Depth of almost strictly sign regular matrices

The concept of depth of an almost strictly sign regular matrix is introduced and used to simplify some algorithmic characterizations of these matrices.     

基于凝固点下降的高纯有机物纯度定值技术研究进展

介绍凝固点测量方法的研究进展。阐述了凝固点下降技术用于高纯有机物纯度定值的基本原理和适用条件,详细介绍了静力学方法和动力学方法这两种凝固点的测量方法,并介绍了两种方法在纯度定值方面的应用情况,且对两种方法的特点进行了对比,展望了凝固点下降纯度定值技术的发展趋势。     

Chemical Remodeling of Cell‐Surface Sialic Acids through a Palladium‐Triggered Bioorthogonal Elimination Reaction

We herein report a chemical decaging strategy for the in situ generation of neuramic acid (Neu), a unique type of sialic acid, on live cells by the use of a palladium‐mediated bioorthogonal elimination reaction. Palladium nanoparticles (Pd NPs) were found to be a highly efficient and biocompatible depropargylation catalyst for the direct conversion of metabolically incorporated N‐(propargyloxycarbonyl)neuramic acid (Neu5Proc) into Neu on cell‐surface glycans. This conversion chemically mimics the enzymatic de‐N‐acetylation of N‐acetylneuramic acid (Neu5Ac), a proposed mechanism for the natural occurrence of Neu on cell‐surface glycans. The bioorthogonal elimination was also exploited for the manipulation of cell‐surface charge by unmasking the free amine at C5 to neutralize the negatively charged carboxyl group at C1 of sialic acids.     

Oxidative Addition of Carbon–Carbon Bonds to Gold

The oxidative addition of strained C? C bonds (biphenylene, benzocyclobutenone) to DPCb (diphosphino‐carborane) gold(I) complexes is reported. The resulting cationic organogold(III) complexes have been isolated and fully characterized. Experimental conditions can be adjusted to obtain selectively acyl gold(III) complexes resulting from oxidative addition of either the C(aryl)? C(O) or C(alkyl)? C(O) bond of benzocyclobutenone. DFT calculations provide mechanistic insight into this unprecedented transformation.     

Kinetic and mechanism of the homogeneous,unimolecular elimination of α,β‐unsaturated aldehydes in the gas phase

The gas phase thermal decarbonylation of α,β‐unsaturated aldehydes E‐2‐butenal and E‐3‐phenyl‐2‐methylpropenal was studied in a static system over the temperature range 380.5–490.0 °C and pressure range 55.5–150 Torr. The reactions are homogeneous and unimolecular and obey a first‐order rate law. The rate coefficient is represented by the following Arrhenius equations: The elimination products of 2‐butenal are propene and CO gas, while 3‐phenyl‐2‐methylpropenal produces α‐methylstyrene, cis‐trans‐β‐methylstyrene, indan, and CO gas. Kinetic and thermodynamic parameters suggest these elimination reactions to proceed through a three‐membered cyclic transition state type of mechanisms. However, a two steps mechanisms for the formation of a carbene type of intermediate through a four‐membered cyclic transition structure can not be overlooked. Copyright © 2007 John Wiley & Sons, Ltd.     

Determination of trace amounts of nitrogen in uranium based samples by ion chromatography (IC) without Kjeldahl distillation

A simple, sensitive and fast ion chromatographic (IC) method with suppressed conductivity detection is described for the determination of traces of nitrogen in uranium based fuel materials. Initially a method was developed to determine nitrogen as NH₄⁺ using cation exchange column after matrix separation by Kjeldahl distillation. The method was then improved by eliminating this distillation. Matrix separation after sample dissolution was done by hydrolyzing and filtering off the polyvalent cations. This had helped in reducing both the sample size and analysis time. Optimization of dissolution conditions for various kinds of uranium based samples was done to keep acid content minimum; a prerequisite chromatographic condition. The calibration plot for nitrogen was linear in the concentration range of 0.02-1 mg L⁻¹ with regression coefficient of 0.9999. The relative standard deviation (R.S.D.) obtained in this method (100 μL injected) was 3% and 2% in 9 replicates at nitrogen level of 28 and 55 ng g⁻¹, respectively. Detection limit based on S/N = 3 (100 μL injected) as well as three times of variation in blank value was 4 ng g⁻¹. The developed method was authenticated by comparison with certified uranium-alloy standard as well as with independent indophenol photometry method. The developed method was applied to uranium-alloy, uranium-metal, sintered UO₂ pellets and sintered UO₂ microspheres samples.     

1,x-elimination reactions: extending the limits of a classical organic reaction

Alpha,omega-dibromo derivatives in which the two terminal carbon atom are separated by an unsaturated spacer unit ("pi spacer") undergo 1,x-elimination reactions (with x=6, 8, 10, and 14), using Mori's reagent (nBu3SnSiMe3/CsF). The resulting cumulenic intermediates cyclodimerize in a subsequent step yielding novel macrocyclic acetylenic and bridged aromatic compounds (cyclophanes). Thus 1,6-eliminations were carried out with dibromide 17 to yield 1,3,7,9-cyclododecatetrayne (20) and with benzylbromide 24 to provide cyclophanes 26 and 27. By 1,8-eliminations the 16-membered macrocycle 33 could be prepared from enediyne 31, the benzannelated 1,5-cyclooctadiyne 41 from dibromide 38, and a mixture of cyclophanes 45 and 46 from the precursor 43. 1,10-Eliminations were carried out successfully with dibromides 47, 50, and 53 yielding the corresponding unsaturated cyclophanes ("cyclophynes") 49, 52, and 55. The influence of the solvent on the cyclodimerization 47-->49 was investigated, with acetonitrile providing the highest yields. The heterophanes 59 a and b were obtained by 1,10-elimination of the precursor dibromides 57 a and b, and in an elimination experiment involving a 1:1 mixture of the dibromides 50 and 57 b the "mixed dimer" 60 was isolated, besides the homodimers 52 and 59 b. The method reached its limits with the 1,14-elimination of 68, 70, and 74 providing the cyclophanes 69, 71, and 75 in varying amounts. Two final debrominations with 76 and 77, which in principle could undergo 1,16- and 1,20-eliminations reactions, respectively, failed. The structures of the new cyclophanes 49, 50, 59 a, and 59 b were established by X-ray structural analysis; all other structure assignments rest on the usual spectroscopic and analytical data.