Vertical charge transfer

Finally, we ask, if the reactive triads in Schemes 1 and 19 are common to both electrophilic and charge-transfer nitration, why is the nucleophilic pathway (k 2) apparently not pertinent to the electrophilic activation of toluene and anisole One obvious answer is that the electrophilic nitration of these less reactive [class (ii)] arenes proceeds via a different mechanism, in which N02 is directly transferred from V-nitropyridinium ion in a single step, without the intermediacy of the reactive triad, since such an activation process relates to the more conventional view of electrophilic aromatic substitution. However, the concerted mechanism for toluene, anisole, mesitylene, t-butylbenzene, etc., does not readily accommodate the three unique facets that relate charge-transfer directly to electrophilic nitration, viz., the lutidine syndrome, the added N02 effect, and the TFA neutralization (of Py). Accordingly, let us return to Schemes 10 and 19, and inquire into the nature of thermal (adiabatic) electron transfer in (87) vis-a-vis the (vertical) charge-transfer in (62). 

FIGURE 6.9 The ground and vertical charge-transfer states in the VBSCD that describes a nucleophilic attack on a carbonyl group. 

Fig. 7.8.4 Interline-transfer charge-coupled device with column light shields for vertical charge transfer...

In general, all reactions between closed-shell electrophiles and nucleophiles are describable by the same diagram type [11] with R and P states, which are vertical charge transfer states that involve an electron transfer from the nucleophile to the electrophile, while coupling the single electron on the oxidized nucleophile to that on the reduced electrophile to form a bond-pair. One of the many examples is the nucleophihc assisted cleavage of an ester where the rate-determining step [62,63] is the formation of a tetrahedral intermediate, as depicted in Fig. 23.8. 

Fig. 23.8. The ground and vertical charge transfer states, of the reactants, in the VBSCD that describes a nucleophihc attack on a carbonyl group (adapted from Ref. [11] with permission of Wiley-VCH STM-Copyright and Licenses).

Table 1. Vertical Charge Transfer Energies for the Gas Phase Reaction, X" + CH3X XCH3 + X ...

Tests using vertical cylindrical silos have shown that the charge transferred by these discharges increases with silo diameter [161]. The maximum observed charge transfer Q varied with silo diameter D according to... 

Fig. 1.20 Cell consisting of two reversible Ag /Ag electrodes (Ag in AgN03 solution). The rate and direction of charge transfer is indicated by the length and arrow-head as follows gain of electrons by Ag -he- Ag—> loss of electrons by Ag - Ag + e- —. (o) Both electrodes at equilibrium and (f>) electrodes polarised by an external source of e.m.f. the position of the electrodes in the vertical direction indicates the potential change. (K, high-impedance voltmeter A, ammeter R, variable resistance)...

Evaluation of the Work Term from Charge Transfer Spectral Data. The intermolecular interaction leading to the precursor complex in Scheme IV is reminiscent of the electron donor-acceptor or EDA complexes formed between electron donors and acceptors (21). The latter is characterized by the presence of a new absorption band in the electronic spectrum. According to the Mulliken charge transfer (CT) theory for weak EDA complexes, the absorption maximum hv rp corresponds to the vertical (Franck-Condon) transition from the neutral ground state to the polar excited state (22). 

Figure 2.5 Charge transfer (CT percentage of e charge) from F to Li+ during ionic-bond formation. For reference, a dotted vertical line marks the equilibrium bond length. Note the steep rise corresponding to the onset of the ionic-covalent transition near R = 1 A.

Since electrophilic and charge-transfer nitrations are both initiated via the same EDA complex and finally lead to the same array of nitration products, we infer that they share the intermediate stages in common. The strength of this inference rests on the variety of aromatic substrates (with widely differing reactivities and distinctive products) to establish the mechanistic criteria by which the identity of the two pathways are exhaustively tested. On this basis, electrophilic nitration is operationally equivalent to charge-transfer nitration in which electron-transfer activation is the obligatory first step. The extent to which the reactive triad in (90) is subject to intermolecu-lar interactions in the first interval (a few picoseconds) following electron transfer will, it is hoped, further define the mechanistic nuances of dissociative electron transfer in adiabatic and vertical systems (Shaik, 1991 Andrieux et al., 1992), especially when inner-sphere pathways are considered (Kochi, 1992). 

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