- Reaction Mechanism and Kinetics of Olefin Metathesis by Supported ReOx/Al2O3 Catalysts
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The self-metathesis of propylene by heterogeneous supported ReOx/Al2O3 catalysts was investigated with in situ Raman spectroscopy, isotopic switch (D-C3= → H-C3=), temperature-programmed surface reaction (TPSR) spectroscopy, and steady-state kinetic studies. The in situ Raman studies showed that two distinct surface ReO4 sites are present on alumina and that the olefins preferentially interact with surface ReO4 sites anchored at acidic surface sites of alumina (olefin adsorption: C4= > C3= > C2=). The isotopic switch experiments demonstrate that surface Re?CH3 and Re?CHCH3 are present during propylene metathesis, with Re? representing activated surface rhenia sites. At low temperatures (3=][Re?]2. At high temperatures (>100 °C), the rate-determining step is the recombination of two surface propylene molecules (rate ≈ [C3=]2[Re?]). To a lesser extent, the recombination of surface Re?CH3 and Re?CHCH3 intermediates also contributes to self-metathesis of propylene at elevated reaction temperatures.
- Lwin, Soe,Wachs, Israel E.
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p. 272 - 278
(2016/01/12)
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- Metal ions do not play a direct role in the formation of carbon-carbon triple bonds during reduction of trihaloalkyls by CrII or V II
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Carbyne radicals: Reactions of trihaloalkyl compounds with Cr2+ or V2+ in aqueous solutions yield alkynes and other products. Stepwise halogen abstractions from the trihaloalkyls form alkyl carbyne triradicals in solution. These radicals undergo coupling reactions, producing triply bonded alkyne molecules (see scheme). This process is not metal-assisted and does not occur in the coordination sphere of the metal ions.
- Levy, Ophir,Bino, Avi
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supporting information
p. 15944 - 15947
(2013/02/23)
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- Efficient functionalisation of cubic monovinylsilsesquioxanes via cross-metathesis and silylative coupling with olefins in the presence of ruthenium complexes
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Monovinylheptaisobutylsilsesquioxane undergoes efficient cross-metathesis and silylative coupling with styrenes. Allyl derivatives were successfully tested in cross-metathesis in the presence of first generation Grubbs' catalyst, while heteroatom-substitu
- Zak, Patrycja,Pietraszuk, Cezary,Marciniec, Bogdan,Spolnik, Brzegorz,Danikiewicz, Witold
-
body text
p. 2675 - 2682
(2010/04/05)
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- Coupling reactions in aldehydes adsorbed on V(100) single-crystal surfaces
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The thermal chemistry of formaldehyde on vanadium (100) single-crystal surfaces was characterized under ultrahigh vacuum (UHV) conditions by using temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) in combination with isotope-labeling experiments. Particular emphasis was placed on establishing a mechanism for the formation of ethylene, which was observed to desorb in two temperature regimes, at 290 and 540 K. The low-temperature reaction was determined to occur via the coupling of methylene groups formed on the surface upon dissociation of the C-O bond in adsorbed formaldehyde. The high-temperature ethylene, on the other hand, was proven to require the prior formation of a diolate, -OCH2CH2O-, intermediate. This chemistry was shown to be quite general, also occuring in cross-coupling mode between two different coadsorbed aldehydes.
- Shen, Min,Zaera, Francisco
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experimental part
p. 8708 - 8713
(2009/10/23)
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- The Yukawa-Tsuno relationship for the β-silicon effect in the solvolysis rates of 2-(aryldimethylsilyl)ethyl chlorides
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Solvolysis rates of 2-(aryldimethylsilyl)ethyl chlorides were determined conductimetrically in 60% (v/v) aqueous ethanol. The effects of aryl substituents at the silyl atom on the solvolysis rates at 50°C were correlated with essentially nonresonant σ parameters of r = 0:10 in terms of Yukawa-Tsuno (Y-T) Eq. 1, giving a ρ value of -1:75. Such a high ρ value may be regarded as the effect of aryl ring on the bridged Si in the rate-determining step. The Si-bridging is consistent with the fact that the solvolysis of the unsubstituted substrate with d2-labeled ethylene moiety gave substitution products with label scrambling. The arylsilyl substituent effects were likewise analyzed for several relevant sets of β-silyl systems in order to ascertain significant variations of ρ values from system to system; Y-T Eq. 1 correlated quite excellently with essentially nonresonant sigmas of negligible resonance demand (r ? 0:10), to exhibit significant variations of ρ from -1:75 to -0:95.
- Fujio, Mizue,Uchida, Mai,Okada, Ayumi,Alam, Md. Ashadul,Fujiyama, Ryoji,Siehl, Hans-Ullrich,Tsuno, Yuho
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p. 1834 - 1842
(2007/10/03)
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- The effect of substituents at silicon on the cross-metathesis of trisubstituted vinylsilanes with olefins
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Efficient cross-metathesis of vinylsilanes, carrying a large spectrum of different substituents at silicon, with various olefins in the presence of the first and second generation Grubbs catalyst and Hoveyda-Grubbs catalyst is described. On the basis of the results of equimolar reactions of vinylsilanes with ruthenium alkylidene complexes and experiments with deuterium-labelled reagents, a general, metallacarbene mechanism for the cross-metathesis of trisubstituted vinylsilanes with olefins has been suggested. Reaction was proved to be a valuable method for synthesis of unsaturated organosilicon derivatives.
- Pietraszuk, Cezary,Fischer, Helmut,Rogalski, Szymon,Marciniec, Bogdan
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p. 5912 - 5921
(2007/10/03)
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- Matrix Isolation Investigation of the Room Temperature and Pyrolytic Reactions of (CH3)2Zn with CH3OH and CH3SH
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Matrix isolation and cryogenic thin film approaches have been employed for the synthesis, isolation, and characterization of 1:1 and 1:2 complexes of (CH3)2Zn with CH3OH and CH3SH.These complexes were characterized by a shifting of certain sensitive vibrational modes of the acid and base subunits in the complex.The ratio of the 1:1 and 1:2 complexes could be altered by changing the relative amounts of the two reagents employed in a given experiment or by warming the cryogenic thin film from 14 K to as high as 200 K.Merged jet mixtures of (CH3)2Zn with either CH3OH and its isotopomers or CH3SH were also pyrolyzed at temperatures as high as 370 deg C.For the (CH3)2Zn/CH3OH pair, significant production of CH4 and CH2O was observed with minor amounts of C2H4.In the (CH3)2Zn/CH3SH experiments a substantial yield of (CH3)2S and CH4 was obtained.These pyrolytic reactions and products have not been reported previously and may have implications for the chemical vapor deposition of ZnO and ZnS.
- Bai, Hebi,Ault, Bruce S.
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p. 10492 - 10497
(2007/10/02)
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- Surface Far-Ultraviolet Photochemistry of Ethyl Chloride on GaAs(1000)
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The photoinduced dissociation at 193 nm of monolayer ethyl chloride (EtCl) was studied on the Ga-rich GaAs(100) (8x2)Ga surface at 90 K.Photodepletion of EtCl is efficient, with a cross section of 6 x 10-19 cm2.Sixty percent of the EtCl molecules depleted by irradiation give ethyl groups bonded to the surface.The ethyl groups are stable up to ca. 500 K, which is the onset of thermal desorption of the products.The thermal desorption products are C2H4, C2H5, C2H6, H2 and GaCl.The hydrocarbon product distribution is approximately 70percent C2H4, 20percent C2H6, and 10percent C2H5 radicals.Experiments with CD3CH2Cl provide evidence that ethylene is formed by β-hydride elimination.The zero-coverage first-order kinetic parameters for this reaction are approximately Ea = 100 kJ/mol and A = 4.4 x 107 s-1.Comparison with the gas-phase photodissociation cross section indicates that the photodepletion is primarily due to substrate-mediated processes rather than to direct photodissociation of EtCl.The most probable mechanism involves photogenerated electron transport to the surface followed by electron dissociative attachment to EtCl.
- Liberman, Vladimir,Nooney, Matthew G.,Amata, Richard J.,Martin, Richard M.
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p. 2261 - 2269
(2007/10/02)
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- Laser-Powered Decomposition of Spiroalkanes (n = 2-5)
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The laser heating of spiroalkanes (n=2-5) and of their 1,1,2,2-tetradeuterated isotopomers reveals dissimilar modes of their thermal decomposition.Spiropentane decomposes into ethene and propadiene via two competing routes: the direct cleavage and the more important cleavage via intermediary methylenecyclobutane.Spirohexane decomposes through two important concurrent pathways which are the expulsions of ethene from the three-membered ring and a more feasible expulsion of ethene from the four-membered ring.Spiroheptane and spirooctane decompose by a radical-chain mechanism and afford complex mixtures of products; upon addition of propene both compounds rearrange into two cycloalkanes wherein the larger ring of the spiroalkane is preserved and substituted with ethylidene and a vinyl group.
- Fajgar, Radek,Pola, Josef
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p. 7709 - 7717
(2007/10/02)
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- The mechanism of the infrared laser induced multiphoton decomposition of ethyl alcohol
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The infrared laser induced multiphoton decomposition of ethyl alcohol has been studied in experiments with iodine as a radical scavenger and the deuterium substituted from CH3CD2OH of the alcohol.These experiments show that the principle product, C2H4, arises from a concerned molecular elimination, as do most of the CH4 and CH3CHO.The C2H6 product results from recombination of nascent methyl radicals formed in the minor bond breaking channel.
- Goodale, J. W.,Evans, D. K.,Ivanco, M.,McAlpine, Robert D.
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p. 1437 - 1439
(2007/10/02)
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- 70. Site-Specific, Oxidative Addition of C-C Bonds by 'Anchored' Bare Fe(I) Cations Prior to C-H Bond Activation
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Evidence is presented for the first time that 'naked' Fe+ ions, complexed to the NH2 group of primary amines, are capable of intramolecularly activating C-C bonds in a highly specific mode without preceding C-H activation.
- Karrass, Sigurd,Schwarz, Helmut
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p. 633 - 637
(2007/10/02)
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- Oxidative Addition of the Carbon-Hydrogen Bond Is Not the Rate-Determining Step in the Remote Functionalization of Nitriles by Bare Fe(I) Ions
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The study of D-labeled 5-cyanononane isotopomers in the gas phase provides evidence that the Fe+-mediated remote functionalization of nitriles, resulting in the regiospecific generation of H2 and C2H4 from the ω and (ω - 1) position of the alkyl chain, can be described as follows: (i) Oxidative addition of a methyl C-H bond is not rate limiting.For the generation of C2H(4-x)Dx it is the elimination of the olefin which is associated with kinetic isotope effects comparable to the ones reported earlier for other systems. (ii) The generation of H(2-x)Dx is affected bytwo isotope effects.One concerns the β-hydrogen transfer (kH/kD = 1.59) and the other the reductive elimination of hydrogen (kH2/kHD = 1.70 and kHD/kD2 = 1.44).
- Czekay, Gregor,Drewello, Thomas,Schwarz, Helmut
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p. 4561 - 4563
(2007/10/02)
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- Deceptive Similarities in the Reactions of Fe+ and Co+ with Linear Nitriles in the Gas Phase
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The gas-phase reactions of the transition-metal ions Fe+ and Co+ with linear C(4)-C(12) nitriles are reported.In spite of an overall similar reactivity pattern, a more detailed analysis, based on the study of labeled nitriles, reveals distinct differences with regard to the mechanisms of elimination of alkenes and alkanes.For both metal ions, hydrogen and alkenes are generated from linear C(4)-C(12) nitriles, and the intermediates are formed via oxidative addition to terminal and internal C-H bonds.For the RCN/Fe+ system insertion in an internal C-H bond commences at position C(8) of the nitrile; for the analogous RCN/Co+ system, the oxidative addition to an internal C-H bond starts at position C(7) of the nitrile.Similarly, alkane formation is different for the two transition-metal ions.For RCN/Fe+ the generation of alkanes is observed for nitriles having at least eight carbon atoms; in contrast, the elimination of alkanes from RCN/Co+ is already observed for C(6) nitriles.Alkane elimination seems to follow the conventional mechanism (i.e., oxidative addition to a C-C bond, β-hydrogen transfer, and reductive elimination) for the RCN/Co+ system, whereas for the RCN/Fe+ complex there exists an additional mechanism.This mechanism corresponds to the loss of H2 from an internal position of the alkyl chain followed by the elimination of an alkene.Some possible origins of the different behavior of Fe+ vs.Co+ are discussed.
- Lebrilla, Carlito B.,Drewello, Thomas,Schwarz, Helmut
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p. 5639 - 5644
(2007/10/02)
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- On the Mechanism of Base-Induced Gas-Phase Elimination Reactions of Ethers
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For the base-induced gas-phase elimination reactions of diethyl ether and cis- and trans-1-tert-butyl-4-methoxy-cyclohexane the kinetic isotope and leaving group effects have been determined as functions of the base strength using the method of Fourier transform ion cyclotron resonance mass spectrometry.The results are interpreted in terms of a variable E2 transition-state structure.Increasing the base strength causes the transition state to shift toward the carbanion or E1cb region of the E2 spectrum, which is also a general phenomenon in the condensed phase.Moreover, it appears that the elimination reactions most readily proceed via a transition state in which the β hydrogen and leaving group are periplanar.If the substrate does not easily allow such a relationship, the transition state is found to shift toward the carbenium ion or E1 region of the E2 spectrum where the geometric restrictions of the substrate are less perceptible.The concept of syn/anti dichotomy is used to explain the formation of tree and solvated alkoxide anions in the reactions induced by OH-.Anti elimination is believed to result in the formation of free alkoxide.Syn elimination, which takes advantage of the electrostatic interaction between the base and leaving group, is held responsible for the formation of solvated alkoxide.The importance of base/leaving group association in the transition state of the syn elimination is demonstrated by the low yield of solvated alkoxide in the reaction of OH-, solvated by a dimethylamine molecule, with diethyl ether.Finally, it seems that the selectivity of gas-phase elimination reactions is determined by not only the relative heights of the intrinsic reaction barriers, but also the relative stabilities of the ion/molecule complexes preceding the reaction barriers.
- Koning, Leo J. de,Nibbering, Nico M. M.
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p. 1715 - 1722
(2007/10/02)
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- Reactions of FeCH3+ and CoCH3+ with Aliphatic Alkenes and Alkynes in the Gas Phase
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The gas-phase reactions of FeCH3+ and CoCH3+ with a variety of alkenes are described by using Fourier transform mass spectrometry (FTMS).FeCH3+ is unreactive with ethene, while CoCH3+ reacts with ethene to yield the allyl product CoC3H53.This reaction presumably proceeds by initial methyl migratory insertion into coordinated ethene to form a Co(propyl)+ complex which subsequently dehydrogenates.Reactions with alkenes containing labile hydrogens α to the double bond proceed by initial elimination of methane to form an activated ?-allyl complex which may subsequently decompose further.Both FeCH3+ and CoCH3+ undergo migratory insertion into coordinated butadiene generating a ?-pentenyl complex which undergoes dehydrogenation.In addition, both FeCH3+ and CoCH3+ appear to insert into ethyne, generating a vinyl species which dehydrogenates to form MC3H3+.These results suggest the following order for methyl migratory insertation into C-C multiple bonds: butadiene ca. ethyne > ethene.Decomposition of M(C5H9)+ species proceeds by initial skeletal rearrangement to a linear structure followed by dehydrocyclization to generate M-c-C5H5+.A stable Co(pentadienyl)+ species was generated and is distinguishable from the corresponding Co(cyclopentyl)+ complex by H/D exchange with deuterium.Finally, D0(Fe+-pentadienyl) is found to exceed 70 +/- 6 kcal/mol and D0(Co+-pentadienyl) exceeds 62 +/- 5 kcal/mol.
- Jacobson, D. B.,Freiser, B. S.
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p. 5876 - 5883
(2007/10/02)
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- Reactions of FeCH2(1+) and CoCH2(1+) with Olefins in the Gas Phase. Studies Involving Olefin Metathesis
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Reactions of the title carbenes with several olefins and alkynes are reported.Ethene reacts with MCH2(1+) yielding exclusively M(1+) formation (C3H6 elimination).Reaction of ethene with MCD2(1+) yields the metathesis products FeCH2(1+) and CoCH2(1+) in 20percent and 2percent yields, respectively.Formation of the metathesis product MC2H4(1+) dominates for propene with no MCH2(1+) produced from MCD2(1+).Formation of MC2H4(1+) is believed to proceed through an ethene-ethylidene intermediate that rearranges to a bis(ethene) complex followed by elimination of ethene.Absence of MCH2(1+) formation from reaction of MCD2(1+) with propene suggests that the alkene-alkylidene conversion is the key step in metathesis of olefins larger than ethene.Several other pathways compete with metathesis such as cyclopropanation, olefin homologation, dehydrogenation, and various C-C bond cleavages.Both carbenes react with butadiene, generating M-c-C5H6(1+) and M-c-C5H5(1+), implying D0(Co(1+)-C5H5) > 76+/- 7 kcal/mol and D0(Fe(1+)-C5H5) > 87 +/- 5 kcal/mol.Finally, ethyne and propyne react with MCH2(1+) to yield M(1+) as the only product.
- Jacobson, D. B.,Freiser, B. S.
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p. 2605 - 2612
(2007/10/02)
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- La photolyse du cyclohexene gazeux a 184,9 nm
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We have studied the 184.9 nm photolysis of gaseous cyclohexene either in the absence or in the presence of radical scavengers such as O2, NO, H2S, or HI at pressures between 1 and 70 Torr.Propane and sulfur hexafluoride has also been used as stabilizing agent.In all cases, ethylene and 1,3-butadiene have rather high quantum yields (Φ>/=0.5).The isotopic analysis of the ethylene formed in the photolysis of cyclohexene-3,3,6,6-d4 shows the high importance of the perhydrogenated species.These results, together with others taken from the literature, favor a one-step fragmentation mechanism or a double-step mechanism involving an intermediate which has a lifetime shorter than that of the photoexcited molecule.
- Collin, Guy J.,Deslauriers, Helene
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p. 1970 - 1972
(2007/10/02)
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- Reactions of Metal-Metal Multiple Bonds. 8. Forming Mo-Mo Quadruple Bonds by Reductive Elimination (Alkyl Group Disproportionation) in the Reactions of 1,2-Mo2R2(NMe2)4 Compounds (M*M) with Carbon Dioxide and 1,3-Diaryltriazines
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Addition of CO2 and 1,3-diaryltriazines to 1,2-Mo2R2(NMe2)4 compounds (M*M), where R = CH3 and CH2Si(CH3)3, yields Mo2R2(O2CNMe2)4 and Mo2R2(NMe2)2(ArN3Ar)2 compounds, respectively, with retention of the Mo*Mo bond, whereas related reactions, where R = C2
- Chetcuti, M. J.,Chisholm, M. H.,Folting, K.,Haitko, D. A.,Huffman, J. C.
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p. 2138 - 2146
(2007/10/02)
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- CO2 LASER-INDUCED DECOMPOSITION OF METHYL IODIDE SENSITIZED BY SULFUR HEXAFLUORIDE
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Gaseous mixtures of methyl iodide and sulfur hexafluoride yield upon irradition by a continuous-wave CO2 laser iodine, methane, ethane, ethylene and acetylene - the products of methyl iodide decomposition.The decomposition rate and the mutual ratio of the products is dependent on the delivered irradiatiion energy, the methyl iodide/sulfur hexafluoride ratio and the energy of the exciting photon.Methyl iodide-d3 in the mixture with SF6 behaves similarly as methyl iodide yielding perdeuterated methane, ethane, ethylene, acetylene and iodine.
- Dathe, Klaus,Engst, Pavel,Pola, Josef,Horak, Milan
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p. 1910 - 1919
(2007/10/02)
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