Trends in Ground-State Entropies for Transition Metal Based Hydrogen Atom Transfer Reactions

Mader, Elizabeth A.; Manner, Virginia W.; Markle, Todd F.; Wu, Adam; Franz, James A.; Mayer, James M.

doi:10.1021/ja8081846

Cited by 84 publications

(119 citation statements)

References 178 publications

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“…First, BDEs are converted to gas phase BDFEs (Eq. 4) by using S ∘ ðH • Þ ¼ 27.42 cal K −1 mol −1 and the common assumption that S ∘ ðXHÞ ∼ S ∘ ðX • Þ for organic compounds (21)(22)(23). Converting these to solution BDFEs (BDFE solv ) requires ΔG ∘ solv ðH • Þ in a given solvent (24) and the difference in the free energy of solvation of XH and X • (Eq.…”

Section: Resultsmentioning

confidence: 99%

Predicting organic hydrogen atom transfer rate constants using the Marcus cross relation

Warren

Mayer

2010

Proc. Natl. Acad. Sci. U.S.A.

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Chemical reactions that involve net hydrogen atom transfer (HAT) are ubiquitous in chemistry and biology, from the action of antioxidants to industrial and metalloenzyme catalysis. This report develops and validates a procedure to predict rate constants for HAT reactions of oxyl radicals (RO • ) in various media. Our procedure uses the Marcus cross relation (CR) and includes adjustments for solvent hydrogen-bonding effects on both the kinetics and thermodynamics of the reactions. Kinetic solvent effects (KSEs) are included by using Ingold's model, and thermodynamic solvent effects are accounted for by using an empirical model developed by Abraham. These adjustments are shown to be critical to the success of our combined model, referred to as the CR/KSE model. As an initial test of the CR/KSE model we measured self-exchange and cross rate constants in different solvents for reactions of the 2,4,6-tri-tert-butylphenoxyl radical and the hydroxylamine 2,2′-6,6′-tetramethylpiperidin-1-ol. Excellent agreement is observed between the calculated and directly determined cross rate constants. We then extend the model to over 30 known HAT reactions of oxyl radicals with OH or CH bonds, including biologically relevant reactions of ascorbate, peroxyl radicals, and α-tocopherol. The CR/KSE model shows remarkable predictive power, predicting rate constants to within a factor of 5 for almost all of the surveyed HAT reactions.free radicals | Marcus theory | proton-coupled electron transfer | reactive oxygen species | oxyl radicals H ydrogen atom transfer (HAT) reactions (Eq. 1) have been a cornerstone of organic and biological chemistry for over a century (1, 2). These reactions are key steps in many important processes, from energy conversion to the chemistry of reactive oxygen species and antioxidants (3). Thus, developing a detailed understanding of the factors that dictate HAT reactivity has long been a goal.A large number of HAT rate constants have been measured in many laboratories with various techniques (4, 5). Rates of HAT reactions have traditionally been understood by using the BellEvans-Polanyi (BEP) relation, which relates the activation energy to the enthalpic driving force (6). Whereas the BEP relation holds well within sets of similar reactions, a comprehensive theoretical understanding of HAT is still emerging (3).Our group has been exploring the use of the Marcus cross relation (CR) Eq. 2 (7), a corollary of the Marcus theory of electron transfer, as the basis for a general model for HAT rates (8)(9)(10)(11)(12). This use of the CR grew out of recognizing HAT as one class of a larger set of reactions in which one electron and one proton are transferred (, termed proton-coupled electron transfer. In this paper, we use "HAT" to refer to any reaction in which H • is transferred from a donor to an acceptor in a single kinetic step (Eq. 1), although other definitions have been used (13).The CR, with respect to HAT reactions, relates the cross rate constant (k XH∕Y• , Eq. 1) to the two degenerate self-exchange rate c...

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Section: Resultsmentioning

confidence: 99%

Predicting organic hydrogen atom transfer rate constants using the Marcus cross relation

Warren

Mayer

2010

Proc. Natl. Acad. Sci. U.S.A.

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137

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“…In addition, some of these non-heme metal couples were found to have very large ground state entropic changes, in contrast to related organic reagents. 117 These studies demonstrated that the historical use of bond dissociation enthalpies (BDEs) in rationalizing hydrogen atom transfer reactivity was not appropriate for closely related reactions of transition metal complexes. We strongly encourage workers to employ bond dissociation free energies (BDFEs) when discussing PCET reaction driving forces.…”

Section: B Non-heme Ironmentioning

confidence: 99%

Moving Protons and Electrons in Biomimetic Systems

Warren

Mayer

2015

Biochemistry

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An enormous variety of biological redox reactions are accompanied by changes in proton content at enzyme active sites, in their associated cofactors, in substrates/products, and between protein interfaces. Understanding this breadth of reactivity is an ongoing chemical challenge. A great many workers have developed and investigated biomimetic model complexes to build new ways of thinking about the mechanistic underpinnings of such complex biological proton-coupled electron transfer (PCET) reactions. Of particular importance are those model reactions that involve transfer of one proton (H+) and one electron (e–), equivalently transfer of a hydrogen atom (H•). In this Current Topic Article we review key concepts in PCET reactivity, and describe important advances in biomimetic PCET chemistry, with special emphasis on research that has enhanced efforts to understand biological PCET reactions.

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“…For instance, the cross relation predicts and explains the inverse temperature dependence of the rate of HAT from [Fe II (H 2 bip) 3 ] 2+ to the stable nitroxyl radical TEMPO • (2,2,6,6-tetramethylpiperidine-1-oxyl radical) 20. Of the various HAT reactions involving TEMPO • / TEMPO-H and transition metal complexes that we have examined,19,20,23,24 the Marcus approach appears to be least accurate for Ru II (acac) 2 (py-imH) + TEMPO • → TEMPO-H and Ru III (acac) 2 (py-im), which has a large KIE 24b. These results prompted our examination of nitroxyl/hydroxylamine self-exchange reactions; the kinetics of 4-oxo-TEMPO • plus TEMPO-H were briefly mentioned in a preliminary communication about the [Fe II (H 2 bip) 3 ] 2+ reaction 20…”

Section: Introductionmentioning

confidence: 98%

Nitroxyl Radical Plus Hydroxylamine Pseudo Self-Exchange Reactions: Tunneling in Hydrogen Atom Transfer

et al. 2009

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Bimolecular rate constants have been measured for reactions that involve hydrogen atom transfer (HAT) from hydroxylamines to nitroxyl radicals, using the stable radicals TEMPO • (2,2,6,6-tetramethylpiperidine-1-oxyl radical), 4-oxo-TEMPO • (2,2,6,6-tetramethyl-4-oxo-piperidine-1-oxyl radical), di-tert-butylnitroxyl ( t Bu 2 NO • ), and the hydroxylamines TEMPO-H, 4-oxo-TEMPO-H, 4-MeO-TEMPO-H (2,2,6,6-tetramethyl-N-hydroxy-4-methoxy-piperidine), and t Bu 2 NOH. The reactions have been monitored by UV-vis stopped-flow methods, using the different optical spectra of the nitroxyl radicals. The HAT reactions all have |∆G°| e 1.4 kcal mol -1 and therefore are close to self-exchange reactions. The reaction of 4-oxo-. Surprisingly, the rate constant for the analogous deuterium atom transfer reaction is much slower: k 2D,MeCN ) 0.44 ( 0.05 M -1 s -1 with k 2H,MeCN /k 2D,MeCN ) 23 ( 3 at 298 K. The same large kinetic isotope effect (KIE) is found in CH 2 Cl 2 , 23 ( 4, suggesting that the large KIE is not caused by solvent dynamics or hydrogen bonding to solvent. The related reaction of 4-oxo-TEMPO • with 4-MeO-TEMPO-H(D) also has a large KIE, k 3H /k 3D ) 21 ( 3 in MeCN. For these three reactions, the E aD -E aH values, between 0.3 ( 0.6 and 1.3 ( 0.6 kcal mol -1 , and the log(A H /A D ) values, between 0.5 ( 0.7 and 1.1 ( 0.6, indicate that hydrogen tunneling plays an important role. The related reaction of t Bu 2 NO • + TEMPO-H(D) in MeCN has a large KIE, 16 ( 3 in MeCN, and very unusual isotopic activation parameters, E aD -E aH ) -2.6 ( 0.4 and log(A H /A D ) ) 3.1 ( 0.6. Computational studies, using POLYRATE, also indicate substantial tunneling in the (CH 3 ) 2 NO • + (CH 3 ) 2 NOH model reaction for the experimental self-exchange processes. Additional calculations on TEMPO( • /H), t Bu 2 NO( • /H), and Ph 2 NO( • /H) self-exchange reactions reveal why the phenyl groups make the last of these reactions several orders of magnitude faster than the first two. By inference, the calculations also suggest why tunneling appears to be more important in the self-exchange reactions of dialkylhydroxylamines than of arylhydroxylamines.

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Trends in Ground-State Entropies for Transition Metal Based Hydrogen Atom Transfer Reactions

Cited by 84 publications

References 178 publications

Predicting organic hydrogen atom transfer rate constants using the Marcus cross relation

Predicting organic hydrogen atom transfer rate constants using the Marcus cross relation

Moving Protons and Electrons in Biomimetic Systems

Nitroxyl Radical Plus Hydroxylamine Pseudo Self-Exchange Reactions: Tunneling in Hydrogen Atom Transfer

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