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991.
Smith AB Freeze BS Lamarche MJ Hirose T Brouard I Xian M Sundermann KF Shaw SJ Burlingame MA Horwitz SB Myles DC 《Organic letters》2005,7(2):315-318
[Structure: see text] The design, syntheses, and biological evaluation of nine totally synthetic analogues of the microtubule-stabilizing agent (+)-14-normethyldiscodermolide (2) are reported. Simplification at the C(21)-C(24) terminal diene and at the C(1)-C(5) lactone moieties reveals significant structure-activity relationships. 相似文献
992.
Zhang W Cozzolino AF Mahmoudkhani AH Tulumello M Mansour S Vargas-Baca I 《The journal of physical chemistry. B》2005,109(39):18378-18384
The wavelength-dependent second-harmonic generation (SHG) efficiency of two simple dipolar chromophores, 4-NO2C6H4N(H)Bun (1) and 4-NO2C6H4SN(H)But (2), was compared in solution and in the solid state. Hyper-Rayleigh scattering measurements at 532 nm provided comparable molecular first hyperpolarizabilities. Both compounds crystallize in non-centrosymmetric space groups, but a more efficient arrangement of dipole moments results in a significantly larger deff value for 2. Kurtz-Perry experiments from 450 to 700 nm revealed an important difference in the resonant component of the nonlinear optical responses of these compounds; the SHG efficiency of crystalline 1 depends more strongly on the incident wavelength than that of 2. This would be in contradiction with the TD-DFT excitation energies calculated for these molecules, but the observation can be explained by the resonant contribution from low-energy interchromophore excitations enabled by pi-stacking in the crystal of 1. 相似文献
993.
Sánchez L Pérez I Martín N Guldi DM 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(11):2457-2468
Novel pi-extended tetrathiafulvalene (exTTF)-based donor acceptor hybrids-dyads and triads-have been synthesized following a multistep synthetic procedure. Cyclic voltammetry and absorption spectroscopy, conducted in room temperature solutions, reveal features that are identical to the sum of the separate donor and acceptor moieties. Steady-state and time-resolved photolytic techniques confirm that upon photoexcitation of the fullerene chromophore, rapid (1.25 x 10(10) s(-1)) and efficient (67 %) charge separation leads to long-lived, charge-separated radical pairs. Typical lifetimes for the dyad ensembles range between 54 and 460 ns, with the longer values found in more polar solvents. This indicates that the dynamics are located in the 'normal region' of the Marcus curve. In the triads, subsequent charge shifts transform the adjacent radical pair into the distant radical pair, for which we determined lifetimes of up to 111 micros in DMF-values never previously accomplished in molecular triads. In the final charge-separated state, large donor-acceptor separation (center-to-center distances: approximately 30 A) minimizes the coupling between reduced acceptor and oxidized donor. Analysis of the charge recombination kinetics shows that a stepwise mechanism accounts for the unusually long lifetimes. 相似文献
994.
Luis Fermín Capitán-Vallvey Natalia Navas Iglesias Ignacio de Orbe Payá Ramiro Avidad Castaneda 《Mikrochimica acta》1997,126(1-2):153-157
A spectrophotometric method for the simultaneous determination of Tartrazine (TT) and Sunset Yellow (SY) in cosmetic products has been developed. An extraction process was carried out using methylene chloride and the colouring matters were measured in the aqueous phase formed, the other components of the sample remaining in the organic phase. The applicable concentration ranges were 0.5–10 ug/ml TT and 0.5–12 g/ml SY. The detection limits were 26 and 11 ng/ml and the relative standard deviations were 1.0 and 0.9% for TT and SY, respectively. The method was applied to the determination of both compounds in cosmetics. 相似文献
995.
Javier Tabera Guillermo Reglero Marta Herraiz Gracia P. Blanch 《Journal of separation science》1991,14(6):392-396
The use of a programmed temperature vaporizing (PTV) injector has been evaluated for the on-line concentration and injection of trace organic compounds either sampled from the head-space above grape juices or purged from solution. The Simplex method was used to improve the sensitivity of the method by optimization of the experimental conditions. 相似文献
996.
Beatriz Giner Begoña Oliver Ignacio Giner Gorka Pera Carlos Lafuente 《Journal of solution chemistry》2007,36(3):375-386
Isentropic and excess isentropic compressibilities of binary mixtures formed by tetrahydrofuran or tetrahydropyran and isomeric
chlorobutanes at 298.15 and 313.15 K have been calculated from experimental measurements of densities and speeds of sounds.
Excess isentropic compressibilities are negative for all the mixtures except for the mixtures containing 1-chlorobutane at
298.15 K at very low concentrations of cyclic ether. Isentropic compressibilities and speeds of sound have been estimated
using the Prigogine-Flory-Patterson theory and satisfactorily compared with the experimental values. 相似文献
997.
Roberto Martínez Micaela Velasco Ignacio Martínez Italo Menconi Alberto Ramfrez Enrique Angeles Ignacio Regla Rafael Lpez 《Journal of heterocyclic chemistry》1997,34(6):1865-1866
The reaction of phenyloxiranes with a clay produces aldehydes, ketones and cyclic ethers. This method has the advantage that experimental conditions are simple and the reagents used are cheaper. 相似文献
998.
An on-column metal coating procedure was developed for sheathless electrospray emitters, based on Justus von Liebig's electroless silver mirror reaction followed by electrochemical deposition of gold onto the silver layer. The coating procedure is straightforward, mild, inexpensive, and can be performed with standard laboratory equipment. A long-term (600 h) stability investigation of the conductive coating was carried out by continuous electrospray in the positive electrospray mode, and no degradation in performance was found. The simplicity of the coating procedure and the robustness of the spray tips makes the spray tips highly suitable to couple delicate wall-coated or monolithic capillary columns to mass spectrometry. Peptide mixtures were separated by capillary electrophoresis and injected into either a Hadamard-transform time-of-flight mass analyzer or a commercial quadrupole mass analyzer using the described sheathless electrospray emitters. The performance was judged to be excellent. 相似文献
999.
Is the CH2OH + O2 → CH2 = O + HO2 Reaction Barrierless? An Ab Initio Study on the Reaction Mechanism
Víctor M. Ramírez-Ramírez Luis Serrano-Andrés Ignacio Nebot-Gil 《Theoretical chemistry accounts》2006,116(4-5):637-640
The reaction mechanism of the CH2OH + O2 gas-phase reaction was investigated by means of ab initio calculations. MP2 and QCISD methodologies were used to obtain the stationary points on the potential energy surface. Single-point high-level QCISD(T) calculations were performed over the QCISD results in order to refine the energy of the transition states and the minima found. A new transition state concerning the initial O2 addition to the CH2OH radical was found, not reported so far for this reaction. Extra CCSD optimisation and single-point high-level CCSD(T) calculations upon the QCISD results confirm this TS. Additional RASSCF calculations show that its wave function has no significant multireferential character.Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users. 相似文献
1000.
Fast-heating programmes for determining titanium and tin in soils, sediments and sludges using electrothermal atomic absorption spectrometry (ETAAS) with slurry sampling are developed. For titanium determination, suspensions are prepared by weighing 5-40 mg of sample and adding 25 ml of a solution containing 50% (v/v) concentrated hydrofluoric acid. For tin determination, suspensions are prepared by weighing up to 300 mg of sample and then adding 1 ml of a solution containing 25% (v/v) concentrated hydrofluoric acid. Palladium (30 μg) and ammonium dihydrogen phosphate (7% w/v) are used as matrix modifiers for titanium and tin, respectively. Prior mild heating in a microwave oven is recommended for titanium determination. Calibration is carried out using aqueous standards. The tin and titanium contents of a number of samples obtained by using the slurry approach agree with those obtained by means of a procedure based on the total dissolution of the samples using microwave oven digestion. The reliability of the procedures is also confirmed by analysing several certified reference materials. 相似文献