A comparison of geometric methods with other methods for the characterization of density matrices

Some recently developed geometric methods for characterizing the subset of density matrices within the space of Hermitian matrices are compared with methods commonly used for the approximate characterization of reduced density matrices. The decomposition of a density matrix into components in terms of the reducing basis set is compared with decomposition in terms of representations of U(r).     

Study of the major fragmentation pathways of cypermethrin and related synthetic insecticides using tandem mass spectrometry

Fragmentation pathways of the synthetic pyrethroid cypermethrin and four structurally related insecticides were investigated using a tandem quadrupole mass spectrometer incorporating a hexapole collision cell under positive-ion electron impact ionization conditions. Conventional mass spectrometry using the first quadrupole analyser only and tandem mass spectrometry on selected precursor ions and product ions, and also constant neutral loss scan experiments, were used. Mechanisms and fragmentation pathways are proposed to explain the inherent stability of ions associated with the benzylphenoxy portion of this class of insecticide.     

A study of molecular one-electron properties in terms of localized molecular orbital components

A number of molecular one-electron progperties have been analyzed by partitioning their electronic components over energy localized molecular orbitals (LMO ). The ammonia and ethane molecules, calculated in an Approximately double zeta qualtiy basis set, were considered. The partitioning of the electronic components of certain one-electron properteies over LMO allows a quantitative rationalization of the sensitivity of certain properties to basis set effects due to the differeing degree of difficulty of accurately determining different LMO as measures of the molecular electron density.     

Quantum scattering calculation of the O(1D)+HBr reaction

Three-dimensional time-dependent quantum wave packet calculation for the O((1)D)+HBr reaction has been carried out using an accurate ab initio global potential energy surface [K. A. Peterson, J. Chem. Phys. 113, 4598 (2000)]. The calculations show that the initial state-selected reaction probabilities are dominated by resonance structures, and the lifetime of the resonance is generally in the subpicosecond time scale. The energy dependence of the reaction cross section is computed, which manifests still resonance structures, and is a decreasing function of the translational energy. The thermal rate constants are also computed, which are nearly independent on the temperature. The calculation results are discussed and compared to similar reaction with deep well.     

Synthesis and structure of pyrazolo[5,1-a]isoindol-8-ones from Aromatic Ortho Diesters

The title compounds 2 , have now been shown to arise in certain cases from condensation of aromatic ortho-diesters with ketone, where formerly only indeno[1,2-c]pyrazol-4(1H)-ones 3 have been reported from such reaction. Heretofore 2 was obtained in a less direct fashion from phthalaldehydic acid esters. When hetero-aromatic diesters were employed, new heterocyclic ring systems as represented by 2a,b ,c were prepared for the first time. Structures of 2 have been verified from detailed ¹H and ¹³C nmr studies, while representative intermediates 4–9 in the condensation of diesters to 2 have been isolated and identified.     

Rapid determination of corticosteroids in pharmaceuticals by flow injection analysis

The proposed method is based on the reduction of blue tetrazolium by the steroid in an alkaline medium to form a highly colored formazan. The effects of reagent concentration, temperature, flow rate, and manifold design on the reaction are discussed for a typical steroid, methylprednisolone acetate. Analytical readout is obtained within 30 s after sample introduction and up to 100 samples/h can be processed with baseline resolution between peaks. Typical relative standard deviations of 0.5% are obtained with 10-μ1 injection volumes. Results obtained by flow injection analysis are similar to those obtained with the AutoAnalyzer technique.     

The Dyeing of Nylon and Cotton Cloth with Azo Dyes: Kinetics and Mechanism

The mechanism of the dyeing of cotton and nylon cloth by the azo dyes Orange G and Sunset Yellow FCF was investigated using a channel flow cell. The variation in dyeing with flow rate was found to proceed via a mechanism in which the flux of dye entering the cloth relative to the flux of dye to the cloth surfacedecreasedwith increasing flow rate. A mechanism is deduced in which the dye passes from bulk solution, through a porous surface layer within the cloth, before passing into the bulk cloth. Adsorption onto surface sites in this porous layer blocks the passage of further dye into the cloth. Kinetic parameters for such a mechanism are given.     

A novel,stereoselective silyl-directed Stevens [1,2]-shift of ammonium ylides

[reaction: see text] The silyl group of 2-silylpyrrolidines such as 1 plays several critical roles: a stereochemical control element in a facially selective carbenoid addition to the ring nitrogen, a stereochemical "placeholder" during regioselective 1,2-migration with retention by the resulting spirocyclic ammonium ylide, and a hydroxyl surrogate for an eventual stereoselective Fleming-Tamao oxidation. This chemistry represents a novel use of the Stevens rearrangement and offers a short, enantioselective route to hydroxylated quinolizidines such as 3 from Boc-pyrrolidine.     

In situ soft X-ray studies of ethylene oxidation mechanisms and intermediates on the Pt(111) surface

In situ studies of ethylene oxidation on Pt(111) have been performed using a powerful combination of fluorescence yield soft X-ray methods for temperatures up to 600 K and oxygen pressures up to 0.01 Torr. Absolute carbon coverages have been determined both in steady-state and dynamic catalytic conditions on the Pt(111) surface. Fluorescence yield near-edge spectroscopy (FYNES) and temperature-programmed fluorescence yield near-edge spectroscopy (TP-FYNES) experiments above the carbon K edge were used to identify the structure and bonding of the dominant surface species during oxidation. TP-FYNES experiments of preadsorbed ethylene coverages in oxygen pressures up to 0.01 Torr indicate a stable intermediate is formed over the 215-300 K temperature range. By comparing the intensity of the C-H sigma resonance at the magic angle with the intensity in the carbon continuum, the stoichiometry of this intermediate has been determined explicitly. Based on calibration with known C-H stoichiometries, the intermediate has a C(2)H(3) stoichiometry for oxygen pressures up to 0.01 Torr, indicating oxydehydrogenation occurs before skeletal oxidation. FYNES spectra at normal and glancing incidences were performed to characterize the structure and bonding of this intermediate. Using FYNES spectra of ethylene, ethylidyne, and acetylene as reference standard, this procedure indicates the oxidation intermediate is tri-sigma vinyl. Thus, oxidation of ethylene proceeds through a vinyl intermediate, with oxydehydrogenation preceding skeletal oxidation.