Structure of Free Radicals

Spin density surface for 2-methyl- 2-propyl radical shows the location of the unpaired electron. 

What is the preferred geometry about the radical center in free radicals Carbocation centers are characterized by a vacant orbital and are known to be planar, while carbanion centers incorporate a nonbonded electron pair and are typically pyramidal (see Chapter 1, Problem 9). 

First examine the geometry of methyl radical. Is it planar or puckered Examine the geometries of 2-methy 1-2-propyl radical, trifluoromethyl radical, trichloromethyl radical and tricyanomethyl radical. Classify each of the substituents (methyl, fluoro, chloro and cyano) as a n-electron donor or as a Tt-electron acceptor (relative to hydrogen). Does replacement of the hydrogens by 7t-donor groups make the radical center more or less puckered Does replacement by Jt-acceptor groups make the radical center more or less puckered Justify your observations. 

Display spin density surfaces for all radicals. For which radical is the unpaired electron least delocalized from the radical center For which is it the most delocalized Is there any relationship between degree of puckering of the radical center and extent of spin delocalization  

Onto which atoms (carbon, nitrogen or both) is the unpaired electron in tricyanomethyl radical delocalized Rationalize your result by drawing resonance contributors. 

---

Since these early experiments, a great deal of additional information about the existence and properties of free-radical intermediates has been developed. In this chapter, we will discuss the structure of free radicals and some of the special properties associated with free radicals. We will also discuss some of the key chemical reactions in which free-radical intermediates are involved. 

The structure of free radical 85 produced by dissociation of a stannyl benzopinacolate, was investigated by ESR spectroscopy and confirmed by ESR simulation. Free radical 85 is a good source for stannyl free radicals, as shown in reaction 10262. 

During the long Antarctic night, appreciable amounts of molecular chlorine, Cl, and hypochlorous acid, HOCl, accumulate within the polar vortex. When the sun returns during the spring (in September in Antarctica), ultraviolet radiation decomposes the accumulated molecular chlorine and hypochlorous acid to produce atomic chlorine. Cl. Atomic chlorine is a free radical. Free radicals are atoms or molecules that contain an unpaired or free electron. The Lewis structures of free radicals contain an odd number of electrons. The unpaired electron in free radicals makes them very reactive. The free radical Cl produced from the decomposition of CI2 and HOCl catalyzes the destruction of ozone as represented by the reaction ... 

Since only free radicals give an esr spectrum, the method can be used to detect the presence of radicals and to determine their concentration. Furthermore, information concerning the electron distribution (and hence the structure) of free radicals can be obtained from the splitting pattern of the esr spectrum (esr peaks are split by nearby protons).141 Fortunately (for the existence of most free radicals is very short), it is not necessary for a radical to be persistent for an esr spectrum to be obtained. Esr spectra have been observed for radicals with lifetimes considerably less than 1 sec. Failure to observe an esr spectrum does not prove that radicals are not involved, since the concentration may be too low for direct observation. In such cases the spin trapping technique can be used.142 In this technique a compound is added that is able to combine with very reactive radicals to produce more persistent radicals the new radicals can be observed by esr. The most important spin-trapping compounds are nitroso compounds, which react with radicals to give fairly stable nitroxide radicals 143 RN=0 + R —> RR N—O. 

II/III Structure of free radicals by ESR spectroscopy, including spin trapping (no direct organometallic content) 71... 

Ultraviolet and visible absorption spectroscopy has been a powerful aid to studies of free radicals in the gas phase but is of less value in the solid phase because of line broadening and consequent lack of resolution. Under suitable conditions, however, solid state infrared spectra can give information on the structure of free radicals and on the forces holding the constituent atoms in their equilibrium positions. The technique is less sensitive than e.s.r. because the extinction coefficients of vibrational transitions are generally low but the ability to observe non-para-magnetic molecules in addition to free radicals is sometimes advantageous. 

In CIDNP studies the pattern of nuclear spin polarizations is used to deduce the structures of free radical intermediates, and if there is a simultaneous NOE this can affect the conclusions drawn. (344) In the photolysis of [10] to yield [12] the biradical intermediate [11] should not cause significant polarization of the olefinic protons of [10] but in fact these are observed to have weak emission. This raises the possibility that the intermediate is really the biradical [13], but a homonuclear double resonance experiment which destroyed the cyclopropyl spin polarization of [10] eliminated the olefinic emission which was evidently solely due to the NOE. Thus it is confirmed that [11] is indeed the intermediate in the reaction. (344)... 

Vibrational Structure of Free Radicals in Dense Media... 

The term vtiamin C refers to ascorbic acid (the fully reduced form of the vitamin) and to dchydroascorbic acid- Removal of one electron from ascorbic acid yields semidehydroascorbic acid (ascorbate radical). This form of the vitamin is a free radical it contains an unpaired electron- The structures of free radicals are written with large dots. The removal of a second electron yields dehydroascorbic acid. Conversion of ascorbate to dehydroascorbate, via the removal of two electrons, can occur under two conditions (1) with use of ascorbic add by ascorbate-dependent enzymes and (2) with the spontaneous reaction of ascorbate with oxygen. Semidehydroascorbate is an intermediate in this conversion palhway... 

Herzberg contributed to the field of atomic and molecular spectroscopy, where he and his colleagues determined the structures of a large number of diatomic and polyatomic molecules, the structures of free radicals, and the identification of certain molecules in planetary atmospheres, in comets, and in interstellar space. In 1971 he was awarded the Nobel Prize in chemistry for his contributions to the knowledge of electronic structure and geometry of molecules, particularly free radicals. ... 

Fig. 3.5 Chemical structures of free radical scavengers that have been used for the treatment of stroke. Edaravone (a) tirilazad (b) ebselen (c) and NXY-059 (d)...

A-1) Chemical structure of free radical is assigned from the magnitude of the hfs which is related to the strength of the interaction between the electron and nuclear spins and from the number of the splittings. 

A-2) Stereo-structure of free radical can be obtained from McConnell s relation [1]. This is the dependence of the isotropic hfs interaction, on dihedral angle, 0, shown in Fig. 7.1 as an example in the case of polyethylene end radical. 

The concentration and structure of free radicals generated in D-glucose, lactose and cellulose by )f-irradiation have been determined by the chemiluminescence produced on contact with distilled water or... 

Table 1.2 Structure of free-radically initiated polydienes...

Figure 7. Structure of free radical interceptors present in green tea and in rosemary.

The investigations carried out earlier characterise the reactivity of different classes of polymers with NO. However, the mechanisms of free-radical processes proposed on the basis of the results considered are rather formal. As a rule, they take account of changing molecular weights and the composition of final molecular products of the nitration. In connection with this, the study of structures of free radicals forming in primary and intermediate stages of polymer conversions attracts special interest. Such research allows drawing conclusions on the mechanism of initiation of free-radical conversions dependent on the nature of functional groups of macromolecules. As... 

By the method of electron spin echo, weak hyperfine interactions of a number of imidazoline nitroxides with the magnetic nuclei of the surrounding molecules of the matrix were investigated (in frozen solution). Analysis of modulation effects in the primary echo made it possible to determine the number oi the nearest molecules of the solvent surrounding the radicals and the effective distances to them the results were compared with the structure of free radicals (Yudanov et a/., 1976 Dikanov et al, 1977). 

---