Radical electron model

Radical IV can be considered as a unique phosphorus radical species. Reduction of the parent macrocycle with sodium naphtalenide in THF at room temperature gave a purple solution. The FPR spectrum displayed a signal in a 1 2 1 pattern, with flp(2P)=0.38 mT. DFT calculations on radical IV models indicated a P-P distance of 2.763 A (P - P is3.256 A in the crystal structure of the parent compound and the average value of a single P-P bond is 2.2 A). According to the authors, the small coupling constant arises from the facts that the principal values of the hyperfine tensor are of opposite sign and that the a P P one electron bond results from overlap of two 3p orbitals [88]. 

The second assumption has been effectively invalidated by the discovery of the hydrated electron. However, the effects of LET and solute concentration on molecular yields indicate that some kind of radical diffusion model is indeed required. Kuppermann (1967) and Schwarz (1969) have demonstrated that the hydrated electron can be included in such a model. Schwarz (1964) remarked that Magee s estimate of the distance traveled by the electron at thermalization (on the order of a few nanometers) was correct, but his conjecture about its fate was wrong. On the other hand, Platzman was correct about its fate—namely, solvation—but wrong about the distance traveled (tens of nanometers). 

Because of the precise control of the redox steps by means of the electrode potential and the facile measurement of the kinetics through the current, the electrochemical approach to. S rn I reactions is particularly well suited to assessing the validity of the. S rn I mechanism and identifying the side reactions (termination steps of the chain process). It also allows full kinetic characterization of the reaction sequence. The two key steps of the reaction are the cleavage of the initial anion radical, ArX -, and conversely, formation of the product anion radical, ArNu -. Modeling these reactions as concerted intramolecular electron transfer/bond-breaking and bond-forming processes, respectively, allows the establishment of reactivity-structure relationships as shown in Section 3.5. 

There is no question that, indirectly or directly, Kirrmann and Prevost were influenced by Lowry s theories for explanation of reaction mechanisms. Another important influence was Dupont, with whom they talked at length in the laboratory and who published a paper in 1927 in which he attempted to combine the electron octet theory of valence and Bohr s hydrogen electron model with classical concepts of stereochemistry. Dupont also adopted without reservation Lowry s application of ionic radicals in hydrocarbon chemistry. 66... 

In the previous four sections, several solvent radical ions that cannot be classified as molecular ions ( a charge on a solvent molecule ) were examined. These delocalized, multimer radical ions are intermediate between the molecular ions and cavity electrons, thereby bridging the two extremes of electron (or hole) localization in a molecular liquid. While solvated electrons appear only in negative-EAg liquids, delocalized solvent anions appear both in positive and negative-EAg liquids. Actually, from the structural standpoint, trapped electrons in low-temperature alkane and ether glasses [2] are closer to the multimer anions because their stabilization requires a degree of polarization in the molecules that is incompatible with the premises of one-electron models. 

This model shows the electron density for the odd electron of the planar methyl radical. The radical electron is in a p orbital perpendicular to the plane of the atoms. 

Extensive investigations on the catalytic mechanism of classical peroxidases resulted in a consensus model involving five different iron species [30, 31], These species are ferrous, ferric, Compound I, Compound II, and Compound III (Fig. 11.1). As discussed in Chap. 5, after the reaction of ground state (GS) Femporphyrin with H202, Compound I (Cl) is formed, a cationic oxob e,vpor-phyrin-based Ji-free radical. Electron paramagnetic resonance (EPR) studies established that, in peroxidases of classes I and III, the second oxidation equivalent in Cl is present as a porphyrin-based free radical [32, 33]. In peroxidases from fungal sources, electron abstraction from the protein results in the formation of a different species with the free radical based in a residue close to the porphyrin. 

Different theoretical models have been adopted to interpret the spectra of TEA(TCNQ)2 salt. More exact description requires regard to the tet-ramerization or dimerization of the chains in TEA(TCNQ)2. Thus the interpretation of TEA(TCNQ)2 spectra should be made in terms of the theory of e-mv coupling in tetramers with two radical electrons [51] or the dimer CT oscillation mechanism [13,20,52]. The application of the latter mechanism to the TEA(TCNQ)2 salt is shown in Fig. 9. 

A very good model system for studying the spectral properties of isolated TCNQ dimers with one radical electron is tetraethylammonium (TeEA) (TCNQ)2 salt [54,55]. The IR reflectivity was studied from various well-developed crystal faces. The fit of a dimer model with inequivalent molecules to the experimental data is shown in Fig. 10. The optimum values for the electronic mode parameters are (Oct = 3222 cm-1, ye = 2216 cm-1, f = 1864 cm-1, and A = 938 cm-1 [55]. 

The first discovery of chemically induced dynamic electron polarization (CIDEP) was made by Fessenden and Schuler in 1963 (58). These authors observed the abnormal spectra of the H atoms produced during the irradiation of liquid methane. The low-field line in the esr spectrum was inverted compared to the corresponding high-field line. The related chemically induced dynamic nuclear polarization effect (CIDNP) was reported independently four years later by Bargon et al. (22) and by Ward and Lawler (134). Because of the wider application of nmr in chemistry, the CIDNP effect immediately attracted considerable theoretical and experimental attention, and an elegant theory based on a radical-pair model (RPM) was advanced to explain the effect. The remarkable development of the radical-pair theory has obviously brought cross-fertilization to the then-lesser-known CIDEP phenomenon. 

Figure 11. Mechanisms for generating a 5 -deoxyadenosyl 5 -radical. (A) Model invoking an iron-carbon bond (B) model invoking a n,3-S-carbon bond (C) model invoking electron transfer from the iron-sulfur cluster to SAM.

In the following we summarize the experimental observations and the apparently contradictory conclusions which arise if the results are interpreted with either of the models outlined above. An alternative model is presented which invokes involvement of two electronic states of differing symmetry for the peroxy radical. The model is compatible with all the available experimental data on ethyl -f O2. 

Mager HIX, Sazou D, Liu YH, Tu S-C, Kadish KM. Reversible one-electron generation of 4 5-substituted flavin radical cations Models for a postulated key intermediate in bacterial bioluminescence. J Am Chem Soc 1988 110 3759-62. 

Spin properties are notoriously difficult to calculate accurately57. Here, we are actually calculating spin populations, with their intrinsic uncertainties, and not the directly observed hyperfine interactions. On the other hand, analyses of the hyperfine interactions in the ESR spectra to give experimental atomic orbital occupancies for the radical electron are based on a simplistic, rigid linear combination of atomic orbitals (LCAO)-MO model with the reference electron-nuclear coupling parameters taken from the free atom. No allowance is made for radial or angular polarization of the atomic orbitals in the molecular environment. Thus agreement at these levels between calculated and experimental values can only be qualitative, at best. 

Yartsev has considered the effects of electron correlation on a and y using a molecular dimer with two radical electrons as a model. Yartsev and Marcano explore the effects of electron correlation on the static y of dimers through the Hubbard model. 

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