Evidence for Inverted Region Behavior in Proton Transfer to Carbanions

Andrieux, C.P.; Gamby, Jean; Hapiot, Philippe; Savéant, Jean‐Michel

doi:10.1021/ja035268f

Cited by 48 publications

(70 citation statements)

References 66 publications

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“…In analogy to electron transfer reactions [37], inverted regime behavior would be the eventual decrease of the rate constant with increasingly negative reaction free energy DG RXN . Such behavior has been reported experimentally [66,67]; indeed agreement has been achieved [66] between the experimental findings and certain of the tunneling rate constant equations given in Sec. 10.3, for both the transfer of a proton and a deuteron.…”

Section: Discussionsupporting

confidence: 80%

Theoretical Aspects of Proton Transfer Reactions in a Polar Environment

Kiefer¹,

Hynes²

2006

Hydrogen‐Transfer Reactions

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OverviewThis chapter reviews some nontraditional theoretical views developed in this group on acid-base proton transfer reactions. Key ingredients in this picture are a completely quantum character for the proton motion, the identification of a solvent coordinate as the reaction coordinate, and attention to the hydrogen (H-) bond vibrational mode in the acid-base complex. Attention is also given to the electronic structure rearrangements associated with proton transfer. A general overview is presented for the rate constants, including the activation free energy, as well as for the associated primary kinetic isotope effects, for both the proton adiabatic (quantum but nontunneling) and nonadiabatic (tunneling) regimes. While the focus is on ground electronic state proton transfers, some aspects of excited state proton transfers are also discussed.

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Section: Discussionsupporting

confidence: 80%

Theoretical Aspects of Proton Transfer Reactions in a Polar Environment

Kiefer¹,

Hynes²

2006

Hydrogen‐Transfer Reactions

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“…Variations in λ and ∆S°will produce scatter. Much smoother free energy correlations were obtained when a series of acids were used to protonate a common aryl anion 4 or carbanion, 6 avoiding the larger changes in λ for various aryls. As has been explained by Eigen 30 and Marcus, 8 for the proton transfer reactions between O and C atoms, a carbon acid or base contributes the most to λ; the present work changes the carbon base, Ar -• .…”

Section: Discussionmentioning

confidence: 99%

Rate and Driving Force for Protonation of Aryl Radical Anions in Ethanol

et al. 2007

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Aryl radical anions created in liquid alcohols decay on the microsecond time scale by transfer of protons from the solvent. This paper reports a 4.5 decade range of rate constants for proton transfer from a single weak acid, ethanol, to a series of unsubstituted aryl radical anions, Ar-*. The rate constants correlate with free energy change, DeltaG(o), despite wide variations in the two factors that contribute to DeltaG(o): (a) the reduction potentials of the aryls and (b) the Ar-H* bond strengths in the product radicals. For aryl radical anions containing CH2OH substituents, such as 2,2'-biphenyldimethanol*- which is protonated with a rate constant of 3x10(9) s(-1), the faster rates do not fit well in the free energy correlation, suggesting a change in mechanism.

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“…As mentioned above, the inverted region behavior has been observed experimentally for PT reactions 3-5. This discrepancy between theory and experiment has been denoted a “conundrum” in the literature 6.…”

mentioning

confidence: 89%

Driving Force Dependence of Rates for Nonadiabatic Proton and Proton-Coupled Electron Transfer: Conditions for Inverted Region Behavior

2009

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The driving force dependence of the rate constants for nonadiabatic electron transfer (ET), proton transfer (PT), and proton-coupled electron transfer (PCET) reactions are examined. Inverted region behavior, where the rate constant decreases as the reaction becomes more exoergic (i.e., as ΔG 0 becomes more negative), has been observed experimentally for ET and PT. This behavior was predicted theoretically for ET but is not well understood for PT and PCET. The objective of this Letter is to predict the experimental conditions that could lead to observation of inverted region behavior for PT and PCET. The driving force dependence of the rate constant is qualitatively different for PT and PCET than for ET because of the high proton vibrational frequency and substantial shift between the reactant and product proton vibrational wavefunctions. As a result, inverted region behavior is predicted to be experimentally inaccessible for PT and PCET if only the driving force is varied. This behavior may be observed for PT over a limited range of rates and driving forces if the solvent reorganization energy is low enough to cause observable oscillations. Moreover, this behavior may be observed for PT or PCET if the proton donor-acceptor distance increases as ΔG 0 becomes more negative. Thus, a plausible explanation for experimentally observed inverted region behavior for PT or PCET is that varying the driving force also impacts other properties of the system, such as the proton donor-acceptor distance.According to standard Marcus theory of nonadiabatic electron transfer (ET), 1 the dependence of the logarithm of the ET rate constant on the driving force ΔG 0 is represented by an inverted parabola. The maximum rate corresponds to the activationless regime with -ΔG 0 = λ, and the inverted region is defined as -ΔG 0 = λ, where λ is the reorganization energy of the reaction. The existence of the inverted region, where the ET rate constant decreases as the reaction becomes more exoergic (i.e., as ΔG 0 becomes more negative), has been confirmed experimentally. 1,2 The inverted region behavior has also been experimentally observed for proton transfer (PT) reactions, 3-5 although the theoretical basis for these experimental observations has not been well understood. 6 To our knowledge, the inverted region behavior has not yet been experimentally observed for proton-coupled electron transfer (PCET) reactions, in which an electron and proton transfer simultaneously with no stable intermediate. [7][8][9] The objectives of this Letter are to elucidate the fundamental differences in the driving shs@chem.psu.edu. Supporting Information Available: Driving force dependence of the nonadiabatic rate constant with λ = 20 kcal/mol and (a) ω = 400 cm −1 and δx = 0.5 Å, (b) ω = 3000 cm −1 and δx = 0.1 Å; driving force dependence of the nonadiabatic rate constant with two uncoupled modes, whereλ = 20 kcal/mol, ω = 3000 cm −1 and δx = 0.5 Å for the first mode, and ω = 400 cm −1 and δx = 0.1 Å for the second mode; illustration of proton vibra...

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Evidence for Inverted Region Behavior in Proton Transfer to Carbanions

Cited by 48 publications

References 66 publications

Theoretical Aspects of Proton Transfer Reactions in a Polar Environment

Theoretical Aspects of Proton Transfer Reactions in a Polar Environment

Rate and Driving Force for Protonation of Aryl Radical Anions in Ethanol

Driving Force Dependence of Rates for Nonadiabatic Proton and Proton-Coupled Electron Transfer: Conditions for Inverted Region Behavior

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