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101.
The specific rates of solvolysis of N,N,N',N'-tetramethyldiamidophosphorochloridate have been measured at 25.0 degrees C in 31 solvents. Analysis with the extended Grunwald-Winstein equation leads to sensitivities toward changes in solvent nucleophilicity (l) of 1.20 +/- 0.07 and toward changes in solvent ionizing power (m) of 0.69 +/- 0.04. The correlation is improved by omission of the four data points for 2,2,2-trifluoroethanol-ethanol mixtures (F-test value from 155 to 320) with very small reductions in both l and m values. Activation parameters are reported for eight of the solvolyses. The l and m values are very similar to those previously reported for solvolyses of several arenesulfonyl chlorides, consistent with a concerted substitution process. This assignment is supported by a large k(Cl)/k(F) ratio for hydrolysis and a corresponding ratio for hydroxide-assisted hydrolysis of 178. The stereochemistry of nucleophilic attack at tetracoordinate phosphorus(V) is discussed. 相似文献
102.
Abstract— The kinetic factors which determine the rate at which Na+ channels in nerve membranes become photochemically modified were studied on giant axons from lobsters using the double sucrose gap voltage clamp technique. Axons were bathed in artificial sea water containing sensitizing dyes and illuminated from a Xe are source with light in the visible region while being repetitively step depolarized. Successive values of peak Na+ current and time-to-peak were monitored and rate constants for their change served as the assay for magnitude of modification. Action spectra for four sensitizers in the fluorescein series exhibited red shifts of roughly 17nm demonstrating that sensitizing species are not simply free in solution. Eosin Y diffuses to its sensitization sites with a half time of 70s indicating the existence of a major diffusion barrier which may mean that dye must penetrate to the interior of the membrane to be effective. Eosin Y is removed from sensitization sites by rinse with the same half time but shows two fractions: a faster fraction comprising 80% of sensitizing effectiveness and a slower fraction comprising 20%. The concentration dependence for Eosin Y is linear below 10 μ M and shows a progressive saturation at higher values, where the relationship is difficult to determine because of shielding. Different sensitizers vary in their ability to sensitize block of channels vs disruption of inactivation, demonstrating separate processes for the two modifications. It is suggested that both modifications proceed from single photon absorption events by individual sensitizer molecules bound or located close to the modification sites on the channels. 相似文献
103.
CELL MEMBRANE PHOTOMODIFICATION: RELATIVE EFFECTIVENESS OF HALOGENATED FLUORESCEINS FOR PHOTOHEMOLYSIS 总被引:1,自引:0,他引:1
Abstract— Relative potency of 10 fluorescein derivatives as sensitizers of delayed photohemolysis of human erythrocytes has been assessed. Dilute suspensions of washed cells were illuminated in the presence of sensitizer at different concentrations for 1 h and analyzed for percent hemolysis following 23 h of dark incubation. Plots of percent hemolysis versus concentration showed a steep dependence on concentration for all sensitizers. Additional measurements of octanol/water partition coefficients, photon absorption in octanol and in saline and photobleach rates were made. After correction for absorption cross section, the effectiveness values ranged over more than three orders of magnitude with fluorescein being the least potent and rose bengal the most. A reasonable prediction of potency is obtained by taking the product of partition coefficient, relative absorption in octanol versus water and molecular weight of the substituents added to the fluorescein skeleton. The results suggest that the influence of halogen substitution on sensitizing potency is exerted by four factors; (1) distribution of sensitizer into a low polarity region of the cell membrane, (2) absorption efficiency in a low polarity region, (3) triplet quantum yield, and (4) photobleach rate. 相似文献
104.
Fiedler D Leung DH Bergman RG Raymond KN 《Journal of the American Chemical Society》2004,126(12):3674-3675
A supramolecular metal-ligand assembly encapsulates a variety of cationic half-sandwich ruthenium complexes. Due to the chirality of both host and guest, chiral recognition is observed with diastereomeric excesses up to 70%. The chiral cavity can be used to carry out a dynamic resolution of the rapidly equilibrating enantiomers of the chiral organometallic guest. 相似文献
105.
Rachael Padman Dennis Bly 《International Journal of Infrared and Millimeter Waves》1992,13(10):1471-1485
High resolution millimetre and submillimetre wave astronomical spectrometers using hot-electron bolometer mixers as detectors often show marked standing wave patterns in the spectral baseline. LO phase noise contributes through two mechanisms: the phase noise side-bands may be converted to amplitude noise in the source because of the power-frequency characteristic of the source, or they can be converted to in-band amplitude noise through the action of the quasi-optical discriminator formed by the mixer, beam-splitter and telescope structure. The baseline ripple components due to each of these mechanisms have different characteristic periods, and under some circumstances can dominate the spectrometer baseline. The ripple levels estimated using the theory agree well with those observed in practice. It is shown that with careful design systematic effects due to this cause can be reduced to a negligible level. 相似文献
106.
We have experimentally investigated a basic ring laser gyroscope (RLG) with carbon dioxide gain, and studied the prospects of developing a practical CO2 RLG. Rotation sensing was demonstrated on a number of transitions in the 9.4 m and 10.4 m vibration-rotation bands. Gyro response is discussed with regard to lock-in, bias, homogeneous broadening effects, and high power operation. We show that such a system may offer important advantages over standard helium-neon RLGs, including reduced quantum limit and backscattering. The prospects and possible approaches for developing a practical high power CO2 RLG are discussed, and a method of eliminating cross-saturation at high pressure is proposed and analyzed. 相似文献
107.
108.
Chen JP Meziani ZE Beck D Boyd G Chinitz LM Day DB Dennis LC Dodge G Filippone BW Giovanetti KL Jourdan J Kemper KW Koh T Lorenzon W McCarthy JS McKeown RD Milner RG Minehart RC Morgenstern J Mougey J Potterveld DH Rondon-Aramayo OA Sealock RM Smith LC Thornton ST Walker RC Woodward C 《Physical review letters》1991,66(10):1283-1286
109.
A nano-electrospray ionisation source has been designed and constructed for a high temperature ion mobility spectrometer. The drift cell was modified by replacement of the 63Ni atmospheric pressure chemical ionisation source with a tube lens/desolvation region and operated using commercial nano-electrospray capillaries. Ions were introduced into the drift region via a Bradbury-Nielson gate (pulse width 50 micros, repetition period 20 ms). A unidirectional flow of nitrogen was used as the drift gas at temperatures in the range 100-150 degrees C to aid desolvation. The performance of the nano-electrospray ion source has been demonstrated for analytes including crown ethers, amino acids and peptides. Reduced mobilities determined by nano-ESI were consistent with those reported using a 63Ni ion source. 相似文献
110.
Jacob J. Habeeb Farouq F. Said Dennis G. Tuck 《Journal of organometallic chemistry》1980,190(4):325-334
The electrochemical oxidation of indium metal in cells of the type
leads to the formation of RInX2 compounds; if 2,2′-bipyridine is also present, the products are the adducts RInX2·bipy (R = CH3, C2H5, C6H5, C6H5CH2, C6F5; X = Cl, Br, I (not all combinations)). When R′4NX is present instead of bipy, the products are the salts R′4N[RInX3]. The electrochemical oxidation apparently proceeds via the general mechanism discussed previously. Anomalous results with CH3I or C2H5I are discussed in the light of the known solution chemistry of organoindium(III) compounds. 相似文献