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101.
Sessler JL Kim SK Gross DE Lee CH Kim JS Lynch VM 《Journal of the American Chemical Society》2008,130(39):13162-13166
An ion-pair receptor, 1, containing both cation- and anion-recognizing sites, has been synthesized and characterized. Single-crystal X-ray diffraction structural studies and (1)H NMR spectroscopic analyses confirmed that 1 forms stable 1:1 complexes with CsF in solution and in the solid state in spite of the large separation enforced between the receptor-bound anion and cation. In 9:1 CDCl3/CD3OD, binding of fluoride anion within the calix[4]pyrrole core of 1 was not observed in the absence of a cobound cesium cation; however, it was seen in this solvent mixture under conditions where a Cs(+) cation was bound to the crown ether-strapped calix[4]arene subunit. 相似文献
102.
103.
Fowler CJ Haverlock TJ Moyer BA Shriver JA Gross DE Marquez M Sessler JL Hossain MA Bowman-James K 《Journal of the American Chemical Society》2008,130(44):14386-14387
In this communication, a new approach to enhancing the efficacy of liquid-liquid anion exchange is demonstrated. It involves the concurrent use of appropriately chosen hydrogen-bond-donating (HBD) anion receptors in combination with a traditional quaternary ammonium extractant. The fluorinated calixpyrroles 1 and 2 and the tetraamide macrocycle 4 were found to be particularly effective receptors. Specifically, their use allowed the extraction of sulfate by tricaprylmethylammonium nitrate to be effected in the presence of excess nitrate. As such, the present work provides a rare demonstration of overcoming the Hofmeister bias in a competitive environment and the first to the authors' knowledge wherein this difficult-to-achieve objective is attained using a neutral HBD-based anion binding agent under conditions of solvent extraction. 相似文献
104.
A reversed-phase liquid chromatographic/tandem mass spectrometric method was developed and validated for the determination of free carnitine (FC) and total carnitine (TC) in infant formula and raw ingredients. Preparation of samples for FC determination required only nonvolumetric dilution in water, followed by filtration. Determination of TC in whey-based samples required a common saponification procedure, which hydrolyzed the acylcarnitines to FC. L-Carnitine standards were prepared in water at 3 concentrations (7.5, 30, and 60 ng/mL). L-Carnitine-d3 was incorporated in the sample and standard preparations as a stable isotope internal standard (ISTD), which helped to normalize signal suppression due to fouling of the ionization source, or matrix effects. A short C8 column (50 x 2.1 mm id) was used for liquid chromatography, with mobile phases consisting of 0.1% heptafluorobutyric acid in water and methanol. L-Carnitine and the ISTD eluted at about 2 min, and the injection-to-injection cycle time was about 11 min. Multiple-reaction monitoring was used for quantitation by monitoring the ion transitions m/z 162 > 103 and 162 > 85 for L-carnitine, and m/z 165 > 103 and 165 > 85 for the ISTD. The values for overall method precision for a whey-based infant formula, a protein hydrolyzate infant formula, and a water-soluble vitamin/amino acid premix were relative standard deviations of 1.32, 1.86, and 3.33%, respectively. The overall mean recoveries were between 97.3 and 102.8%. Additionally, the method results showed good correlation with those obtained for a radioenzymatic assay done in-house. 相似文献
105.
W. S. Johnson's total synthesis of the sesquiterpenoid longifolene is a classic example of the power of cationic polycyclizations for constructing complex molecular architectures. Herein we revisit the key polycyclization step of this synthesis using hybrid Hartree-Fock/density functional theory calculations and validate the feasibility of Johnson's proposed mechanism. We also explore perturbations to the 3-center 2-electron bonding array in a key intermediate that result from changing the polycyclic framework in which it is embedded. 相似文献
106.
Halton B Dixon GM Jones CS Parkin CT Veedu RN Bornemann H Wentrup C 《Organic letters》2005,7(5):949-952
Reaction of 1,1-dichloro-2,5-diphenylcyclopropabenzene 6 with Meldrum's acid 8 in the presence of pyridine leads to coupling of the cycloproparenyl cation 7 with the stabilized diketo anion 9. Subsequent, spontaneous, base-induced dehydrochlorination gives the alkylidenecyclopropabenzene 11 in a one-pot reaction. Flash vacuum thermolysis of 11 at 650 degrees C ejects acetone and carbon dioxide, giving cyclopropabenzenylidenethenone 12 that is isolated in an Ar matrix at 20 K and characterized by a strong ketene band at 2107 cm(-1) in the IR spectrum. [reaction: see text] 相似文献
107.
F. Henz A. Fischer A. Gutbier G. Brunner J. A. Muller O. Klenker H. Baubigny H. Ost W. Klapproth J. M. M. Dormaar J. Langness E. F. Smith H. D. Law F. M. Perkin M. Heidenreich F. Beilstein O. v. Blaese A. van Bylert G. Denigès C. J. Brooks O. Schmatolla J. P. Longstaff Noaillon Ch. Ratner G. Bornemann J. C. Mackintosh W. R. Lang C. M. Carson J. Walker 《Analytical and bioanalytical chemistry》1907,46(9):593-606
108.
Automatic Deformation of Riemann–Hilbert Problems with Applications to the Painlevé II Transcendents
The stability and convergence rate of Olver’s collocation method for the numerical solution of Riemann–Hilbert problems (RHPs) are known to depend very sensitively on the particular choice of contours used as data of the RHP. By manually performing contour deformations that proved to be successful in the asymptotic analysis of RHPs, such as the method of nonlinear steepest descent, the numerical method can basically be preconditioned, making it asymptotically stable. In this paper, however, we will show that most of these preconditioning deformations, including lensing, can be addressed in an automatic, completely algorithmic fashion that would turn the numerical method into a black-box solver. To this end, the preconditioning of RHPs is recast as a discrete, graph-based optimization problem: the deformed contours are obtained as a system of shortest paths within a planar graph weighted by the relative strength of the jump matrices. The algorithm is illustrated for the RHP representing the Painlevé II transcendents. 相似文献
109.
110.