncey P DeStaebler HC Dorfan DE Dorfan JM Drewer DC Elia R Feldman GJ Fernandes D Field RC Ford WT Fordham C Frey R Fujino D Gan KK Gero E Gidal G Glanzman T Goldhaber G Gomez Cadenas JJ Gratta G Grindhammer G Grosse-Wiesmann P Hanson G Harr R Harral B Harris FA 《Physical review letters》1989,63(22):2447-2451
93.
94.
Isocratic hydrophobic interaction chromatography of five proteins has been carried out using mobile phases containing the surfactant 3-(3-cholamidopropyl)-dimethylammoniopropane sulfonate (CHAPS). Linear relationships were found between log k' and ammonium sulfate concentrations for all the proteins with CHAPS in the submicellar concentration range. The slope of such a plot decreases monotonically as CHAPS concentration is increased. To a first approximation, the effect of CHAPS on protein retention can be explained in terms of a competitive binding model. However, CHAPS does show differential effects on the elution of proteins, substantially altering selectivity. The use of a normalized capacity factor, k'/k'o, proves useful for comparing retention times of different proteins as a function of CHAPS concentration. The magnitudes of k'/k'o were found to be inversely correlated with the slopes of plots of log k' vs. ammonium sulfate concentration in the absence of CHAPS. Adsorption isotherms for CHAPS were determined over the working range of ammonium sulfate. The binding of CHAPS to the SynChropak Propyl stationary phase and its effects on retention were found to be readily reversible. For each protein, plots of k'/k'o vs. surface concentration of CHAPS were superposable for data obtained at different salt concentrations. These findings support a competitive binding model. A simple geometric argument for stationary phase occupancy provides a qualitative explanation for the observed surfactant selectivity. 相似文献
95.
96.
Factors affecting the cleavage of the carbon-oxygen bond in linear and cyclic aldehyde hydrates, heimacetals, acetals, and methyl ribosides and glucosides have been investigated using semiempirical calculations (AM1 and PM3). (For some systems, low- and high-level ab initio energies are available for comparison with the semiempirical results. With one exception, the results obtained by the two methods show excellent agreement in relative energies and trends in reactivity.) The effects on reactivity and stability caused by substituting a sulfur for the alpha oxygen in the oxocarbenium ion were also studied. In general, systems that can have an antiperiplanar alignment of lone pairs on the leaving group and potential oxocarbenium ion oxygens undergo spontanteous cleavage. An examination of various conformers of the leaving group relative to the potential oxocarbenium oxygen shows, however, that lone pair repulsion and steric factors for MeOH as the leaving group are more important than the antiperiplanar effect for bond cleavage. All compounds in which the alpha-oxygen in the potential carbenium ion is replaced by sulfur undergo spontaneous cleavage regardless of the leaving group or structure of the compound. Energy profiles, DeltaH(), and DeltaH(R) values show that linear and cyclic thiocarbenium ions are much more stable than the corresponding oxocarbenium ions. Comparison of results for methyl ribosides and glucosides with results for corresponding pyridinium substrates suggests that both should hydrolyze through an A-1 mechanism. General-acid catalysis with hydronium as the acid was studied. With solution results, the computations suggest that substrates with either a good leaving group or stable oxocarbenium ion react with rate-limiting proton transfer from the acid to the leaving group but that substrates with both a good leaving group and stable carbenium ion react with concerted proton transfer and bond cleavage. 相似文献
97.
Holographic projector using one lens 总被引:1,自引:0,他引:1
Buckley E 《Optics letters》2010,35(20):3399-3401
It is shown that the lens count in a Fourier holographic projector can be reduced by encoding the equivalent lens power in sets of Fresnel holograms. By using appropriately calculated Fresnel holograms in a reflective configuration to effectively share a lens between the beam-expansion and demagnification stages of a holographic projector, a reduction in lens count from four to one is demonstrated. 相似文献
98.
We demonstrate the use of a prism-grating sequence to reduce third-order dispersion inside a mode-locked Yb fiber laser. This laser generates pulses as short as 33 fs with extremely clean temporal and spectral profiles. Nanojoule pulse energies are possible. 相似文献
99.
100.
AM1-calculated energy profiles for dissociation of (methoxymethyl)pyridinium and dimethylanilinium ion substrates show that the methoxymethyl carbenium ion is not sufficiently stable to exist as an intermediate on the reaction coordinate for this model reaction. [(Thiomethoxy)methyl]pyridinium ion, however, has a distinct transition state because of the stability of the resulting ion-neutral complex. The complete potential energy surfaces for water displacement on the methoxymethyl substrate with either pyridine or dimethylaniline as the leaving group show distinct transition states and very flat surfaces for the ion-neutral complexes in which interaction of the carbenium ion with both leaving group and nucleophile is stabilizing. Secondary systems studied, including linear methoxy and thiomethoxy substrates, 5- and 6-membered cyclic oxo and thio substrates, and ribosyl-, xylopyranosyl-, and glucopyranosylpyridinium ions yield ion-neutral complexes with sufficient intrinsic stability to exist as intermediates. Comparison with solution data, primarily activation entropy and Br?nsted coefficients, suggests that the sugar oxocarbenium ions, either as distinct, solvent-equilibrated intermediates or elements of ion-neutral complexes, are formed by unimolecular dissociation of the respective substrates in solution. 相似文献