17217-66-2

Upstream product

• 143-33-9 sodium cyanide 
• 118-75-2 chloranil 
• 34537-54-7 2,6-dichloro-p-benzoquinone anion radical 
• 87-87-6 2,3,5,6-tetrachlorobenzene-1,4-diol 
• 67146-57-0 1,1'-dibenzyl-1,4,1',4'-tetrahydro-[4,4']bipyridinyl-3,3'-dicarboxylic acid diamide 

Downstream Products

• 670-54-2 tetracyanoethylene anion 
• 118-75-2 chloranil 
• 198-55-0 PERYLENE 
• 87-87-6 2,3,5,6-tetrachlorobenzene-1,4-diol 
• 55976-90-4 p-chloranil dianion 

Relevant academic research and scientific papers

Autooxidation of tetrachlorohydroquinone in aqueous media

The oxidation of tetrachlorohydroquinone in an aqueous solution at pH 7.40 is an autocatalytic reaction (sigmoid kinetic curves). The interaction of the tetrachloro-1,4-semiquinone radical anion with dioxygen occurs with the rate constant k2 equal to 9±3 L mol-1 s-1 (22-37 °C). Superoxide dismutase does not affect the maximum rate of tetrachlorohydroquinone oxidation.

Electron-transfer studies of a peroxide dianion

A peroxide dianion (O22-) can be isolated within the cavity of hexacarboxamide cryptand, [(O2)∪mBDCA-5t-H 6]2-, stabilized by hydrogen bonding but otherwise free of proton or metal-ion association. This feature has allowed the electron-transfer (ET) kinetics of isolated peroxide to be examined chemically and electrochemically. The ET of [(O2)∪mBDCA-5t-H6] 2- with a series of seven quinones, with reduction potentials spanning 1 V, has been examined by stopped-flow spectroscopy. The kinetics of the homogeneous ET reaction has been correlated to heterogeneous ET kinetics as measured electrochemically to provide a unified description of ET between the Butler-Volmer and Marcus models. The chemical and electrochemical oxidation kinetics together indicate that the oxidative ET of O22- occurs by an outer-sphere mechanism that exhibits significant nonadiabatic character, suggesting that the highest occupied molecular orbital of O 22- within the cryptand is sterically shielded from the oxidizing species. An understanding of the ET chemistry of a free peroxide dianion will be useful in studies of metal-air batteries and the use of [(O 2)∪mBDCA-5t-H6]2- as a chemical reagent.

A radical intermediate in the conversion of pentachlorophenol to tetrachlorohydroquinone by sphingobium chlorophenolicum

Pentachlorophenol (PCP) hydroxylase, the first enzyme in the pathway for degradation of PCP in Sphingobium chlorophenolicum, is an unusually slow flavin-dependent monooxygenase (kcat = 0.02 s-1) that converts PCP to a highly reactive product, tetrachlorobenzoquinone (TCBQ). Using stopped-flow spectroscopy, we have shown that the steps up to and including formation of TCBQ are rapid (5-30 s-1). Before products can be released from the active site, the strongly oxidizing TCBQ abstracts an electron from a donor at the active site, possibly a cysteine residue, resulting in an off-pathway diradical state that only slowly reverts to an intermediate capable of completing the catalytic cycle. TCBQ reductase, the second enzyme in the PCP degradation pathway, rescues this nonproductive complex via two fast sequential one-electron transfers. These studies demonstrate how adoption of an ancestral catalytic strategy for conversion of a substrate with different steric and electronic properties can lead to subtle yet (literally) radical changes in enzymatic reaction mechanisms.

Structure and function of quinones in biological solar energy transduction: A high-frequency d-band EPR spectroscopy study of model benzoquinones

Quinones are utilized as charge-transfer cofactors in a wide variety of reactions that are crucial for photosynthesis and respiration. In photosynthetic protein complexes, both Type I and Type II, including oxygenic and anoxygenic reaction centers contain quinone cofactors that are known to participate in electron- and proton-transfer processes. Type II reaction centers, purple bacterial reaction centers, and photosystem II utilize benzoquinone molecules, ubiquinone, and plastoquinone, respectively, to facilitate proton-coupled electron transfer reactions. Here, we report a systematic study of the principal components of the g-tensor of an extensive library of model benzosemiquinone anion radicals in both protic (2-isopropanol) and aprotic (dimethyl sulfoxide) solvents using high-frequency EPR spectroscopy. A detailed comparison of the experimental g-values of the benzosemiquinone models at D-band EPR frequency allows for the discrimination of substituent effects and solvent hydrogen bonds on the principal components of the g-tensor. Further, we compare the primary plastosemiquinone, QA-, of photosystem II with the substituent and solvent hydrogen bond effects of benzosemiquinone models in vitro. This study significantly extends the experimental basis for elucidating the role of both molecular structure and interactions with environment on the functional tuning of quinone cofactors in biological solar energy transduction.

Substituent effects in oxime radical cations. 1. Photosensitized reactions of acetophenone oximes

A variety of ortho-, meta-, and para-substituted (-H, -F, -Cl, -CF 3, -CN (meta and para only), -CH3, -OCH3, and -NO2) acetophenone oximes were synthesized and studied using laser flash photolysis (LFP) and steady-state photolysis experiments in acetonitrile with chloranil as the photosensitizer. In addition, semi-empirical (AM1) calculations were performed on the neutral species, the radical cations, and the corresponding iminoxyl radicals. The data was analyzed in terms of the electrochemical peak potentials of the oximes, the quenching rates of triplet chloranil (LFP), the calculated ionization potentials, and the measured conversions of the oximes in the steady-state photolysis experiments. Photolysis of the oximes in the presence of chloranil results in the formation of the chloranil radical anion, which reacts rapidly with the oxime radical cation to form the semiquinone radical and an iminoxyl radical. Evidence for the formation of the chloranil radical anion and the semiquinone radical was obtained from LFP studies. The measured quenching rates from the LFP studies represent the rates of electron transfer from the oximes to triplet chloranil. This data was correlated to various radical and polar substituent constants. The Hammett studies suggest that steric, polar, and radical effects are important for ortho-substituted acetophenone oximes, polar effects are important for parasubstituted oximes, and radical stabilization is more important than polar effects for the meta-substituted substrates. The calculated ionization potentials of the oximes show an excellent correlation with the measured quenching rates supporting the electron transfer pathway. On the basis of calculated charge densities, we conclude that the measured substituent effects are transition state effects rather than ground state effects. At this point all of the available data suggests that the conversion of the oximes is controlled by two energetically opposing reactions, namely oxidation of the neutral oxime, which is favorable for oximes with electron-donating substituents, and deprotonation of the oxime radical cation, which is favorable for oximes with electron-withdrawing substituents. The overall result is a reaction with little selectivity as far as substituent effects are concerned.

In Situ Electrochemical EPR Studies of Charge Transfer across the Liquid/Liquid Interface

The in situ measurement of EPR spectra of radical ions generated at the polarized liquid/liquid interface is described in relation to the 7,7,8,8-tetracyanoquinodimethane (TCNQ), 2,3,5,6-tetrachloro-p-benzoquinone (TCBQ), and 2,3,5,6-tetrafluoro-p-benzoquinone (TFBQ) radical anions and the tetrathiafulvalene (TTF) radical cation. TCNQ and TTF were chosen as model compounds with which to quantify the performance of a novel liquid/ liquid electrochemical EPR cell. The anion radical of TCNQ and the cation radical of TTF in 1,2-dichloroethane (DCE) were produced at the water interface by electron transfer from/to the aqueous-phase ferricyanide/ferrocyanide redox couple by applying a potential difference between the two phases with a four-electrode potentiostat. The EPR signal intensity (at constant magnetic field) of the resultant organic radicals was monitored during potential step experiments which indicated that the EPR data could be modeled in terms of diffusional transport. TCBQ and TFBQ were chosen as compounds to model the electron transfer behavior of biologically important quinones at the oil/water interface. The EPR and voltammetric data obtained from TCBQ/TCBQ-· and TFBQ/ TFBQ-· indicated unambiguously that the two semiquinones are stable at the DCE/water interface and do not undergo immediate protonation.

Oxidation mechanism of NAD dimer model compounds

The oxidation of a dimeric N-benzyldihydronicotinamide with various oxidants such as quinones, triphenyl carbenium ions and a triplet exited tris(bipyridine) ruthenium(II) complex occurs via initial outer-sphere electron transfer followed by fast C-C bond cleavage and second electron transfer. The kinetic studies allow the determination of the oxidation potential of this compound.

C60 as a Photocatalyst of Electron-Transfer Processes: Reactions of Triplet C60 with Chloranil, Perylene, and Tritolylamine Studied by Flash Photolysis and FT-EPR

Photoprocesses in benzonitrile solutions of C60 and chloranil (CA) have been studied by complementary techniques of nanosecond laser photolysis and Fourier transform EPR.Direct oxidation of 3C60 by CA is slow (k = (2.0 +/- 0.3)E7 M-1 s-1), consistent with the high oxidation potential of 3C60.However, the formation rate and yield of CA(1-) are much increased by addition of perylene (Pe) or tritolylamine (TTA) via the fast reactions 3C60 + Pe -> C60 + 3Pe, followed by 3Pe + CA -> Pe(1+) + Ca(1-), or 3C60 +TTA -> C60(1-) + TTA(1+), followed by C60(1-) + CA -> C60 + CA(1-).These reactions utilize the broad absorption and initial high triplet yield of C60, as well as the low oxidation potential of 3Pe or high reduction potential of 3C60, to catalyze efficient formation of CA(1-) and enhance separation of radicals.Triplet C60 also reacts with Pe by electron transfer, forming Pe(1+) and C60(1-) with rate one-third that of energy transfer.However, the Ca(1-) formed in the Pe-catalyzed reaction is strongly spin-polarized, indicating that it is formed primarily via the 3Pe pathway.The extinction coefficient of C60(1-) at 1080 nm is measured (ε = 18300 +/- 1100 M-1 cm-1) using the TTA reaction.

Chronoamperometry at Channel Electrodes. Experimental Applications of Double Electrodes

Applications of transient experiments using double-channel electrodes are reported.The current response of a downstream detector electrode to a potential step at an upstream generator electrode is found to be in good agreement with theory.Such experiments are used to (i) demonstrate that the diffusion coefficient of the electrogenerated radical anion of p-chloranil in acetonitrile solution is 0.94 x 10-5 cm2 s-1, which contrasts markedly with that of the parent material (1.75 x 10-5 cm2 s-1), and (ii) characterize the nature of electrolytic processes occuring at the surface of porous silicon electrodes.The latter are shown to be electroactive throughout the depth of the porous surface layer.