Photo‐reductive fading of disperse azo dyes on nylon substrate: thermochemical analyses of reactions of photo‐induced radicals using the semiempirical molecular orbital method†

When the excited triplet states of disperse azo dyes with nitro groups abstract hydrogen to generate hydrazinyl (from azo groups) and nitrosyl hydroxide radicals (from nitro groups), both the radicals as H‐acceptors carry out azo scission, conversion to nitrogen dioxide via disproportionation reactions and self‐decomposition via rearrangement. A kinetic equation was formulated by the sum of these reactions, which describes the initial rates (K_PA) of reductive fading. The K_PA values were controlled by the rate constants of the reactions of hydrazinyl and nitrosyl hydroxide radicals as H‐acceptors, which were estimated by thermochemical analyses of the reactants, intermediates and end products using the semiempirical molecular orbital PM5 method, and by the concentrations of the reactants: H‐acceptors and H‐donors. The K_PA values observed for 12 dyes were explained semi‐quantitatively by multiple routes of reactions depending upon to what extent each radical reaction was thermochemically favoured.     

Solvent effects on the rates and mechanisms of reaction of phenols with free radicals

The rates of formal abstraction of phenolic hydrogen atoms by free radicals, Y* + ArOH --> YH + ArO*, are profoundly influenced by the hydrogen-bond-accepting and anion-solvation abilities of solvents, by the electron affinities and reactivities (Y-H bond dissociation enthalpies) of radicals, and by the phenol's ring substituents. These apparently simple reactions can occur by at least three different, nonexclusive mechanisms: hydrogen atom transfer, proton-coupled electron transfer, and sequential proton-loss electron transfer. The delicate balance among these mechanisms depends on both the environment and the reactants. The main features of these mechanisms are described, together with some interesting kinetic consequences.     

Studies leading to the development of a single-electron transfer (SET) photochemical strategy for syntheses of macrocyclic polyethers, polythioethers, and polyamides

Organic photochemists began to recognize in the 1970s that a new mechanistic pathway involving excited-state single-electron transfer (SET) could be used to drive unique photochemical reactions. Arnold's seminal studies demonstrated that SET photochemical reactions proceed by way of ion radical intermediates, the properties of which govern the nature of the ensuing reaction pathways. Thus, in contrast to classical photochemical reactions, SET-promoted excited-state processes are controlled by the nature and rates of secondary reactions of intermediate ion radicals. In this Account, we discuss our work in harnessing SET pathways for photochemical synthesis, focusing on the successful production of macrocyclic polyethers, polythioethers, and polyamides. One major thrust of our studies in SET photochemistry has been to develop new, efficient reactions that can be used for the preparation of important natural and non-natural substances. Our efforts with α-silyl donor-tethered phthalimides and naphthalimides have led to the discovery of efficient photochemical processes in which excited-state SET is followed by regioselective formation of carbon-centered radicals. The radical formation takes place through nucleophile-assisted desilylation of intermediate α-silyl-substituted ether-, thioether-, amine-, and amide-centered cation radicals. Early laser flash photolysis studies demonstrated that the rates of methanol- and water-promoted bimolecular desilylations of cation radicals (derived from α-silyl electron donors) exceeded the rates of other cation radical α-fragmentation processes, such as α-deprotonation. In addition, mechanistic analyses of a variety of SET-promoted photocyclization reactions of α-silyl polydonor-linked phthalimides and naphthalimides showed that the chemical and quantum efficiencies of the processes are highly dependent on the lengths and types of the chains connecting the imide acceptor and α-silyl electron donor centers. We also observed that reaction efficiencies are controlled by the rates of desilylation at the α-silyl donor cation radical moieties in intermediate zwitterionic biradicals that are formed by either direct excited-state intramolecular SET or by SET between the donor sites in the intervening chains. It is important to note that knowledge about how these factors govern product yields, regiochemical selectivities, and quantum efficiencies was crucial for the design of synthetically useful photochemical reactions of linked polydonor-acceptor substrates. The fruits of these insights are exemplified by synthetic applications in the concise preparation of cyclic peptide mimics, crown ethers and their lariat- and bis-analogs, and substances that serve as fluorescence sensors for important heavy metal cations.     

Electrochemical examination of the ascorbic acid radical anion in non-aqueous electrolytes

A quasi-reversible redox reaction involving ascorbic acid was observed in non-aqueous electrolytes at conductive diamond electrode. The chemical reversibility of these reactions is consistent with ascorbic acid being reduced to the ascorbic acid radical anion in a one-electron process, with subsequent reoxidation to ascorbic acid. This is the first report on the electrochemical production of the ascorbic acid radical anion in non-aqueous electrolytes. Ascorbyl 6-stearate and 4-hydroxy 2(5H)-furanone, which have somewhat similar structures as ascorbic acid, also showed one-electron transfer reduction reaction producing radicals with a single negative charge, suggesting that these compounds follow the same electrochemical behavior as ascorbic acid. The double bond and hydroxyl substituent on the five-membered ring are shown to be necessary for the stabilization of the radical anions. It was confirmed by the calculation of the total energy using molecular orbital methods that resonance structures involving the double-bond and hydroxyl group provide significant stabilization of the radical anions. Electrochemical preparation may be a useful method for the detailed study of radicals, their molecular structure and reactivity.     

Intramolecular 1,2- and 1,3-Hydrogen Transfer Reactions of Thiyl Radicals

This review article summarizes recent experiments on 1,2- and 1,3-hydrogen transfer reactions in thiyl radicals from cysteine and related compounds. Pulse radiolysis in combination with time-resolved UV spectroscopy was applied to monitor the equilibration of initial thiyl radicals with carbon-centered radicals at both the C_α and C_β positions of cysteine. Experiments with thiyl radicals from penicillamine and cysteamine confirmed the formation of carbon-centered radicals at these positions. Complementary evidence for the intermediary formation of carbon-centered radicals was obtained from mass spectrometry and ¹H NMR spectroscopy experiments, both of which indicated covalent H/D exchange at original C H bonds when thiyl radicals were generated in D₂O. The 1,2- and 1,3-hydrogen transfer reactions can have profound consequences for the integrity of proteins when Cys residues are oxidized to Cys thiyl radicals, which subsequently equilibrate with carbon-centered radicals.     

NIEDER- UND HOCHMOLEKULARE RADIKALKATIONEN VON CHINOXALINDERIVATEN

Quinoxaline derivatives are used as sensitizing agents in colour-photography and for photoinduced polymerization reactions. The reactivity of these molecules depends on their preference for hydrogen transfer. As models for investigation of these reactions quinoxaline radical cations were prepared, using both monomer derivatives and polymers with analoguous side chains. The radicals are generated by reduction or by UV-radiation and are characterized by their ESR-spectra and their decay during radiation.     

Electrochemical oxidation of aliphatic hydrocarbons promoted by inorganic radicals. II. NO3 radicals

The anodic electrolysis of linear alkanes in tert-butanol/H₂O solutions containing HNO₃ and saturated with oxygen at P_{O 2}=1 atm results in the partial oxidation of the hydrocarbons to the corresponding isomeric ketones. The experimental data are in support of a mechanism in which the first steps are: (1) one-electron transfer from the nitrate ion to the anode to give a nitrate radical; (2) hydrogen abstraction by the nitrate radical on the aliphatic substrate to give an alkyl radical; (3) molecular oxygen addition by the alkyl radical and formation of peroxy radicals. These species decay by bimolecular reactions and a carbonyl function is formed. The isomer distribution is in excellent accord with a statistical H abstraction from the secondary C–H bonds.     

Relationship between photofading and chemical structure of disperse azo dyes on nylon fabric†

The initial rates of photofading for 30 disperse azo dyes on nylon fabric upon exposure to a carbon arc in air have been analysed by formulating a kinetic equation that describes azo scission via the disproportionation reactions and intramolecular H‐transfer by two kinds of hydrazinyl radicals and the conversion of a nitro group to a nitroso group via the disproportionation reaction of nitrosyl hydroxide radicals. The five reaction rate constants are discussed in terms of the heats of reaction by calculating the heats of formation for the reactants, intermediates and products of each reaction using the PM5 method. Phenylazo‐ and thiazoleazo‐N,N‐substituted anilines and phenylazo‐pyridones exhibited large rate constants of multiple terms, while phenylazo‐phenols had the highest light fastness and very small rate constants for the disproportionation reactions of hydrazinyl radicals. Photofading on nylon fabric was primarily controlled by the thermal reactions of photo‐induced monohydrogenated dye radicals, which occurred via one or two primary multiple routes.     

Redox properties and bond dissociations energies of phenoxyl radicals

Reduction half-wave potentials of several phenoxyl radicals have been measured by photomodulated voltammetry in acetonitrile. Two of the investigated radicals exhibit a reversible behavior, but for the others a rather quasi-reversible nature of the heterogeneous electron transfer has to be considered. The measured reduction half-wave potentials are within 70 mV of reported values for formal one-electron potentials of the phenolate-phenoxyl couples. That is why one can assume that the values of half-wave reduction potentials are close to the standard potentials of the electrochemical reactions. Differences of the bond dissociation energy (BDE)(OH), for the substituted phenols relative to the BDE(OH) of the phenol were calculated and compared with corresponding data in the gas-phase.     

The Charge Transfer Photochemistry of Coordination Complexes in Aqueous Solution

Irradiation of charge transfer absorption bands of coordination complexes can result in net oxidation reduction or ligand exchange reactions. The photoredox processes generally involve the formation of reactive radicals in the primary photochemical act. The identification of these primary radicals is essential to the specification of the primary photochemical act since radical reactions with substrate, other radicals, etc. can make the net products and yields appear vastly different from the nature of the primary act. For example, in a few documented cases, the metal ion fragment formed in a photoredox process has characteristics of a radical and can couple with ligand radicals giving rise to a net photochemical process which appears to be ligand exchange rather than photoredox. A variety of radicals, organic as well as inorganic, can be generated and studied in aqueous solution. Some information has also been obtained concerning the nature of the photochemical precursors to the radicals formed in photoredox processes.     

Kinetics of,free radical modification of polyolefins in extruders – chain scission,crosslinking and grafting

Free radical reactions, carried out in polymer melts, have become a popular method of chemically modifying polyolefins. The elementary free-radical reactions which are relevant in the chemical modification of polyolefins at high temperatures in extruders, such as chain transfer to polymer via primary and polymer radicals, β-scission, double bond addition and bimolecular termination, affect chain scission, long chain branching, crosslinking and grafting. This brief review will discuss some of the work focused on the kinetics and mathematical modelling of these reactions.     

Short Stopping of Runaway Methyl Methacrylate Polymerizations

Polymerization reactions are generally risky reactions for two reasons. On the one hand, they are strongly exothermic reactions; and on the other hand, the viscosity of the reaction masses can strongly increase during the course of the reaction. Especially in homogeneous systems, the increase of viscosity causes a sharp decrease of the heat transfer coefficient. Moreover, there are several polymerization systems which show strongly autocatalytic behavior at higher polymer volume fractions. In the case of loss of agitation or cooling, it is necessary to bring the reaction back to safe operating conditions. The addition of a so-called stopping agent is one of the possibilities to prevent a reaction system from runaway under such circumstances. These stoppers are radical-trapping agents which can react with free radicals by formation of a terminated product with respect to the polymerization process. With respect to the free radical, inhibition and chain growth are the two reactions which compete with each other. According to the reaction rate ratio of these two reactions, these agents are classified into retarders and true inhibitors. In the case of an inhibitor the rate of inhibition is much faster than the rate of chain growth, in the case of a retarder both reaction rates are of nearly the same order of magnitude. Surveys of the literature with respect to the inhibition of polymerization reactions are given in [1,3]. The present paper deals with the inhibition of radical solution and suspension polymerizations of methyl methacrylate using 4-hydroxy-2,2,6,6-tetramethylpiperidinoxyl in an adiabatic reaction calorimeter.     

Fourier Transform EPR Measurement of Homogeneous Electron Transfer Rates

Fourier-Transform Electron Paramagnetic Resonance has measured the rates of homogeneous electron transfer reactions involving duroquinone radical anions. The radicals are generated by laser photolysis of duroquinone in methanol solution containing 10% triethylamine. The duroquinone concentration was varied over a factor of 1000. The rate constant for electron transfer between the radical anion and the neutral duroquinone is 1.5×10⁸M⁻¹s⁻¹ and the intrinsic spin relaxation rates, T⁻¹₁ and T⁻¹₂ are 0.32 MHz and 0.44 MHz, respectively. Two-Dimensional magnetization transfer spectra show that the reaction is a homogeneous electron transfer reaction. The electron transfer rates are measured by two novel pulse sequences designed for more efficient data acquisition in samples with no unresolved inhomogeneous broadening. These two sequences result in a more rapid, accurate determination of the second-order chemical rate constant since the second-order rate constant is obtained directly and is not derived from a single measurement of a pseudo-first-order rate. The transient duroquinone radical anions studied here appear to have the same T₁, T₂ and electron transfer rates as stable duroquinone radical anions in alkaline solutions.     

Electron Transfer Reactions in Pulping Systems (II): Electrochemistry of Anthraquinone/Lignin Model Quinonemethides

Abstract

Electron transfer reactions have been observed during the electrolyses of solutions containing anthraquinone and 8-aryl ether lignin model quinonemethides. In dry acetonitrile at a reduction potential of -0.9V (vs. Ag/AgCl) electrons are transferred from the electrode to anthraquinone (AQ) to form stable anthrahydroquinone radical anions (AHQ^?). The lignin model quinonemethides are not reduced directly at the electrode at this potential but are reduced by AHQ^? to give quinonemethide radical anions (QM^?) and AQ. The QM^? species rapidly fragment at their β-aryl ether bond to give phenolate ions and radicals; the latter further reduces to another phenolate ion. For example, the β-methyl lignin model QM 1 gives guaiacol and isoeugenol upon electrolysis at -0.9V in the presence of AQ. In wet acetonitrile, reduction of AQ at -0.9V leads to both anthrahydroquinone radical anion and dianion; the dianion is formed by direct electrolysis of the radical anion and by disproportlonation of the radical anion. Under all conditions and substrates examined, electron transfer reactions proceeded in preference to bond formation reactions which would generate “adducts.” The implication of these results Is that it should be possible to delignlfy wood by electron transfer reactions and that anthraquinone probably functions this way.     

Oxidation of guanine by carbonate radicals derived from photolysis of carbonatotetramminecobalt(III) complexes and the pH dependence of intrastrand DNA cross-links mediated by guanine radical reactions

The carbonate radical anion CO(3)(*-) is a decomposition product of nitrosoperoxycarbonate derived from the combination of carbon dioxide and peroxynitrite, an important biological byproduct of the inflammatory response. The selective oxidation of guanine in DNA by CO(3)(*-) radicals is known to yield spiroiminodihydantoin (Sp) and guanidinohydantoin (Gh) products, and also a novel intrastrand cross-linked product: 5'-d(CCATCG*CT*ACC), featuring a linkage between guanine C8 (G*) and thymine N3 (T*) atoms in the oligonucleotide (Crean et al., Nucleic Acids Res. 2008, 36, 742-755). Involvement of the T-N3 (pK(a) of N3-H is 9.67) suggests that the formation of 5'-d(CCATCG*CT*ACC) might be pH-dependent. This hypothesis was tested by generating CO(3)(*-) radicals through the photodissociation of carbonatotetramminecobalt(III) complexes by steady-state UV irradiation, which allowed for studies of product yields in the pH 5.0-10.0 range. The yield of 5'-d(CCATCG*CT*ACC) at pH 10.0 is approximately 45 times greater than at pH 5.0; this is consistent with the proposed mechanism, which requires N3(H) thymine proton dissociation followed by nucleophilic addition to the C8 guanine radical.     

Nitrosation of N-terminally blocked tryptophan and tryptophan-containing peptides by peroxynitrite

Tryptophan is known to be a major target of oxidative stress and to take part in electron transfer. In proteins, its fluorescence is extinguished after treatment with oxidative agents, like peroxynitrite (ONOO(-)/ONOOH) - the product of the reaction of NO* and superoxide anion (O*(2)(-)) radicals. The main reactions of N-blocked tryptophan derivatives (melatonin or N-acetyl-L-tryptophan) exposed to peroxynitrite at physiological pH are oxidation to formylkynuramine or formylkynurenine, respectively, and nitrosation, which leads to substituted 1-nitrosoindoles. Here we show that peroxynitrite-induced nitrosation is specific to N-blocked L-tryptophan derivatives and is not obtained with free L-tryptophan. Such a nitrosation can be evaluated by using 4,5-diaminofluorescein (DAF-2), which is converted to the fluorescent triazolofluorescein by NO* donors and nitrosating agents. N-acetyl-L-tryptophan was shown to be twice as efficient as melatonin in transferring NO from peroxynitrite to DAF-2. DAF-2 responses were then used to assess the ability of a series of L-tryptophan-containing peptides to give transient N-nitrosoindoles upon treatment with peroxynitrite. Many peptides proved not to be susceptible to nitrosation under these conditions. However, the N-terminally blocked peptide of endothelin-1 (Ac-Asp-Ile-Ile-Trp) reacted in a very similar fashion to melatonin; this shows that tryptophan residue nitrosation could occur when it was exposed to peroxynitrite.     

Kinetic Study of Hydroperoxide Degradation in Edible Oils Using Electron Spin Resonance Spectroscopy

Lipid oxidation is a complex phenomenon involving free radicals which are highly reactive molecular species. The life-time of these radical species is extremely short and their detection is therefore difficult. Several electron spin resonance (ESR) spectroscopy methodologies make it possible to identify, quantify and measure the reactivity of radical species formed during oxidation–reduction reactions. In this study we took advantage of the specificity of ESR spectroscopy to detect radical compounds in order to determine the rate constants of hydroperoxide degradation, a key reaction involved in lipid oxidation. The interaction of 5-doxyl stearic acid and lipid-derived radicals was studied by following the intensity of ESR spectra. A kinetic model was developed to simulate data analysis obtained by ESR and values of rate constants for hydroperoxide degradation were determined at 100 and 110 °C. This quantitative approach of ESR spectroscopy has produced useful information about new rate estimates for hydroperoxide degradation in edible oils.     

Understanding hydrogen atom transfer: from bond strengths to Marcus theory

Hydrogen atom transfer (HAT), a key step in many chemical, environmental, and biological processes, is one of the fundamental chemical reactions: A-H + B → A + H-B. Traditional HAT involves p-block radicals such as tert-BuO(?) abstracting H(?) from organic molecules. More recently, the recognition that transition metal species undergo HAT has led to a broader perspective, with HAT viewed as a type of proton-coupled electron transfer (PCET). When transition metal complexes oxidize substrates by removing H(?) (e(-) + H(+)), typically the electron transfers to the metal and the proton to a ligand. Examples with iron-imidazolinate, vanadium-oxo, and many other complexes are discussed. Although these complexes may not "look like" main group radicals, they have the same pattern of reactivity. For instance, their HAT rate constants parallel the A-H bond strengths within a series of similar reactions. Like main group radicals, they abstract H(?) much faster from O-H bonds than from C-H bonds of the same strength, showing that driving force is not the only determinant of reactivity. This Account describes our development of a conceptual framework for HAT with a Marcus theory approach. In the simplest model, the cross relation uses the self-exchange rate constants (k(AH/A) for AH + A) and the equilibrium constant to predict the rate constant for AH + B: k(AH/B) = (k(AH/A)k(BH/B)K(eq)f)(1/2). For a variety of transition metal oxidants, k(AH/B) is predicted within one or two orders of magnitude with only a few exceptions. For 36 organic reactions of oxyl radicals, k(AH/B) is predicted with an average deviation of a factor of 3.8, and within a factor of 5 for all but six of the reactions. These reactions involve both O-H or C-H bonds, occur in either water or organic solvents, and occur over a range of 10(28) in K(eq) and 10(13) in k(AH/B). The treatment of organic reactions includes the well-established kinetic solvent effect on HAT reactions. This is one of a number of secondary effects that the simple cross relation does not include, such as hydrogen tunneling and the involvement of precursor and successor complexes. This Account includes a number of case studies to illustrate these and various other issues. The success of the cross relation, despite its simplicity, shows that the Marcus approach based on free energies and intrinsic barriers captures much of the essential chemistry of HAT reactions. Among the insights derived from the analysis is that reactions correlate with free energies, not with bond enthalpies. Moreover, the radical character or spin state of an oxidant is not a primary determinant of HAT abstracting ability. The intrinsic barriers for HAT reactions can be understood, at least in part, as Marcus-type inner-sphere reorganization energies. The intrinsic barriers for diverse cross reactions are accurately obtained from the HAT self-exchange rate constants, a remarkable and unprecedented result for any type of chemical reaction other than electron transfer. The Marcus cross relation thus provides a valuable new framework for understanding and predicting HAT reactivity.     

Wasserlösliche disulfonierte aromatische Ketone — Photophsikalische und photochemische Eigenschaften

Water Soluble Disulphonated Aromatic Ketones — Photophysical and Photochemical Properties By the sulphonation of benzophenone, xanthone, thioxanthone, acridone, and methylacridone the disulphonated derivatives BPS, XS, TXS, AcS and MAcS are produced. Their disodium salts are well soluble in water. The absorption and the emission spectral data of these ketones are similar to those of the respective parent compounds. The S₁ state of BPS alone is of n — π* type, in all the other cases it possesses mainly π — π* character. The S₁ states can serve as donors in electron transfer reactions with iodonium salts. The kinetic curves of flash photolysis of these ketones in water show two decay processes, which are attributed to the triplet state and the long living ketyl radical. The triplet absorption spectra of the disulphonated ketones are very close to those of the unsubstituted ketones, but the life times of the former are much higher. The T₁ are quenched through onium salts by electron transfer reactions. Acryl amide, too, acts as a quencher for the triplets.