Radical Cations and Radical Anions of Three Bridgehead Azopolycycloalkanes: A fluid-solution ESR study

The radical mono-ions of three azoalkanes in which the azo group is connected to the polycyclic alkane moieties at the bridgehead C-atoms, i.e. 1,1′-azonorbornane ( 1 ), 1,1′-azotwistane ( 2 ), and 1,1′-azobicyclo[3.2.1]octane ( 3 ), were studied in fluid solution by ESR spectroscopy. According to the ESR parameters and MO models, the radical cations of 1–3 should be considered as σ radicals, whereas the corresponding radical anions are π radicals. INDO calculations point to a a remarkable dependence of the ¹⁴N-coupling constants on the geometry at the N-atoms in the radical cations of aliphatic azo compounds.     

ESR. Study of an X-Irradiated Single Crystal of Methylene Diarsonic Acid

X-Irradiation of single crystals of methylene diarsonic acid gives rise to a variety of radicals which have been identified by ESR. spectroscopy; (OH)₂As(O)CHAs(O) (OH)₂, ^?OÅs(OH)₂CH₂As(O)(OH)₂, AsO₃^– and (OH)Ås(O)CH₂As(O)(OH)₂. The spin densities have been obtained from the ⁷⁵As-hyperfine tensors. An additional species - probably a radical anion localized on a pentacoordinated arsenic - is also detected after annealing at room temperature. The radiation damage process is discussed.     

Catalysis of Radical Reactions: A Radical Chemistry Perspective

The area of catalysis of radical reactions has recently flourished. Various reaction conditions have been discovered and explained in terms of catalytic cycles. These cycles rarely stand alone as unique paths from substrates to products. Instead, most radical reactions have innate chains which form products without any catalyst. How do we know if a species added in “catalytic amounts” is a catalyst, an initiator, or something else? Herein we critically address both catalyst‐free and catalytic radical reactions through the lens of radical chemistry. Basic principles of kinetics and thermodynamics are used to address problems of initiation, propagation, and inhibition of radical chains. The catalysis of radical reactions differs from other areas of catalysis. Whereas efficient innate chain reactions are difficult to catalyze because individual steps are fast, both inefficient chain processes and non‐chain processes afford diverse opportunities for catalysis, as illustrated with selected examples.     

Radical Cations and Radical Anions of Three Bridgehead Azopolycycloalkanes: A Fluid-Solution ESR Study

The radical mono-ions of three azoalkanes in which the azo group is connected to the polycyclic alkane moieties at the bridgehead C-atoms, i. e. 1,1′-azonorbornane ( 1 ), 1,1′-azotwistane ( 2 ), and 1,1′-azobi-cyclo[3.2.1]octane ( 3 ), were studied in fluid solution by ESR spectroscopy. According to the ESR parameters and MO models, the radical cations of 1 – 3 should be considered as σ radicals, whereas the corresponding radical anions are π radicals. INDO calculations point to a remarkable dependence of the ^l4N-coupling constants on the geometry at the N-atoms in the radical cations of aliphatic azo compounds.     

A single-crystal ESR study of bis(1,1-diisobutyl-3-benzoyl-selenoureato)-copper(II)

A single-crystal ESR study of bis(1,1-diisobutyl-3-benzoyl-selenoureato)copper(II) is reported. From the ⁷⁷Se-hfs the Se atoms of the CuSe₂O₂ coordination sphere were found to be in cis-position. In addition covalency considerations of the Cu-chalcogeno bonds have been made.     

Oxidation of aromatic compounds: XVI. Radical cations derived from acetylenic compounds with electron-withdrawing groups: Reactions and ESR parameters

One-electron oxidation of aryl-substituted acetylenes ArC≡CX where X is an electron-withdrawing group gives different products, depending on the X substituent. Acetylenic substrates with medium-strength electron-withdrawing substituents, X = CO₂R, COAr, COR, PO(OEt)₂, give rise to tetrasubstituted ethenes X(ArCO)C=C(COAr)X. Compounds with strong electron-withdrawing groups (X = COCF₃, COCO₂R, CN) are converted into furan derivatives. Probable mechanisms of transformations of ArC≡CX radical cations into the final products are discussed. Radical cations derived from disubstituted acetylenes were characterized by ESR spectroscopy.     

The Radical Cation of Azatriquinane: An ESR Study

Azatriquinane (=10-azatricyclo[5.2.1.0^1,10]decane; 1 ) was oxidized to its radical cation 1 ^.+ by γ- irradiation in a CF₂ClCFCl₂ matrix at 77 K. A prominent feature of the ESR spectrum of 1 ^.+ is the ¹⁴N-hyperfine anisotropy which broadens the components with M_I(¹⁴N)=±1. The observed coupling constants are a_N=+ 2.5 and a_H=+ 4.0 mT for the ¹⁴N nucleus and the three methine β-protons, respectively. While the a_N value points to pyramidalization at the N-atom comparable to that in the radical cation of quinuclidine (=1-azabicyclo[2.2.2]octane; 4 ), an eclipsing of the singly occupied orbital at this atom by the three C−H_β bonds is indicated in view of the large a_H value. Theoretical calculations on the geometry of 1 ^.+ are in accord with the conclusions drawn from the experimental findings.     

Reactions of H atoms and OH radicals with ascorbic acid: A pulse radiolysis Fourier transform ESR study

Using pulse radiolysis, free radicals of ascorbic acid were generated by reactions of the primary radicals H and OH in acidic and basic aqueous solutions. The formation and the decay of several radicals of ascorbic acid were detected by time resolved Fourier transform electron spin resonance within a time interval of 100 ns to 1 ms. The rate constant of addition of H atoms to ascorbic acid (1.3·10⁸ dm³· mol⁻¹·s⁻¹) was directly determined by the change of line width of the low field line of the H atom in the presence of ascorbic acid. The addition of OH radicals to ascorbic acid results in different radical structures, detected by highly resolved Fourier transform ESR spectra.     

Cations of aldehydes and ketones: an ESR study

Exposure of dilute solutions of acetaldehyde, acetone and cyclohexanone in fluorotrichloromethane to ⁶⁰Co γ-rays at 77 K gave the corresponding radical cations, characterised by ESR spectroscopy.     

A single-crystal and solution ESR study of bis(N,N-diethyl-N′-benzoylthioureato)-copper(II)

A single-crystal and solution ESR study of bis(N,N-diethyl-N′-benzoylthioureato)Cu(II) is reported. The bonding situation in this complex, which contains a S₂O₂ coordination sphere with S atoms in cis-position, is considered.     

自由基三氟甲基化反应进展

近年来,三氟甲基化反应得到了快速的发展和广泛的关注,由于三氟甲基本身的一些特殊化学和物理性质,使得其在医药、农药和材料等领域发挥着越来越重要的作用。 随着有机氟化学的发展,对于自由基三氟甲基化反应也有了新的认识。 本文通过对不同的三氟甲基试剂作为三氟甲基自由基的前体,综述了近年来自由基三氟甲基化反应的研究进展,并予以展望。     

水介质中的有机自由基反应

化学反应溶剂的绿色化是绿色化学发展的必然趋势,利用储量丰富、廉价易得、无毒性、无污染的水替代传统有机溶剂作为反应介质是化学家们追求的目标。自由基反应由于反应活性高、反应条件温和,逐渐成为有机合成的一种重要策略。本文依据所构建化学键类型的不同,对近5年来水介质中的有机自由基反应研究进展进行了综述。     

Sodium p-Toluenesulfinate in Free Radical Reactions

A sulfonyl radical induced addition reaction of p-tolyl p-toluenethiosulfonate with olefins by using sodium p-toluenesulfinate as sulfonyl radical precursor is described.     

Radical Anions of Polyalkylazulenes: An ESR and ENDOR Study

Proton-hyperfine data are reported for the radical anions generated from azulene ( 1 ) and its alkyl derivatives 2 – 11 in 1,2-dimethoxyethane both ‘chemically’ with K and electrolytically. The alkyl derivatives are 1,3-dimethyl- ( 2 ), 5,7-dimethyl- ( 3 ), 1,3,5,7-tetramethyl- ( 4 ), 2-methyl- ( 5 ), 4,6,8-trimethyl- ( 6 ), 2,4,6,8-tetramethyl-( 7 ), 1,3,4,6,8-pentamethyl- ( 8 ), 1,3,4,8-tetramethyl-6-propyl- ( 9 ), 6-(tert-butyl)-1,3,4,8-tetramethyl- ( 10 ), and 1,2,3,4,6,8-hexamethylazulene ( 11 ). Alkyl substituents at the odd-numbered centers μ = 1, 3, 5, and 7 partly shift the π-spin population from the seven- to the five-membered ring, whereas those at the even-numbered centers μ = 4, 6, and 8 exert an opposite effect on the π-spin distribution.     

有机化学教学中有关芳基重氮盐的游离基反应

芳基重氮盐是应用广泛的一类有机中间体,芳基重氮盐的亲核取代反应在历程上一般可分为3种类型:一种是经芳基正离子的历程;另一种是经中间加成物分解的历程;还有一种是经芳基游离基的历程。本文着重讨论了芳基重氮盐经芳基游离基转化的反应及历程。     

A single-crystal and powder ESR study of copper(II)-bis(diethyl-thioselenocarbamate). Noncoinciding principal axes of g and A

Single-crystal ESR studies of Cu(II)bis(diethyl-thioselenocarbamate) (Cu(et₂tsc)₂), diluted in the corresponding Zn(II) complex, and ESR studies on Cu(et₂tsc)₂ in polycrystalline Ni(et₂tsc)₂ are reported. The g tensor and the Cu and Se hyperfine coupling tensors having been obtained. From the measured ⁷⁷Se hfs it can be concluded that the guest molecules, when doped in Zn(et₂tsc)₂, do not accept the structure of the host crystal but have a structure similar to that of the pure copper complex. The principal axes of the g and the Cu hfs tensor do not coincide. The large spin-orbit coupling of the Se atoms, participation of Se 4d orbitals and the low symmetry of the complex are considered as the reasons for this behaviour.     

ESR Study of a Nitroxide Radical in Sol-Gel Glasses

A spin probe TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidinyl-1-oxy) was dissolved in a tetraethyl orthosilicate sol-gel reaction system and measured by electron spin resonance spectroscopy at 295 K. The nitrogen hyperfine coupling constant was from 1.64–1.66 mT in the sol-gel solutions. The values were sensitive to the ethanol-to-water ratio of the solutions. The hyperfine coupling constant in the xerogels was 1.70 mT, which was almost the same as that in water, indicating that the probe molecules were trapped in silica pores with water adsorbed on the silica surfaces. The motion of TEMPOL in the xerogels was considerably slower than in the sol-gel solutions. The local viscosity estimated was from 70–90 cP. The ESR spectra of TEMPOL were altered during the sol-gel process, indicating that adsorbed water on the silicas surfaces has an important role for trapping organic molecules in sol-gel glasses.