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991.
Daschbach JL Dohnalek Z Liu SR Smith RS Kay BD 《The journal of physical chemistry. B》2005,109(20):10362-10370
The adsorption of water on FeO(111) is investigated using temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRAS). Well-ordered 2 ML thick FeO(111) films are grown epitaxially on a Pt(111) substrate. Water adsorbs molecularly on FeO(111) and desorbs with a well resolved monolayer peak. IRAS measurements as a function of coverage are performed for water deposited at 30 and 135 K. For all coverages (0.2 ML and greater), the adsorbed water exhibits significant hydrogen bonding. Differences in IRAS spectra for water adsorbed at 30 and 135 K are subtle but suggest that water adsorbed at 135 K is well ordered. Monolayer nitrogen TPD spectra from water covered FeO(111) surfaces are used to investigate the clustering of the water as a function of deposition or annealing temperature. Temperature dependent water overlayer structures result from differences in water diffusion rates on bare FeO(111) and on water adsorbed on FeO(111). Features in the nitrogen TPD spectra allow the monolayer wetting and 2-dimensional (2D) ordering of water on FeO(111) to be followed. Voids in a partially disordered first water layer exist for water deposited below 120 K and ordered 2D islands are found when depositing water above 120 K. 相似文献
992.
993.
Various approaches to the syntheses of 5-substituted-3-carbomethoxy-2,5-dihydrothiophenes and their product sulfolenes, required as synthetic precursors for tangutorine, are described. An efficient route to 3,5-disubstituted-4,5-dihydrothiophenes and hence 3,5-disubstituted-2-sulfolenes by radical chemistry is also described. 相似文献
994.
Kimberly A. Carter-Fenk Kevin Carter-Fenk Michelle E. Fiamingo Heather C. Allen John M. Herbert 《Chemical science》2021,12(24):8320
Surface-sensitive vibrational spectroscopy is a common tool for measuring molecular organization and intermolecular interactions at interfaces. Peak intensity ratios are typically used to extract molecular information from one-dimensional spectra but vibrational coupling between surfactant molecules can manifest as signal depletion in one-dimensional spectra. Through a combination of experiment and theory, we demonstrate the emergence of vibrational exciton delocalization in infrared reflection–absorption spectra of soluble and insoluble surfactants at the air/water interface. Vibrational coupling causes a significant decrease in peak intensities corresponding to C–F vibrational modes of perfluorooctanoic acid molecules. Vibrational excitons also form between arachidic acid surfactants within a compressed monolayer, manifesting as signal reduction of C–H stretching modes. Ionic composition of the aqueous phase impacts surfactant intermolecular distance, thereby modulating vibrational coupling strength between surfactants. Our results serve as a cautionary tale against employing alkyl and fluoroalkyl vibrational peak intensities as proxies for concentration, although such analysis is ubiquitous in interface science.Coupling between surfactant molecules at the air/water interface bleeds intensity into a diffuse background, such that single-wavelength vibrational intensity is effectively depleted at high surface coverage. 相似文献
995.
Reaction of Cu(ClO(4))(2) x 6H(2)O with a racemic mixture of the novel chiral ligand N-(1,2-bis(2-pyridyl)ethyl)pyridine-2-carboxamide (PEAH) affords only the homochiral dimeric copper(II) complexes [Cu(2)((R)()PEA)(2)](ClO(4))(2) and [Cu(2)((S)()PEA)(2)](ClO(4))(2) in a 1:1 ratio. The phenomenon of molecular self-recognition is also observed when a racemic mixture of the monomeric copper(II) complex [Cu((R(S))()PEA)(Cl)(H(2)O)] is converted into the homochiral dimeric species [Cu(2)((R(S))()PEA)(2)](ClO(4))(2) via reaction with Ag(+) ion. This is the first report of direct conversion of a racemic mixture of a chiral monomeric copper(II) complex to a mixture of the homochiral dimers. 相似文献
996.
Jin-Lin Shen Christian Hummert B. Luckas 《Fresenius' Journal of Analytical Chemistry》1997,357(1):101-104
Extracts containing the diarrhetic shellfish poisoning (DSP) toxins okadaic acid (OA), dinophysistoxin-2 (DTX2), and dinophysistoxin-1 (DTX1) were purified on a silica gel cartridge and derivatized with 4-bromomethyl-7 methoxycoumarin (BrMmc). After pre-column derivatization
the BrMmc derivatives of the DSP toxins were directly injected into an HPLC system, isocratically eluted, and quantified by
fluorescence detection. The signals of the esters showed good linearity in the fluorescence detector within the examined contamination
range of 0.03 mg DSP/kg to 2.5 mg DSP/kg. The detection limits for the DSP toxins as 7-Mmc esters were 0.04 ng (corresponding
to 0.05 mg DSP/kg). The chromatographic conditions allow to couple the HPLC device with mass spectrometry. The method was
tested with various mussel tissue samples.
Received: 14 December 1995/Revised: 26 January 1996/Accepted: 30 January 1996 相似文献
997.
998.
Pyrazines and quinoxalines bearing 2-substituents that direct ortho metalation reacted with lithium 2,2,6,6-tetramethylpiperidide to produce 2-substituted-3-lithiopyrazines and quinoxalines. These lithio reagents reacted with N-methoxy-N-methylbenzamide to give good to moderate yields of 3-substituted pyrazinyl or quinoxalinylphenylmethanones. The 3-methylthio substituents of some ketone products were oxidized to methylsulfonyl groups that were susceptible to nucleophilic displacement. 相似文献
999.
Rolf Gleiter Jens Spanget-Larsen Erik W. Thulstrup Ichiro Murata Kazuhiro Nakasuji Christian Jutz 《Helvetica chimica acta》1976,59(5):1459-1468
The photoelectron (PE.) spectra of azuleno[l, 2, 3-cd]phenalene ( 1 ) and azuleno- [5,6,7-cd]phenalene( 2 ) have been recorded. The first five bands of both compounds could be assigned to transitions corresponding to removal of electrons from 4a2, 6b1, 5b1, 3a2 and 4bl orbitals. This assignment is based mainly on a comparison between the observed ionization potentials and orbital energies calculated in a HMO and a PPP model. The UV./VIS. polarized absorption spectrum of 1 in the region 10000–45000 cm?1 has been measured by means of the stretched film technique. The measurements were performed in polyethylene sheets at 77°K. Several bands could be assigned to π* ← π transitions calculated by a PPP-CI method. A comparison between the electronic structures of 1 and 2 is made by means of a simple HMO diagram. 相似文献
1000.
A numerical model for calculating the electrostatic interaction between two particles of arbitrary shape and topology is described. A key feature of the model is a generalized discretization program, capable of simulating any desired analytical shape as a set of flat, triangular elements. The relative sizes of the elements are adjusted using a density function to better match the desired shape and the spatial variation of the electrical surface properties on each particle. The distribution of either surface potential or surface charge density is then calculated using a boundary element approach to solve the linearized Poisson-Boltzmann equation. Example interaction energy profiles are calculated for three different types of roughness-bumps, pits, and surface waves. It is found that the interaction energy between rough particles remains different from that between two equivalent smooth spheres at all separations, even for gap widths much larger than either the solution Debye length or the characteristic roughness size. This behavior at large gap widths arises from the nature of the decay of the electric potential away from each particle. In addition, the magnitude of the roughness effect is found to depend greatly on the size and shape of the nonuniformity as well as the electrostatic boundary conditions. For example, for a sphere containing asperities of height equal to 0.2 times the particle radius, the interaction energy can be as much as 50% greater than that between two equivalent spheres under the condition of constant surface potential. At constant surface charge density, the ratio of the interaction energies between rough and smooth spheres was found to either diverge or become zero as contact between the two particles is approached, depending on the nature of the roughness. Changes of this magnitude could clearly have a substantial impact on the stability behavior of a dispersion of such particles. Copyright 2001 Academic Press. 相似文献