STRUCTURE OF RADICALS

Most radicals located on saturated bonds are jt-radicals with a planar configuration and may be depicted with the free spin located in a p-orbital (1). Because such radical centers are achiral, stereochemical integrity is lost during radical formation, A new configuration will be assumed (or a previous configuration resumed) only upon reaction. Stereoselectivity in radical reactions is therefore dependent on the environment and on remote substituents. 

Radicals with very polar substituents e.g. trifluoromethyl radical 2), and radicals that arc part of strained ring systems (e.g. cydopropyl radical 3) arc ct-radicals. They have a pyramidal structure and are depicted with the free spin resident in an spJ hybrid orbital. nr-Radicals with appropriate substitution are potentially chiral, however, barriers to inversion are typically low with respect to the activation energy for reaction. 

Most radicals located on double bonds (e.g. 4, 5) or aromatic systems (e.g. 6) are a-radicals. The free spin is located in an orbital orthogonal to the it-bond system and it is not delocalized. The orbital of the vinyl radical (4) containing the free spin can be cis- or trans- with respect to substituents on the double bond. The barrier for isomerization of vinyl radicals can be significant with respect to the rate of reaction. 

Radicals with adjacent Jt-bonds [e.g. allyl radicals (7), cyclohexadienyl radicals (8), acyl radicals (9) and cyanoalkyl radicals (10)] have a delocalized structure. They may be depicted as a hybrid of several resonance forms. In a chemical reaction they may, in principle, react through any of the sites on which the spin can be located. The preferred site of reaction is dictated by spin density, steric, polar and perhaps other factors. Maximum orbital overlap requires that the atoms contained in the delocalized system are coplanar. 

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In an electron spin resonance spectrometer, transitions between the two states are brought about by the application of the quantum of energy hv which is equal to g H. The resonance condition is defined when hv = g H and this is achieved experimentally by varying H keeping the frequency (v) constant. Esr spectroscopy is used extensively in chemistry in the identification and elucidation of structures of radicals. 

A. Klein, E.J.L. Mclnnes, T. Scheiring and S. Zalis, Electronic structure of radical anionic binuclear organoplatinum complexes. A multiple frequency EPR investigation, J. Chem. Soc., Faraday Trans., 1998, 94, 2979. 

Summarizing, this example provides several take-home lessons complete sets of hypersurface calculations for main-frame models of compounds can be quite helpful in close correlation to experimental data. Obviously, both the radical cation ground state structure and the angular dependence of the coupling constants are correctly predicted. In return, by introducing experimental data into the established correlations, the structure of radical cations in solution may be cautiously approximated. Altogether, this example teaches another lesson on how drastic those structural changes may be, which accompany even one-electron redox reactions. 

The structure of radicals and radical ions can also readily be predicted by treating an unpaired electron in the same manner as a free valence... 

Electron spin resonance (ESR) spectroscopy can be advantageously used to measure the radical concentrations of the nitroxide radicals (XV and XVI) produced, since these are much more stable then the R- radicals. Of greater importance, ESR can be used to determine the structure of R% since the ESR of the nitroxide radical is quite sensitive to the structure of R. (For this purpose, nitroso spin traps are more useful, since the R group in the nitroxide radical is nearer to the lone electron.) This can allow a determination of the structures of radicals first formed in initiator decomposition, the radicals that actually initiate polymerization (if they are not identical with the former) as well as the propagating radicals [Rizzardo and Solomon, 1979 Sato et al 1975],... 

The utility of ESR spectra for determining the structures of radicals is demonstrated by considering some examples. Methyl group substitution for hydrogen in the methyl radical ultimately results in slight deviation from planarity with a low inversion barrier. The a values for methyl, ethyl, isopropyl, and terf-butyl are 38.3, 39.1, 41.3, and 45.2 G, respectively. The ferf-butyl radical is indicated to have 10° deviation from planarity, which is confirmed by infrared (IR) and Raman spectroscopy. ... 

Heberger K, Lopata A (1998) Assessment of nudeophilicity and electrophilicity of radicals, and of polar and enthalpy effects on radical addition reactions. J Org Chem 63 8646-8653 HerakJN, Behrens G (1986) Formation and structure of radicals from D-riboseand 2-deoxy-D-ribose by reactions with SO4 radicals in aqueous solution. An "in-situ" electron spin resonance study. Z Naturforsch 41c 1062-1068... 

The long-standing interest of Boyd and his coworkers in radicals and radical ions has led to many papers since 1993 on hyperfine structures. These papers have pushed the conventional multireference configuration interaction methods to the limits of the available computers, tested the predictive ability of various functionals commonly used in DFT calculations, and, among other topics, modeled the effect of a noble gas matrix on the hyperfine structures of radicals. Recent research focused primarily on radicals formed as a consequence of radiation damage to DNA. 

Radical cation structure types can be classified according to the nature of the donor molecules, viz., it-, n-, or cr-donors, from which they are generated. Radical cations derived from typical it-donors may be closely related to the structure of their precursors, whereas substantial differences may be observed between the structures of radical cation and precursor for cr-donors. The potential surfaces of radical cations and their parents may differ in three features reaction barriers may be reduced, free energy differences between isomers may be reduced or reversed and energy minima on the radical cation surface may have geometries corresponding to transition structures on the parent potential surface. The pursuit of such novel structure types has given new direction to radical cation chemistry. Representative radical cation structures are discussed to document their rich variety and to illustrate the molecular features that determine their structures. 

The final contribution is focussed on organic radical cations in a comprehensive and fundamental manner. It starts out with experimental methods of generation and characterization followed by a discussion of various types of electron transfer induced reactions. In the last section unusual structures of radical cations are described. 

Fig. 7.8. Calculated dependencies (DFT) of the iso(29Sic) (a) and a oC S ) (b) for (Y Si -O-) 8 Sic radical (Y = F, H, OFI, hydroxyl groups are in /rani-configuration) on the Si,OSi/ bond angles. The spatial structure of radicals possessed the point symmetry group C3v.

T etrahy dropyran PM3 Ab initio and DFT with several basis sets Thermodynamic properties Structure of radical cation 1997PCA2471... 

All of the mechanisms that have been presented so far have involved the reaction of electrophiles with nucleophiles. Carbocations and carbanions have been encountered as intermediates. In this chapter the chemistry of a new reactive intermediate, called a radical (or free radical), is presented. A radical is a species with an odd number of electrons. After a discussion of the structure of radicals, including their stability and geometry, various methods of generating them are described. Next, the general reactions that they undergo are presented. Finally, specific reactions involving radical intermediates are discussed. 

There are two reasons why some radicals are more persistent than others (1) steric hindrance and (2) electronic stabilization. In the four extreme cases above, their exceptional stability is conferred by a mixture of these two effects. Before we can analyse the stability of other radicals, however, we need to look at what is known about the shape and electronic structure of radicals. 

How to analyse the structure of radicals electron spin resonance... 

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