In situ sum frequency generation vibrational spectroscopy observation of a reactive surface intermediate during high-pressure benzene hydrogenation

Sum frequency generation surface vibrational spectroscopy and kinetic measurements using gas chromatography have been used to identify a reactive surface intermediate in situ during hydrogenation of benzene on a Pt(111) single crystal surface at Torr pressures. Upon adsorption at 310 K, both chemisorbed and physisorbed benzene coexist on the surface, a result which has not previously been observed. Kinetic measurements show a linear compensation effect for the production of both cyclohexane and cyclohexene. From these data the isokinetic temperature was identified and correlated to the chemisorbed benzene species, which were probed by means of vibrational spectroscopy. Additionally, chemisorbed benzene was determined to be a reactive intermediate, which is critical for hydrogenation.     

Characteristic Numbers of Multiple-Point Manifolds

We give a general method for expressing various characteristicnumbers of the multiple-point manifolds of an immersion. 2000Mathematics Subject Classification 57R42 (primary), 57R20, 57R75(secondary).     

Mechanistic study of electrocatalytic oxidation of formic acid at platinum in acidic solution by time-resolved surface-enhanced infrared absorption spectroscopy

Surface-enhanced infrared absorption spectroscopy (SEIRAS) combined with cyclic voltammetry or chronoamperometry has been utilized to examine kinetic and mechanistic aspects of the electrocatalytic oxidation of formic acid on a polycrystalline Pt surface at the molecular scale. Formate is adsorbed on the electrode in a bridge configuration in parallel to the adsorption of linear and bridge CO produced by dehydration of formic acid. A solution-exchange experiment using isotope-labeled formic acids (H(12)COOH and H(13)COOH) reveals that formic acid is oxidized to CO(2) via adsorbed formate and the decomposition (oxidation) of formate to CO(2) is the rate-determining step of the reaction. The adsorption/oxidation of CO and the oxidation/reduction of the electrode surface strongly affect the formic acid oxidation by blocking active sites for formate adsorption and also by retarding the decomposition of adsorbed formate. The interplay of the involved processes also affects the kinetics and complicates the cyclic voltammograms of formic acid oxidation. The complex voltammetric behavior is comprehensively explained at the molecular scale by taking all these effects into account.     

Guest Editors' Foreword

This issue of Discrete & Computational Geometry contains the detailed proof by T. Hales and S.P. Ferguson of the Kepler conjecture that the densest packing of three-dimensional Euclidean space by equal spheres is attained by "cannonball" packing. This is a landmark result. This conjecture, formulated by Kepler in 1611, was stated in Hilbert's formulation of his 18th problem [8]. The proof consists of mathematical arguments and a massive computer verification of many inequalities.     

Synthesis and application of a fluorous Lawesson's reagent: convenient chromatography-free product purification

[reaction: see text] A fluorous analogue of Lawesson's reagent for thionation of carbonyl compounds has been developed and its use demonstrated on a series of amides, esters, and ketones. The separation of the Lawesson's reagent-derived byproducts can be achieved by a simple fluorous solid-phase extraction.     

How ∗ not ∗ to solve a Sudoku

We prove the NP-hardness of a consistency checking problem that arises in certain elimination strategies for solving Sudoku-type problems.     

A global bifurcation index for set-valued perturbations of Fredholm operators

In the paper the homotopy invariant detecting global bifurcations of solutions to multi-parameter equations involving general set-valued perturbations of linear Fredholm operators of non-negative index is introduced. Some applications to the existence problems for differential inclusions are provided.     

Endor for characterizing transition metal centres in LiNbO3

Abstract

ENDOR investigations show that in nearly stoichiometric LiNbO₃ Mn²⁺ substitutes for Li with a Li vacancy in the first Li shell for most centres. In heavily Mg codoped LiNbO₃ the Cr³⁺ dopant, characterized by a new isotropic ESR signal, is shown to substitute for Nb. Information on local disorder is obtained.     

Development of a validated liquid chromatography/tandem mass spectrometry method for the distinction of thyronine and thyronamine constitutional isomers and for the identification of new deiodinase substrates

Thyronines (THs) and thyronamines (TAMs) are two groups of endogenous iodine-containing signaling molecules whose representatives differ from each other only regarding the number and/or the position of the iodine atoms. Both groups of compounds are substrates of three deiodinase isozymes, which catalyze the sequential reductive removal of iodine from the respective precursor molecule. In this study, a novel analytical method applying liquid chromatography/tandem mass spectrometry (LC-MS/MS) was developed. This method permitted the unequivocal, simultaneous identification and quantification of all THs and TAMs in the same biological sample. Furthermore, a liquid-liquid extraction procedure permitting the concurrent isolation of all THs and TAMs from biological matrices, namely deiodinase (Dio) reaction mixtures, was established. Method validation experiments with extracted TH and TAM analytes demonstrated that the method was selective, devoid of matrix effects, sensitive, linear over a wide range of analyte concentrations and robust in terms of reproducible recoveries, process efficiencies as well as intra-assay and inter-assay stability parameters. The method was applied to study the deiodination reactions of iodinated THs catalyzed by the three deiodinase isozymes. With the HPLC protocol developed herein, sufficient chromatographic separation of all constitutional TH and TAM isomers was achieved. Accordingly, the position of each iodine atom removed from a TH substrate in a Dio-catalyzed reaction was backtracked unequivocally. While several established deiodination reactions were verified, two as yet unknown reactions, namely the phenolic ring deiodination of 3',5'-diiodothyronine (3',5'-T2) by Dio2 and the tyrosyl ring deiodination of 3-monoiodothyronine (3-T1) by Dio3, were newly identified.     

Highly selective synthesis of catalytically active monodisperse rhodium nanocubes

Monodisperse sub-10 nm Rh nanocubes were synthesized with high selectivity (>85%) by a seedless polyol method. The {100} faces of the Rh NCs were effectively stabilized by chemically adsorbed Br- ions from trimethyl(tetradecyl)ammonium bromide (TTAB). This simple one-step polyol route can be readily applied to the preparation of Pt and Pd nanocubes. Moreover, the organic molecules of PVP and TTAB that encapsulated the Rh nanocubes did not prevent catalytic activity for pyrrole hydrogenation and CO oxidation.