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We have investigated the cannibalistic self‐trapping reaction of an ortho‐benzyne derivative generated from 1,11‐bis(p‐tolyl)undeca‐1,3,8,10‐tetrayne in an HDDA reaction. Without adding any specific trapping agent, the highly reactive benzyne is trapped by another bisdiyne molecule in at least three different modes. We have isolated and characterized the resulting products and performed high‐level calculations concerning the reaction mechanism. During the cannibalistic self‐trapping process, either a C≡C triple bond or an sp–sp3 C−C single bond is cleaved. Up to seven rings and nine C−C bonds are formed starting from two 1,11‐bis(p‐tolyl)undeca‐1,3,8,10‐tetrayne molecules. Our experiments and calculations provide considerable insight into the variety of reaction pathways which the ortho‐benzyne derivative, generated from a bisdiyne, can take when reacting with another bisdiyne molecule., Go for the throat: The benzyne derivative generated from a linked bisdiyne in a hexadehydro‐Diels–Alder reaction undergoes cannibalistic self‐trapping by at least three different pathways. Three highly conjugated products resulting from self‐trapping were isolated, fully characterized and their photophysical properties examined. The mechanisms leading to their formation were investigated by high‐level calculations.

Spiro-substituted nitroxyl biradicals are widely used as reagents for dynamic nuclear polarization (DNP), which is especially important for biopolymer research. The main criterion for their applicability as polarizing agents is the value of the spin–spin exchange interaction parameter (J), which can vary considerably when different couplers are employed that link the radical moieties. This paper describes a study on biradicals, with a ferrocene-1,1′-diyl-substituted 1,3-diazetidine-2,4-diimine coupler, that have never been used before as DNP agents. We observed a substantial difference in the temperature dependence between Electron Paramagnetic Resonance (EPR) spectra of biradicals carrying either methyl or spirocyclohexane substituents and explain the difference using Density Functional Theory (DFT) calculation results. It was shown that the replacement of methyl groups by spirocycles near the N-O group leads to an increase in the contribution of conformers having J ≈ 0. The DNP gain observed for the biradicals with methyl substituents is three times higher than that for the spiro-substituted nitroxyl biradicals and is inversely proportional to the contribution of biradicals manifesting the negligible exchange interaction. The effects of nucleophiles and substituents in the nitroxide biradicals on the ring-opening reaction of 1,3-diazetidine and the influence of the ring opening on the exchange interaction were also investigated. It was found that in contrast to the methyl-substituted nitroxide biradical (where we observed the ring-opening reaction upon the addition of amines), the ring opening does not occur in the spiro-substituted biradical owing to a steric barrier created by the bulky cyclohexyl substituents.

The redox behaviors of macrocyclic molecules with an entirely π-conjugated system are of interest due to their unique optical, electronic, and magnetic properties. In this study, defect-free cyclic P3HT with a degree of polymerization (DPn) from 14 to 43 was synthesized based on our previously established method, and its unique redox behaviors arising from the cyclic topology were investigated. Cyclic voltammetry (CV) showed that the HOMO level of cyclic P3HT decreases from –4.86 eV (14 mer) to –4.89 eV (43 mer), in contrast to the linear counterparts increasing from –4.94 eV (14 mer) to –4.91 eV (43 mer). During the CV measurement, linear P3HT suffered from electro-oxidation at the chain ends, while cyclic P3HT was stable. ESR and UV–Vis–NIR spectroscopy suggested that cyclic P3HT has stronger dicationic properties due to the interactions between the polarons. On the other hand, linear P3HT showed characteristics of polaron pairs with multiple isolated polarons. Moreover, the dicationic properties of cyclic P3HT were more pronounced for the smaller macrocycles.

The one‐electron reduction of a cyclic (alkyl)(amino)carbene (CAAC)‐stabilized arylborylene carbonyl complex yields a dimeric borylketyl radical anion, resulting from an intramolecular aryl migration to the CO carbon atom. Computational analyses support the existence of a [(CAAC)B(CO)Ar].− radical anion intermediate. Further reduction leads to a highly nucleophilic dianionic (boraneylidene)methanolate., The one‐electron reduction of a cyclic (alkyl)(amino)carbene (CAAC)‐stabilized borylene carbonyl complex yields a dimeric borylketyl radical anion, resulting from an intramolecular aryl migration to the CO carbon atom. Computational analyses support the existence of a [(CAAC)B(CO)Ar].− radical anion intermediate. Further reduction leads to a highly nucleophilic dianionic (boraneylidene)methanolate.

Presented here is a comprehensive study of highly oxidized multiple‐decker complexes composed of TbIII and CdII ions and two to five phthalocyaninato ligands, which are stabilized by electron‐donating n‐butoxy groups. From X‐ray structural analyses, all the complexes become axially compressed upon ligand oxidation, resulting in bowl‐shaped distortions of the ligands. In addition, unusual coexistence of square antiprism and square prism geometries around metal ions was observed in +4e charged species. From paramagnetic 1H NMR studies on the resulting series of triple, quadruple and quintuple‐decker complexes, ligand oxidation leads to a decrease in the magnetic anisotropy, as predicted from theoretical calculations. Unusual paramagnetic shifts were observed in the spectra of the +2e charged quadruple and quintuple‐decker complexes, indicating that those two species are actually unexpected triplet biradicals. Magnetic measurements revealed that the series of complexes show single‐molecule magnet properties, which are controlled by the multi‐step redox induced structural changes., This work was financially supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Numbers JP14J02656 (Y.H.), JP20225003 (M.Y.), JP15K05467 (K.K.), JP24750119 (K.K.), 18K14242 (Y.H.), 19K05401(Y.K), Tohoku University Molecule & Material Synthesis Platform in Nanotechnology Platform Project, CREST, JST JPMJCR12L3 (M.Y.), PhD scholarship from the Beilstein‐Institut zur Förderung der Chemischen Wissenschaften (M.D.), the German‐Japanese University Consortium (HeKKSaGOn), the state of Baden‐Württemberg through bwHPC and the German Research Foundation (DFG) through grant no INST 40/467‐1 FUGG (JUSTUS cluster), European Union (ERC StG 2012–306826 e‐GAMES), Networking Research Center on Bioengineering, Biomaterials, and Nanomedicine (CIBER‐BBN), the Spanish Ministry of Economy and Competitiveness (projects FANCY CTQ2016‐80030‐R, MOTHER MAT2016‐80826‐R and the “Severo Ochoa” Programme for Centers of Excellence in R&D, SEV‐2015‐0496), Generalitat de Catalunya (2017SGR918), 111 Project (B18030) from China (M.Y.), the scientific grants R‐143‐000‐A80‐114, R‐143‐000‐A65‐133 from the National University of Singapore. We thank Dr. Norihisa Hoshino for EPR measurements, Dr. Tetsuko Nakaniwa and Prof. Dr. Genji Kurisu and Dr. Takefumi Yoshida for SCXRD measurements. This paper is Contribution No. 62 from the Research Center for Thermal and Entropic Science.

In this work, biradical boron containing systems with various structures are investigated to reveal the dependency of the biradical character on the ligated carbene (NHC, CAAC) and the related steric demands of the substituents., In dieser Arbeit werden biradikalische Bor-Systeme mit unterschiedlichen Strukturen untersucht, um die Abhängigkeit des biradikalischen Charakter von der Carben-Einheit, die am Bor (NHC, CAAC) gebunden ist, und den sterischen Anspruch der Substituenten zu analysieren.

Photoinitiated radical chemistry has proven to be useful for breaking covalent bonds within many biomolecules in the gas phase. Herein, we demonstrate that radical chemistry is useful for bond synthesis in the gas phase. Single peptides containing two cysteine residues capped with propylmercaptan (PM) often form disulfide bonds following ultraviolet excitation at 266 nm and loss of both PM groups. Similarly, noncovalently bound peptide pairs where each peptide contains a single cysteine residue can be induced to form disulfide bonds. Comparison with disulfide bound species sampled directly from solution yields identical collisional activation spectra, suggesting that native disulfide bonds have been recapitulated in the gas phase syntheses. Another approach utilizing radical chemistry for covalent bond synthesis involves creation of a reactive diradical that can first abstract hydrogen from a target peptide, creating a new radical site, and then recombine the second radical with the new radical to form a covalent bond. This chemistry is illustrated with 2-(hydroxymethyl-3,5-diiodobenzoate)-18-crown-6 ether, which attaches noncovalently to protonated primary amines in peptides and proteins. Following photoactivation and crosslinking, the site of noncovalent adduct attachment can frequently be determined. The ramifications of these observations on peptide structure and noncovalent attachment of 18-crown-6-based molecules is discussed.

Visible-light sensitized photoswitches have been paid particular attention in the fields of life sciences and materials science because long-wavelength light reduces photodegradation, transmits deep inside of matters, and achieves the selective excitation in condensed systems. Among various photoswitch molecules, the phenoxyl-imidazolyl radical complex (PIC) is a recently developed thermally reversible photochromic molecule whose thermal back reaction can be tuned from tens of nanoseconds to tens of seconds by rational design of the molecular structure. While the wide range of tunability of the switching speed of PIC opened up various potential applications, no photosensitivity to visible light limits its applications. In this study, we synthesized a visible-light sensitized PIC derivative conjugated with a benzil unit. Femtosecond transient absorption spectroscopy revealed that the benzil unit acts as a singlet photosensitizer for PIC by the Dexter-type energy transfer. Visible-light sensitized photochromic reactions of PIC are important for expanding the versatility of potential applications to life sciences and materials science.

This dissertation will examine the electronic properties of several materials used in state-of-the art technology and ongoing scientific investigation by means of high-level ab initio quantum chemical calculations. In this way, the application of such calculations to graphene reactivity and defects, organic photovoltaic processes, and excited states and biradical character of polycyclic aromatic hydrocarbons (PAH) is presented. Graphene reactivity and defect characterization is currently the source of intense scrutiny. The reactivity of pristine graphene and single and double carbon vacancies toward a hydrogen radical was assessed using complete active space self-consistent field theory (CASSCF) and multireference configuration interaction singles and doubles (MR-CISD) calculations using a pyrene model. While all carbon centers formed stable, novel C-H bonds, the single carbon defect was found to be most stabilizing thanks to the dangling bond at the six-membered ring. Bonding at this carbon center shows interaction with several closely-spaced electronic excited states. A rigid scan was also used to show the dynamic landscape of the potential energy surfaces of these structures. Double carbon vacancy pyrene and circumpyrene were used to investigate the structure and electronic states of an intrinsic silicon impurity in graphene by density functional theory (DFT) methods. The ground state was found to be low spin, non-planar and D2 symmetric with a nearby low spin, planar state with D2h symmetry. Characterization with natural bond orbital (NBO) analyses showed that the hybridization in the non-planar structure is sp3 and sp2d in the planar structure. Charge transfer from the silicon dopant to the surrounding carbon atoms is demonstrated via analysis with natural charges and molecular electrostatic potential (MEP) plots. Experiment shows drastically enhanced power conversion efficiency (PCE) in a bulk heterojunction (BHJ) device consisting of poly(3-hexylthiophene-2, 5-diyl) (P3HT), terthiophene (T3), and fullerene (C60). This BHJ scheme was investigated using DFT and several environmental models. The state-specific (SS) environment was found to best reproduce the experimental findings showing that initial photoexcitation of a local, bright exciton at P3HT was able to decay through several charge transfer (CT) bands of P3HT→C60 and T3→C60 character explaining the increased PCE. The spectra calculated for the isolated system and linear response (LR) environment were very similar to each other and showed the initial photoexcitation at P3HT decayed instead into bands of local C60 excitonic states quenching the CT states. Crucial benchmark calculations of the structure and electronic excited states missing from the current literature were performed for oligomers of poly(p-phenylenevinylene) (PPV) dimers using high-level scaled opposite-spin (SOS) second-order Møller-Plesset (SOS-MP2) and second-order algebraic diagrammatic construction (SOS-ADC(2)) calculations. Comparisons were also made to DFT methods. The dimer structures were studied in two conformations: sandwich-stacked and displaced; the latter exhibited greater stacking interactions. Good agreement was found between the cc-pVQZ and aug-cc-pVQZ basis sets and the cc-pVTZ basis. Comparisons of the interaction energies were made to the complete basis set limit. Basis set superposition error was also accounted for. The excited state spectra presented a varied and dynamic picture that was analyzed using transition density matrices and natural transition orbitals (NTOs). The assessment of the biradical character of several trans-diindenoacenes, cis-diindenoacenes, and trans-indenoindenodi(acenothiophenes) was accomplished utilizing the multireference averaged quadratic coupled-cluster (MR-AQCC) method. These systems have been previously identified as highly reactive in experiments, often subject to unplanned side-reactions. The biradical character was characterized using the singlet-triplet splitting energy (∆ES-T), effective number of unpaired electrons (NU), and unpaired electron density. For all structures, low spin ground states were found. In addition, increasing the number of core benzene rings decreased the ∆ES-T, increased the NU, and increased the unpaired electron density. Good agreement to experimental values for the ∆ES-T is also demonstrated.

The formal cyclopentane-1,3-diyl derivatives [E1(μ-NTer)2({E2C} = NDmp)] (Ter = 2,6-dimesityl-phenyl, Dmp = 2,6-dimethylphenyl) were prepared by 1,1-insertion of CNDmp into the N–E2 bond of [E1(μ-NTer)2E2] (E1 = N, P; E2 = P, As)., The formal cyclopentane-1,3-diyl derivatives [E1(μ-NTer)2({E2C} = NDmp)] (Ter = 2,6-dimesityl-phenyl, Dmp = 2,6-dimethylphenyl) were prepared by 1,1-insertion of CNDmp into the N–E2 bond of [E1(μ-NTer)2E2] (E1 = N, P; E2 = P, As). The insertion does not occur for E1 = E2 = As or E2 = Sb. Dependent on the choice of formal radical centres E, either a biradicaloid or a zwitterion was obtained. The biradicaloid features a P and an As radical center and its biradical character was established by computations as well as characteristic reactivity with respect to the formation of a housane derivative and the activation of molecules bearing multiple bonds, which was demonstrated using the example of PCtBu. In contrast, the formally N,As- and N,P-centered biradicaloids are better regarded as zwitterionic species in accord with computations and diminished reactivity, as neither housane formation nor activation of multiple bonds could be observed.

International audience; A dinitroxide biradical (dCdO) has the peculiar tendency to crystallize in non-centrosymmetric polar space groups either as a stable dichloromethane solvate or as a pure phase. The molecules arrange with no compensation of their dipolar moment resulting in net macroscopic polar order.; Un biradical dinitroxyde (dCdO) possède une tendance singulière à cristalliser en groupes spatiaux polaires non centrosymétriques, soit sous forme de solvate stable avec le dichlorométhane ou sous forme pure. Les molécules s’organisent sans compensation de leur moment dipolaire, produisant un ordre polaire macroscopique net.

N-Heterocyclic carbenes are able to form covalent adducts with nitrous oxide (N2O) under ambient conditions. This thesis summarizes the studies of these adducts and their reactivity. A ditopic carbanionic N-heterocyclic carbene was found to react with nitrous oxide, resulting in a stable covalent adduct with two N2O groups attached to the heterocycle. Mesoionic N-heterocyclic carbenes derived from C2-arylated 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, as well as a triazolylidene, also react with N2O. Crystallographic analyses of all complexes reveal bent N2O groups, which can adopt a cis or trans configuration. Resulting complexes of imidazole- or triazole-based mesoionic carbenes and nitrous oxide can be converted into azo dyes by reaction with arenes in the presence of AlCl3 or HCl. The azo coupling can be achieved with electron-rich aromatic compounds such as mesitylene, trimethoxybenzene, azulene, or dibenzo-18-crown-6. The latter coupling reaction allows for an easy preparation of a colorimetric sensor for potassium or sodium ions. The putative intermediate of the reaction with acid, a rare diazohydroxide, has been isolated and structurally characterized. Using imidazolium compounds with two N2O groups, it is possible to prepare amine-substituted azo dyes, or dyes with two azoarene groups attached to the heterocycle. The synthesis and the characterization of a new class of neutral aminyl radicals is also reported. Monoradicals were obtained by the reduction of azoimidazolium dyes with potassium. Structural, spectroscopic and computational data suggest that the spin density is centered on one of the nitrogen atoms of the former azo group. The reduction of a dimeric dye with an octamethylbiphenylene bridge between the azo groups resulted in the formation a biradical with largely independent unpaired electrons. Both, the monoradicals and the biradical were found to display high stability in solution as well as in the solid state.

金沢大学薬学部, 金沢大学大学院自然科学研究科生理活性物質科学, Photolysis of 2-phenylcyclododecanone (2PCDD, ring size: C12) has been investigated in an air-saturated solution. The ﾎｱ-photocleavage of 2PCDD (Norrish type I) has led to the formation of triplet acyl-benzyl biradical (3BR) which reacts with O2 to produce a peroxyester intermediate. A major photoproduct of 2-phenyl-1-oxacyclododecane is formed by a stepwise decarboxylation from the peroxyester intermediate. The peroxyester is also a common intermediate for minor products of benzaldehyde, cyclodecane, and acetophenone. 1-Phenylcycloundecane is produced from decarbonylation of 3BR. On the other hand, 3BR undergoes intersystem crossing to the singlet biradical which in turn recombines to form the parent 2PCDD or to afford a cage product of cyclophane. The recombination yield to the starting species is estimated to be 0.5 by measuring a time evolution of enantiomer concentration of 2PCDD starting from one optically pure enantiomer. Photolysis mechanism of 2PCDD in the presence of molecular oxygen has been proposed and discussed. © 2006 Elsevier B.V. All rights reserved.

Coupled-cluster calculations of increasing accuracy (approximate doubles: CC2; doubles: EOM-CCSD; connected triples: CR-EOM-CCSD(T)) for CIS-optimized potential energy profiles of adenine and its derivatives indicate that the ultrafast internal conversion of the optically excited π π* state occurs through a state switch to a biradical state, which intersects the ground state at a lower energy. The electronic nature of the biradical state is defined by an electronic configuration in which one unpaired electron occupies a π* orbital confined to the five-membered ring. The second unpaired electron is localized very strongly on a p-type C2 atomic orbital of the six-membered ring. The biradical state minimum has a strongly puckered six-membered ring and a C2–H bond, which is twisted nearly perpendicular to the average ring plane. Consistent with the biradical-mediated internal conversion, the π π* state lifetime is extremely short in adenine and 9-methyladenine, which have barrierless crossing to the biradical state. The lifetime is slightly longer in N,N-dimethyladenine, which has a small barrier for the state switch. In 2-aminopurine the biradical state is found above the π π* state, preventing the biradical state switch and dramatically increasing the lifetime. These results, combined with an earlier work on pyrimidine bases, strongly suggest the importance of a direct decay of the doorway π π* state via a biradical state switch in the photophysics of DNA, even though the nature of the biradical state is somewhat different in purines and pyrimidines.Key words: adenine, guanine, DNA damage, radiationless decay, biradical, ab initio, coupled clusted.

Magic Angle Spinning (MAS) combined with Dynamic Nuclear Polarization (DNP) has been proven in recent years to be a very powerful method for increasing solid-state NMR signals. Since the advent of biradicals such as TOTAPOL to increase the nuclear polarization new classes of radicals, with larger molecular weight and/or different spin properties have been developed. These have led to unprecedented signal gain, with varying results for different experimental parameters, in particular the microwave irradiation strength, the static field, and the spinning frequency. Recently it has been demonstrated that sample spinning imposes DNP enhancement processes that differ from the active DNP mechanism in static samples as upon sample spinning the DNP enhancements are the results of energy level anticrossings occurring periodically during each rotor cycle. In this work we present experimental results with regards to the MAS frequency dependence of the DNP enhancement profiles of four nitroxide-based radicals at two different sets of temperature, 110 and 160 K. In fact, different magnitudes of reduction in enhancement are observed with increasing spinning frequency. Our simulation code for calculating MAS-DNP powder enhancements of small model spin systems has been improved to extend our studies of the influence of the interaction and relaxation parameters on powder enhancements. To achieve a better understanding we simulated the spin dynamics of a single three-spin system {ea-eb-n} during its steady state rotor periods and used the Landau-Zener formula to characterize the influence of the different anti-crossings on the polarizations of the system and their necessary action for reaching steady state conditions together with spin relaxation processes. Based on these model calculations we demonstrate that the maximum steady state nuclear polarization cannot become larger than the maximum polarization difference between the two electrons during the steady state rotor cycle. This study also shows the complexity of the MAS-DNP process and therefore the necessity to rely on numerical simulations for understanding parametric dependencies of the enhancements. Finally an extension of the spin system up to five spins allowed us to probe the first steps of the transfer of polarization from the nuclei coupled to the electrons to further away nuclei, demonstrating a decrease in the spin-diffusion barrier under MAS conditions.

The spin of an electron often misleadingly interpreted as the classical rotationof a particle. The quantum spin distinguishes itself from classicalrotation by possessing quantized states and can be detected by its magneticmoment. The properties of spin and its collective behavior with otherfundamental properties are fascinating in basic sciences. In many aspectsit offers scope for designing new materials by manipulating the ensemblesof spin. In recent years attention towards high density storage devices hasdriven the attention to the fundamental level were quantum physics rules.To understand better design of molecule based storage materials, studies onspin degrees of freedom and their coupling properties can not be neglected. To account for many body effect of two or more electrons consistent withrelativity, an approximation like the Breit Hamiltonian(BH) is used in modernquantum chemical calculations, which is successful in explaining the splitin the spectra and corresponding properties associated with it. Often differenttactics are involved for a specific level of computations. For example themulti-configurational practice is different from the functional based calculations,and it depends on the size of the system to choose between resourcesand accuracy. As the coupling terms offers extra burden of calculating theintegrals it is literally challenging. One can readily employ approximations as it suits best for the applicationoriented device computations. The possible methods available in the literatureare presented in chapter 2. The theoretical implementations of couplingfor the multi-reference and density functional method are discussed in detail.The multi-reference method precedes the density functional methodin terms of accuracy and generalizations, however it is inefficient in dealingvery large systems involving many transition elements, which is vital formolecule based magnets as they often possess open shell manifolds. On theother hand existing density functional method exercise perturbations techniqueswhich is extremely specialized for a specific system - highly coupledspins. The importance of spin-spin coupling(SSC) in organic radical-Oxyallyl(OXA)was systematically studied with different basis sets and compared with asimilar isoelectronic radical(TMM). The method of spin-spin coupling implementationsare also emphasized. Similar coupling studies were carriedivout for the species HCP and NCN along with spin-orbit coupling(SOC).The splitting of the triplet states are in good agreement with experiments QC 20110210

Petroleum fuels are the world's most important primary energy source and the need to maintain their supply is a major actual challenge involving both economical and environmental features. Decreasing fuels consumption is one of the more efficient ways to reconcile the goals of energy price and environmental protection. Numerical simulations become therefore a very important tool to optimize fuels and motors. Detailed chemical kinetic models are required to reproduce the reactivity of fuels and to characterize the amount of emitted pollutants. Such models imply a very large number of chemical species and elementary reactions, for a given species, and the determination of thermodynamic and kinetic data is a critical problem. Nowadays, quantum chemistry methods are able to calculate accurately thermodynamic data for a large number of chemical systems and to elucidate the reactivity of these systems. In this work we have used quantum chemistry to study the unimolecular reactions (initiation, molecular reactions, ß-scissions, cyclic ethers formations) involved in the decomposition of monocyclic and polycyclic hydrocarbons. From the results of quantum chemical calculations, a detailed chemical kinetic mechanism of the pyrolysis of a polycyclic alkane has been developed and validated against experimental data, Les carburants dérivés du pétrole constituent la première source mondiale énergétique et leur approvisionnement constitue un défi actuel majeur impliquant des enjeux économiques et environnementaux cruciaux. Une des voies les plus efficaces pour peser simultanément sur ces deux enjeux passe par la diminution de la consommation en carburant. La simulation numérique constitue dès lors un outil précieux pour améliorer et optimiser les moteurs et les carburants. Les modèles chimiques détaillés sont nécessaires pour comprendre les phénomènes d'auto-inflammation et caractériser la nature et les quantités de polluants émis. Ces modèles mettent en jeu un nombre très important d'espèces et de réactions élémentaires, pour une espèce donnée et pour lesquelles la détermination des données thermodynamiques et cinétiques est un problème crucial. La chimie quantique constitue un outil précieux permettant d'une part de déterminer de façon précise les données thermocinétiques pour bon nombre de systèmes chimiques et d'autre part de mieux comprendre la réactivité de ces systèmes. Dans ce travail, les réactions unimoléculaires de décomposition d'hydrocarbures monocycliques et polycycliques (amorçages, réactions moléculaires, ß-scissions, formations d'éthers cycliques) ont été étudiées à l'aide des méthodes de la chimie quantique. Un mécanisme détaillé de pyrolyse d'un alcane polycyclique a été développé à partir des données thermodynamiques et cinétiques et des corrélations entre structure et réactivité déterminées pour les cyclanes à partir des calculs quantiques. Les simulations effectuées à partir de ce modèle sont en très bon accord avec les résultats expérimentaux de la littérature

The thermal decomposition of the exo-tricyclo[5.2.1.02,6]decane, a tricyclic alkane which ispart of some fuels used in aeronautics, was studied at the DCPR in the scope of thedevelopment of the cooling system of the engine of hypersonic aircrafts.The experimental study of the thermal decomposition of this alkane was performed with a jetstirred reactor, at atmospheric pressure, at high dilution, at residence times between 0.5 and 6sand for a wide range of temperatures (673-1173 K). The formation of 53 products of thereaction was observed during this study.A mechanism of the pyrolysis of the exo-tricyclo[5.2.1.02,6]decane was developed (2789reactions ; 929 molecular, radical et biradical species) and was validated from experimentalresults. The kinetic analysis of the mechanism highlighted the importance of the reactions ofunimolecular initiation on the consumption of the reactant., La décomposition thermique de l'exo-tricyclo[5.2.1.02,6]décane, un alcane tricyclique quirentre dans la composition de carburants utilisés en aéronautique, a été étudiée aux DCPRdans le cadre du développement du système de refroidissement du moteur de véhiculeshypersoniques.L'étude expérimentale de la décomposition thermique de cet alcane a été réalisée avec unréacteur auto agité par jets gazeux, à pression atmosphérique, à haute dilution, pour des tempsde passage compris entre 0,5 et 6s et sur une large gamme de température (673-1173 K). Cetteétude a permis de mettre en évidence la formation de 53 produits de la réaction.Un mécanisme de la pyrolyse de l'exo-tricyclo[5.2.1.02,6]décane a été développé (2789réactions ; 929 espèces moléculaires, radicalaires et biradicalaires) et a été validé à partir desrésultats expérimentaux. L'analyse cinétique du mécanisme a permis de soulignerl'importance des réactions d'amorçage unimoléculaire sur la consommation du réactif.

Funding Information: This research was supported by the grant NWO–CW 711.014.006 from the Council for Chemical Sciences of The Netherlands Organization for Scientific Research. S.T. acknowledges the support from the Project LISBOA-01-0145-FEDER-007660 (Microbiologia Molecular, Estrutural e Celular) funded by FEDER funds through COMPETE 2020─Programa Operacional Competitividade e Internacionalização (POCI) and by national funds through FCT─Fundação para a Ciência e a Tecnologia and from the European Union’s Horizon 2020 Research and Innovation Program through TIMB3 (grant agreements no 810856). This work is dedicated to the late Prof. Simon de Vries, who pioneered the development of the rapid kinetic techniques that have been seminal in this study. Publisher Copyright: © 2021 The Authors. Published by American Chemical Society. The heme enzyme chlorite dismutase (Cld) catalyzes O-O bond formation as part of the conversion of the toxic chlorite (ClO2-) to chloride (Cl-) and molecular oxygen (O2). Enzymatic O-O bond formation is rare in nature, and therefore, the reaction mechanism of Cld is of great interest. Microsecond timescale pre-steady-state kinetic experiments employing Cld from Azospira oryzae (AoCld), the natural substrate chlorite, and the model substrate peracetic acid (PAA) reveal the formation of distinct intermediates. AoCld forms a complex with PAA rapidly, which is cleaved heterolytically to yield Compound I, which is sequentially converted to Compound II. In the presence of chlorite, AoCld forms an initial intermediate with spectroscopic characteristics of a 6-coordinate high-spin ferric substrate adduct, which subsequently transforms at kobs = 2-5 × 104 s-1 to an intermediate 5-coordinated high-spin ferric species. Microsecond-timescale freeze-hyperquench experiments uncovered the presence of a transient low-spin ferric species and a triplet species attributed to two weakly coupled amino acid cation radicals. The intermediates of the chlorite reaction were not observed with the model substrate PAA. These findings demonstrate the nature of physiologically relevant catalytic intermediates and show that the commonly used model substrate may not behave as expected, which demands a revision of the currently proposed mechanism of Clds. The transient triplet-state biradical species that we designate as Compound T is, to the best of our knowledge, unique in heme enzymology. The results highlight electron paramagnetic resonance spectroscopic evidence for transient intermediate formation during the reaction of AoCld with its natural substrate chlorite. In the proposed mechanism, the heme iron remains ferric throughout the catalytic cycle, which may minimize the heme moiety's reorganization and thereby maximize the enzyme's catalytic efficiency. publishersversion published

A four-pulse version of the pulse double electron–electron resonance (DEER) experiment is presented, which is designed for the determination of interradical distances on a nanoscopic lengthscale. With the new pulse sequence electron–electron couplings can be studied without dead-time artifacts, so that even broad distributions of electron–electron distances can be characterized. A version of the experiment that uses a pulse train in the detection period exhibits improved signal-to-noise ratio. Tests on two nitroxide biradicals with known length indicate that the accessible range of distances extends from about 1.5 to 8 nm. The four-pulse DEER spectra of an ionic spin probe in an ionomer exhibit features due to probe molecules situated both on the same and on different ion clusters. The former feature provides information on the cluster size and is inaccessible with previous methods.