Radical cations conformations

Aromatic sulfides analogous to thiophenols constitute a group of molecules that fulfils the structural conditions necessary for the observation of FET (Sec. 2.4), i.e. they exhibit a low barrier to rotation about the Qp2 S bond. Thus, the torsion motions of the substituents can be accompanied by considerable fluctuation of the electrons in the highest molecular orbitals with two extreme examples of conformers, planar and vertical. The presence of two radical cation conformers was deduced as primary products of the bimolecular free electron transfer (FET) from aromatic sulfides PhSCH2Ph, PhSCHPhj, and PhSCPhg to w-butyl chloride radical based on the nanosecond pulse radiolysis experiments. ... 

H321). Tliese radical cations exist in a nonplanar conformation. With fra s-3,4-dimethyl substituents, the barrier to ring flip was estimated to be >5 kcal/mol. In connection with reactions of thiirane with the thiirane radical cation, ab initio calculations were reported on the 1,2-dithietane radical cation (93JA12510). 

Later we will describe both oxidation and reduction processes that are in agreement with the electrochemically stimulated conformational relaxation (ESCR) model presented at the end of the chapter. In a neutral state, most of the conducting polymers are an amorphous cross-linked network (Fig. 3). The linear chains between cross-linking points have strong van der Waals intrachain and interchain interactions, giving a compact solid [Fig. 14(a)]. By oxidation of the neutral chains, electrons are extracted from the chains. At the polymer/solution interface, positive radical cations (polarons) accumulate along the polymeric chains. The same density of counter-ions accumulates on the solution side. 

Direct esr evidence for the intermediacy of radical-cations was obtained on flowing solutions of Co(III) acetate and a variety of substituted benzenes and polynuclear aromatics together in glacial acetic acid or trifluoroacetic acid solution . A p value of —2.4 was reported for a series of toluenes but addition of chloride ions, which greatly accelerated the reaction rate, resulted in p falling to —1.35. Only trace quantities of -CH2OAC adducts were obtained and benzyl acetate is the chief product from toluene, in conformity with the equation given above. 

Conformational isomerism in linear conjugated polyene radical cations... 

The present volume deals with the properties of dienes, described in chapters on theory, structural chemistry, conformations, thermochemistry and acidity and in chapters dealing with UV and Raman spectra, with electronic effects and the chemistry of radical cations and cations derived from them. The synthesis of dienes and polyenes, and various reactions that they undergo with radicals, with oxidants, under electrochemical conditions, and their use in synthetic photochemistry are among the topics discussed. Systems such as radialenes, or the reactions of dienes under pressure, comprise special topics of these functional groups. 

The suprafacial shift along the carbon framework is not restricted to cyclic systems but may also prevail in acyclic cases. In the example given in Scheme 11, minimization of dipolar repulsion between the two C-0 bonds mandates a preferred conformation of the initial radical, leading to a stereo-chemically defined alkene radical cation and, ultimately, to a single diastere-omer of the product [119]. 

Recently, the radical cation of PBN has been characterized by matrix spectroscopy and its reactivity has been studied by fast spectroscopic methods (Zubarev and Brede, 1994), and found to conform to the behaviour deduced from the OsCU and TBPA + studies. y-Radiolysis of PBN in a glassy matrix of isobutyl chloride or Freon-113 (CF2C1CFC12) at 77 K produced an intensely green glass containing PBN +, the epr spectrum of which had an anisotropic nitrogen coupling constant Ay = 2.75 mT and gy = 2.0037. Tlie mechanism of the radiolysis reaction is well established (Neta, 1976) and involves the formation of solvated electrons (e ), which add to the matrix species and produce chloride ion, and positive holes (h+) which eventually come to rest at the matrix component of lowest Ip (Symons, 1997), in this case PBN (see reactions (30) and (31)). 

Mattay et al. examined the regioselective and stereoselective cyclization of unsaturated silyl enol ethers by photoinduced electron transfer using DCA and DCN as sensitizers. Thereby the regiochemistry (6-endo versus 5-exo) of the cyclization could be controlled because in the absence of a nucleophile, like an alcohol, the cyclization of the siloxy radical cation is dominant, whereas the presence of a nucleophile favors the reaction pathway via the corresponding a-keto radical. The resulting stereoselective cis ring juncture is due to a favored reactive chair like conformer with the substituents pseudoaxial arranged (Scheme 27) [36,37]. 

Figure 7. Calculated bond lengths (pm) and hyperfine couplings (G) for the syn- and ant/-conformers of vinylcyclopropane radical cationic states (MP4SDQ/6-31G )." ...

Figure 6.9. Top to bottom Two possible Jahn-Teller distorted geometries for the methane radical cation and calculated geometry for its energy minimum schematic representation of the SOMO for ethane radical cation SOMOs of three different propane radical cations observed by ESR the SOMO of butane radical cation and SOMOs for two conformers of pentane radical cation.

The radical cation (19 " ) of the strained bicyclo[2.1.0]pentane also has a puckered conformation, supported by one strongly coupled (flagpole) proton (flsyn = 4.49 mT). Ab initio calculations indicate that the transannular bond retains some bonding and that the bridgehead carbons remain pyramidal. ... 

---