Radial Cation

For a review, see Taylor, in Bamford Tipper, Ref. 53, vol. 13, 1972, pp. 186-194. An alternative mechanism, involving radial cations, has been reported Courtneidge Davies McGuchan Yazdi J Organomet. Chem. 1988,341, 63. 

Significant insights into the nature of the nucleophilic capture of radial cations is provided by the regiochemistry of the attack on the bridged norcaradiene radical cation, 18 +. The products suggested regiospecific attack of methanol on 18"+ with capture at C2 and C5 generating 104" and 105". The attack occurs with limited stereoselectivity, because products derived from 104" and 105" were formed in comparable yields [131]. 

V. Cyclobutadienyl Radical tons A. Cyclobutadienyl Radial Cation... 

Photocycloaddition. Cationic Ru complexes were found to be useful visible-light photocatalysts for cycloaddition reactions with al-kenes. Upon visible-light irradiation a mixture of p-tolylethene with l-(p-methoxyphenyl)propene (142) in the presence of [Ru(bpm)3] + (bpm = 2,2 -bipyrimidine) as a photocatalyst under air as a co-oxidant led to the intermolecular [2 + 2] cycloadduct (143) in 86% yield. In this reaction, the radical cation (142) is a key intermediate in the chain re-action. [Ru(bpz)3] + (bpz = 2,2 -bipyrazine) was a good visible-light photocatalyst for the intermolecular [3 - - 2] cycloaddition of cyclopropy-lamines and alkenes. [Ru(bpz)3] " ", excited by visible light, oxidized the 2-azabicyclo[3.1.0]pentane (144) to afford the radial cation (142)" ". ... 

The spatial extension of polarons has been calculated for various 7r-conjugated polymer [25-27]. In the case of polythiophene, it corresponds to five monomer units. In a short oligomer, the polaron can no longer be considered as a charge free to move along the polymer chain. Hence, the concept of polaron has to be changed to that of radial cation, in which the geometrical structure modification extends all over the molecule. 

The spectral characteristics of a radical cation % dimer are also noted for a polyester that contains oxidized quaterthiophene units (Fig. 7) [52]. The isolated quater-thiophene units of this polymer are too short for the formation of a stable dicationic oxidation state. However, oxidation of the polymer affords a material that has a strong optical absorption spectrum that resembles that of a radieal cation % dimer, a weak ESR signal, and a conductivity of 0.8 S cm-. Thus, it is posited that spinless charge carriers arise from the association of the mono(radial cation)s derived from oxidation of the quaterthiophene units within the solid polymeric material. 

Fig. 10. Unpolarized Raman spectra (T = 300 K) for solid Ceo, KaCeo, RbsCeo, NaeCeo, KaCco, RbeCeo and CseCeo [92, 93], The tangential and radial modes of Ag symmetry are identified, as are the features associated with the Si substrates. From the insensitivity of these spectra to crystal structure and specific alkali metal dopant, it is concluded that the interactions between the Cao molecules are weak, as are also the interactions between the Cao anions and the alkali metal cations.

FIG. 10 Cation (full), anion (dashed), and oxygen (dotted) radial density distributions in nonpolar pores. Top NaCl solution bottom KCl solution. 

As indicated earlier, protective oxide scales typically have a PBR greater than unity and are, therefore, less dense than the metal from which they have formed. As a result, the formation of protective oxides invariably results in a local volume increase, or a stress-free oxidation strain" . If lateral growth occurs, then compressive stresses can build up, and these are intensified at convex and reduced at concave interfaces by the radial displacement of the scale due to outward cation diffusion (Fig. 7.7) . 

The reduced oxidation near sample corners is related to these stress effects, either by retarded diffusion or modified interfacial reactionsManning described these stresses in terms of the conformational strain and distinguished between anion and cation diffusion, and concave and convex surfaces. He defined a radial vector M, describing the direction and extent of displacement of the oxide layer in order to remain in contact with the retreating metal surface, where ... 

Physically meaningful ionic radii may be obtained from Poisson equation for anions, and from electrostatic potentials defined in the the context of DFT for cations [17,18], However, there remains the problem of being forced to use different mathematical criteria in both cases, because the electrostatic potential of anions and cations display a different functional behaviour with respect to the radial variable. 

From eq (7) it may be concluded that the charge normalization condition is never satisfied for cations. As a result, the functional dependence of (r) with the radial variable is quite different in this case. For instance, it may be easily shown that 0(r) displays a monotonic decreasing behavior without extrema points along the complete domain of the r variable. As a result, expression (8) for O(r) is not longer valid for singly positive charged atomic systems. 

The use of electrostatic potentials, defined in the context of DFT, for the calculation of ion solvation energies has been reviewed. It has been shown that physically meaningful ionic radii may be obtained from this methodology. In spite of the fact that the electrostatic potentials for cations and anions display a quite different functional dependence with the radial variable, we have shown that it is still possible in both cases to build up a procedure consistent with the Bom model of ion solvation. 

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