Lichtinitiierte Polymer- und Polymerisationsreaktionen. 14. Radikalische Photopolymerisation mit Kaliumtrisoxalatocobaltat-Aryloniumsalz-Systemen

Light Induced Polymer and Polymerization Reactions. 14. Radical Photopolymerization by Potassium Trisoxalato Cobaltate Arylonium Salts Systems. The photopolymerization of acrylamide (AA) by trisoxalato cobaltate ([Co(ox)₃]³⊖) is inefficient because the produced nucleophilic ·CO₂⊖-radicals are oxidized or they terminate with polymer radicals. If onium compounds are added to this system the initiation efficiency is increased as follows : p-CH₃OC₆H₄N₂⊕>Ph₂I⊕>PhCOCH₂S(CH₃)₂⊕>Ph₃S⊕. By means of quantum yield measurements a mechanism was proposed including the electron transfer from ·CO₂⊖ to onium compounds. The produced radicals also initiate the polymerization of AA. The exponent m of the polymerization rate (v_p ≈︁ [AA]^m) is decreased from 1.4 to 1.1 in the presence of Ph₂I⊕, deciding the decrease of termination reaction between ·CO₂⊖ and polymer radicals. In comparison with K₃[Fx(ox)₃] the corresponding cobalt complex possesses higher polymerization quantum yields for AA.     

Acylierung von Heterocyclen mit Kohlensäurederivaten. I. Kinetik und Mechanismus der Reaktion von 2-Aminobenzimidazolen mit Arylcyanaten

Acylation of Heterocycles with Carbonic Acid Derivatives. I. Kinetics and Mechanism of the Reaction of 2-Aminobenzimidazoles with Aryl Cyanates The second order rate constants for the reaction of 2-amino-benzimidazoles (2-ABI) with aryl cyanates forming 2-amino-benzimidazole aryl ester imide 3 have been determined in dependence on substituent effects by u. v. measurements. The results are interpreted by a six-membered cyclic transition state in which the electrophilic attack of the cyanate on the endocyclic N atom is catalyzed by an H bridge interaction of the exocyclic amino group of 2-ABI with the OCN group.     

Fast multilevel implementation of recursive spectral bisection for partitioning unstructured problems

If problems involving unstructured meshes are to be solved efficiently on distributed-memory parallel computers, the meshes must be partitioned and distributed across processors in a way that balances the computational load and minimizes communication. The recursive spectral bisection method (RSB) has been shown to be very effective for such partitioning problems compared to alternative methods, but RSB in its simplest form is expensive. Here a multilevel version of RSB is introduced that attains about an order-of-magnitude improvement in run time on typical examples.     

Photokatalytische Bildung von Wasserstoff aus Thiolen in Gegenwart von Vitamin B12-Modellverbindungen mit Azid als photochemischem Opferliganden

Photocatalytic Hydrogen Formation from Thioles in the Presence of Vitamin-B₁₂ Model Complexes with Azide as Photochemical Sacrificial Ligand The photolysis of [N₃Co(chelat)B] complexes ( 1–3 ) (chelat = dimethylglyoxime, dmg; N,N′-o-phynylenebis(salicylidenimine), salphen; N,N′-ethylene-bis(salicylidenimine), salen; B = pyridine) leads by homolytic cleavage of the Co–N₃ bond to both coordinatively unsaturated cobalt(II) chelates [Co(chelat)B] and N₃ ligand radicals that undergo fast decay to dinitrogen. The photolysis of the cobalt (III) complexes 1–3 in the presence of thiophenole and other thioles proceeds catalytically and yields the corresponding disulphides and dihydrogen. The mechanism of this photocatalytic generation of dihydrogen is due to the catalytic activity of the coordinatively unsaturated cobalt(II) species formed photochemically. A photocatalytic cycle is proposed describing the generation of hydrogen. Possible photochemical and thermal steps of that cycle are discussed.     

在铅焊料的标准流程中用无铅元件

随着欧洲的RoHS和WEEE指令已于2006年7月1日开始执行,许多元件供应商面临着使其产品符合无铅环境的挑战。两个指令都要解决将铅从元件中去除的问题。半导体公司必须选择高性价比、与含铅焊料逆向兼容,并与无铅焊料前向兼容的无铅镀层策略。逆向兼容表示无铅元件能够用锡铅（SnPb）焊料回流工艺安放在PCB上。前向兼容表示目前的锡铅工艺能够在面板组装中,与无铅焊料一起使用。     

Photochemische Untersuchungen von Vitamin B12-Modellverbindungen mit Azid bzw. Thiolat als Axialliganden – Precursorverbindungen für koordinativ ungesättigte Cobalt(II)-Komplexe

Photochemistry of Azido and Thiolato Vitamin-B₁₂ Model Complexes as Precursor Compounds for Coordinatively Unsaturated Cobalt(II) Complexes The photolysis of [LCo(chelat)B] complexes ( 1–3 ) (L = azide, N; thiolate, RS⁻; chelat = dimethylglyoxime, dmg; N,N′-o-phenylene-bis(salicylidenimine), salphen; N,N′-ethylene-bis(salicylidenimine), salen; B = pyridine, imidazole, triphenylphosphine) leads upon the homolytic cleavage of the Co L bond to both coordinatively unsaturated reactive cobalt(II) chelates [Co(chelat)B] and ligand radicals L^•. The efficiency of these photochemical redox reactions is described in relation to the structure of the cobalt(III) chelates, the wavelength of irradiation, the light-intensity as well as the solvents and substrates used during the photochemical experiments. Further, sensitization experiments using [Ru(bipy)₃]Cl₂ as sensitizer are described and the redox potentials of the investigated complexes are discussed.