- Theory of Intramolecular Electron Transfer Reactions in Anion Radicals of Nitrobenzyl Halides
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Recently, we have suggested a lattice-based theory of chemical reactivity in which quantum and statistical mechanical arguments are combined to yield a method for calculating rate constants for certain classes of reactions.We present here the first concrete application of this theory and calculate the first-order rate constants for dehalogenation of the ortho-, meta-, and para-substituted nitrobenzyl chloride anion radicals.Our theoretical results are obtained by calculating the ? orbital spin populations of the anion radicals (MO calculations using the GAUSSIAN 82 series of programs), using the consequent values to distinguish the ortho, meta, and para sites, and then calculating the rate constants using a lattice statistical approach, the latter based on a theory presented recently.Apart from the limitations of the MO calculations and the neglect of solvent effects, the method involves no further assumptions or parameters.The theoretical estimates are in good agreement with the experimental rate constants.
- Miller, Kristine E.,Kozak, John J.
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- Kinetic Study of the Phthalimide N-Oxyl Radical in Acetic Acid. Hydrogen Abstraction from Substituted Toluenes, Benzaldehydes, and Benzyl Alcohols
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The phthalimide N-oxyl (PINO) radical was generated by the oxidation of N-hydroxyphthalimide (NHPI) with Pb(OAc)4 in acetic acid. The molar absorptivity of PINO. is 1.36 × 103 L mol -1 cm-1 at λmax 382 nm. The PINO radical decomposes slowly with a second-order rate constant of 0.6 ± 0.1 L mol-1 s-1 at 25°C. The reactions of PINO . with substituted toluenes, benzaldehydes, and benzyl alcohols were investigated under an argon atmosphere. The second-order rate constants were correlated by means of a Hammett analysis. The reactions with toluenes and benzyl alcohols have better correlations with σ+ (ρ = -1.3 and -0.41), and the reaction with benzaldehydes correlates better with σ (ρ = -0.91). The kinetic isotope effect was also studied and significantly large values of kH/kD were obtained: 25.0 (p-xylene), 27. 1 (toluene), 27.5 (benzaldehyde), and 16.9 (benzyl alcohol) at 25°C. From the Arrhenius plot for the reactions with p-xylene and p-xylene-d10, the difference of the activation energies, EaD - E aH, was 12.6 ± 0.8 kJ mol-1 and the ratio of preexponential factors, AH/AD, was 0.17 ± 0.05. These findings indicate that quantum mechanical tunneling plays an important role in these reactions.
- Koshino, Nobuyoshi,Saha, Basudeb,Espenson, James H.
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p. 9364 - 9370
(2007/10/03)
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- Secondary α-Deuterium Kinetic Isotope Effects Signifying a Polar Transition State in the Thermolysis of Ring-Substituted tert-Butyl Phenylperacetates
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Several ring-substituted tert-butyl phenylperacetates (YC6H4CH2CO3But) and their deuterated versions (YC6H4CD2CO3But) were prepared (Y: p-OCH3, p-CH3, p-H, and p-NO2). Thermolyses at 80°C in CDCl3 showed excellent first-order kinetics. The rates have been measured as kYH × 104 and kYD × 104 s-1: 11.9 and 9.20 (p-OCH3), 2.64 and 2.22 (p-CH3), 1.06 and 0.93 (p-H), 0.164 and 0.156 (p-NO2). Hammett correlations were derived to yield ρYH+ = -1.17 and ρYD+ = -1.12. However, better Hammett plots were obtained with three points (p-OCH3, p-CH3, and p-H) showing ρYH+ = -1.35 and ρYD+ = -1.28. SDKIE was calculated as 1.293 (p-OCH3), 1.189 (p-CH3), 1.140 (p-H), and 1.051 (p-NO2), showing substantial substituent effects. Values of kYH/kYD for p-NO2 showed little temperature dependence. Hammett correlations and SDKIE were derived from the same kinetic entity that is the bond cleavage.
- Kim, Sung Soo,Tuchkin, Alexey
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p. 3821 - 3824
(2007/10/03)
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- Evaluation of dissociation energies of S-H bonds in thiophenols and thioalcohols on the basis of kinetic measurements
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Kinetic data on the reactions of alkyl and benzyl radicals with thiophenol C6H5SH were analyzed within the framework of the parabolic model of transition state. The values of the parameter that establishes a correlation between the activation energy of a reaction and its enthalpy were calculated for reactions of alkyl and benzyl radicals with the C6H5SH. The equations of the parabolic model were used to calculate the bond dissociation energies for 11 thiophenols and 4 thioalcohols. The activation energies for reactions of 12 thiophenoxy radicals with cumene and of C6H5S? radical with several alkyl-aromatic hydrocarbons were obtained.
- Denisov
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p. 238 - 241
(2007/10/03)
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- Rapid Cleavage Reactions of Haloaromatic Radical Anions Measured with Fast-Scan Cyclic Voltammetry
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Cyclic voltammetry at scan rates from 10 mV/s to 1E6 V/s has been used to characterize the reduction of aryl and benzyl halides in acetonitrile solutions containing 0.6 M tetraethylammonium perchlorate.The use of very rapid scan rates is made possible with electrodes of micrometer dimensions.The kinetics and mechanism of the reduction process have been evaluated by digital simulation of the recorded voltammograms.The radical anion generated at the electrode surface is found to have a half-life ranging from less than 100 ns in the case of 4'-bromoacetophenone to 70 ms for the case of m-nitrobenzyl chloride.The reduction mechanism for the aryl halides is consistent with the ECE-DISP1 mechanism.Thus following initial one-electron reduction, halogen bond cleavage occurs resulting in a radical that is subsequently reduced.For the compounds with relatively long half-lives (>1ms) the results in thus work are in good agreement with prior investigations with cyclic voltammetry.Good agreement is also obtained with compounds that have a short half-life ( ca. 1 μs) which have been characterized by the technique of redox catalysis.However, the measured rate constants differ with previously reported values which were estimated or measured at the extreme time limits of classical electrochemical techniques.
- Wipf, David O.,Wightman, R. Mark
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p. 4286 - 4291
(2007/10/02)
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- Kinetic Study for Reactions of Nitrate Radical (NO3.) with Substituted Toluenes in Acetonitrile Solution
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The absolute rate constants for the reactions of the nitrate radical (NO3.) with substituted toluenes in acetonitrile have been determined by the flash photolysis method.From the plots of the rate constants against the ionization energies, it was revealed that the reaction path for toluene derivatives with low ionization energies is different from that for toluene derivatives with high ionization energies.For toluene, a deuterium isotope effect was observed to be ca. 1.6, suggesting the direct hydrogen atom abstraction reaction; in this group, xylenes and p-chlorotoluene belong.For toluene derivatives with electron-withdrawing substit uents, NO3. may add to the phenyl rings followed by successive reactions.For both groups, linear correlations against ionization energies with negative slopes show that NO3. is highly electrophilic and that strong polar effects exist in the transition states of both reactions.For toluenes with methoxy groups, the electron-transfer reaction from methoxytoluene to NO3. is a main initial path, since the transient absorption band due to the cation radical of methoxytoluene was detected.
- Ito, Osamu,Akhido, Seiji,Iino, Masashi
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p. 2436 - 2440
(2007/10/02)
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- Temperature Effects on Rates of Dehalogenation of Aromatic Anion Radicals
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The temperature dependence of the unimolecular dehalogenation of radical anions of nitrobenzyl halides and haloacetophenones was measured between -7 and 70 deg C.Activation parameters range from Ea=11.2-16.9 kcal/mol and log A=12.7-17.1.Both Ea and log A increase from p- to o-nitro radicals and from chloro to bromo radicals.Unfavorable steric effects that move the halogen atom out of the aromatic plane result in lowered A factors.In general, the variation of k294 with structure depends in a complex way on the combination of Ea and log A factors, which suggests caution in the evaluation of rate constants at one temperature.The fast unimolecular dissociation of (p-NO2C6H4CH2Br)(1-). (k294=4.6*105 s-1) allows measurement of the slower bimolecular electron transfer (C6H5NO2)(1-).+p-NO2C6H4CH2Br->(p-NO2C6H4CH2Br)(1-).+C6H5NO2 (k294=1.9*106 M-1 s-1).Both the activation energy and probability factor contribute to the slow rate, possibly due to a geometry change upon the reduction of ArNO2.Extending the temperature studies to supercooled solutions shows no discontinuity of the unimolecular rate constants near the phase transition temperatures.
- Meot-Ner (Mautner), M.,Neta, P.,Norris, Robert K.,Wilson, Karen
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p. 168 - 173
(2007/10/02)
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- INTRAMOLECULAR ELECTRON TRANSFER AND DEHALOGENATION OF NITROAROMATIC ANION RADICALS.
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A series of nitroaromatic compounds, containing Cl, Br or tosyl groups at various positions, were synthesized and studied by pulse radiolysis in aqueous alcohol solutions. One-electron reduction of the compounds produces the anion radicals which then undergo an intramolecular electron transfer and eliminate X** minus (Cl** minus , Br** minus , or TsO** minus ). The rates of X** minus elimination vary over six orders of magnitude and are affected by the C-X bond dissociation energies, the size and nature of the group bridging the X with the pi system, and the relative positions of these groups. Intramolecular electron transfer through space is also demonstrated. Refs.
- Bays,Blumer,Baral-Tosh,Behar,Neta
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p. 320 - 324
(2007/10/02)
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- Intramolecular Electron Transfer in the Anion Radicals of Nitrobenzyl Halides
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One-electron reduction of nitrobenzyl halides produces the anion radicals which subsequently undergo intramolecular electron transfer and decompose into nitrobenzyl radicals and halide ions.The optical absorption spectra of the initial anion radicals (λmax ca. 300-310 nm) and the subsequently formed nitrobenzyl radicals (λmax = 359 and 400 for the para and ortho, respectively) are quite intense (ε ca. 104 M-1 cm-1 in most cases) and significantly different.This enables identification of the various species and measurement of the rates of intramolecular electron transfer or C-X bond scission.The rates are 4*103, 1.7*105, and 5.7*105 s-1 for p-nitrobenzyl chloride, bromide, and iodide, respectively.The ortho derivatives decomposed nearly twice as rapidly while the meta decomposed much more slowly.The anion radical of p-nitrobenzyl bromide has pKa = 2.8, and the protonated form is found to undergo the intramolecular transfer ca. 60 times more slowly than the anion radical.The pattern of reactivity of the various anion radicals is rationalized in terms of spin density and charge distribution at the various positions on the ring and in terms of the electrophilicities of the halogens.
- Neta, P.,Behar, D.
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p. 4798 - 4802
(2007/10/02)
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- Proton-Transfer and Electron-Transfer Processes in Reaction of p-Nitrotoluene with Bases. A Spectrophotometric Study
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The interaction of p-nitrotoluene (PNT) with a variety of basic systems has been studied spectrophotometrically.The bases employed, in decreasing order of base strength, were potassium hydride, dimsyl potassium, triphenylmethylpotassium, fluorenylpotassium, and potassium p-nitroanilide, while the solvents used were dimethyl sulfoxide, tetrahydrofuran, and dimethoxyethane, with 18-crown-6 ether present in the latter two cases.The major species produced in the reactions were p-nitrobenzyl anion (PNT-), p-nitrotoluene radical anion (PNT-.), and p,p'-dinitrostilbene radical anion (DNS-.).The principal processes in the PNT-base systems are believed to be (i) PNT + B- PNT- + BH; (ii) PNT + B- PNT-. + B.; (iii) PNT- + PNT PNT. + PNT-..DNS-. is formed through secondary processes via p,p'-dinitrobibenzyl.Discussion of process (i) is given in terms of the relative pKas of PNT and the respective BH species, while (ii) is analysed on the basis of the reduction potential of PNT and the oxidation potential of B-.For the case of the PNT/Ph3C- system, the electron-transfer process (ii) is kinetically preferred but this equilibrium is rapidly established.On the other hand the proton-transfer process (i) is greatly favored thermodynamically and is effectively irreversible; as a result the proton transfer predominates.This is in accord with observation of PNT- as the initially formed species in this system.The processes occurring in the other base systems are similarly analysed.The assignment of the absorption due to the p-nitrobenzyl anion is at variance with an earlier literature report deduced from examination of the PNT/t-BuOK/t-BuOH system.Electron-transfer and proton-transfer processes in the PNT/t-BuO- system are also discussed.
- Buncel, E.,Menon, B. C.
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p. 3499 - 3507
(2007/10/02)
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