Formation Of Biradicals Research Articles

Aromaticity of the Bergman, Myers-Saito, Schmittel, and directly related cyclizations of enediynes.

Aromaticity criteria (magnetic susceptibility exaltations, nucleus independent chemical shifts (NICS), and aromatic stabilization energy (ASE) evaluations) for enediyne and enyne-allene cyclizations evaluated at (UBS)-BLYP/6-31G* all agree that the degrees of cyclic electron delocalization of the benzenoid systems formed by the Bergman (3) and Myers-Saito reactions (5) are comparable to benzene. The reaction enthalpy differences between the parent cyclizations and their benzannelated analogues are not entirely due to disparities in gained ASE during the reactions. The alternative formation of fulvene biradicals is not accompanied by favorable aromatic effects.

The first steps of the light-induced biradical and zwitterion formation from the clusters [Os 3(CO) 10(α-diimine)] studied with ultrafast time-resolved absorption spectroscopy

The (sub)picosecond time-resolved transient absorption spectra of two triangular [Os 3(CO) 10(α-diimine)] clusters have been studied to establish the primary photoprocesses responsible for the formation of biradicals and zwitterions. The TA spectra of [Os 3(CO) 10( iPrAcPy)] obtained by excitation into its visible absorption band, show a bleach due to the disappearance of the parent cluster and a new absorption with a maximum at 630 nm. In a non-coordinating solvent the bleach and absorption decay with a lifetime of 25±2 ps but do not disappear completely. The bleach decays to approximately 30% of the initial signal and the transient absorption changes into a much broader absorption without a distinct maximum. The initial transient absorption is assigned to the excited state of the cluster having predominant σ(OsOs)→π*( iPrAcPy) character. From the relaxed excited state the cluster partially decays to the ground state and partially produces biradicals. The lifetime of the excited state does not depend on the solvent as long as it is non-coordinating, but it depends on the energy of this 3σπ* excited state, as observed for [Os 3(CO) 10(dmb)]. This effect is attributed to a lowering of the barrier for the reaction from the 3σπ* state. In coordinating acetonitrile (MeCN) the excited state of [Os 3(CO) 10( iPrAcPy)] decays double-exponentially. The longer lifetime ( τ=21.4 ps) matches that observed in non-coordinating solvents and is assigned to biradical formation. In agreement with previous observations that zwitterion formation in coordinating solvents must occur in the picosecond time domain, the second and faster process ( τ=2.9 ps) is assigned for zwitterion formation. These zwitterions are formed by heterolytic splitting of an OsOs bond induced by coordination of MeCN to the Os(CO) 2( iPrAcPy) moiety in the excited state of the cluster. Time-resolved absorption studies in the microsecond time domain showed that the MeCN-coordinated biradicals convert with a lifetime of 13.7 μs into zwitterions. The unique result of this study is that coordinating solvents such as MeCN may induce both homolytic and heterolytic cleavage of a metalmetal bond in such clusters.

DNA interstrand cross-link induced by estrogens as well as their complete and synergic carcinogenesis

The estrogens show negative activity in Ames test, but estrodiol and diethylstilbestrol in estrogens both are carcinogens based upon animal experiments and epidemiological investigation. It is concluded from the di-region theory, a mechanism conception put forward by one of the present authors, that the carcinogenesis of estrogens is switched on by the covalent cross-link between complementary DNA bases induced by them. We verified for the first time by the DNA alkaline elution method that both estrodiol and diethylstilbestrol cause covalent cross-link between DNA-protein and DNA interstrands after metabolic activation with dosage correlation, but neither the non-carcinogens cholesterol nor pyrene can lead to these sorts of cross-link in the same condition. It has been known that there is a synergetic effect between estrogen and pollution of polycyclic aromatic hydrocarbons. Although non-carcinogenic pyrene alone cannot induce cross-link, its addition with equal molar quantity to estrodiol culture causes synergically the total and DNA interstrand cross-link ratios to be respectively four and three times more than the ones in the cultivation with estrodiol only. It is shown that not only the estrodiol set off the formation of pyrene bi-radicals, but also the pyrene radicals arouse conversely the production of estrodiol bi-radicals.

A computational study of the factors controlling triplet-state reactivity in 1,4-pentadiene

The triplet-state reactions of 1,4-pentadiene have been investigated using density functional theory (UB3LYP) and ab initio (CASSCF) calculations with a 6-31G basis set. Intramolecular [2 + 2] photocycloadditions and three different reaction pathways leading to vinylcyclopropane have been examined. The computed results are in good agreement with the experimental observations, predicting the dominant product to be vinylcyclopropane produced by a di-pi-methane rearrangement, and the favored [2 + 2] cycloaddition product to be bicyclo[2.1.0]pentane. Reaction pathways involving initial C-C or C-H bond cleavage were found to be too high in energy to be significant. Both the [2 + 2] cycloadditions and the di-pi-methane rearrangement proceed through cyclic biradical intermediates formed on the triplet surface. The relative rates of formation of these triplet biradicals are found to depend on three factors: biradical stability, the geometry of the transition structure, and orbital interactions through bonds.

Photophysics and photochemistry of poly (4′-ethoxyacrylophenone)

The photophysics and the long wave ( λ≥300 nm) photochemistry of poly (4′-ethoxyacrylophenone) were investigated. The quantum yield for fluorescence is low (10 −4 mol (einstein) −1), and the principal excited species is the carbonyl triplet. The quantum yield for phosphorescence is greater and the λ max value is 445 nm. The triplet lifetime at 77 K is 0.41 s, but 8.7 μs at 300 K. These observations are consistent with the formation of a low-lying triplet, which, in turn is the result of substitution of the aromatic ring with the electron donating ethoxy group. The principal photoprocess occurring in dilute solution (in CH 2Cl 2) of the polymer is a Norrish Type II decomposition, which results in random chain scission and a rapid decrease in the molecular weight. The relatively low quantum yield (6×10 −2 mol (einstein) −1) for the scission process reflects the low reactivity of a triplet (which has some π, π ∗ character) in abstraction reactions. In the case of PEAP, H-abstraction is a prerequisite for biradical formation, which, in turn leads to chain scission. Chain scission was subject to inhibition by triplet quenchers, such as naphthalene and cis-1,3-pentadiene, and the data conformed to Stern–Volmer kinetics. However, the quenching rate constants, k Q were lower in magnitude than the corresponding diffusion controlled values, and it is proposed that this is principally due to a steric effect, the actual encounter of the quencher with the carbonyl triplet being subject to adverse steric interactions associated with the proximity of the relatively bulky polymer chain.

Electronic Decoupling in Ground and Excited States of Asymmetric Biaryls

New asymmetric biaryls have been synthesized in order to clarify the conditions necessary for charge-transfer transitions in photoexcited neutral compounds and biradical formation in doubly charged ground-state species. A parallel behavior for both types of approaches is observed and explained with a simple coupling model allowing prediction of the intermoiety coupling strength. It is shown that for weakly coupled biaryls the monoions are connected with charge localization and the dications and dianions form thermally excited biradicals while the fluorescence data indicate biradicaloid excited states connected with partial charge separation. More strongly coupled biaryls, on the other hand, result in diamagnetic species for the doubly charged ground-state ions, and a charge-transfer contribution in the fluorescence spectra of the neutral compound is absent.

Reductive cleavage of dichalcogenide bonds

Electrochemical reduction of 1,2-di(2,4,6-triphenylpyridinium-l)ethane, 1,4-di(2,4,6-triphenylpyridinium-1)butane, 1,6-di(2,4,6-triphenylpyridinium-l)hexane, 1,4-di(N-methyl-4,6-diphenylpyridinium-2)benzene, and 2,7-di(N-methyl-4,6-diphenylpyridinium-2)fluorene in solutions of DMF and MeCN in the potential range from−0.8 to −1.7 V (SCE) was studied by CV. The successive formation of stable radical cations, biradicals, and dianions was investigated by semiempirical MO calculations at the PM3 level.

Synthesis of Dibenzodiynes: Exceptionally Easy Formation of a Chrysene Biradical

Synthesis of Dibenzodiynes: Exceptionally Easy Formation of a Chrysene Biradical

ESR Determination of Vitamin K1 Following Photoexitation of a Frozen Solution

It was found that the photoexitation of a frozen solution of vitamin K1 (phyllonaphtoquinone) gave rise to the formation of biradicals with a very bright spectra (ΔH ∼ 200 G), typical for randomly oriented radical pairs with a summary spin of S = 1. This spectra was not overlaped by those of the other radical products generated following the photolysis of vitamin K1 in different matrices. This method is proposed for the determination of vitamin K, in biological samples and was successful in following the accumulation of vitamin K1 in mouse liver following subcutaneous injection.

Photochemistry of the Triangular Clusters Os3(CO)10(α-diimine): Homolysis of an Os−Os Bond and Solvent-Dependent Formation of Biradicals and Zwitterions

ADVERTISEMENT RETURN TO ISSUEPREVAddition/CorrectionPhotochemistry of the Triangular Clusters Os3(CO)10(α-diimine): Homolysis of an Os−Os Bond and Solvent-Dependent Formation of Biradicals and ZwitterionsJos Nijhoff, Maarten J. Bakker, František Hartl, Derk J. Stufkens, Fu, and Rudi van EldikCite this: Inorg. Chem. 1998, 37, 5, 1136Publication Date (Web):February 18, 1998Publication History Published online18 February 1998Published inissue 9 March 1998https://doi.org/10.1021/ic980143aCopyright © 1998 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views168Altmetric-Citations1LEARN 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 (8 KB) Get e-Alerts Get e-Alerts

Effect of Polymer Matrix on Kinetics and Mechanism of Carbene Reactions

The features of kinetics and mechanism of the cage radical reactions initiated by carbenes in solid polymers has been considered. The peculiar low temperature reaction of arylcarbenes is their interaction in clusters with formation of biradicals (BR). The yield of BR is a measure of the distribution non-homogeneity of the carbene precursors in polymers. The reaction of BR formation allows us to study the molecular dynamics at cryogenic temperatures. The main factor determining the kinetic non-equivalence of triplet carbenes in low temperature reaction of the hydrogen atom abstraction is the dispersion of the cage distances between carbenes and C—H bonds of macromolecules. The strong effect of structural modifications in filled polymers on the thermal decay of carbenes has been established. The matrix effect on the mechanism of the product formation in the carbene reactions, associated with the relaxation processes, has been recognized by the example of the polyvinylalcohol cross-linkage. The advantage of polyisoprene stabilization against thermoxidation by the phenol grafting in carbene synthesis has been shown.

Molecular Control of Solid-State Reactivity and Biradical Formation from Crystalline Ketones

A growing number of solid-state reactions with high selectivities and specificities have been reported in recent years.1 As the potential of highly selective transformations where solvents can be omitted is increasingly appreciated, it has become desirable to search for reliable methods to carry out reactions in the crystalline solid state.2 In this Communication, we analyze some of the factors that affect the photochemical reactivity of crystalline ketones capable of forming biradicals and radical pairs.3 Our interest in biradicals comes from the fact that they occur in a large number of organic reactions, and strategies for their generation may help develop protocols where selectivity and specificity may be achieved by means of crystal control. The main challenge in the preparation and use of biradicals in solid-state reactions comes from the fact that biradical formation requires cleavage of a σ bond under conditions where separation of the radical termini is essentially impossible (Scheme 1). It is known that photochemical excitation (step 1) to dissociative surfaces in solution and in the gas phase leads to separation of atoms A and B (step 2) to form extended biradicals that explore bimolecular reactions and unimolecular rearrangements.4 In contrast, the two radical centers in the solid state are held close together by their rigid environment, and the two termini are likely to collapse to remake the bond that was originally broken (step 3). For biradical products to form in the solid state, chemical reactions that are comparable to or faster than step 3 will be required. We suggest that proper conditions may be facilitated by molecular design. For instance, in the case of ketone precursors, we propose that dissociative triplet state surfaces and irreversible radical-radical reactions may be formulated with some certainty from known substituent effects.4 A triplet surface will slow down step 3, and a rapid decarbonylation reaction may make up for the lack of biradical separation in a sequence that should lead to products from dialkyl biradicals (Schemes 2 and 3).5 To test our hypothesis, we analyzed the solid-state reactivity of several known 2-phenyland 2,6-diphenylcyclohexanones with 2-hydroxy, 2-methyl, 2,6-dihydroxy, or 2,6-dimethyl substituents. The selection of compounds 1-6 (Scheme 2) was based on the expected photochemical effects of their R-substituents and on their abilities to form good crystals. We have elucidated all their X-ray structures.6 The positions of their R-substituents (axial/equatorial) are represented in the scheme. The conformation about the phenyl groups is such that good benzylic stabilization is expected. If the orientation of the ring is maintained though the reaction, the p-orbitals of the aromatic systems should be kept in close alignment with the benzylic radical p-orbital (see Supporting Information). Several reports on the solution photochemistry of most of the compounds in Scheme 2 are available.7 Although all the compounds in the scheme have high and very similar reaction yields in solution (Φ > 0.6), variations in their solid-state reactivity are very large. Crystals of 1 are photostable. Relative yields of solid-state reaction for the other compounds are shown in parentheses in Scheme 2. These were determined in side-

QSAR Study of Phenols: Relation between the Phenoxyl Radical Formation and the Antiinflammatory Activity via an Antioxidant Mechanism

AbstractThe antiinflammatory mechanism of phenols as radical scavengers was analyzed. The biradical formation process Ph‐OH → Ph‐O + H was studied by the reaction coordinate method using the semiempirical AM1 quantum‐mechanical method. The enthalpies of formation of ground, transition, and radical states were determined; other parameters like the reaction, stabilization and activation enthalpies for phenoxyl radical formation of fifteen phenolic compounds were also calculated and correlated with their antiinflammatory activities.The results suggest that the ability of phenols to form the phenoxyl radical and the stability of this derived radical are important in the antiinflammatory activities of phenols via neutralization of harmful radicals.

Novel Oligoradicals and High Spin Formation

Abstract Orthogonal alignment and variation of the MO-coefficients at the bridgehead postion has been tested for high spin formation in directly linked arylenes. In 2,21-bipyrenyl and para-terpyrenyl possessing very small MO-coefficients but also small steric indrance bi- and triradicalanionss are found to be in low spin groundstate, with thermally excited triplet and quartet states 100-1000 calmol−1 higher in energy, respectively. For 9,91-bianthryl biradical formation is well established, such that higher oligomers have been charged resulting in a S = 3/2 for the trimer, a S = 2 for the tetramer and a polyanionradical with descrete S = 1, S = 3/2, and S = 2 states in the polymer, where this highly charged material becomes insoluble. The ground state is again low spin due to increased deviation from orthogonality at low temperatures. The thermally activated higher spin multiplicities are found with AEST= 60 cal/mol (S=1), AEDQua = 120 Cal/mol (S=3/2), and AESQuin = 180 cal/mol (s=2). In another approach ...

Photodegradation of poly(3,5-dimethoxyacrylophenone) in the long-wave UV region

The photophysics and long-wave (λ ≥ 300 nm) photochemistry of poly(3,5-dimethoxyacrylophenone) (P3,5DMAP) were investigated. The quantum yield for fluorescence is low (< 10 −4) and the principal excited species is the triplet carbonyl. Emission (phosphorescence) occurs at 450 nm, the triplet energy (i.e. E T (0-0)) is 270 kJ mol −1 and the phosphorescence lifetime is long (23 μs). These observations are consistent with the formation of a low-lying π,π ∗ triplet which, in turn, is the result of substitution of the aromatic ketone by electron donating methoxy groups. The principal photoprocess is a Norrish type II decomposition which results in random chain scission. The low value of the quantum yield, i.e. Φ cs = 10 4 , is further evidence for a π,π ∗ triplet which is typically unreactive in abstraction reactions. In the case of P3,5DMAP, H-abstraction is a prerequisite for biradical formation (and for chain scission). Inhibition of chain scission was investigated, using naphthalene and cis-1,3-pentadiene as quenchers, and the data could be represented by Stern-Volmer kinetics. However, the quenching rate constants ( k O ) were lower than the corresponding diffusion controlled values, and it was proposed that the actual encounter of triplet and quencher is subject to adverse steric interactions associated with the proximity of the polymer chains.

ChemInform Abstract: Biradical Formation from Molecules with (Z)‐7‐Sulfonyl‐3‐hexen‐1,5‐ diyne Functionalities.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Ab Initio Study of the Bergman Reaction: The Autoaromatization of Hex-3-ene-1,5-diyne

The Bergman reaction of the hex-3-ene-1,5-diyne molecule (enediyne) to form the singlet biradical p-benzyne structure under ring closure has been studied by ab initio methods. The complete active space (CAS) SCF method was used for geometry optimizations at five points along the reaction path of the Bergman reaction. Subsequent calculations using the multireference second-order perturbation theory based on a CASSCF reference (CASPT2) established the energetics along the reaction path, especially for the reactant, transition state, and product. The energetics were further corrected for zero point vibrational energy at the CASSCF level of theory. The study incorporated four different basis sets: a double-zeta plus polarization (DZP), a triple-zeta plus double polarization (TZ2P), and two different average atomic natural orbital (ANO) basis sets. The study predicts the energy barrier to ring closure to be 25.0 +/- 3.1 kcal/mol and the enthalpy of reaction to be 4.9 +/- 3.2 kcal/mol. The latter value deviates significantly from the value estimated on the basis of group additivity and the Born-Haber cycle. The enediyne moiety is the main functional group of a new class of anticancer agents. The activation of the Bergman reaction in these drugs has been argued to be due to a small structural change in the terminal-terminal carbon distance of about -0.5 Angstrom originating from an epoxide conversion in the rest of the drug. The present study indicates that such a structural change is not sufficient for the Bergman autoaromatization to proceed at an appreciable rate. A new reaction path involving no biradical formation, resulting in the same products, is suggested.

Preparation and Thermal Behavior of Fluorine-Containing Phenylmaleimides

Fluorine content dependency on the melting point and thermal polymerization reactivity of fluorine-containing phenylmaleimides were investigated by differential scanning calorimetry and are discussed on the basis of molecular structure calculations. The syntheses of fluorine-containing maleimides were carried out by thermal dehydration. Their yields tended to be lower with increasing fluorine content. This was due to lowered nucleophilic reactivity of the amino group. The fluorine-containing maleimides tended to have a high melting point with increasing fluorine content. But symmetry in the chemical structure was a more important factor influencing the melting point. The thermal polymerization temperature of double bonds became higher with increasing fluorine content. This thermal polymerization reactivity could be explained from results of molecular structure calculations. The decreased thermal polymerization reactivity of fluorine-containing maleimides could be attributed to molecular stabilization or the difficulty of activated biradical formation as shown by MNDO or MM method calculations. This is in agreement with that of fluorine-containing bismaleimides.

Biradical Intermediates in Tetrafluoroethylene Dissociation and Difluorocarbene Recombination

Tetrafluoroethylene pyrolysis has been studied under the strictly homogeneous conditions of a free-piston monopulse adiabatic compression set-up, and biradical intermediate C ˙ F 2 − C ˙ F 2 is shown to precede CF 2 formation. The rate constant for biradical decomposition into two CF 2 species is estimated to be K 4 =10 13.5±0.1 exp(–201 ± 8 kJ mol −1 / RT ) s −1 . Based on the detailed equilibrium principle CF 2 recombination is proposed to take place through biradical formation.

Spin-coupled description of organic reaction pathways: the cycloaddition reaction of two ethene molecules

Spin-coupled (SC) theory is applied to the description of one of the simplest cycloaddition reactions: the orbital-symmetry-forbidden ground-state dimerization of two ethene molecules to cyclobutane: 2 C2H4→ C4H8. The aim is to show that the easy-to-visualize and easy-to-interpret features of the SC wavefunction, namely, the form of the valence orbitals and the nature of the spin-coupling pattern within the active space, provide important guiding information, which helps make qualitative predictions about the reaction pathway without carrying out a detailed investigation of the related potential surface.It is demonstrated that the SC orbitals from a four-orbital active space, corresponding to the forbidden concerted face-to-face approach, bend outwards of the C4H8 ring, which suggests that optimum overlap can be achieved only through a non-concerted approach, passing either through a cis, or through a trans transition state. A re-examination of the size of the active space needed for the proper valence-bond (VB) type analysis of the reaction shows that eight valence orbitals are required in order to achieve correlated descriptions of the two carbon–carbon double bonds in the reacting ethenes, of the three carbon–carbon single bonds and of the two unpaired electrons in the tetramethylene biradical intermediates, as well as of the four equivalent carbon–carbon single bonds in the final product, cyclobutane. SC calculations for the model reaction pathways leading to the formation of cis and trans tetramethylene biradicals, performed within an eight-orbital active space indicate that the trans pathway has a lower potential barrier and leads to a biradical lower in energy; the minimum corresponding to the formation of a cis biradical is rather shallow. The electronic structure of the tetramethylene biradicals is rationalized in easily conceivable terms, such as orbital shapes and spin-coupling patterns.