- Efficient Photocatalysis of the irreversible One-Electron and Two-Electron Reduction of Halothane on Platinized Colloidal Titanium Dioxide in Aqueous Suspension
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The irreversible one- and two-electron reductions of halothane (2-bromo-2-chloro-1,1,1-trifluoroethane) by conduction band electrons, eCB-, photogenerated in aqueous colloidal suspensions of platinized titanium dioxide (TiO2/Pt) have been investigated.Both bromide and fluoride ions, i.e., the respective products of the one-electron and two-electron transfer to halothane, are formed with high quantum yields Φ (ΦF- = ΦBr = 0.43 for photoplatinized TiO2) provided efficient hole scavengers, e.g., methanol, are present.The intermediacy of a carbon-centered radical, CF3C.HCl, resulting from the initial reaction of halothane with eCB-, is strongly indicated.However, this radical is apparently well stabilized on the oxide surface and does not diffuse into the aqueous phase to an appreciable extent since typical product distributions for homogeneous free-radical reactions are not observed.Adsorption of the parent molecule halothane onto TiO2/Pt also,appears to be an efficient process as deduced from several experimental results.Induction periods between 15 and 35 min, apparent in the concentration vs. illumination time profiles, are explained by photoinduced rearrangements of the Pt deposits resulting in better electron relay properties of the metal.Implications of the observed results on the design of suitable catalysts for synthetic organic photochemistry are discussed.
- Bahnemann, Detlef W.,Moenig, Joerg,Chapman, Rita
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- Internal excitation in the products of nucleophilic substitution from the dissociation of metastable ion complexes
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The relative kinetic energy distributions for the products of the dissociation of four metastable gasphase ion clusters have been analyzed by means of ion kinetic energy spectroscopy, and the results modeled using statistical phase space theory. The systems studied represent reaction intermediates in bimolecular nucleophilic substitutions (S(N)2). These studies build on previous investigations that demonstrated vibrational excitation in the products of the substitution reactions of halide ions with methyl halides. The present studies explore the effects of molecular structure, reaction exothermicity, and nucleophile and leaving group variation. The experimental kinetic energy distributions are compared with theoretical distributions calculated for statistical partitioning of energy among internal modes and relative kinetic energy of the products. In each reaction, the calculated distributions agree with the experimental distributions only if a significant fraction of the energy released in the exothermic reactions is assumed to be unavailable for randomization in the dissociation. The results suggest that the products of these S(N)2 reactions are internally excited.
- Graul, Susan T.,Carpenter, Catherine J.,Bushnell, John E.,Van Koppen, Petra A. M.,Bowers, Michael T.
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- Kinetics of radical heterolysis reactions forming alkene radical cations
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Rate constants for heterolytic fragmentation of β-(ester)alkyl radicals were determined by a combination of direct laser flash photolysis studies and indirect kinetic studies. The 1,1-dimethyl-2-mesyloxyhexyl radical (4a) fragments in acetonitrile at ambient temperature with a rate constant of khet > 5 × 109 s-1 to give the radical cation from 2-methyl-2-heptene (6), which reacts with acetonitrile with a pseudo-first-order rate constant of k = 1 × 106 s-1 and is trapped by methanol in acetonitrile in a reversible reaction. The 1,1-dimethyl-2-(diphenylphosphatoxy)hexyl radical (4b) heterolyzes in acetonitrile to give radical cation 6 in an ion pair with a rate constant of khet = 4 × 106 s-1, and the ion pair collapses with a rate constant of k ≤ 1 ± 109 s -1. Rate constants for heterolysis of the 1,1-dimethyl-2-(2,2- diphenylcyclopropyl)-2-(diphenylphosphatoxy)ethyl radical (5a) and the 1,1-dimethyl-2-(2,2-diphenylcyclopropyl)-2-(trifluoroacetoxy)ethyl radical (5b) were measured in various solvents, and an Arrhenius function for reaction of 5a in THF was determined (log k = 11.16-5.39/2.3RT in kcal/mol). The cyclopropyl reporter group imparts a 35-fold acceleration in the rate of heterolysis of 5a in comparison to 4b. The combined results were used to generate a predictive scale for heterolysis reactions of alkyl radicals containing β-mesyloxy, β-diphenylphosphatoxy, and β-trifluoroacetoxy groups as a function of solvent polarity as determined on the ET(30) solvent polarity scale.
- Horner, John H.,Bagnol, Laurent,Newcomb, Martin
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p. 14979 - 14987
(2007/10/03)
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- Gas-phase ionic reactions of benzyl and methoxide anions
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Gas-phase reactions of benzyl and methoxide anions with alkyl formate and other esters were compared using Fourier transform io cyclotron resonance spectroscopy. Although these anions have similar basicities, in many cases the reaction pathways differ.
- Gatev, Geo G.,Zhong, Meili,Brauman, John I.
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p. 531 - 536
(2007/10/03)
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- Electron transfer as a possible initial step in nucleophilic addition elimination reactions between (radical) anions and carbonyl compounds in the gas phase
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The reactions of the HO-, CH3S-, CH2S- and CH2=C(CH3)-CH2- ions with three ketones (CF3COR; R=CH3, CF3, C6H5) and three esters of trifluoroacetic acid (CF3CO2R; R=CH3, C2H5 and C6H5) have been studied with use of Fourier Transform Ion Cyclotron Resonance (FT-ICR) mass spectrometry. All four negative ions react exclusively by proton transfer with CF3COCH3. With the other substrates, the HO- ion reacts by various pathways, such as proton transfer, SN2 substitution, E2 elimination and attack on the carbonyl group. The CH3S- ion is unreactive towards CF3COC6H5 but is able to react by hydride transfer, SN2, E2 and/or carbonyl attack with the remaining neutral species. The CH2S- radical anion reacts by electron transfer to afford stable molecular radical anions of CF3COCF3 and CF3COC6H5, whereas the main reaction with the two esters, CF3CO2CH3 and CF3CO2C2H5, is dissociative electron transfer leading to CF3CO2- and CF3- ions. The CH2=C(CH3)-CH2- anion displays a more complex reactivity pattern involving electron transfer, SN2, E2 as well as attack on the carbonyl group. Direct evidence for the occurrence of electron transfer as the initial step in an overall BAC2 type process has not been obtained for the systems studied. The reaction of the CH2S- ion with CF3CO2C6H5 was observed, however, to yield exclusively a CF3COCHS-. radical anion. Based upon the absence of a BAC2 process in the reaction of CH2S- with the methyl and ethyl esters of trifluoroacetic acid in combination with the facile occurrence of electron transfer from this radical anion, it is suggested that the CF3COCHS-. ion is formed by an initial electron transfer followed by coupling between the CH2S molecule and the CF3CO2C6H5- radical anion and subsequent loss of C6H5OH from the collision complex.
- Staneke, Paul O.,Ingemann, Steen,Nibbering, Nico M. M.
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p. 179 - 184
(2007/10/03)
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- Gas-Phase Nucleophilic Displacement Reactions
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Displacement reactions of each of a variety of anionic nucleophiles reacting with each of a variety of neutrals have been studied by pulsed ion cyclotron resonance (ICR) spectroscopy.Rate constants for these reactions are interpreted in terms of a three-step reaction sequence.RRKM calculations are used to obtain information about the energy of transition states.The origin of the barrier to reaction in solution is discussed.
- Olmstead, William N.,Brauman, John I.
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p. 1653 - 1662
(2007/10/03)
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- Proton transfers among oxygen and nitrogen acids and bases in DMSO solution
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Rate constants for the proton-transfer reactions between conjugate acids and bases of several amines, phenols, carboxylic acids, and the solvated proton in DMSO-d6 at 20 °C have been determined by the use of NMR line-shape analysis. Equilibrium constants for the same reactions are obtained from the pKa's of the acids in dimethyl sulfoxide, some of which have been reported in earlier work and the rest obtained in the present work by use of Bordwell's indicator techniques. All of the reactions have rale constants considerably below expected diffusion-controlled limits for the proton transfers in the thermodynamically favorable direction, and several of the reactions, including the identity reactions of carboxylic acids, have kinetic deuterium isotope effects, kH/kD, between 0.8 and 1.3. For reactions of N,N-dimethylbenzylammonium ion with several phenoxides, carboxylates, and solvent, the rate constants for transfers in the unfavorable directions show a reasonable Bronsted correlation with β ≈ 1 and a reasonably constant reverse rate constant of ≈3 × 106 M-1 s-1. The data clearly indicate that the proton-transfer step is not rate-limiting in these reactions. Most likely, desolvation is involved in the rate-limiting steps, but the rate constants are not simple functions of acidities as might have been expected if hydrogen bonding of acid to solvent were the major factor involved in the solvation Other factors, particularly dispersion interactions of solvent with solutes, are discussed. We suggest that the formation of an acid-base complex with proper orientation to allow contact between the proton and the basic site is rate-determining and involves desolvation along with detailed steric interactions of the acid-base pair.
- Ritchie, Calvin D.,Lu, Shanzheng
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p. 7748 - 7756
(2007/10/02)
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- Generation, Thermodynamics, and Chemistry of the Diphenylcarbene Anion Radical (Ph2C.-)
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Dissociative electron attachment with Ph2C=N produced Ph2C.- (m/z 166).The reactions of Ph2C.- with potential proton donors of known gas-phase acidity were used to bracket PA(Ph2C.-) = 380 +/- 2 kcal mol-1 from which ΔHf0(Ph2C.-) = 81.8 +/- 2 kcal mol-1 was calculated.The reactions of Ph2C.- with CH3OH and C2H5OH proceeded with major and minor amounts, respectively, of a H2.+-transfer channel, forming Ph2CH2, RCHO, and an electron.The kinetic nucleophilicity of Ph2C.- in SN2 displacement reactions with CH3X and C2H5X molecules was shown to be medium, which requires a significant intrinsic barrier in these reaction.The reactions of Ph2C.- with various aldehydes, ketones, and esters were fast and established two principal product-forming channels: (1) H+ transfer if the neutral reactant contains activated C-H bonds and (2) carbonyl addition followed by radical β-fragmentation of one of the groups originally attached to the carbonyl carbon.The order for the ease of radical β-fragmentation in the tetrahedral intermediates was RO > alkyl >> H, and CO2CH3 > CH3.Since the reactions of Ph2C.- with the simple esters HCO2CH3 and CH3CO2CH3 were fast, it should now be possible to examine the reactions of carbonyl-containing organic molecules, which are expected to react slower than these esters and obtain their relative reactivities.
- McDonald, Richard N.,Gung, Wei Yi
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p. 7328 - 7334
(2007/10/02)
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- Catalysis of Anilide Hydrolysis by Poly(ethylenimine) Derivatives
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Amide hydrolysis in 4-nitro-3-carboxytrifluoroacetanilide (1) is catalyzed by poly(ethylenimines).The rate-controlling step is the formation of a tetrahedral intermediate, breakdown of which is catalyzed by the conjugate acid and base forms of the amine g
- Suh, Junghun,Klotz, Irving M.
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p. 2373 - 2378
(2007/10/02)
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- The Effect of Both ?-Donor Substituents and Acid Media on the Formation of Trityl Cations, Studied by Relaxation Methods and NMR-Spectroscopy
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Trityl trifluoroacetates were taken to study why ion formation from ionogens is favoured by acid media.From relaxation measurements, NMR-spectroscopy and conductivity studies it was found that this effect is due to anion solvation by acid, namely a specific solvation of the free anions and a statistical solvation of the anions bound in the ionogen.This leads to a drastically reduced ion recombination rate constant even at low concentrations of acid and an increased dissociation rate constant at rather high concentrations of acid. -.Keywords: Kinetics/ Ion solvation/ Carbenium ions/ NMR-spectroscopy
- Blumenstock, H.,Dickert, F.,Fackler, H.,Hammerschmidt, A.
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p. 157 - 170
(2007/10/02)
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- Nucleophilic Reactions of F3C- at sp2 and sp3 Carbon in the Gas Phase. Characterization of Carbonyl Addition Adducts
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The reactions of F3C- with CH3Br and CH3Cl established the medium kinetic nucleophilicity of F3C- on Bohme's reactivity scale for gas-phase SN2 reactions.The reactions of F3C- with (CH3)2C=O and CH3CO2CH3 proceeded by competitive bimolecular H+ transfer and termolecular carbonyl addition giving the corresponding adducts anions m/z 127 and 143, respectivaly.F3C- reacted with esters C6H5CO2CH3, CF3CO2CH3, and (CH3O)2C=O both by SN2 displacement forming the corresponding carboxylate anions and by carbonyl addition yielding the adduct anions; with CF3CO2C2H5 and CF3CO2C(CH3)3, the competitive bimolecular reaction channel involved E2 elimination giving CF3CO2-.The major reaction channel of F3C- with HCO2CH3 was the Riveros reaction that produced the series of cluster ions F3C-(HOCH3), F3C-(HOCH3)2, CH3O-(HOCH3), and CH3O-(HOCH3)2, along with a minor amount of carbonyl addition.The fast termolecular reaction of F3C- with (CF3)2C=O exclusively formed the adduct (CF3)3CO- (m/z 235) which was characterized as the bound, tetrahedral structure by bracketing its proton affinity.The reaction of F3C- with CO2 giving CF3CO2- was established as a termolecular process when the "apparent" bimolecular rate constant was shown to be PHe dependent.These results demonstrate unequivocally that the reactions of gas-phase nucleophiles with the carbonyl group of ketones and esters proceed by addition yielding the corresponding adduct oxyanions which is analogous to the related process in the condensed phase.
- McDonald, Richard N.,Chowdhury, A. Kasem
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p. 7267 - 7271
(2007/10/02)
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- Some perfluoroalkyliminosulfur derivatives
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Trifluoromethyliminosulfur dichloride and pentafluoroethyliminosulfur dichloride are prepared by reaction of aluminum trichloride with trifluoromethyliminosulfur difluoride and pentafluoroethyliminosulfur difluoride, respectively. These imino dichlorides
- Lustig, Max
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p. 1317 - 1319
(2007/10/12)
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