Electron spin resonance radical conformation

The electron spin resonance and conformations of cation radicals from 1,4-... 

Electron paramagnetic resonance spectroscopy (HER), also called electron spin resonance spectroscopy (ESR), may be used for direct detection and conformational and structural characterization of paramagnetic species. Good introductions to F.PR have been provided by Fischer8 and I.effler9 and most books on radical chemistry have a section on EPR. EPR detection limits arc dependent on radical structure and the signal complexity. However, with modern instrumentation, radical concentrations > 1 O 9 M can be detected and concentrations > I0"7 M can be reliably quantified. 

D. J. Edge and J. K. Kochi, Effects of halogen substitution on alkyl radicals conformational studies by electron spin resonance. J. Am. Chem. Soc. 94(18), 6485-6495 (1972). 

This different behavior can be explained by the conformation of the radicals and by stereoelectronic effects [6]. Electron spin resonance (ESR) investigations have revealed that the D-glucopyranosyl radical 7 does not adopt the 4C1 conformation 7a, but is distorted into the B2 5 shape 7b (Scheme 5) [7,8], The equatorial-like attack at the boat conformer... 

H.-G. Korth, R. Sustmann, J. Dupuis, and B. Giese, Electron spin resonance spectroscopic investigation of carbohydrate radicals. Part 2. Conformation and configuration in pyranos-l-yl radicals, J. Chem. Soc. Perkin Trans. 2 1453 (1986). 

TL4833). The radical was characterized by infrared (IR) spectroscopy, electron spin resonance (ESR), and mass spectrometry (MS), and proposed as a possible biological marker. A subsequent X-ray analysis showed 36 and 38 to have chair conformations—somewhat flattened at P in 36 and at N in 38 (83CJC427). In the phosphine oxide 38, the P—O bond is axial and the P—Ph bond is equatorial. 

The molecular and electronic structures of cyclic disulfide cation radicals of 1,2-dithietane 6 and 1,2-dithiete 7, and radical cations of 1,2-dithiolane 2 (2a-c represent stable conformations determined in terms of the symmetry restriction of Cs, Cz, and Czv), with emphasis on the nature of a two-center three-electron (Zc-ie) sulfur-sulfur bond have been examined by ab initio molecular orbital (MO) calculations <1997JMT(418)171>. Unrestricted Hartree-Fock (UHF)/ MIDI-4(d) computations showed that this bond in organodisulfide radical cation 2 is shorter in comparison to 1,2-dithiolane 2 and possesses partial Jt-bond character (structure A), as previously implied by electron spin resonance (ESR) spectroscopy <1982JA2318>, which correlates best with the form as the most favorable conformation of the cation radical 2. Contrary to the repulsive S-S interaction in the parent 1,2-dithiolane arising from the lone pairs of electrons, the hemi-7t-bond formed by one-electron oxidation should stabilize the five-membered ring of 2, or, for example, a similar cation radical of LA 3 which is involved in diverse biochemical reactions. 

The conformation of (aziridinyl)methyl radical has been studied by electron spin resonance (ESR) spectroscopy (71TL2247, 74JA2447, 74PMH(6)95>. An ESR study of the radical cation of aziridine has appeared <86JPC2292>. 

The radical anions of (43) and (44) have been examined by electron spin resonance spectroscopy <86JCS(P2)185i>. The ESR spectrum of the radical anion of (43) has been interpreted in terms of the presence of both (E,E)- and (Z, )-conformers and the activation parameters for the interconversion have been determined. 

Within the last two decades Electron Spin Resonance-(ESR) spectroscopy has become a standard experimental technique in electrochemical research. The main interest was in the field of electrochemical generation of radicals to characterize their structure by ESR spectroscopy or to prove their presence in electrode reactions. The studies have been extended to the kinetics of radical reactions and the set up of reaction mechanism, to the solvation phenomena in radical electron densities and to radical conformation and ion complex structure. The latest development is the study of the electrode materials and their surface layers in electrochemical systems by simultaneous ESR spectroscopic and electrochemical measurements, e.g., of polymer modified electrodes. 

Korth H, Sustmann R, Groninger KS, Witzel T, Giese B (1986) Electron spin resonance spectroscopic investigation of carbohydrate radicals. Part 3. Conformation in deoxypyranosan-2-, 3-, and 4-yl radicals. J Chem Soc Perkin Trans 2 1461-1464... 

Also, it was demonstrated that acyclic radicals can react with high stereoselectivity [45]. In order for the reactions to be stereoselective, the radicals have to adopt preferred conformations where the two faces of the prochiral radical centers are shielded to different extents by the stereogenic centers. Giese and coworkers [49] demonstrated with the help of Electron Spin Resonance studies that ester-substituted radicals with stereogenic centers in (3-positions adopt preferred conformations that minimize allylic strain [49] (shown below). In these conformations, large (L) and medium sized substituents (M) shield the two faces. The attacks come preferentially from the less shielded sides of the radicals. Stereoselectivity, because of A-strain conformation, is not limited to ester-substituted radicals [50]. The strains and steric control in reactions of radicals with alkenes can be illustrated as follows [50] ... 

Matsumoto, A. Giese, B. Conformational structure of methacrylate radicals as studied by electron spin resonance spectroscopy From small molecule radicals to polymer radicals. Macromolecules 1996,29, 3758-3772. 

The electron spin resonance spectrum of the radical ion complex of pyracene in tetrahydrofuran (Fig. 15) revealed that the structure of the complex is d)mamic in nature. A marked line-width alternation is observed 112-114), which arises from a d)mamic equilibrium existing between the two possible conformations of the ion pair with the counter ion in position A and in position B. 

Kochi, J.K. Configurations and Conformations of Transient Alkyl Radicals in Solution by Electron Spin Resonance Spectroscopy. Adv. Free-Radical Chem. 5 (1975) 189. Williams, G.H., Editor London Elek Science. 

An electron spin resonance study of intermediates in radical addition of halogens to olefins suggests an intermediate radical having a preference for the conformation (641) at low temperatures. ... 

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