- Dynamic Spectral Shifts of Molecular Anions in Organic Glasses
-
Time-dependent spectra of the radical anions of pyromellitic dianhydride and p-dinitrobenzene have been observed after formation by pulse radiolysis in frozen 2-methyltetrahydrofuran and triacetin glasses.At temperatures near the glass transition, the spectra shift toward the blue over the entire observed time range (100 ns - 100 s), while at temperatures well below the glass transition, the spectral shifts can be stopped or greatly slowed.The magnitudes of the shifts are not large (typically ca. 10 nm), but because they are larger than the vibrational line widths, dramatic kinetics may be observed: the absorbance grows or decays by more than a factor of five at some wavelengths.The observations are consistent with a solvent molecule reorientation mechanism for spectral shifts of molecular ions in low-temperature organic glasses.
- Huddleston, R. Kurt,Miller, John R.
-
-
Read Online
- Photochemical properties of excited triplet state of 6H-purine-6-thione investigated by laser flash photolysis
-
Photochemical reactions of 6H-purine-6-thione (PuT) via the excited triplet state [3(PuT)*] have been studied by means of laser flash photolysis in organic solvents. Transient absorption bands at 475 and 690 nm were assigned to 3(PuT)*. Intersystem quantum yield and the lowest triplet energy of 3(PuT)* were evaluated to be 0.99 and 63 kcal/mol, respectively. The self-quenching rate constant is quite large (2.3×109 M-1 s-1 in THF). In photoinduced electron transfer, 3(PuT)* acts as electron acceptor for tetramethylbenzidine, while 3(PuT)* acts as electron donor for p-dinitrobenzene. Rate constants for H-atom abstraction (khT) of 3(PuT)* from benzenethiols, tocopherol, and 1,4-cyclohexadiene are on the order of 108 M-1 s-1. From the Hammett plots of khT for substituted benzenethiols, a negative ρ value indicates that 3(PuT)* has electrophilic character. In the addition reaction of 3(PuT)* toward various alkenes, the electrophilic character of 3(PuT)* was also confirmed. By steady-light photolysis of PuT, purine was produced via 3(PuT)* after H-atom abstraction. On combination of these results, the character of the lowest 3(PuT)* was presumed.
- Alam,Fujitsuka,Watanabe,Ito
-
p. 1338 - 1344
(2007/10/03)
-
- Resonance Electron Capture Rate Constants for Substituted Nitrobenzenes
-
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.
-
p. 3770 - 3776
(2007/10/02)
-
- Entropy Changes and Electron Affinities from Gas-Phase Electron-Transfer Equilibria: A(-) + B = A + B(-)
-
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
-
p. 2747 - 2752
(2007/10/02)
-
- Electron Affinities of Di- and Tetracyanoethylene and Cyanobenzenes Based on Measurements of Gas-Phase Electron-Transfer Equilibria
-
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
-
p. 5453 - 5459
(2007/10/02)
-
- Electron affinity of SF6 and perfluoromethylcyclohexane. The unusual kinetics of electron transfer reactions A- + B = A + B-, where A = SF6 or perfluorinated cyclo-alkanes
-
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.
-
p. 1059 - 1068
(2007/10/02)
-
- Electron Affinities from Electron-Transfer Equilibria : A(-) + B = A + B(-)
-
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
-
p. 4627 - 4634
(2007/10/02)
-
- One-Electron Reduction of Nitrobenzenes by α-Hydroxyalkyl Radicals via Addition/Elimination. An Example of an Organic Inner-Sphere Electron-Transfer Reaction
-
The reaction in aqueous solution of α-hydroxyalkyl radicals with para-substituted nitrobenzenes were studied by using product analysis, electron spin resonance, and pulse radiolysis techniques.At neutral pH the α-hydroxyalkyl radicals are quantitatively oxidized to yield the corresponding ketones or aldehydes and H+, and the nitrobenzenes are reduced to the radical anions.The mechanism of this redox reaction depends strongly on the substituents on the α-hydroxyalkyl radical (the electron donor) and on the nitrobenzene (the electron acceptor).In case of α-hydroxymethyl radical, the reaction proceeds by addition to the nitro group to produce an alkoxynitroxyl radical which can undergo an OH--catalyzed heterolysis to give formaldehyde and the radical anion of the nitrobenzene.With the α-hydroxyethyl radical, both addition and "electron transfer" take place, the fraction of electron transfer increasing with increasing electron-withdrawing power of the substituent.The nitroxyl-type adducts undergo a spontaneous unimolecular heterolysis to give acetaldehyde, H+, and nitrobenzene radical anion.The rate constants ks (from 2 to 5*104 s-1) for this heterolysis increase with increasing electron-withdrawing strength of the substituent if it is on the benzene, and they decrease if the substituent is on the methyl carbon of the nitroxyl.The heterolysis reaction is characterized by low (5-10 kcal/mol) activation enthalpies and strongly negative (-5 to -25 eu) activation entropies, which originate from hydration of a proton in the transition state.From the effect on the activation parameters exerted by substituents on the electron acceptor and on the electron donor parts of the nitroxyl radical it is concluded that the heterolysis reaction proceeds by a push-pull mechanism and is entropy controlled.In the α-hydroxyprop-2-yl radical with substituted nitrobenzenes, the lifetimes of potential adducts of the nitroxyl type are s=2.1*103 s-1.The heterolysis reaction can also be slowed down by making the solvent less polar than water: in 95percent propan-2-ol/5percent water ks=1.5*104 s-1 for R=CN as compared to >106 s-1 in water.
- Jagannadham, V.,Steenken, S.
-
p. 6542 - 6551
(2007/10/02)
-
- Relative Electron Affinities of Substituted Nitrobenzenes in the Gas Phase
-
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.
-
p. 2993 - 2995
(2007/10/02)
-
- Reduktion der Dinitrobenzole an Kohlenstoff-Faser-Elektroden in Acetonitril ohne und mit Protonendonatoren
-
The reduction of the isomeric dinitrobenzenes at carbon fibre electrodes in acetonitrile in the absence and in the presence of proton donors is studied by means of cyclic voltammetry and constant potential electrolysis.In the absence of proton donors a di
- Jannakoudakis, P. D.,Theodoridou, E.
-
p. 167 - 180
(2007/10/02)
-