Birefringence induite dans les directions remarquables d'une suspension de particules rigides en ecoulement

A theoretical formula is derived for the flow birefringence exhibited along the streamlines and in the direction of the shear rate (G) by dilute suspensions of rigid spheroid-like macromolecules; numerical values are computed up to σ = G/D = 900 (D = rotary diffusion coefficient). These results provide two tests for identifying prolate and oblate particles; when combined with the data for observation perpendicular to the plane of flow, they confirm the connection between non-Newtonian viscosity and the Maxwell effect for monodisperse samples; for vanishing rate of shear, they allow a mixture of particles to be characterized by means of a mean rotary diffusion constant ($\text{D}$) and a mean shape coefficient ($\text{R}$).     

Time-dependent pump-probe spectra of NeBr2

Time- and frequency-resolved pump-probe measurements on NeBr2 have been performed to better characterize its fragmentation dynamics on the B electronic state for vibrational levels in the energy region of the transition from direct vibrational predissociation to intramolecular vibrational relaxation dynamics. Above nu'=20 of the Br2 stretching mode, it was observed that the dependence of lifetime on the vibrational quantum number deviates from the energy-gap law by leveling off in the range of 10 psE transitions of the complex. These transitions are shifted 20 cm(-1) to lower energy from the free Br2 resonances, indicating an E state Ne-Br2 bond energy of 82 cm(-1). Measurements of NeBr2 vibrational predissociation via the delta nu=-2 channel were also performed for nu'=27, 28, and 29. The closing of the delta nu=-1 channel leads to an increase in the lifetimes of these vibrational levels. A new Nd:yttrium aluminum garnet pumped dual optical parametric oscillator/optical parametric amplifier system is described that allows us to conveniently record time-delayed pump-probe spectra with 2-cm(-1) spectral resolution and 15-ps time resolution.     

Association of Diphenylguanidine and Quantum-Chemical Calculations of the Structure of Its Cyclic Dimers

The frequencies and intensities of the two tautomeric structures of the N, -diphenylguanidine monomer and its hydrogen-bonded cyclic dimers were calculated for structure elucidation of the monomer and dimer forms of this compound and identification of the stretching vibration band NH of this molecule in solution. Ab initio HF/3-21G and B3LYP/6-31G(d,p) calculations of DPG monomers and cyclic associates suggest that the asymmetric tautomer is dominant, proving that the dimer structures with two C=(Ph)N...H–NPh hydrogen bonds prevail in solution. An assignment of IR absorption frequencies of DPG in solution is suggested.     

Isolation of pure (2S,1'S,2'S)-2-(2'-carboxycyclopropyl)glycine from Blighia sapida (Akee)

The isolation of (2S,1'S,2'S)-2-(2'-carboxycyclopropyl)glycine (CCG I, 2) from Blighia sapida (Akee) was achieved through column chromatography on deactivated silica gel followed by ion-exchange chromatography. A HPLC method has also been devised in order to assess the purity of the isolated product.     

Zur Ermittlung und rechnerischen Überprüfung thermodynamischer Daten aus experimentell gefundenen Werten, 10. Mitt.: Zur Frage der Parameterzahl und Fehlerverteilung bei einigen Rechenansätzen für binäre thermodynamische Systeme

Zusammenfassung Es wird eine vergleichende Untersuchung über die Anwendungsmöglichkeit zweier dreiparametriger und zweier zweiparametriger Ansätze angestellt. Die Bestimmung der Konstanten erfolgt nach der Methode der kleinsten Quadrate, die entstehenden Fehlerquadratsummen werden als Funktion der Iterationszahl dargestellt und verglichen. Ebenso wird mit den Verbesserungen _i und den mittleren Fehlern der einzelnen Parameter verfahren. Es ergibt sich, daß der Ansatz vonWilson für stark assoziierende Systeme mit nahezu horizontaler Grenztangente die kleinste Fehlerverteilung aufweist. Die beiden dreiparametrigen Ansätze vonRedlich-Kister und vonMusil-Breitenhuber zeigen sich für das Systemn-Hexan-Methylamin besser geeignet als für die Systeme Benzol-Methanol und CCl₄-Methanol. Die Problematik der Parameterzahl wird an Hand der Fehlerverteilung bei den Ansätzen vonMB undRK aufgezeigt und diskutiert.Das Programm wurde in der Programmiersprache FORTRAN erstellt und mit der UNIVAC-490-Computer-Anlage des Grazer Rechenzentrums gerechnet.

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Four thermodynamical statements are tested with regard to their applicability to binary systems, two statements having three and two only two parameters. The least square method ofGauss shows that theWilson statement has the smallest dispersion of errors for systems with nearby horizontal boundary tangent. [vv], _i and are shown as functions of the number of iterations. The problem of the parameters' number is shown and discussed by means of the dispersion of errors for theMusil-Breitenhuber (MB) andRedlich-Kister (RK)-statements.The programme is written in FORTRAN and has been used the UNIVAC-490-Computer of the Graz Computer Centre.

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Mit 5 Abbildungen

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Herrn Professor Dr.Robert Fischer zum 65. Geburtstag in Dankbarkeit und Verehrung gewidmet.

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bis 6. Mitt.:F. Gölles, Mh. Chem.92, 981 (1961);93, 191, 201 (1962);94, 1108 (1963);95, 1656 (1964);96, 1366 (1965);7. bis 9. Mitt.:F. Gölles undA. Höpfner, Mh. Chem.97, 368, 911 (1966);99, 230 (1968).     

Verwendung schneller Reaktorneutronen zur raschen und zerst?rungsfreien Spurenanalyse, insbesondere von Sauerstoff

Zusammenfassung Der intensive Fluß schneller Neutronen, wie er von Kernreaktoren geliefert wird, kann für eine rasche und zerstörungsfreie, aktivierungs-analytische Bestimmung einer Vielzahl von Elementen verwendet werden. Bei manchen Elementen läßt sich sogar bei der Aktivierung mit schnellen Reaktorneutronen eine im Vergleich zur Aktivierungsanalyse mit thermischen Neutronen tiefere Erfassungsgrenze erreichen. Werden die Proben in einem schnellen, halbautomatischen Rohrpostsystem zum Reaktorkern befördert, können auch sehr kurzlebige Radionuklide gemessen werden. Besonders Sauerstoff läßt sich über die Kernreaktion ¹⁶O(n, p)¹⁶N sehr empfindlich bestimmen, wobei die hochenergetischen -Quanten des 7,2 sec Stickstoff-16 gemessen werden. Es können so bis zu 10 ppm Sauerstoff in vielen Arten von Analysenproben erfaßt werden. Das halbautomatische Analysensystem, das mit Hilfe einer schnellen Rohrpost die Durchführung rascher und zerstörungsfreier Aktivierungs-analysen im Forschungsreaktor München erlaubt, wird beschrieben. Die Verwendung des Systems zur Sauerstoffanalyse und die damit zusammenhängenden Probleme werden genauer besprochen.

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Summary The intense fast-neutron fluxes, which are available in nuclear reactors, can be used for rapid and non-destructive activation analysis of many elements. Some elements have even superior detection limits for activation with fast reactor neutrons compared to reactor thermal-neutron activation. Short-lived radionuclides can also be measured using a fast pneumatic tube system. Oxygen can be determined by the nuclear reaction ¹⁶O(n, p)¹⁶N initiated by fast reactor neutrons counting the high-energy gamma rays of the 7.4 sec nitrogen-16. The method is suitable for determining as little as 10 ppm of oxygen in many types of analytical samples. The fast transfer system, which is used in the Munich research reactor for rapid and non-destructive activation analysis is described. The use of the transfer-system for oxygen activation analysis and problems associated with oxygen analysis are discussed.

     

Product structure in the reaction of dimethyl acetylenedicarboxylate with 2-furyl-1,2,3,4-tetrahydroquinolines

Dimethyl acetylenedicarboxylate reacts with 4-substituted and 3,4-fused 2-furyl-1,2,3,4-tetrahydroquinoline derivatives at the furan fragment according to the [4 + 2]-cycloaddition pattern. The reaction is not stereoselective, and it yields two diastereoisomeric 7-oxabicyclo[2.2.1]hepta-2,5-dienes whose structure was determined by X-ray analysis and NMR spectroscopy.     

Jet cooled NO2 intra cavity laser absorption spectroscopy (ICLAS) between 11200 and 16150 cm

We have combined the high sensitivity of the ICLAS technique with the rotational cooling effect of a slit jet expansion in order to observe and to understand the visible and near infrared NO₂ spectrum. By this way, an equivalent absorption pathlength of several kilometers through rotationally cooled molecules has been achieved. Due to the vibronic interaction between the two lowest electronic states, ²A₁ and à ²B₂, this spectrum is vibronically dense and complex. Moreover, the dense room temperature rotational structure is perturbed by additional rovibronic interactions. In contrast, the rotational analysis of our jet cooled spectrum is straightforward. The NO₂ absorption spectrum is vanishing to the IR but, owing to the high sensitivity of the ICLAS technique, we have been able to record the NO₂ spectrum down to 11200 cm⁻¹ with a new Ti:sapphire ICLAS spectrometer. As a result 249 ²B₂ vibronic bands have been observed (175 cold bands and 74 hot bands) in the 11200–16150 cm⁻¹ energy range. Due to the cooling effect of the slit jet we have reduced the rotational temperature down to about 12 K and at this temperature the K = 0 subbands are dominant. Consequently, we have analysed only the K = 0 manifold for N 7 of each vibronic band. The dynamical range of the band intensities is about one thousand. Due to the strong vibronic interaction between the ²A₁ and à ²B₂ electronic states, we observed not only the a₁ vibrational levels of the à ²B₂ state but also the b₂ vibrational levels of the ²A₁ state interacting with the previous ones. By comparison with the calculated density of states, we conclude that we have observed about 65% of the total number of ²B₂ vibronic levels located in the studied range. However, there are more missing levels in the IR because of the weakness of the spectrum in this range. The correlation properties of this set of vibronic levels have been analysed calculating the power spectrum of the absorption stick spectrum which displays periodic motions: the dominant period, at 714 ± 20 cm⁻¹, corresponds to the bending motion of the à ²B₂ state. The other observed periods remain unassigned. In contrast the next neighbor spacing distribution (NNSD) shows a strong level repulsion, i.e. a manifestation of quantum chaos. These two observations, apparently contradictory, can be rationalized as follows: the short time dynamics, for t < 10⁻¹² s, is “regular” while for longer times the dynamics becomes “chaotic”. We suggest that this behavior may be observed directly with a pump and probe fs laser experiment.     

Palladium-induced cyclizations for the synthesis of cis-2,5-disubstituted-3-methylenetetrahydrofurans: studies of the C7-C22 core of amphidinolide K

[formula: see text] The diastereoselective synthesis of cis-2,5-disubstituted-3-methylenetetrahydrofurans via Pd(0)-catalyzed cyclization of 2-methylene-1,4-diols is described. Investigations into the scope of the reaction and its application toward the synthesis of amphidinolide K is reported.     

THE EFFECTS OF PORPHYRIN STRUCTURE AND AGGREGATION STATE ON PHOTOSENSITIZED PROCESSES IN AQUEOUS AND MICELLAR MEDIA

The efficiency of several porphyrins at 10 μM and 83 μM as sensitizers of the photooxidation of 0.1 mM tryptophan and histidine via a singlet oxygen-mechanism was studied in pH 7.4-buffered aqueous solutions and in aqueous dispersions of Triton X-100 micelles. The porphyrins were either solubilized in the bulk aqueous medium or associated with the micellar phase, whereas the amino acids were always located in the aqueous phase. With those porphyrins, such as uroporphyrin I, meso-tetra (4-sulfonatophenyl)porphine, meso-tetra(4-carboxyphenyl)porphine and meso-tetra)N,N,N-trimethylanilinium)porphine, which are > 98% monomeric in both media, the efficiency of histidine photooxidation was independent of the site of O₂(¹Δ_g) generation, as shown by the closely similar values for the photooxidation rate constant and oxygen-consumption quantum yield in the presence and absence of Triton micelles; the same indications were provided by photokinetic experiments with tryptophan. Actually, laser flash photolysis studies showed that the micelle-incorporation of the above mentioned porphyrins brought about only minor changes in their photophysical properties, including the relative yield of O₂(¹Δ_g) generation. On the other hand, hematoporphyrin IX, its Zn²⁺-complex, and coproporphyrin III are largely aggregated in homogeneous aqueous solution; their incorporation into Triton micelles caused an increase of the triplet quantum yield and an enhancement of the oxygen-consumption quantum yield and photooxidation rate constant for both histidine and tryptophan. The lower photosensitizing efficiency of aggregated porphyrin species in comparison with the corresponding monomeric porphyrin was confirmed by measuring the initial rate and quantum yield of oxygen consumption upon irradiation of 1 mM histidine and tryptophan in the presence of different hematoporphyrin concentrations within the 0.3-100μM range.