Radicals three-electron bonded

Addition of small amounts of (CH3)2S (5 x 10 5 — 5 x 10"4m) to deoxygenated solutions of 3m HC104 and 0.5m DMSO leads to replacement of the 285 nm absorption by 465 nm absorption which is known to belong to the complexed three-electron-bonded radical cation of the disulfide47. 

In sharp contrast to the stable [H2S. .SH2] radical cation, the isoelectron-ic neutral radicals [H2S.. SH] and [H2S. .C1] are very weakly-bound van der Waals complexes [125]. Furthermore, the unsymmetrical [H2S.. C1H] radical cation is less strongly bound than the symmetrical [H2S.. SH2] ion. The strength of these three-electron bonds was explained in terms of the overlap between the donor HOMO and radical SOMO. In a systematic study of a series of three-electron bonded radical cations [126], Clark has shown that the three-electron bond energy of [X.. Y] decreases exponentially with AIP, the difference between the ionisation potentials (IP) of X and Y. As a consequence, many of the known three-electron bonds are homonuclear, or at least involve two atoms of similar IP. 

Fig. 39). The ESR data for dimethylselenide dimer cation radical 107c are consistent with two-center three-electron bonding, in which two electrons are in a cr bond between the two chalcogens, while the third electron is in a o orbital. 

A review considering the generation and characterization of radical ions, their reactions, formation of species with three-electron bonds, and radical cations of strained systems has been published." The redox and acidity properties of a number of substituted benzene radical cations were smdied by pulse radiolysis. ... 

Because the three-electron-bonded radicals are formed at the cost of the removal of the nitrogen p-electron, such cation-radicals should be considered as p-acids. Of course, the formation and behavior of these p-acids have to be dependent on steric factors. Works by Tomilin et al. (1996, 2000), Bietti et al. (1998), Dombrowski et al. (2005), and Yu et al. (2007) should be mentioned as describing stereoelectronic requirements to formations and configurational equilibria of A-alkyl-substituted cation-radicals. 

One-electron oxidation of l,6-diazabicyclo[4.4.4]tetradecane proceeds at a remarkably low rate. The cation-radical obtained contains a three-electron o bond between the two nitrogen atoms (Alder and Sessions 1979). In this case, the three-electron bond links the two nitrogens that are disjoined in the initial neutral molecule, at the expense of one electron from the lone electron pair of the first nitrogen and the two electrons of the second nitrogen, which lasts as if it is unchangeable. The authors named such a phenomenon as strong inward pyramidalization of the nitrogens with remarkable flexibility for the N—N interaction. This interaction results in 2a-la bond formation (Scheme 3.21). 

The molecular geometry, which allows optimal p orbital interaction to yield a three-electron bond, presumes an orientation of p orbitals belonging to each sulfur atom along the S S axis. This is the case of the chair-boat conformer of the 1,5-dithiacyclooctane cation-radical, the first structure in Scheme 3.22.In the 1,3-dithiacyclopentane cation-radical, the sulfurp orbitals are aligned almost perpendicular to the ring plane, and this prevents stabilization by the transannular interaction between the two sulfur atoms in the cycle. This unreal structure (the second structure in brackets in Scheme 3.22) cannot exist. However, the cation-radical of bis(2-methyl-1,3-dithianyl)methanol (the third structure in Scheme 3.22) was predicted to exist Li and Kutateladze (2003) calculated this structure as the most stable because it differs by a special orbital pattern from the structure in brackets. 

There are three possible types of three-electron bonds. Oxidation of a u bond leads to a cation-radical with a, u three-electron bond. This bond contains no antibonding electrons, and the total bond strength exceeds that of a double bond by the energy of half a n bond. Olefins can acquire the 2a—In bond on one-electron oxidation, the bond constructed from the electrons 2a and In. Oxidation of organic disulfides, RSSR, to their cation-radicals (RSSR) yields species in which the unpaired electron from the oxidized sulfur interacts with the unbound p-electron pair of the second sulfur (Glass 1999). This establishes a 2n-In bond on top of the already existing o bond. The overall bond strength of this five-electron (2a—2n-In ) bond also exceeds that of the normal... 

Scheme 3.30 depicts an intriguing case, when one-electron oxidation of the conformationally constrained exo-2-(carboxy)-en(i(9-2-(ammo)-en(i(9-6-(methylthio)-bicyclo[2.2.1]heptane gives rise to a cation-radical in which an amino- and not a carboxylate group participates in the three-electron bond with sulfur (Glass 1995). 

The onium form of the ethyl acetate cation-radical is more stable by 50 kJ moH than the corresponding carbonyl form (Rhodes 1988). The CHj fragment is stabilized by the three-electron bonding with the neighboring oxygen in the following manner -O.. CH2. Oxidation of the carbonyl... 

Oxidation of iodoalkanes involves removal of an electron from the halogen nonbonding orbital. The radical-cations of primary and secondary alkyl iodides can be identified in aqueous solution by their absorption spectra and have half-lives of microseconds [1]. They are formed during pulse radiolysis of the iodoalkane in aqueous solution in the presence of nitrous oxide. This system generates hydroxyl radicals, which remove an electron from the iodine atom lone pair. Iodoalkane radical-anions complex with the lone-pair on other heteroatoms to form a lollo three-electron bond. In aqueous solution, the radical-cation of iodomethane is involved in an equlibrium indicated by Equation 2.1. 

Related three-electron bond radical-cations 1 are formed from dialkyl sulphides by oxidation with hydroxyl radicals generated using pulse radiolysis [2]. An isoe-lectronic three-electron bond between two nitrogen atoms in 2 is formed by reduc-... 

Diamines with a cage-like structure in which the nitrogen atoms are separated by a three carbon chain, form radical-cations where the non-bonding orbital from one nitrogen atom interacts with the radical-cation on the other nitrogen atom to form a three-electron bond [76]. Cyclic voltammetry of 16 in acetonitrile shows two reversible waves with E° = 0.11 and 0.72 V vs. see. The second wave is due to the formation of a dication with a tw o-electron bond between the nitrogen atoms. 

In the absence of a three-electron bond, it is possible that some interaction (Van der Waals, electrostatics, etc.) between the product radical and anion exists. This situation has been discussed in some detail for the interaction between a halide ion and an alkyl radical generated in the gas phase by dissociative electron attachment to an alkyl halide. " It is expected that these interactions will be more important in the gas phase, as a solvent tends to screen charge. Wentworth suggested that an appropriate potential... 

Fig. 3 Bonding and antibonding combinations of a carbon-centered radical and a nucleophile to form a two-center, three-electron bond. In this simple description, the a anion radical has a bond order of one-half.

The lifetime of the RSSR radical anions is usually very short on the microsecond timescale in water. However, they have been detected and characterized by time-resolved optical methods. In one early study, y irradiation of matrices containing alkyl and aryl disulfides provided spectroscopic evidence for the formation of the corresponding radical anions. Subsequently, the formation of RSSR radical anions has been well documented, particularly by EPR, flash photolysis, and pulse radiolysis. In fact, 2a/ a three-electron bonded radical anion species, particularly from sulfur compounds, constitute significant and interesting intermediates. The RSSR radical anions may be obtained from different approaches. One is by one-electron reduction of disulfides (equation 75), such as by pulse radiolysis. However, the most common approach is by association of RS and RS (equation 79). ... 

REACTION WITH FREE RADICALS HYDROGEN ATOM ABSTRACTION AND ONE- OR THREE-ELECTRON BONDING... 

A study by Abu-Raqabah and Symons (Abu-Raqabah, A. Symons, M. C. R., J. Am. Chem. Soc., 1990, 112, 8614) has characterized the pyridine-chlorine atom three-electron bonded species Py <— Cl by ESR and UV spectroscopy. In an earlier paper, Breslow and co-workers (Breslow, R. Brandi, M. Hunger, J. Adams, A. D., J. Am. Chem. Soc., 1987,109, 3799) considered ring acylated pyridine-chlorine radicals to be n radicals and anticipated special stability for the 4-carboalkoxypyridine-chlorine radical. 

The difference between the lengths of the central CC bond in the equilibrium geometries of the radical cation of [ 1.1.1 Jpropellane (a one-electron bond, nearly the same length as in the neutral molecule) and of its radical anion (a three-electron bond, much longer than in the neutral molecule), well reproduced by ab initio calculations, emphasizes the inadequacy of the usual simplistic concepts of bonding based on Hiickel theory and neglect of overlap. An explicit introduction of overlap, however, permits a qualitative rationalization of the difference. 

The energetics of free radicals1,2 and estimation of their heats of formation by kinetic methods have been reviewed,3 along with the mechanism of magnetic isotope effects in radical reactions.4 Three-electron-bonded radicals (or a radicals) are the subject of a review which details methods of preparation and detection as well as examples of homo- and hetero-nuclear a radicals.5... 

Section 3.2 (Principle of the Detained Electron That Controls Ion Radical Reactivity) includes an extensive body of phenomena, from the formation of three-electron bonds, to ion pairing, to the distonic stabilization of ion radicals at the expense of separation between their spins and charges. 

Removal of an electron from a hydrazine unit changes the lone pair orbital occupancy from four to three, which has a large effect on the preferred geometry with respect to the nitrogens. The formation of a three-electron bond has also been demonstrated in the cation radical of octamethyl-l,2,4,5-tetra-aza-3,6-disilacyclohexane. In this cation radical, the spin density is distributed between only two of the four nitrogen atoms. There is a pronounced interaction of the unpaired electron with protons of the methyl groups joined to these two nitrogen atoms. 

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