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We have studied the kinetics of the Diels-Alder reactions of cyclopentadiene, sorbyl alcohol, and sorbyltrimethylammonium bromide with a series of N-substituted maleimides in micellar media. Micellar rate constants have been determined and were found to be 20-40 times lower than the respective aqueous rate constants. Nevertheless, it was found that upon addition of sodium dodecyl sulfate the observed rate constants could be enhanced up to a factor of about 4.5. The low micellar rate constants can be attributed to the relatively apolar (water-poor) region of the micelle, in which the reactions take place. NMR experiments indicate that the reactants usually reside near the alpha- or beta-CH2 groups of the surfactant molecules in the micelle. Comparison of the micellar rate constants with rate constants in water/1-propanol mixtures suggests a concentration of water of 10-15 M in the micellar region where the diene and dienophile react.  相似文献   
455.
The substitution of chloro or bromo groups in tetracene gives rise to the change of crystal structure, having a substantial effect on carrier transport. Halogenated tetracene derivatives were synthesized and grown into single crystals. Monosubstituted 5-bromo- and 5-chlorotetracenes have the herringbone-type structure, while 5,11-dichlorotetracene has the slipped pi stacking structure. Mobility of 5,11-dichlorotetracene was measured to be as high as 1.6 cm2/V.s in single-crystal transistors. The pi stacking structure, which enhances pi orbital overlap and facilitates carrier transport, may thus be responsible for this high mobility.  相似文献   
456.
Rate constants are reported for the pH-independent hydrolysis of 4-methoxyphenyl-2,2-dichloroethanoate in aqueous solution as a function of the concentration of added cyanomethane (acetonitrile), polyethylene glycol (PEG 400) and tetrahydrofuran (THF). The concentration of water was varied between ca. 25 and 55.5 M. It was found that the variation in water activity yields only a minor contribution to the observed variation in rate constants. Interestingly, for both cyanomethane and PEG 400 log(k) varies approximately linearly with the molar concentration of water. Medium effects in highly aqueous solutions ([H(2)O] > 50 M) of ethanol, 1-propanol, 2-propanol, 1-butanol and 2-methyl-2-propanol have also been determined. Unexpectedly, in this concentration range the alcohols induce significantly smaller effects per unit volume than cyanomethane. The present results are discussed in terms of pairwise interaction parameters. Isobaric activation parameters have been determined and reveal remarkable differences in the nature of the induced medium effects.  相似文献   
457.
Thermal analyses of synthetic and natural vivianite (Fe2+)3(PO4)2·8H2O) were determined using a high-resolution thermal analyser coupled to a mass spectrometer.Five dehydration weight loss steps were observed for the natural vivianite at 105, 138, 203, 272 and 437 °C. The first weight loss step involves the reaction (Fe2+)3(PO4)2·8H2O→(Fe2+)3(PO4)2·3H2O+5H2O. The TGA/MS for the synthetic vivianite gave similar results to that of the natural sample. Mass spectrometry shows that water is lost up to 450 °C and after this temperature oxygen is lost. Changes in the structure of vivianite were followed using infrared emission spectroscopy. A model is proposed for the dehydration of vivianite.  相似文献   
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The kinetics of the 1,3-dipolar cycloadditions (DC) of benzonitrile oxide with a series of N-substituted maleimides in micellar media have been investigated. Surfactants studied include anionic sodium dodecyl sulfate, cationic cetyltrimethylammonium bromide, and a series of nonionic alkyl poly(ethylene oxide) surfactants (CxEy). The kinetic data have been analyzed by using the pseudo-phase model for bimolecular reactions. Much larger micellar accelerations (up to a factor of 17) were observed for these reactions than was previously found for Diels-Alder (DA) reactions (J. Org. Chem. 2002, 67, 7369-7377). This is explained by the smaller solvent sensitivity of DC reactions, which causes the micellar rate constants to be much closer to the value of water (km/kw approximately 0.25-0.45 for DC reactions vs 0.02-0.05 for DA reactions). Further evidence is presented, that a water/1-propanol mixture ([H2O] ca. 15 M) is a fairly good mimic of the micellar reaction environment for these reactions. Isobaric activation parameters have been determined for the reaction in the micellar phase of C16E20, using micellar rate constants. They correspond well to values obtained for the aforementioned micelle mimic.  相似文献   
460.
We have developed a united atom force field able to accurately describe the adsorption properties of linear alkanes in the sodium form of FAU-type zeolites. This force field successfully reproduces experimental adsorption properties of n-alkanes over a wide range of sodium cation densities, temperatures, and pressures. The force field reproduces the sodium positions in dehydrated FAU-type zeolites known from crystallography, and it predicts how the sodium cations redistribute when n-alkanes adsorb. The cations in the sodalite cages are significantly more sensitive to the n-alkane loading than those in the supercages. We provide a simple expression that adequately describes the n-alkane Henry coefficient and adsorption enthalpy as a function of sodium density and temperature at low coverage. This expression affords an adequate substitute for complex configurational-bias Monte Carlo simulations. The applicability of the force field is by no means limited to low pressure and pure adsorbates, for it also successfully reproduces the adsorption from binary mixtures at high pressure.  相似文献   
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