Radical ions electronic structures

Ignasiak, T., Kemp-Jones, A. V. and Strausz, 0. P., "The Molecular Structure of Athabasca Asphaltenes. Cleavage of the Carbon-Sulfur Bonds by Radical Ion Electron Transfer Reactions," J. Org. Chem., 1977, 42(2), 312-320. 

Free Radicals. In the formula of a polyatomic radical an unpaired electron(s) is(are) indicated by a dot placed as a right superscript to the parentheses (or square bracket for coordination compounds). In radical ions the dot precedes the charge. In structural formulas, the dot may be placed to indicate the location of the unpaired electron(s). 

Exceeding the limitation of molecular dynamics, the steric requirement of trimethylsilyl groups can cause drastic changes both in structure and of molecular properties of organosilicon compounds. For illustration, the so-called "Wurster s-Blue11 radical ions are selected On one-electron oxidation of tetramethyl-p-phenylenediamine, its dark-blue radical cation, detected as early as 1879 [11a], is gene-... 

It has been pointed out already that formation of a radical anion by a redox process in solution produces an ion pair and that any hopping of the electrons will thus be bound to the migration of the cation, which then becomes rate-limiting (Gerson et al., 1972, 1974, 1990). The, ion-pair structure of the radicals is mainly affected by the size of the counterions and the ion-solvating capability of the solvent (Hogen-Esch, 1977 Szwarc, 1968). 

For many years, investigations on the electronic structure of organic radical cations in general, and of polyenes in particular, were dominated by PE spectroscopy which represented by far the most copious source of data on this subject. Consequently, attention was focussed mainly on those excited states of radical ions which can be formed by direct photoionization. However, promotion of electrons into virtual MOs of radical cations is also possible, but as the corresponding excited states cannot be attained by a one-photon process from the neutral molecule they do not manifest themselves in PE spectra. On the other hand, they can be reached by electronic excitation of the radical cations, provided that the corresponding transitions are allowed by electric-dipole selection rules. As will be shown in Section III.C, the description of such states requires an extension of the simple models used in Section n, but before going into this, we would like to discuss them in a qualitative way and give a brief account of experimental techniques used to study them. 

The third ligand was assumed to be coordinated to the metal center via the deprotonated 3-hydroxy and 4-carbonyl groups. This coordination mode allows delocalization of the electronic structure and intermolecu-lar electron transfer from the ligand to Cu(II). The Cu(I)-flavonoxy radical is in equilibrium with the precursor complex and formed at relatively low concentration levels. This species is attacked by dioxygen presumably at the C2 carbon atom of the flavonoxyl ligand. In principle, such an attack may also occur at the Cu(I) center, but because of the crowded coordination sphere of the metal ion it seems to be less favourable. The reaction is completed by the formation and fast rearrangement of a trioxametallocycle. 

The broad emission and low-fluorescence quantum yield of PPS suggested a distribution of trapping sites in the Si skeleton, which were also considered responsible for the lower-than-expected conductivity. The far-IR spectrum of PPS suggested the existence of cyclohexasilane rings connected by linear chains.361,362 Subsequent investigations by Irie et al. on the electronic absorption spectra of radical ions of poly(alkylsilyne)s were taken to indicate the presence of various cyclic silicon species, in corroboration of this conclusion.363 The large Stokes shift and broadness of the fluorescence emission indicate a range of fluorophore structures, different from the chromophore structures. This is... 

This general approach has, however, serious limitations. The position of the site for attack (and therefore the electron transfer distance involved) is very conjectural. In addition, the vexing possibility, which we have encountered several times, of a dead-end mechanism (Sec. 1.6.4) is always present. One way to circumvent this difficulty, is to bind a metal complex to the protein at a specific site, with a known (usually crystallographic) relationship to the metal site. The strategy then is to create a metastable state, which can only be alleviated by a discernable electron transfer between the labelled and natural site. It is important to establish that the modification does not radically alter the structure of the protein. A favorite technique is to attach (NH3)5Ru to a histidine imidazole near the surface of a protein. Exposure of this modified protein to a deficiency of a powerful reducing agent, will give a eon-current (partial) reduction of the ruthenium(III) and the site metal ion e.g. iron(III) heme in cytochrome c... 

Radical ions - charged species with unpaired electrons - are easily generated by a number of methods that are discussed in more detail below. Their properties have been characterized by several spectroscopic techniques, and their structures and spin density contributions have been the subject of molecular orbital calculations at different levels of sophistication. The behaviour of radical ions in rearrangement and isomerization reactions as well as in bond-cleavage reactions has been extensively studied [for recent reviews see Refs. 11-13 and references cited therein]. Useful synthetic applications, such as the radical-cation-catalyzed cycloaddition [14-20] or the anfi-Markovnikov addition of nucleophiles to alkenyl radical cations [21-25], have been well documented. In... 

I Nature of Organic Ion-Radicals and Their Ground-State Electronic Structure... 

The concept of molecular orbitals (MOs) helps to explain the electron structure of ion-radicals. When one electron abandons the highest occupied molecular orbital (HOMO), a cation radical is formed. HOMO is a bonding orbital. If one electron is introduced externally, it takes the lowest unoccupied molecular orbital (LUMO), and the molecule becomes an anion-radical. LUMO is an antibonding orbital. Depending on the HOMO or LUMO involved in the redox reaction, organic donors appear as n, a, or n species, whereas organic acceptors can be tt or a species. Sometimes, a combination of these functions takes place. 

Connections between Ion-Radicae Reactivity and Electronic Structure oe Ion-Radical Products... 

However, there are some cases when an unpaired electron is localized not on the n, but on the o orbital of an anion-radical. Of course, in such a case, a simple molecular orbital consideration that is based on the n approach does not coincide with experimental data. Chlorobenzothiadiazole may serve as a representative example (Gul maliev et al. 1975). Although the thiadiazole ring is a weaker acceptor than the nitro group, the elimination of the chloride ion from the 5-chlorobenzothiadiazole anion-radical does not take place (Solodovnikov and Todres 1968). At the same time, the anion-radical of 7-chloroquinoline readily loses the chlorine anion (Fujinaga et al. 1968). Notably, 7-chloroquinoline is very close to 5-chlorobenzothiadiazole in the sense of structure and electrophilicity of the heterocycle. To explain the mentioned difference, calculations are needed to clearly take into account the o electron framework of the molecules compared. It would also be interesting to exploit the concept of an increased valency in the consideration of anion-radical electronic structures, especially of those anion-radicals that contain atoms (fragments) with available d orbitals. This concept is traditionally derived from valence-shell expansion through the use of d orbital, but it is also understandable in terms of simple (and cheaper for calculations) MO theory, without t(-orbital participation. For a comparative analysis refer the paper by ElSolhy et al. (2005). Solvation of intermediary states on the way to a final product should be involved in the calculations as well (Parker 1981). 

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