- Photocatalytic reduction of nitroorganics over illuminated titanium dioxide: Electron transfer between excited-state TiO2 and nitroaromatics
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The present study investigates the steady-state photocatalytic reduction of methyl viologen and a suite of monosubstituted nitrobenzenes. Reduction was carried out in deoxygenated, illuminated aqueous slurries of titanium dioxide (Degussa P25) in the presence of a sacrificial electron donor, 2-propanol. Langmuir-Hinshelwood plots were obtained for the reduction of each compound and found to be linear, with an average correlation of 0.98 and with a standard deviation on the correlations of 0.02. The concentration independent rates for nitroaromatic reduction obtained from these plots were normalized against the rate of methyl viologen reduction and the ratio was used to solve for the rate constant of nitroaromatic reduction, assuming a bimolecular model. The assumptions behind this procedure were tested by the use of the Marcus expression. Using the reorganization energy for the reaction as the fitting variable, it was possible to fit the measured rates to the predicted rates with a reorganization energy of 138 kJ/mol.
- Ferry, John L.,Glaze, William H.
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p. 2239 - 2244
(2007/10/03)
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- Resonance Electron Capture Rate Constants for Substituted Nitrobenzenes
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We report here a new method for the determination of electron capture (EC) rate constants that utilizes a pulsed electron beam mass spectrometer.The method is first tested by measurements of the known dissociative electron capture rate constants for several halogenated methanes that have been extensively studied by other techniques.The resonance electron capture (REC) rate constants of nitrobenzene (NB) and 23 substituted nitrobenzenes (SNB's) are then determined for the first time at 125 deg C in 10 Torr of methane buffer gas.The SNB's studied here include several sets of closely related structural isomers whose electron affinities (EA's) have been previously determined.It is shown that the REC rate constants of these compounds bear little systematic relationship with the EA's of these compounds.The REC rate constants of the SNB's are also compared with other previously reported characteristics associated with the negative ionization of these compounds, including their entropies of negative ionization, the lifetimes against autodetachment of their initially formed molecular anions, and the rates of autodetachment from electronically excited states of their molecular anions.
- Knighton, W. B.,Mock, R. S.,McGrew, D. S.,Grimsrud, E. P.
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p. 3770 - 3776
(2007/10/02)
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- Entropy Changes and Electron Affinities from Gas-Phase Electron-Transfer Equilibria: A(-) + B = A + B(-)
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By measuring the electron-transfer equilibria 1, A(-) + B = A + B(-), at 150 deg C with a pulsed electron high-pressure mass spectrometer we determined the ΔGo1 values involving 12 new compounds.Measurements of the temperature dependence of K1 for 21 reactions involving some of the new compounds and many compounds whose ΔGo1 had been determined previously led, via van't Hoff plots, to ΔHo1 and ΔSo1 values.These were interconnecting such that ΔHo and ΔSo continuous scales (ladders) were obtained.These were anchored to SO2 whose electron affinity is accurately known.Available geometries and vibrational frequencies for SO2 and SO2(-) permit the evaluation of So(SO2(-)) - So(SO2).Through the ΔSo scale the So(B(-)) - So(B) for the other compounds B could be obtained also.Certain regularities in the So(B(-)) - So(B) data permitted entropy estimates to be made also for compounds for which no van't Hoff plots were made.In this manner a table of ΔHo, ΔSo, and ΔGo data for the electron capture e + B = B(-) was obtained, which contains some 50 compounds B.Most of the compounds are substituted benzenes, quinones, conjugated acid anhydrides, and perfluorinated organics.
- Chowdhury, Swapan,Heinis, Thomas,Grimsrud, Eric P.,Kebarle, Paul
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p. 2747 - 2752
(2007/10/02)
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- Electron Affinities of Di- and Tetracyanoethylene and Cyanobenzenes Based on Measurements of Gas-Phase Electron-Transfer Equilibria
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The electron affinities of tetracyanoethylene, trans-1,2-dicyanoethylene, and eleven substituted benzonitriles as well as two naphthonitriles were determined by measurement of the electron-transfer equilibria A-+B=A+B- with a pulsed electron high ion source pressure mass spectrometer.Rate constants for exothermic electron transfer involving the cyano compounds were found to be near unit collision efficiency.The EA (tetracyanoethylene)=3.17 eV obtained in the present work is considerably higher than the 2.3 eV photodetachment value of Palmer and Lyons.The electron affinities of benzene and benzonitrile substituted by CN, CHO, and NO2 increase in the given order, while the order for nitrobenzene is CHO, CN, NO2.This reversal of order is explained on the basis of a larger attenuation of the ?-withdrawing effect relative to the field effect of substituents when the electron density in the ?* single-electron orbital is decreased.
- Chowdhury, Swapan,Kebarle, Paul
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p. 5453 - 5459
(2007/10/02)
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- Electron affinity of SF6 and perfluoromethylcyclohexane. The unusual kinetics of electron transfer reactions A- + B = A + B-, where A = SF6 or perfluorinated cyclo-alkanes
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Measurements of the equilibria ( 1 ): A- + B = A + B- with a pulsed electron high pressure mass spectrometer lead to ΔG, ΔH, and ΔS.Equilibria involving SF6 as A and perfluoromethylcyclohexane C7F14 provide electron affinities E.A.(SF6) =1.05 +/- 0.1 eV and E.A.(C7F14) =1.06 +/- 0.15 eV.The kinetics of reactions (1) involving the above two compounds were studied.The rate constants kj for SF6- + B = SF6 + B- were found to increase with exothermicity of the reaction.The temperature dependence for k f was determined.B compounds leading to high exothermicities were associated with kj at the collision limit and essentially no temperature dependence.B of progressively lower electron affinity led to kf below the collision limit and negative temperature dependence, while B with the lowest E.A. but still leading to exothermic reaction produced lowest kf with positive temperature dependence.A model is provided explaining the above behavior.The known large change of geometry between SF6- and SF6 introduces an internal barrier in the reaction coordinate for the reactions ( 1 ) involving SF6.
- Grimsrud, E. P.,Chowdhury, S.,Kebarle, P.
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p. 1059 - 1068
(2007/10/02)
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- Electron Affinities from Electron-Transfer Equilibria : A(-) + B = A + B(-)
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Determination of the equilibrium constants K1 for gas-phase electron-transfer equilibria with a pulsed electron beam high ion source pressure mass spectrometer led to the electron affinities of 34 compounds with EA's between 0.5 and 3eV.The compounds are mostly substituted nitrobenzenes, substituted quinones, and conjugated molecules containing oxygen atoms.The EA of smaller molecules like SO2, NO2, CS2, and CH3NO2 also were determined.The method is very well suited for rapid, accurate, routine determinations of electron affinities.A comparison with EA's determined with other gas-phase methods and EA's evaluated from polarographic half-wave reduction potentials and charge-transfer spectra in solution is made.The rate constants for a number of exothermic electron-transfer reactions were determined.Most of these proceed at near collision rates.Electron-transfer reactions involving perfluorinated compounds like perfluoromethylcyclohexane, perfluorocyclohexane, and sulfurhexafluoride do not follow this behavior.While the perfluoro compounds have high thermal electron capture cross sections, they do not accept electrons from A(-) of compounds A with lower electron affinity.The perfluoro anions do transfer electrons to compounds A with higher electron affinity, and the rate constants increase with EA(A) - EA(perfluoro compound).
- Grimsrud, Eric P.,Caldwell, Gary,Chowdhury, Swapan,Kebarle, Paul
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p. 4627 - 4634
(2007/10/02)
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- Relative Electron Affinities of Substituted Nitrobenzenes in the Gas Phase
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Using pulsed ion cyclotron resonance mass spectrometry, we have determined the relative electron affinities of 12 substituted nitrobenzenes in the gas phase by measuring equilibrium constants for electron transfer reactions of the type C6H5NO2- + X-C6H4NO2 = X-C6H4NO2- + C6H5NO2, where X is a substituent group.An excellent correlation is found between the relative electron affinities of the substituted nitrobenzenes and the relative gas-phase acidities of substituted anilines and phenols.
- Fukuda, Elaine K.,McIver, Robert T.
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p. 2993 - 2995
(2007/10/02)
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