Symmetrical radical ions

The true E c contains a self-interaction correction that exactly cancels the selfinteraction energy in j//p ri)p r2)fn dri dr2, but most currently used functionals are not completely free of self-interaction. Because of the self-interaction error, most currently available functionals give very incorrect U(R) curves at large intemuclear distances for symmetrical radical ions such as Hj, HeJ, and Fj and overestimate the intermolecular interaction in some charge-transfer complexes [Y. Zhang and W. Yang, J. Chem. Phys., 109, 2604 (1998)]. 

This failure of most DFT exchange functionals, which can be traced back to an incorrect dissociation behavior of symmetric radical ions, is unfortunate, because the inability of DFT methods to localize spin and charge means that they are unlikely to give highly accurate transition state geometries for reactions of radical ions where this occurs. Until this problem of DFT methods has been remedied, the use of conventional ab initio methods will be necessary for geometry optimizations of such transition states. 

Attempts have been made by various groups to detect EPR signals from this radical ion. On ZnO, a symmetric EPR spectrum with g = 2.003 and A H s3G can be observed on adsorption of oxygen. When 170-enriched oxygen is used to produce this EPR spectrum, no hyperfine structure is observed (3). It is possible that this spectrum can be assigned to OJ which is undergoing a rapid exchange reaction ... 

In aqueous solutions, it has been shown that the solvated electron e" circulates in the solvation shell until it is captured by I2 to given an I, radical ion which is finally stabilized to I . The solvated electrons have a characteristic absorption band near 700 nm which has been detected in flash pbotolytic studies of aqueous KI. The orbital of the excited electron may be considered to be spherically symmetric like that of a hydrogen atom, with its centre coinciding with the centre of the cavity containing the ion. 

Other alkyl hypohalites usually add to carbon-carbon multiple bonds in a free-radical process.155-158 Ionic additions may be promoted by oxygen, BF3, or B(OMe)3.156-160 While the BF3-catalyzed reaction of alkyl hypochlorites and hypo-bromites gives mainly halofluorides,159 haloethers are formed in good yields but nonstereoselectively under other ionic conditions.156-158 160 In contrast, tert-BuOI reacts with alkenes in the presence of a catalytic amount of BF3 to produce 2-iodoethers.161 Since the addition is stereoselective, this suggests the participation of a symmetric iodonium ion intermediate without the involvement of carbocationic intermediates. 

Triarylphosphines were prepared by the reaction between lithium diphenylphosphide in THF and m-and p-iodotoluene (or the corresponding bromo compounds), 4-bromobiphenyl and p-dibromobenzene in yields of 70-80% (isolated after oxidation, as the phosphine oxides).143 The absence of cine substitution products is a synthetic advantage and would have been taken as a prima facie indication that the displacements are examples of the 5rn1 reaction, had the mechanism been recognized at the time. Operation of the radical ion mechanism in DMSO, or liquid ammonia, in which marginally improved yields are obtained, was confirmed by Swartz and Bunnett,48 but no extension to the scope of the reaction was made. Rossi and coworkers have developed a procedure for one-pot preparation of triarylphosphines starting from elemental phosphorus (Scheme 6).146 As an example of the synthesis of a symmetrical tri-arylphosphine, triphenylphosphine (isolated as its oxide) was obtained in 75% yield, with iodobenzene as the aryl halide (ArX in Scheme 6, steps i-iii only). Unsymmetrical phosphines result from the full sequence of reactions in Scheme 6, and p-anisyldiphenylphosphine (isolated as its oxide) was produced in 55% yield, based on the phosphorus used, when chlorobenzene (ArX) and p-methoxyanisole (AiOC) were used. 

This reaction involves the reductive homo-coupling of a carbonyl compound to produce a symmetrically substituted 1,2-diol. TheJQrst step is single electron transfer of the carbonyl bond, which generates radical ion intermediates that couple via carbon-carbon bond formation to give a 1,2-diol. The example depicted above shows the preparation of pinacol itself. 

For cyclopropane, substituents at a single carbon might most effectively stabilize the antisymmetrical HOMO, whereas substitution at two carbons is expected to stabilize the symmetrical orbital. Since there is ample evidence for radical ions derived from the prototype of 2At symmetry (vide supra), cyclopropane radical cations with the alternative, antisymmetrical singly occupied (SO) MO appeared be of particular interest. We have identified two substrates, benzonorcaradiene (105) [229] and spiro[cyclopropane-l,9 -fluorene] (106), whose radical cations belong to this category [299, 300]. Interestingly, there is, as yet, no theoretical support for such species. 

The structurally similar radical ion, CS, has also been examined. The thermal behaviour of a strong band at 1161 cm correlated very well with that of the e.s.r. spectrum and it is attributed to a fundamental vibration of CS. As e.s.r. studies (Bennett et al., 1967c) show that the radical ion is non-linear, this band is probably due to the anti-symmetric stretching mode of CS. When potassium, instead of sodium, was used to prepare the radical ion, the band shifted to 1180 cm , which indicates that there is a perturbation of the CS radical ion by the counter-ion. This is in agreement with the e.s.r. results which show that the gr-tensor is affected by the counter-ion. 

Polymerization of butane-1,4-diol dimethacrylate, sensitized by benzophenone in the presence of three different sulfides, has been described by Andrzejewska et al. [190]. The measurements show that in the absence and in the presence of propyl sulfide and 2,2 -thiobisethanol no polymer was formed. This can be explained by the effective back electron transfer process that occurs in the radical-ion pair in organic solvents. Effective polymerization was observed only in the presence of TMT. Laser flash photolysis studies performed for the benzophenone-TMT pair allow one to construct a scheme (Scheme 23) explaining characteristic features of the mechanism of polymerization initiated by the system. The results prompted the authors to study other symmetrically substituted 1,3,5-trithianes as electron donors for benzophenone-sensitized free-radical polymerization (Figure 38 Table 12) [191]. 

Kebarle, Hiraoka, and coworkcrs applied pulsed electron-beam MS to study the existence and structure of CH5 (CH4) cluster ions in the gas phase. These CHs iCIF), clusters were previously observed by MS by Field and Beggs. Enthalpy and free energy changes they determined are compatible with the Cj symmetrical structure. Recently, Jung and coworkers utilizing electron ionization MS explored ion-molecule reactions within ionized methane clusters including CI I, (CH4), which was found to be the most abundant. This cluster is believed to be the product of the intracluster ion-molecule reaction depicted in Equation (5.6), which includes the methane dimer radical ion 8 ... 

Comparing the experimental ESR spectra with the simulated ones the following conclusions were suggested. (1) The principal values of the /i/tensor are almost axially symmetric with the perpendicular component A( A2 A3) being less than the linewidth of ca. 3 G. (2) Two different parallel components of hf splittings were observed for both ( O— 0) and — 0) radical ions with... 

The e.s.r. spectra of radical ions generated from several symmetrically substituted thiathiophthens, either by electrolytic reduction or by reaction with metallic potassium, suggest that these species have symmetric structures. ... 

Transition state geometries for reactions of open-shell molecules, particularly radical ions, pose special problems for DFT methods. In contrast to closed-shell systems whose ground state wavefunction is always totally symmetric, rearrangements of radicals and radical ions frequently involve crossings of states of different symmetry. In this situation, the molecule must lose symmetry to effect an adiabatic passage from reactants to products. In radical ions this loss often involves a localization of spin and charge, and it seems that DFT methods tend to oppose this localization. 

Kolbe electrolysis is a powerful method of generating radicals for synthetic applications. These radicals can combine to symmetrical dimers (chap 4), to unsymmetrical coupling products (chap 5), or can be added to double bonds (chap 6) (Eq. 1, path a). The reaction is performed in the laboratory and in the technical scale. Depending on the reaction conditions (electrode material, pH of the electrolyte, current density, additives) and structural parameters of the carboxylates, the intermediate radical can be further oxidized to a carbocation (Eq. 1, path b). The cation can rearrange, undergo fragmentation and subsequently solvolyse or eliminate to products. This path is frequently called non-Kolbe electrolysis. In this way radical and carbenium-ion derived products can be obtained from a wide variety of carboxylic acids. 

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. 

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