Electron-cation

The Hiickel rule predicts aromaticity for the six-7c-electron cation derived from cycloheptatriene by hydride abstraction and antiaromaticity for the planar eight-rc-electron anion that would be formed by deprotonation. The cation is indeed very stable, with a P Cr+ of -1-4.7. ° Salts containing the cation can be isolated as a product of a variety of preparative procedures. On the other hand, the pK of cycloheptatriene has been estimated at 36. ° This value is similar to those of normal 1,4-dienes and does not indicate strong destabilization. Thus, the seven-membered eight-rc-electron anion is probably nonplanar. This would be similar to the situation in the nonplanar eight-rc-electron hydrocarbon, cyclooctatetraene. 

Figure 17.15 The structure of (a) the nonlinear p" cation in laAsFg and (b) the weaker cation-anion interactions along the chain (cf Fig. 17.13). For comparison, the dimensions of (c) the linear 22-electron cation L" and (d) the nonlinear 20-electron cation Te3 are given. The data for this latter species refer to the compound [K(crypt)]2Te3.en in K2Tc3 itself, where there are stronger cation-anion interactions, the dimensions are r = 280 pm and angle = 104.4°).

In the very early stages of oxidation the oxide layer is discontinuous both kinetic and electron microscope" studies have shown that oxidation commences by the lateral extension of discrete oxide nuclei. It is only once these interlace that the direction of mass transport becomes of importance. In the majority of cases the metal then diffuses across the oxide layer in the form of cations and electrons (cationic diffusion), or as with the heavy metal oxides, oxygen may diffuse as ions with a flow of electrons in the reverse direction (anionic diffusion). The number of metals oxidising by both cationic and anionic diffusion is believed to be small, since a favourable energy of activation for one ion generally means an unfavourable value for the other... 

Similar arguments can be used to predict the relative stabilities of the cyclo-heptatrienyl cation, radical, and anion. Removal of a hydrogen from cyclohepta-triene can generate the six-77-electron cation, the seven-77-electron radical, 01 the eight-77-elec iron anion (Figure 15.6). All three species again have numerous resonance forms, but HiickeTs rule predicts that only the six-7r-electron cyclohep-tatrienyl cation should be aromatic. The seven-77-electron cycloheptatrienyl radical and the eight-77-electron anion are antiaromatic. 

Figure 15.6 Generation of the cycioheptatrienyl cation, radical, and anion. Only the six---electron cation is aromatic.

Both the cycioheptatrienyl radical and the anion are reactive and difficult to prepare. The six-Tr-electron cation, however, is extraordinarily stable. In fact, the cycioheptatrienyl cation was first prepared more than a century ago by reaction of Br2 with cycloheptatriene (Figure 15.7), although its structure was not recognized at the time. 

The polymerization of ethylene by group 4 metallocenes is widely recognized to proceed via 14-electron cationic intermediates 31,32 Cationic zirconocene33-35 and titanocene36-38 complexes, (1-3), were first isolated in 1986 by Jordan and Bochmann, respectively. Both (1) and (2) are active for ethylene polymerization in the absence of a cocatalyst.34... 

In catalytic reaction conditions (H2 pressure), by interaction of a solvent such as THF or acetone, the 16-electron cationic [Rh(diphos)(NBD)]+ affords a 12-electron unsaturated diphosphine intermediate, which is the real active species. The catalytic cycle begins with alkene binding, followed by oxidative addition of H2. These cationic catalysts can reduce alkenes to... 

With an increase of E beyond a certain value specific to the liquid, the free-ion yield increases sublinearly with the field, eventually showing a saturation trend at very high fields (see Mathieu et al.,1967). Freeman and Dodelet (1973) have shown that a fixed electron-ion initial separation underestimates the free-ion yield at high fields, and that a distribution of thermalization distance must be used to explain the entire dependence of Pesc on E. Therefore, the theoretical problem of the variation of free-ion yield with external field is inextricably mixed with that of the initial distribution of electron-cation separation. 

The enantiomeric excess (ee) obtained under catalytic conditions was similar to that found when the hydride transfer was carried out in a stoichiometric reaction (Eq. (40)) these stoichiometric reactions were carried out in the presence of excess CH3CN, which captures the 16-electron cationic Ru complex following hydride transfer. 

Neutral dienes have been reacted with a large variety of ions in the gas phase. Besides the cases concerning the same reactants discussed above but with reversed charge distribution, e.g. those of neutral 1,3-butadiene with ionized alkenes, there are interesting studies of reactions of 1,3-dienes with even-electron cations and studies on ion/molecule... 

Unsaturated even-electron cations have been used in the gas phase to react with olefins, including dienes, in a way that characterizes their structure. In most cases, these ion/molecule reactions take place by [4 + 2] cycloadditions followed by specific elimination of even-electron neutrals. A most suitable instrumental setup for these studies are triple-quadrupole and pentaquadrupole mass spectrometers in which the ion/molecule addition reactions take place subsequent to the selection of the reagent ion. In most... 

Even-electron rule Odd-electron ions (such as molecular ions and fragment ions formed by rearrangements) may eliminate either a radical or an even-electron neutral species, but even electron ions (such as protonated molecules or fragments formed by a single bond cleavage) will not usually lose a radical to form an odd-electron cation. In other words, the successive loss of radicals is forbidden. [12]... 

The protonation of a number of azaindolizines occurs at the additional non-bridgehead nitrogen (whether in the 5-membered or in the 6-membered ring). The sole exception is 5-azaindolizinc [182], in which protonation at carbon leads to the establishment of a six Tt-electron cation in the 6-membered ring ([183], [184]). Several... 

Returning to ion-pair zirconocene catalysts, the initiation of the polymerisation process requires the displacement of the anion so that the alkene can be coordinated. The mobility of the anion is therefore an important factor and has become the focus of a number of detailed investigations. The original mechanistic scheme of alkene insertion and polymer chain growth (Scheme 8.4) implied dissociation of the anion and formation of a 14-electron cationic intermediate, which then reacted... 

Figure 1 Electron escape probability as a function of the applied electric field. The solid lines are obtained from Eq. (23) for different values of the initial electron cation distance ro- The broken lines are calculated for ro = 1 nm from the numerical solution of Eq. (16) with the sink term given by k r) = A exp[—a(r—

Figure 2 Survival probability of geminate ion pairs as a function of time. The two solid lines correspond to two different values of the initial electron-cation distance. The broken lines show the asymptotic kinetics calculated from Eq. (25). The value of the escape probability for Tq = O.Sr is indicated by

Figure 3 Escape probability as a function of the initial electron-cation distance. The lower broken curve is calculated from the Onsager equation [Eq. (15)]. The numerical results for different mean free times x were taken from Ref. [22]. The unit of x is rJ(ksTlmf, where m is the electron mass. The upper broken curve was calculated using the energy diffusion model. (From Ref. 23.)...

If the excited molecule formation is mainly because of electron-cation recombination the concentration dependence of quenching of the precursor charged species may follow some form of the modified Warman-Asmus-Schuler (WAS) semiempirical equation [8], e.g. ... 

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