Spin Density Distribution in 2,4,6-Triphenylphenoxyl

The distribution of the unpaired electron over the oxygen and the 24 carbon atoms in the free 2,4,6-triphenylphenoxy radical was determined by electron spin resonance spectroscopy and quantum-mechanical approximation methods. The hyperfine splitting was evaluated with the aid of the spectra of triphenylphenoxyls deuterated in some or all of the substituent phenyl groups. The results of the quantum-mechanical approximations were checked by recording the ESR spectra of triphenylphenoxyls labeled with ¹³C in positions 1,2,3, or 4 of the central ring. The spin density distribution permits a first discussion of the ¹⁷O-coupling constants of correspondingly labeled triphenylphenoxyl and other organic free radicals.     

Automated carbohydrate synthesis to drive chemical glycomics

This feature article describes the development of the first automated solid-phase oligosaccharide synthesizer. A series of chemical challenges had to be addressed to accomplish this breakthrough and provide rapid access to oligosaccharides of biological significance. Accelerated synthesis of glycoconjugates promises to greatly impact the emerging field of glycobiology. Chemical glycomics uses synthetic carbohydrates and analogs to study their role in recognition, signal transduction pathways and other events of fundamental biomedical significance and shapes up to become the next major wave in biomedical research. The automated synthesis of a novel malaria vaccine candidate is discussed to illustrate the medical potential of chemical glycomics.     

Dimethylzinc-mediated addition of alkenylzirconocenes to alpha-keto and alpha-imino esters

[reaction: see text] Hydrozirconation of alkynes followed by in situ transmetalation to dimethylzinc and 1,2-addition to activated ketones and N-diphenylphosphinoylimines leads to tertiary allylic alcohols and amines in high overall yield. With 8-phenylmenthol as the chiral auxiliary, si-face attack proceeds in good to excellent diastereoselectivities.     

Field ionization mass spectrometry—IV. Isotopically labeled heptanals

The field ionization (FI) mass spectra of n-heptanal and a series of deuterium labeled analogs have been studied, with the objectives of initiating systematic investigations of reaction mechanisms of FI produced ions and to permit comprison with those found for other ionization processes. It is now recognized that FI ions have: (a) lower average internal energies and (b) shorter residence times than similar ions generated by electron-impact (EI), and the possibility exists of H/D-randomization occuring in ions formed by desorption from the emitter, by unimolecular decomposition close to the emitter and by either ‘fast’ or ‘slow’ metastable decompositions. In this study only the peak shifts of normal ions could be utilized; accurate mass measurements of all major ions revealed elemental compositions similar to EI. A site-specific McLafferty rearrangement gave the base peak at m/e 44 ([C₂H₄O]^+.), although the apparently complementary ion at m/e 70 ([C₅H₁₀]^+.) arose in a less specific process. Ions at m/e 43 ([C₃H₇]⁺) and 71 ([C₅H₁₁]⁺ 80%; [C₄H₇O]⁺ 20%) were apparantly generated without significant H/D-scrambling. Of special interest was the observation of the rearrangement ion at m/e 86 ([C₅H₁₀O]^+.) caused by loss of C-2 and C-3 as C₂H₄, as found for EI. It is concluded that at least in this system, decomposing molecular ions formed: (a) in the gas phase extremely close to the emitter and/or (b) on the emitter surface, have lifetimes sufficiently short to preclude complete H/D randomization. The results also provide evidence for common fragmentation mechanisms for heptanal molecular ions at both the low end and the high end of the energy distribution.     

Equilibria in complexes of N-heterocyclic molecules,part XII. The reaction of hydroxide and alkoxide Ions with platinum(II) complexes of 5-nitro-1,10-phenanthroline and 2,2′-bipyridyl

Summary Bis-(5-nitro-1,10-phenanthroline)platinum(II) dichloride and diperchlorate have been prepared. The reaction between the parent cation and hydroxide ion has been studied using 1 FI n.m.r. spectroscopy and found to involve attack at the ligand. The bis-(2,2-bipyridyl)platinum(II) ion has been shown to be highly reactive towards methoxide ion. The dissociation of a 2,2-bipyridyl ligand is preceded by attack at the ligand.Part XI: R. D. Gillard, t.. A. P. Kane-Maguire and P. A. Williams,Transition Met. Chem., 2, 47 (1977).On leave from the University of Baghdad, Iraq.     

Kinetic studies in mass spectrometry—IV. The [M — Cl] reaction in substituted chlorobenzenes and the question of molecular ion isomerization.

The [M]⁺˙ → [M ? Cl]⁺ reaction in a series of m- and p-X substituted chlorobenzenes has been studied, utilizing a simple kinetic approach, comparison of metastable ion relative abundances, and by measurement of ionization and appearance potentials. All evidence obtained is consistent with rearrangement prior to cleavage in the molecular ions, in which substituent position becomes effectively randomized. These findings are related to known hydrogen randomization reactions occurring in either the molecular ion or [M ? Cl] ion of chlorobenzenes. Mechanisms involving carbon scrambling via such species as ionized benzvalenes or prismanes, or ring-opening to isomeric acyclic molecular ions in which hydrogen randomization might occur can be entertained, but mechanisms involving simple hydrogen shifts in the intact benzene ring appear less likely.     

Beitrag zur Kenntnis des Systems Vanadin—Chrom—Stickstoff

The System V–Cr–N has been investigated at 1100 and 1400 °C and at nitrogen pressures between 1 and 1000 atmospheres by X-ray techniques. VN and CrN on the one hand and V₂N and Cr₂N on the other are forming complete series of solid solutions. The phase field of the mononitride solid solution is dependent on nitrogen pressure and temperature conditions.

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Auszug aus der Diplomarbeit des Herrn Dipl.-Ing. Dr.W. Schebesta.     

On-line preconcentration of arsenic from water samples with an anion-exchanger column coupled to a hydride generation ICP system

Summary An on-line anion-exchange preconcentration hydride generation ICP system for the determination of total inorganic arsenic in water is described. The column was packed with strongly basic anion-exchange resin (AG 1-X8). Experimental conditions including pH of the sample solution, eluent, flow rate of eluent, oxidation states of arsenic and competing anion ions were studied. Compared with the conventional continuous hydride generation ICP, a 9.2-fold improvement in sensitivity was obtained with RSD 1–2% at 100 ng/ml. The detection limit (3) was 0.08 ng/ml. The recoveries in water samples were satisfactory. The system provides complete automation of sample loading, eluting and regenerating of the resin.On leave from Shanghai Institute of Metallurgy, Chinese Academy of Sciences, Shanghai, China 200050     

The electronic structure of benzvalene : Photoelectronspectroscopic studies

The photoelectron(PE)spectrum of tricyclo[3.1.0.0^2,6] hex-2-ene(benzvalene 1) has been recorded. The first four bands in the PE spectrum of 1 can be assigned to transitions to ²B₂, ²A₁, ²A₂ and ²B₁ states of 1¹. This assignment is discussed in terms of the results of semiempirical and ab initio calculations on 1. Furthermore the highest occupied MO's of 1 are derived qualitatively from an interaction diagram between a distorted bicyclobutane and an ethylene moiety.     

Syntheses of 3,7-dimethylpentadecan-2-ols from intermediates formed from co-oligomerisation reactions of butadiene with diethyl malonate and acetaldehyde

The products of the reaction of butadiene with diethyl malonate catalysed by a palladium complex and of butadiene with acetaldehyde catalysed by a nickel(0) complex have been used in two syntheses of 3,7-dimethylpentadecan-2-ol.