Radical pair formation due to compression-induced electron transfer in crystals of energetic salts.

The kinetics of the photochemical hydrogen abstraction in crystals of fluorene doped with acridine

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTRadical pair formation in phenyl ring containing molecules: crystal structure of [(methylthio)methyl]triphenylphosphonium chloride and ESR study of the x-irradiatedM. Geoffroy, M. V. V. S. Reddy, and G. BernardinelliCite this: J. Phys. Chem. 1986, 90, 8, 1557–1560Publication Date (Print):April 1, 1986Publication History Published online1 May 2002Published inissue 1 April 1986https://doi.org/10.1021/j100399a020RIGHTS & PERMISSIONSArticle Views37Altmetric-Citations4LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (458 KB) Supporting Info (1)»Supporting Information Supporting Information Get e-Alerts

Radical pair formation in dihalogenated benzene derivatives at 77K

The review summarises the results of studies on the redox reactions involving sterically hindered quinones and pyrocatechols (phenols) occurring in glassy and single-crystal matrices. Exposure of these systems to light results in the transfer of a hydrogen atom or an electron from the phenol to the quinone accompanied by the formation of radical pairs, viz., two paramagnetic species existing in the triplet state. Studies of the transformations in single-crystal matrices enabled separation of the mechanisms of electron and hydrogen atom transfer. Synchronous transfer of two hydrogen atoms from two sterically hindered phenol molecules to o-benzoquinone is also possible. The bibliography includes 39 references.

Bond cleavage and formation are key steps in chemistry and biochemistry. The present work investigates the generation of diphenylmethyl cations (Ph2CH+) via photoinduced bond cleavage of diphenylmethyl derivatives with a cationic or neutral leaving group. The resulting Ph2CH+ cations and its numerous derivatives serve as reference electrophiles for one of the most extensive reactivity scales covering 40 orders of magnitude. In chapter 1, the focus is on the initial bond cleavage of diphenylmethyltriphenylphosphonium ions (Ph2CH−PPh3+) exhibiting a cationic leaving group. With the help of state-of-the-art quantum chemical and quantum dynamical methods, the reaction mechanism of the bond cleavage is revealed. Using a reduced model system, the potential energy surfaces can be calculated at the ONIOM level of theory along specially designed reactive coordinates. Two competing reaction channels emerge: a homolytic one in the S1 state and a heterolytic one in the ground state. They are connected via an energetically accessible conical intersection which makes an efficient generation of the observed Ph2CH+ cations feasible. In contradiction with the experiment in polar or moderately polar solvents, quantum dynamical calculations for the isolated molecule reveal the formation of Ph2CH• radicals. While electrostatic solvent effects are negligible in this system, dynamic solvent effects emerge as being essential to explain the molecular mechanism. Two methods with increasing complexity to describe the dynamic impact of the solvent environment are developed. The first approach, the dynamic continuum ansatz, treats the environment implicitly. It uses Stokes’ law and the dynamic viscosity of the solvent in combination with quantum chemically and dynamically evaluated quantities to obtain the decelerating force exerted on the dissociating fragments. The ansatz does not require any fitting of parameters. The second method, the QD/MD approach, is based on an explicit treatment of the solvent surrounding. It combines molecular dynamics (MD) simulations of the reactant in a box of solvent molecules with quantum dynamics (QD) calculations of the reactant’s dynamics. In this way, a more detailed microscopic picture of the molecular process can be derived taking into account individual arrangements of the solvent. Both methods unveil the crucial impact of the solvent cage on the bond cleavage mechanism. It hinders the free dissociation in the S1 state and guides the molecular system to the conical intersection. QD simulations including the non-adiabatic coupling around the conical intersection show the formation of Ph2CH+ within ∼400 fs which compares well with the initial rise of the cation absorption in the experiment. Chapter 2 deals with the position of the counterion X– in the ion pairs Ph2CH−PPh3+ X–, PhCH2−PPh3+ X–, and (p-CF3-C6H4)CH2−PPh3+ X– in solution with X– being Cl–, Br–, BF4–, and SbF6–. These structures are essential to clarify the role of oxidizable counterions like e.g. Cl– during the initial bond cleavage in dichloromethane. The structures determined quantum chemically in dichloromethane show a similar counterion position than in the crystal. They are confirmed by the good accordance of the calculated and measured 1H NMR shifts. The C(α)–H···X– hydrogen bonds account for the pronounced counterion-dependent 1H NMR shifts of the C(α)–H in CD2Cl2. The strong downfield shift of the signals increases according to SbF6– < BF4– << Br– < Cl–. The last part (chapter 3) focuses on the secondary processes within a few picoseconds to several nanoseconds after the C-Cl bond cleavage in diphenylmethylchloride in solution. Initially, the neutral leaving group Cl leads mainly to the formation of radical pairs; only a minor fraction of ion pairs is generated in the beginning. A combined Marcus-Smoluchowski model is used to simulate the interplay between geminate recombination, diffusional separation, and electron transfer of the radical and ion pair populations. The distance-dependent rates of the three processes together with broad distance-dependent population distributions faithfully reproduce the spectroscopically observed dynamics. The majority of Ph2CH+ cations is generated via electron transfer from the radical pairs. The detailed understanding of the secondary processes shows that a high Ph2CH+ cation yield can be expected if the radicals within a pair stay nearby for a long time to achieve an efficient electron transfer and if the resulting ions are separated fast to prevent geminate recombination.

In order to check the validity of several basic assumptions of protein photochemically induced nuclear polarization (protein photo-CIDNP), we have investigated the quenching processes of the dye triplets by the side chains of tyrosine, histidine, and tryptophan in a variety of molecular systems and environments. The quenching (H atom or electron transfer) is the generating process of the triplet electron-spin-correlated radical pair, the evolution of which gives rise to nuclear polarization. At pH 7 the quenching of 10-(carboxyethyl)flavin triplets by tyrosine and tryptophan is almost diffusion controlled. Quenching by histidine is slower. We have also investigated the slow quenching (by electron transfer) by the side chains of methionine and could show that quenching by cysteine S derivatives is negligible. Quenching by tyrosine and histidine peptides and by the tyrosines of the pancreatic trypsin inhibitor protein is slightly slower than by free side chains. Quenching is strongly viscosity controlled, to be expected of a process requiring bimolecular contact. Reactivity trends at high viscosities resemble those observed in fluid aqueous solutions. Activation energies of quenching by tyrosine, tryptophan, and histidine are similar. No difference could be detected in the mechanism of quenching by these side chains. No fast static quenching was observed that could compete with the diffusional process.

Rose-bengal-photosensitized formation of hydrogen peroxide and hydroxyl radicals

Exact analytical expressions for the evolution of a non-diffusing spin-correlated radical pair in a strong magnetic field are presented. The recombination of singlet and triplet pairs at different rates, and the coherent evolution caused by magnetic interactions, lead to radical pair concentrations which are the sum of four time-dependent terms : two decaying exponentials and two exponentially damped oscillations. The predicted time dependence is used to illuminate the recombination dynamics of the primary radical pair in photosynthetic bacterial reaction centres. Although mono-exponential decay is expected ˜ 20 ns after the formation of each individual radical pair, the inhomogeneous distribution of Larmor frequencies of the two electron spins leads to non-exponential kinetics and different time constants for the disappearance of singlet and triplet pairs, and the formation of singlet and triplet recombination products.

ESR studies have been made with single crystals of sodium 1-oximinopropionate irradiated at −196°C and at room temperature. The principal values and axes of the g and hyperfine tensors of the trapped radicals have been determined. The analysis of the observed data has indicated that the σ-electron radicals CH3C(COO−)=NO are trapped at −196°C and at room temperature. It has been found that the electronic structures of the trapped radicals at these two temperatures are essentially the same, but that the environments of the radicals are very different from each other. After the crystal had been heated at 50°C for a week, the σ-electron radical vanished and the π-electron radicals, CH3C(OH)(COO−)NHO· and CH3CR’-(COO−)NRO·, appeared. The former may be formed in the solid state by adding a crystal water molecule to the unsaturated radical. The trapped radicals in irradiated single crystals of potassium 1-oximinopropionate were also studied at room temperature and after having been heated at 50°C. They are the σ-electron radical and the π-electron radical respectively, with structures similar to those of the sodium salt. The radical pairs have been observed in irradiated single crystals of the sodium salt at −196°C.

Deposition of radiation energy in solids as visualized by the distribution, structure and properties of alkyl radicals in γ-irradiated n-alkane single crystals

The photosensitized monomerization of the cyclobutane dimers of 1,3-dimethylthymine by p-chloranil was investigated by means of steady-state irradiation and laser-flash photolysis. Quantum yields for the monomerization are 0.34 for the cis,syn dimer, 0.39 for the trans,syn dimer, and much less than 10(-2) for the cis,anti isomer. Formation of the chloranil anion radical associated with quenching of triplet chloranil by the dimers demonstrates that electron transfer from dimers to triplet chloranil occurs to initiate the monomerization. Kinetic analysis suggested that the syn-dimer cation radicals undergo the ring cleavage at greater than or equal to 10(9) s-1 before escaping from the solvent cage, while the reactivity of the anti-dimer cation radical is very low. The different reactivities of the syn and anti dimer cation radicals are discussed in terms of through-bond coupling between the n orbitals of N(1) and N(1') involving the cyclobutane-ring sigma orbitals. In the cases of the syn-dimers, the sensitizer-dimer ion-radical pairs undergo the rapid geminate recombination that works as a major energy dissipating channel responsible for the lower-than-unity quantum yields. It has been found that the presence of Mg(ClO4)2 at 0.1 M enhances approximately 1.5 times either the monomerization of the syn dimers or the formation of the chloranil anion radical. A laser-flash photolysis study shows that Mg2+ forms a complex with either the triplet or the anion radical of chloranil. The net salt effects are attributed to the retardation of the rapid geminate recombination by the participation of Mg2+ in the sensitizer-dimer ion-radical pairs.

Within the framework of the radical pair mechanism, magnetic fields may alter the rate and yields of chemical reactions involving spin-correlated radical pairs as intermediates. Such effects have been studied in detail in a variety of chemical systems both experimentally and theoretically. In recent years, there has been growing interest in whether such magnetic field effects (MFEs) also occur in biological systems, a question driven most notably by the increasing body of evidence for the involvement of such effects in the magnetic compass sense of animals. The blue-light photoreceptor cryptochrome is placed at the centre of this debate and photoexcitation of its bound flavin cofactor has indeed been shown to result in the formation of radical pairs. Here, we review studies of MFEs on free flavins in model systems as well as in blue-light photoreceptor proteins and discuss the properties that are crucial in determining the magnetosensitivity of these systems.

Multi-step electron transfer is increasingly recognized as a means for moving charge in biological systems over long distances rapidly. Many postulated multi-step mechanisms rely on the formation of organic radicals (amino-acid radicals, nucleobase radicals) as intermediate electron or ?hole? carriers. In this thesis, the multi-step mechanism for electron transfer in both proteins and DNA is investigated. These two systems form a natural complement; the role of electron transfer in DNA with regard to lesion repair is still unknown, as are the electron transfer events in the proteins that mediate the repair. Rhenium-labeled mutant Pseudomonas aeruginosa azurins serve as model systems for this phenomenon in proteins. The photo-active rhenium label in these azurins can be oxidized by a flash/quench reaction to provide a potent oxidant capable of generating a variety of radicals in the protein matrix. Three mutants (with one tryptophan residue each) were constructed to investigate the effect of tryptophan radicals on charge transfer in proteins. The properties of tryptophan radicals in three protein environments have been investigated; including a kinetically stable tryptophan radical that persists for more than 5 hrs at room temperature. The variation in these radicals plays a significant role in their effect on the oxidation of the remote copper center in azurin. The stable radical greatly reduces the rate of electron transfer from copper relative to the rhenium-labeled wild-type analogs, while another radical plays no role in copper oxidation. In order to examine multi-step electron transfer in DNA, a series of photo-active ruthenium and rhenium-thymine complexes were constructed. By attaching the metal complexes at the sugar of the deoxyribonucleic acid, they were incorporated into DNA strands by solid-state synthetic techniques. Two different ruthenium-labeled DNA strands were produced in this way; one with a single guanine base and one with two side by side guanine bases. The strand containing a guanine-guanine sequence showed formation of a guanine radical by EPR under flash/quench conditions, while the strand containing a single guanine remained EPR silent. These strands represent an excellent template to examine a system which may or may not exhibit multi-step charge transfer.

The bimolecular electron transfer from p‐amino benzhydrazide (PABH) and its meta‐derivative (m‐amino benzhydrazide (MABH)) to specific one‐electron oxidant results in the formation of anilino radical. In case of PABH, reaction with •OH radicals results in the synchronous formation of adduct as well as anilino radical. The interesting observation is that the yield of the anilino radical increases with increase in pH. The effect of substitution also has a significant effect on the formation of adduct. In case of MABH, significant yield of anilino radical gets form on reaction with •OH radical. The free radical electron transfer from PABH and MABH to parent radical cation of non‐polar solvent also results in the formation of anilino radical only suggesting that the radical cation of PABH and MABH has short life time. The above results were supported by quantum chemical calculations. Copyright © 2013 John Wiley & Sons, Ltd.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTEnergetics of formation for conjugate xanthyl carbenium ions, carbanions, and radicals by hydride, proton, and electron transfer in solution and their reactions to give symmetrical bixanthylsEdward M. Arnett, Robert A. Flowers II, Alison E. Meekhof, and Leanne MillerCite this: J. Am. Chem. Soc. 1993, 115, 26, 12603–12604Publication Date (Print):December 1, 1993Publication History Published online1 May 2002Published inissue 1 December 1993https://doi.org/10.1021/ja00079a057RIGHTS & PERMISSIONSArticle Views240Altmetric-Citations20LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (308 KB) Get e-Alerts Get e-Alerts