- Activation and Chain-carrying CH2 Species in Terminal Alkene Metathesis on Molybdena-Titania Catalysts
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The metathesis reaction of terminal alkenes other than isobutene took place on MoO3-x/TiO2, whereas that of isobutene proceeded in the presence of ethylene and following treatment of the catalyst with SnMe4 at room temperature.These results infer that chain-carrying CH2 species are generated to only a small extent by the adsorption of isobutene on MoO3-x/TiO2, although they are formed by the adsorption of alk-1-enes and SnMe4.The metathesis-inactive material, fully oxidized MoO3/TiO2, was changed to a metathesis-active catalyst by treating it with SnMe4.This suggests that MoO3/TiO2 has no ability to yield CH2 species with terminal alkenes but that these species can be supplied with SnMe4.An analysis by X-ray photoelectron spectroscopy infers that SnMe4 adsorbed on MoO3/TiO2 releases methyl groups concomitant with the oxidation of Sn.
- Tanaka, Katsumi,Tanaka, Ken-ichi
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- Infrared Multiphoton Decomposition of Highly Excited tert-Butyl-d9 Bromide
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The infrared multiphoton decomposition (IR MPD) of tert-butyl-d9 bromide as a function of pressure and fluence is reported.The major decomposition route was molecular elimination of DBr in accord with earlier pyrolysis studies.Evidence for a second channel comprising Br and tert-butyl radicals was found at high fluence or high pressure.The pressure dependence of this second channel suggests that through energy pooling collisions the molecules can reach an otherwise inaccessible channel.The effect of added gases (O2, N2, He, Ar, DBr, and 2-methylpropene-d8) on the MPD is presented.This study is a further indication of the utility of a new method of analysis of MPD experiments.
- Toselli, B. M.,McRae, Glenn A.,Ivanco, M.,McAlpine, Robert D.
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- Tuning Catalytic Sites on Zr6O8 Metal-Organic Framework Nodes via Ligand and Defect Chemistry Probed with tert-Butyl Alcohol Dehydration to Isobutylene
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Metal-organic frameworks (MOFs) have drawn wide attention as candidate catalysts, but some essential questions about their nature and performance have barely been addressed. (1) How do OH groups on MOF nodes act as catalytic sites (2) What are the relationships among these groups, node defects, and MOF stability, and how do reaction conditions influence them (3) What are the interplays between catalytic properties and transport limitations To address these questions, we report an experimental and theoretical investigation of the catalytic dehydration of tert-butyl alcohol (TBA) used to probe the activities of OH groups of Zr6O8 nodes in the MOFs UiO-66 and MOF-808, which have different densities of vacancy sites and different pore sizes. The results show that (1) terminal node OH groups are formed as formate and/or acetate ligands present initially on the nodes react with TBA to form esters, (2) these OH groups act as catalytic sites for TBA dehydration to isobutylene, and (3) TBA also reacts to break node-linker bonds to form esters and thereby unzip the MOFs. The small pores of UiO-66 limit the access of TBA and the reaction with the formate/acetate ligands bound within the pores, whereas the larger pores of MOF-808 facilitate transport and favor reaction in the MOF interior. However, after removal of the formate and acetate ligands by reaction with methanol to form esters, interior active sites in UiO-66 become accessible for the reaction of TBA, with the activity depending on the density of defect sites with terminal OH groups. The number of vacancies on the nodes is important in determining a tradeoff between the catalytic activity of a MOF and its resistance to unzipping. Computations at the level of density functional theory show how the terminal OH groups on node vacancies act as Br?nsted bases, facilitating TBA dehydration via a carbocation intermediate in an E1 mechanism; the calculations further illuminate the comparable chemistry of the unzipping.
- Babucci, Melike,Gaggioli, Carlo Alberto,Gagliardi, Laura,Gates, Bruce C.,Ray, Debmalya,Yang, Dong
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p. 8044 - 8056
(2020/04/30)
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- Characterization of the non-uniform reaction in chemically amplified calix[4]resorcinarene molecular resist thin films
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The ccc stereoisomer-purified tert-butoxycarbonyloxy-protected calix[4]resorcinarene molecular resists blended with photoacid generator exhibit a non-uniform photoacid-catalyzed reaction in thin films. The surface displays a reduced reaction extent, compared with the bulk, with average surface-layer thickness 7.0±1.8nm determined by neutron reflectivity with deuterium-labelled tert-butoxycarbonyloxy groups. Ambient impurities (amines and organic bases) are known to quench surface reactions and contribute, but grazing-incidence X-ray diffraction shows an additional effect that the protected molecular resists are preferentially oriented at the surface, whereas the bulk of the film displays diffuse scattering representative of amorphous packing. The surface deprotection reaction and presence of photoacid were quantified by near-edge X-ray absorption fine-structure measurements.
- Prabhu, Vivek M.,Kang, Shuhui,Kline, R. Joseph,Delongchamp, Dean M.,Fischer, Daniel A.,Wu, Wen-Li,Satija, Sushil K.,Bonnesen, Peter V.,Sha, Jing,Ober, Christopher K.
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experimental part
p. 1065 - 1073
(2011/12/16)
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- Deuterium kinetic isotope effects in gas-phase SN2 and E2 reactions: Comparison of experiment and theory
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The competition between bimolecular nucleophilc substitution and base-induced elimination is investigated through kinetic isotope effect measurements for gas-phase reactions of RCl + ClO- (R = methyl, ethyl, isopropyl, and tert-butyl) utilizing a FA-SIFT instrument. The overall reaction rate constants and the kinetic isotope effect for the reaction of C2H5Cl + ClO- are compared to computational results. [Hu, W. P.; Truhlar, D. G. J. Am. Chem. Soc. 1996, 118, 860.] Experimental results show that as the degree of substitution in the neutral reactant increases the E2 channel becomes dominant. The systematic change in the overall kinetic isotope effects indicates that, for the reaction of ClO- with C2H5Cl, both the SN2 and E2 pathways do occur, as predicted by computation; however the experimental reaction rate constants and KIE deviate strongly from the computational result. Copyright
- Villano, Stephanie M.,Kato, Shuji,Bierbaum, Veronica M.
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p. 736 - 737
(2007/10/03)
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- Synthesis and EPR and ENDOR Investigations of Coppinger's Radical with Perdeuteriated tert-Butyl Groups
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The synthesis of Coppinger's radical (galvinoxyl) with perdeuteriated tert-butyl groups is described.Its EPR spectrum exhibits markedly decreased line widths (0.015 mT) compared with the unlabelled radical (0.039 mT).The correspondingly higher resolution
- Gersdorff, J. von,Kirste, B.,Kurreck, H.
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p. 1077 - 1079
(2007/10/02)
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- An Electron Spin Resonance Study of the Photolysis of Azide Ions in Liquid Ammonia: Formation of Imine Radical Anions in the Presence of Alkenes
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U.v. photolysis of ammonia or ammonia-ethereal solutions containing sodium or tetra-n-butylammonium azide leads to the formation of the diazene radical anion -., which is detectable by e.s.r. spectroscopy.The e.s.r. spectrum of the ammonia-solvated electron is also observed and the primary photochemical process is probably ejection of an electron from N3- to give the azido radical N3..Addition of N3. to ammonia and subsequent loss of nitrogen accompained or followed by a hydrogen shift would give the hydrazyl radical, deprotonation of which gives -..In the presence of mono- or 1,1-di-alkylated ethenes, R1R2C=CH2, the spectrum of -. is replaced by one assigned to the imine radical anion 1R2CH-C(H)=NH>-.; possible routes for formation of such species following addition of N3. to the alkene are discussed.
- Brand, John C.,Roberts, Brian P.,Strube, Roland
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p. 1659 - 1664
(2007/10/02)
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- REACTIONS OF 2-METHYLPROPENE AND OTHER ALKENES ON ZINC OXIDE
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Studies have been made on the hydrogenation, deuteration and exchange with deuterium of ethene, propene and 2-methylpropene on zinc oxide and Arrhenius parameters for the reactions have been determined.Some general conclusions are reached about the catalytic behaviour of alkenes on zinc oxide from these and earlier results.Exchange occurs readily with alkenes which can dissociate to allyl intermediates but can also take place via the formation of vinyl intermediates with other alkenes, particularly at higher temperatures.Similar rates of alkane formation are found with different alkenes at temperatures at which the strengths of adsorption of the alkene are equivalent.The mechanism of alkane formation does not contribute to exchange of the alkene nor give rise to double-bond movement and the probable rate-determining step is the formation of adsorbed alkyl species which are then rapidly converted to alkane.
- Brown, Ronald,Kemball, Charles,Taylor, Duncan
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p. 2899 - 2914
(2007/10/02)
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