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  • General Chemistry  (9)
  • Inorganic Chemistry  (6)
  • Analytical Chemistry and Spectroscopy  (3)
  • 78.55.-m  (2)
  • 68.55.+b  (1)
  • 1980-1984  (21)
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Keywords
Publisher
Year
  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Applied physics 33 (1984), S. 97-105 
    ISSN: 1432-0630
    Keywords: 78.65.-s ; 71.70.-d ; 68.55.+b
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract Low-temperature photoluminescence measurements on nominally undoped AlxGa1−xAs/GaAs quantum well heterostructures (QWHs) grown by molecular beam epitaxy (MBE) exemplified the exclusivelyintrinsic free-exciton nature of the luminescence under moderate excitation conditions. Neither any spectroscopic evidence for alloy clustering in the AlxGa1−xAs barriers nor any extrinsic luminescence due to recombination with residual acceptors has been detected in single and double QWHs when grown at 670 °C under optimized MBE growth conditions. Carrier confinement in AlxGa1−xAs/GaAs QWHs starts at a well width ofL z≌30 nm when x≌0.25. The minor average well thickness fluctuation ofΔL z=4×10−2nm as determined from the excitonic halfwidth allowed the realization of well widths as low asL z=1 nm and thus a shift of the free-exciton line as high as 2.01 eV which is close to the conduction band edge of the employed Al0.43Ga0.57As confinement layer. The measurements further revealed a strongly enhanced luminescence efficiency of the quantum wells as compared to bulk material which is caused by the modified exciton transition probabilities due to carrier localization.
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  • 2
    ISSN: 1432-0630
    Keywords: 68.55.+ b ; 71.55.-i ; 72.20.-i ; 78.55.-m
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract Hall effect, DLTS and low-temperature photoluminescence measurements were used to study the effect of dimeric (As2) vs tetrameric (As4) vapour species on the electrical and optical properties of nominally undoped and of Ge-doped GaAs layers grown by molecular beam epitaxy (MBE). The arsenic molecular beam was generated from separate As2 and As4 sources, respectively, and from a single source providing an adjustable As2/As4 flux ratio. The occurence of the previously described defect related bound exciton lines in the luminescence spectra at 1.504–1.511 eV was found to be directly correlated with the presence of three deep states (M1, M3, M4) which are characteristic of MBE grown GaAs. The intensity of the extra luminescence lines and simultaneously the concentration of the deep electron traps can be reduced substantially simply by decreasing the As4/As2 flux ratio. The incorporation of defect related centers as well as of amphoteric dopants like Ge strongly depends on the surface chemistry involved. Therefore, a considerably lower autocompensation ratio in Ge-dopedn-GaAs is obtained with As2 molecular beam species which provide a higher steady-state arsenic surface population.
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Applied physics 33 (1984), S. 9-17 
    ISSN: 1432-0630
    Keywords: 81.15.-z ; 78.55.-m ; 78.20.-e
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract Nominally undoped AlxGa1−xAs grown by molecular beam epitaxy from As4 species at elevated substrate temperatures of 670°C exhibits well-resolved excitonic fine structure in the low-temperature photoluminescence spectra, if the effective As-to-(Al+Ga) flux ratio on the growth surface is kept within a rather narrow range of clearly As-stabilized conditions. In contrast to previous results on AlxGa1−xAs of composition 0.15〈x〈0.25, the prominent carbon-related recombination at ∼23 meV below the bound exciton line was foundnot to shift in energy by changing the excitation intensity. This implies a simple freeelectron carbon-acceptor recombination mechanism for the line without any participation of a donor. In AlxGa1−xAs of composition close to the direct-to-indirect cross-over point, two distinct LO-phonons separated by 34 and 48 meV from the (D 0,C 0) peak position at x=0.43 were observed which were before only detectable by Raman scattering experiments. The intensity of the carbon-impurity related luminescence lines in bulk-type AlxGa1−xAs and GaAs layers was found to be strongly reduced, as compared to the excitonic recombination lines, if the respective active layer was covered by a very thin confinement layer of either GaAs on top of AlxGa1−xAs or vice versa grown in the same growth cycle.
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  • 4
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 115 (1982), S. 804-807 
    ISSN: 0009-2940
    Keywords: Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Acetylenedicarbonyl DichlorideIn situ prepared carbonyl chloride 2 can be used as „activated acetylenedicarboxylic acid“ in Diels-Alder syntheses.
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  • 5
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 114 (1981), S. 2834-2843 
    ISSN: 0009-2940
    Keywords: Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Glycosylhydrazines, 5. Synthesis of pyrazole-, Pyrazolo[3,4-d]pyrimidine-, and 1H-1,2,4-Triazol-Nucleoside 5′-amide Derivatives via Riburonamide HydrazonesIn a single step reaction the 3,5-dimethylpyrazole nucleoside-5′-amides 6a,c, the functionally substituted pyrazole nucleoside-5′-amides 9a and 10b, and the 1H-1,2,4-triazol-nucleoside-5′-amide 18 were obtained regio- and stereoselectively from the riburonamide hydrazones 4a-c.9a and 10b were transformed into the unprotected pyrazolo[3,4-d]pyrimidine nucleoside-5-amide 14a and 15b, derivatives of allopurinol riboside. likewise the ethylamide 20 of virazole-5′-acid was synthesized from 18.
    Notes: Aus den Riburonamid-hydrazonen 4a-c wurden in einer Eintopfreaktion regio- und stereoselektiv die 5′-Säureamid-Derivate des 3,5-Dimethylpyrazol-nucleosids 6a,c, der funktionell substituierten Pyrazol-nucleoside 9a) und 10n) und des 1H-1,2,4-Triazole-nucleosids 18 direkt aufgebaut. Als Allopurinolribosid-Derivate wurden aus 9a und 10b die ungeschützten pyrazolo[3,4-d]pyrimidin-nucleosid-5′-säureamide 14a und 15b hergestellt. Entsprechend wurde aus 18 das Ethylamid 20, der Virazol-5′-säure erhalten.
    Additional Material: 1 Tab.
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  • 6
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 113 (1980), S. 1761-1767 
    ISSN: 0009-2940
    Keywords: Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Reactions of Acetals of L-Sorbose with n-Butyllithium2,3:4,6-Di-O-isopropylidene-α-L-sorbose (1a) gives with n-butyllithium the known 6-deoxy-2,3-O-isopropylidene-β-D-threo-hex-5-enulofuranose (2), while the corresponding benzylidene compound 4 yields the glycal derivative 2,5-anhydro-4,6-O-benzylidene-3-deoxy-L-threo-hex-2-enitol (5). The 1-benzyl ether 1b is converted into 3 by Wittig rearrangement. Under analogous conditions both possible Wittig rearrangement products 7a and b are obtained from methyl 6-O-benzyl-2,3,4-tri-O-methyl-α-D-glucopyranoside (6). Their configuration is established by 1H NMR spectroscopy.
    Notes: 2,3:4,6-Di-O-isopropyliden-α-L-sorbose (1a) ergibt mit n-Butyllithium die bekannte 6-Desoxy-2,3-O-isopropyliden-β-D-threo-hex-5-enulofuranose (2), während die entsprechende Benzylidenverbindung 4 das Glycal-Derivat 2,5-Anhydro-4,6-O-benzyliden-3-desoxy-L-threo-hex-2-enitol (5) liefert. Aus dem 1-Benzylether 1b entsteht u. a. durch Wittig-Umlagerung das Produkt 3. Unter den gleichen Bedingungen werden aus Methyl-6-O-benzyl-2,3,4-tri-O-methyl-α-D-glucopyranosid (6) beide möglichen Wittig-Umlagerungsprodukte 7a und 7b erhalten. Ihre Konfigurationszuordnung erfolgt aus dem 1H-NMR-Spektrum.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 114 (1981), S. 740-745 
    ISSN: 0009-2940
    Keywords: Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Reactions of Acetals of L-Rhamnose with Lithium-organic Compounds New Syntheses of Mycaral, Olivomycal, and Methyl 2,3,6-Trideoxy-α-L-erythro-hex-2-enopyranosideMethyl α-L-mycaroside (6a) is obtained from the dideoxy-uloside 2) by Grignard addition. the benzylidene rhamnoside (1a) reacts with methyllithium to give in only one step mycaral (4) (21%) and olivomycal (5) (72%). With lithiumdiisopropylamide 1a yields the unsaturated sugar 3a.
    Notes: Methyl-α-L-mycarosid (6a) wird aus dem Didesoxy-ulosid 2 durch Grignard-Addition erhalten. Das Benzylidenrhamnosid 1a reagiert mit Methyllithium in einem schritt zu Mycaral (4) und Olivomycal (5). Mit Lithiumdiisopropylamid erhält man hingegen aus 1a den ungesättigten Zucker 3a.
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  • 8
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 113 (1980), S. 1775-1789 
    ISSN: 0009-2940
    Keywords: Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Stereospecific Synthesis of Nucleoside-5′-carboxylic Acid Derivatives with Pyrimidines as Hetero-cyclic BasePyrimidine nucleoside-5′-carboxylic acid derivatives were synthesized by condensation of riburonic acid glycosyl halides with heterocyclic bases via different methods. According to the investigations, the benzoyl-protected riburonic acid glycosyl halides 3b and 16b are valuable starting materials for the synthesis of β-nucleosides. Silver perchlorate proved to be the best catalyst in these condensation reactions with respect to stereoselectivity and yield. The methyl esters and the ethyl amides of uridine-, cytidine-, and thymidine-5′-carboxylic acid 8b, 11b, 18b, 19b and 20b were synthesized by this method stereospecifically and in good yield.
    Notes: Pyrimidinnucleosid-5′-carbonsäure-Derivate wurden durch Kondensation von Riburonsäure-glycosylhalogeniden mit heterocyclischen Basen nach verschiedenen Methoden hergestellt. Nach den Untersuchungen eignen sich als Ausgangskomponenten zur Synthese der β-Nucleoside am besten die benzoyl-geschützten Riburonsäure-glycosylhalogenide 3b und 16b. Die Verwendung von Silberperchlorat als Katalysator für die Kondensation ergab insgesamt die besten Ergebnisse bezüglich Stereoselektivität und Ausbeute. Die Methylester bzw. Ethylamide 8b, 11b, 18b, 19b und 20b der Uridin-, Cytidin- und Thymidin-5′-carbonsäure wurden auf diese Weise stereospezifisch und mit guten Ausbeuten synthetisiert.
    Additional Material: 2 Tab.
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  • 9
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 114 (1981), S. 1192-1195 
    ISSN: 0009-2940
    Keywords: Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Reactions of Acetals of Sugars with Organolithium Compounds A Simple Synthesis of 4,6-Dideoxy-α-D-erythro-hexopyranos-3-ulose1,2:3,4-Di-O-isopropylidene-β-L-arabinose (3) reacts with lithium diisopropylamide to give the 4-deoxy-4-eno sugar 4. After its oxidation a stereospecific 1,4-addition by methyllithium/copper (I) iodide yields the 4,6-dideoxy-3-ulose 6.
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  • 10
    Electronic Resource
    Electronic Resource
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 17 (1981), S. 303-305 
    ISSN: 0030-4921
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The two possible conformations of 13-R-substituted oxyprotoberberines are enantiomeric. The racemization barriers are determined for three of these compounds (R=OMe, OCOMe, OCOPh) using variable temperature 1H NMR. It was found that ΔG203
    Additional Material: 1 Ill.
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