Untersuchung der Thermolysereaktion von Malonsäuremonoarylestern

Thermolysis of malonic acid monoaryl esters gives rise to the formation of a steady-state equilibrium with diaryl malonates and free malonic acid, the latter being decarboxylated to form acetic acid. The yields of diaryl malonates are lowered by a concurrent reaction, namely the decarboxylation of the monoesters, giving aryl acetates and CO₂.

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H. Junek, E. Ziegler, U. Herzog undH. Kroboth, Synthesis1976, 332.     

Discriminating bacteria from other atmospheric particles using femtosecond molecular dynamics

We investigated femtosecond pump–repump depletion schemes in biological fluorophors (tryptophan and riboflavin) in order to discriminate bioaerosols from organic interferents emitted by combustion (traffic related urban aerosols). Although fluorescence depletion is significative for riboflavin (Rbf, Vitamin B2), the most striking results have been obtained for the amino acid tryptophan (Trp). By using a 270 nm-pump 810 nm-repump femtosecond excitation, we showed that Trp exhibits fluorescence depletion up to 50%, contrary to naphthalene (<2%), despite almost identical absorption/emission spectra. We demonstrate that this process in Trp is so robust that it still occurs in living bacteria (Bacillus subtilis, Escherechia coli and Enterococcus fæcalis) but is absent for pure diesel fuel. This remarkable difference between biological and organic aerosols can be exploited to discriminate among them.     

Structure of solvated metal ions in nitrate and chloride solutions of erbium(III) and yttrium(III) in dimethyl sulfoxide

X-ray diffraction measurements on 1M yttrium(III) and erbium(III) nitrate and chloride solutions in dimethyl sulfoxide (DMSO) have shown that Er(III) and Y(III) solutions of equal compositions are isostructural. The intensity difference functions can then be used to derive the detailed structure of the coordination sphere around the metal ions. The DMSO molecules are coordinated over oxygen with average M-O-S bond angles of about 130°. Two different conformations, corresponding to different relative orientations of the M-O and O-S bonds, seem to be prevalent. In the nitrate solutions an average of about 1.5 nitrate ions are coordinated as bidentate ligands to each metal ion. In the chloride solutions about 1.3 chloride ions belong to the inner-coordination sphere.On leave from the Department of Inorganic Chemistry, Royal Institute of Technology, S-10044 Stockholm, Sweden     

On the relation between a common gauge origin formulation and the GIAO formulation of the NMR shielding tensor

In this article the formal equivalence between the “gauge including atomic orbitals” (GIAO) and the simpler common gauge (cg) formulations of the nuclear magnetic resonance (NMR) shielding tensor is proven. To be able to give this proof, one has to assume exactly solved zeroth order quantum mechanical equations and complete basis sets. The proof of the equivalence has been known in the literature for some time (e.g. S.T. Epstein `The variation method in quantum chemistry', Academic Press, 1974). However, our approach has the advantage that, by comparing the GIAO and cg methods, it provides insight into the para- and diamagnetic contributions of the shielding. Thus, it yields justification for qualitative analysis schemes that have been used to explain trends in chemical shifts. Our formal proof is based on density functional theory. However, it is argued that very similar arguments should apply to other levels of theory as well. Received: 25 June 2002 / Accepted: 18 July 2002 / Published online: 15 October 2002 Acknowledgments. The author would like to acknowledge Tom Ziegler, Calgary, for numerous discussions on the subject. The reviewer is acknowledged for the insightful and detailed comments. Correspondence to: G. Schreckenbach e-mail: schrecke@alcor.concordia.ca     

Stereo- and regioselectivity in dynamic gas-phase thermoisomerization (DGPTI): novel route to alpha-campholanic acid and derivatives

[reaction: see text] Dynamic gas-phase thermoisomerization (DGPTI) of (-)-2-phenylisoborneols effects stereo- and regioselective ring opening under formation of (+)-trans-alpha-campholanic acid derivatives. Similarly, (-)-alpha-2-phenylfenchol underwent under DGPTI conditions ring opening to (-)-fencholic acid derivatives. In both cases, DGPTI led to cleavage of the weakest bond in the isomeric bicyclic structures. A reaction mechanism involving a diradical intermediate is supported by a deuterium labeling study.     

Determination of Rare Earth Elements in Acid Mine Drainage by Inductively Coupled Plasma Mass Spectrometry

A method was developed for the direct determination of 14 naturally occuring rare earth elements (REE, La–Lu) by means of inductively coupled plasma mass spectrometry in seepage waters caused by acid mine drainage. The analytical procedure includes the additional determination of oxide and hydroxide formation by Ba and REE. The determined polyatomic ion formation can be used to correct spectral interferences of oxides and hydroxides of Ba and the light REE on middle and heavy REE. The total REE content in the acidic seepage waters investigated is up to 220µgL^–1. The concentration of the REE normalized to Post Archean Australian Shale (PAAS) is shown, and fractionation of the REE is discussed. The PAAS-normalized REE-patterns feature an enrichment of middle and heavy REE as opposed to the light ones. For one seepage water sampled over a period of about half a year, the REE patterns show very high reproducibility. The accuracy of the method was demonstrated by the analysis of two water samples containing certified concentrations of REE.     

Electron delocalization in the radical cation of 1,3,6,8-tetraazatricyclo[4.4.1.1(3,8)]dodecane, a 4-nitrogen-7-electron system.

The radical cation of 1,3,6,8-tetraazatricyclo [4.4.1.1(3,8)]dodecane (TTD) has been studied using magnetic resonance and optical spectroscopic methods and computational techniques. With the help of deuterated isotopomers, assignments of EPR and resonance Raman spectra could be unequivocally established. The results demonstrate that the radical cation has D(2d) symmetry, and instantaneous electron delocalization over the four equivalent nitrogen atoms occurs. This extensive delocalization in a completely saturated system is a unique feature of the TTD radical cation. The spectroscopy of TTD, in contrast to that of simpler diamines such as 1,4-diaza[2.2.2]bicyclooctane, simultaneously reveals the consequences of orbital interactions through space and through bonds. The relationship between nitrogen pyramidalization and hyperfine coupling constants in nitrogen-centered radical cations with a number of different bonding arrangements is reviewed.