Substituent effects on the stabilities of phenoxyl radicals and the acidities of phenoxyl radical cations

Bordwell, F. G.; Cheng, Jin‐Pei

doi:10.1021/ja00005a042

Cited by 489 publications

(457 citation statements)

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“…6 ] in acetonitrile in a 1 cm pathlength cuvette. Sample 1-pink, sample 2-red; sample 3-yellow; sample 3-green; sample 4-cyan; sample 5-blue; sample 4-black.…”

Section: Absorption Spectramentioning

confidence: 99%

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Mechanism of H₂ Evolution from a Photogenerated Hydridocobaloxime

2010

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Proton transfer from the triplet excited state of brominated naphthol to a difluoroboryl bridged Co I -diglyoxime complex, forming Co III H, was monitored via transient absorption. The second-order rate constant for Co III H formation is in the range (3.5-4.7) × 10 9 M -1 s -1 , with proton transfer coupled to excited-state deactivation of the photoacid. Co III H is subsequently reduced by excess Co I -diglyoxime in solution to produce Co II H (k red ) 9.2 × 10 6 M -1 s -1 ), which is then protonated to yield Co II -diglyoxime and H 2 .Scheme 1

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“…6 ] in acetonitrile in a 1 cm pathlength cuvette. Sample 1-pink, sample 2-red; sample 3-yellow; sample 3-green; sample 4-cyan; sample 5-blue; sample 4-black.…”

Section: Absorption Spectramentioning

confidence: 99%

“…6 The pK a of 6-bromo-2-naphthol in CH 3 CN can be estimated from the DMSO value by the method of Koppel and coworkers, shown in equation 1.…”

Section: Calculations Of Ground State and Excited State Pk A Values Pmentioning

confidence: 99%

Mechanism of H₂ Evolution from a Photogenerated Hydridocobaloxime

2010

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“…S2) which is formed with a significant H/D KIE in the case of R = Br, Cl, H, CN. However, we also note that the error associated with ∆G CPET is estimated to be on the order of ±0.6 eV (see SI), hence more quantitative considerations would be inappropriate.Given the large uncertainty in ∆G CPET it appears worthwhile to consider yet another PCET reaction mechanism that would be compatible with the experimental observation of reduced ruthenium and the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 8 and *Ru(bpz) 3 2+ producing photoexcited *Ru(bpz) 2 (bpzH) 3+ can be ruled out based on energetic grounds: Based on published pK a values for the phenols and the respective ruthenium complex, 3,26,27,38,40 we estimate that this particular PT event is exergonic by more than 1.3 eV for all 6 reaction couples (see SI; estimated error is ±0.5 eV). However, there is also the possibility of an initial PT event which is coupled to electronic relaxation of the photoexcited ruthenium complex, i. e., a process leading to the Ru(bpz) 2 (bpzH) 3+ species in its electronic ground state.…”

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confidence: 99%

Kinetic Isotope Effects in Reductive Excited-State Quenching of Ru(2,2′-bipyrazine)₃²⁺ by Phenols

Bronner

Wenger

2011

J. Phys. Chem. Lett.

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Electron transfer from phenol molecules to a photoexcited ruthenium(II) complex was investigated as a function of the para-substituent (R = OCH 3 , CH 3 , H, Cl, Br, CN) attached to the phenols. For phenols with electron-donating substituents (R = OCH 3 , CH 3 ), the rate-determining excited-state deactivation process is ordinary electron transfer (ET). For all other phenols, significant kinetic isotope effects (KIEs) (ranging from 2.91 ± 0.18 for R = Br to 10.18 ± 0.64 for R = CN) are associated with emission quenching, and this is taken as indirect evidence for transfer of a phenolic proton to a peripheral nitrogen atom of a 2,2'-bipyrazine ligand in the course of an overall protoncoupled electron transfer (PCET) reaction. Possible PCET reaction mechanisms for the various phenol / ruthenium couples are discussed. While 4-cyanophenol likely reacts via concerted proton-electron transfer (CPET), a stepwise proton transfer-electron transfer mechanism cannot be excluded in the case of the phenols with R = Br, Cl, H. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 2 Table of contents graphic: Keywords: Electron transfer, proton transfer, proton-coupled electron transfer, kinetic isotope effect, luminescence The conversion of small inert molecules such as N 2 , CO 2 or H 2 O to more energy-rich species is an important target of much contemporary research, and proton-coupled electron transfer (PCET) has been recognized to play a key role in this context. 1-2 This has stimulated much experimental and theoretical work on the fundamentals of PCET. [3][4][5][6][7][8] In many investigations, kinetic isotope effects (KIEs) play a pivotal role in distinguishing between concerted proton-electron transfers (CPETs) and stepwise reaction mechanisms. [9][10][11] The majority of mechanistic investigations of PCET have focused on reactions between species in their electronic ground states, [3][4][5][6][7][8][12][13] but recently PCET reactions involving photoexcited molecules or complexes have received increasing attention. [14][15][16][17][18][19][20] In the context of our work presented herein, particularly noteworthy prior studies include the investigation of reductive quenching of the lowest triplet excited state of C 60 by hydrogen-bonded phenols, 21 and the study of a ruthenium(II) complex with a deprotonated pyridylbenzimidazole ligand as a combined electron-proton acceptor in PCET chemistry with an ubiquinol analogue. 22 Building on prior research by Meyer and coworkers on the subject of excited-state quenching by PCET, 23,24 we investigated the luminescence quenching of Ru(bpz) 3 2+ (bpz = 2,2'-bipyrazine) by 6 different phenol molecules. Meyer and coworkers demonstrated that a related ruthenium complex, 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 ...

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“…3 Consequently, much effort has been put into understanding the factors governing the O-H bond dissociation energies, O-H BDE, both in the solution and the gas phase. [4][5][6] There are many experimental studies for the determination of the O-H BDE of substituted phenols. [4][5][6] These studies were generally carried out in solutions (such as water, DMSO, etc.…”

Section: Introductionmentioning

confidence: 99%

“…), and subsequently gas-phase O-H BDE values were determined under some assumptions. 5,6 Unfortunately, the O-H BDE values obtained from different experimental studies vary in a wide range. For example, different experimental studies suggested the O-H BDE for phenol was from 83.3 kcal/mol to 89.6 kcal/mol.…”

Section: Introductionmentioning

confidence: 99%

An Accurate QSPR Study of O−H Bond Dissociation Energy in Substituted Phenols Based on Support Vector Machines

Xue

Zhang

Liu

et al. 2004

J. Chem. Inf. Comput. Sci.

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The support vector machine (SVM), as a novel type of learning machine, was used to develop a Quantitative Structure-Property Relationship (QSPR) model of the O-H bond dissociation energy (BDE) of 78 substituted phenols. The six descriptors calculated solely from the molecular structures of compounds selected by forward stepwise regression were used as inputs for the SVM model. The root-mean-square (rms) errors in BDE predictions for the training, test, and overall data sets were 3.808, 3.320, and 3.713 BDE units (kJ mol -1 ), respectively. The results obtained by Gaussian-kernel SVM were much better than those obtained by multiple linear regression, radial basis function neural networks, linear-kernel SVM, and other QSPR approaches.

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Substituent effects on the stabilities of phenoxyl radicals and the acidities of phenoxyl radical cations

Cited by 489 publications

References 4 publications

Mechanism of H₂ Evolution from a Photogenerated Hydridocobaloxime

Mechanism of H₂ Evolution from a Photogenerated Hydridocobaloxime

Kinetic Isotope Effects in Reductive Excited-State Quenching of Ru(2,2′-bipyrazine)₃²⁺ by Phenols

An Accurate QSPR Study of O−H Bond Dissociation Energy in Substituted Phenols Based on Support Vector Machines

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Substituent effects on the stabilities of phenoxyl radicals and the acidities of phenoxyl radical cations

Cited by 489 publications

References 4 publications

Mechanism of H2 Evolution from a Photogenerated Hydridocobaloxime

Mechanism of H2 Evolution from a Photogenerated Hydridocobaloxime

Kinetic Isotope Effects in Reductive Excited-State Quenching of Ru(2,2′-bipyrazine)32+ by Phenols

An Accurate QSPR Study of O−H Bond Dissociation Energy in Substituted Phenols Based on Support Vector Machines

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Mechanism of H₂ Evolution from a Photogenerated Hydridocobaloxime

Mechanism of H₂ Evolution from a Photogenerated Hydridocobaloxime

Kinetic Isotope Effects in Reductive Excited-State Quenching of Ru(2,2′-bipyrazine)₃²⁺ by Phenols