Dissociative intermediate 16-electron

Comments on the thermal nitration of enol silyl ethers with TNM. The strikingly similar color changes that accompany the photochemical and thermal nitration of various enol silyl ethers in Table 2 indicates that the preequilibrium [D, A] complex in equation (15) is common to both processes. Moreover, the formation of the same a-nitroketones from the thermal and photochemical nitrations suggests that intermediates leading to thermal nitration are similar to those derived from photochemical nitration. Accordingly, the differences in the qualitative rates of thermal nitrations are best reconciled on the basis of the donor strengths of various ESEs toward TNM as a weak oxidant in the rate-limiting dissociative thermal electron transfer (kET), as described in Scheme 4.40... 

While the two-state XT-CT model accounts for some of the essential features of the exciton dissociation process, a more detailed, molecular-level picture should include (i) intermediate electronic states that feature a non-negligible coupling to the XT and/or CT states, (ii) an ensemble of relevant interface configurations, as illustrated in Figs. 3 and 6. In Refs. [52, 53], these two aspects have been addressed. This analysis further allows one to verify whether the conclusions of the two-state analysis are corroborated by a more realistic treatment. 

An electron-transfer reduction of RX can he dissociative (no intermediate anion-radical RX ) or noil-dissociative (intermediate RX ). Where the noil-dissociative pathway is well documented (e.g.. aryl halides), only two reactions of the intermediate RX- arc generally posited, dissociation and electron-transfer oxidation [I7.18. However, product evidence suggests that R-X may undergo other reactions, including reduction. 

It may be significant that, except for the cases of catalytic inducement, the best authenticated examples of the fully developed dissociative intermediate are for anionic complexes, such as [Co(CN)5(H20)] . Here the water molecule dipole will normally be directed unfavourably for coordination and becomes more part of the solvent continuum than an integral part of the complex. Thus the retardation of entry, leading to the full formation of the five-coordinated intermediate, is probably the result of the strong solvent coherence even in the solvation sphere of the complex rather than something inherently different in the electronic properties of the complex itself. 

The ligand field activation energy (LFAE) is defined as the difference in the LFSE for the TS minus that of the reactants. Assuming a square pyramidal dissociative intermediate, the LFAEs for the different d" electron configurations are listed in Table 17.5. 

Initiation of the reaction of l-chloro-2-naphthoxide anion with NajSO, has been proposed to occur by ET between the excited triplet state of the substrate and its ground state. This reaction can be dye-photoinitiated-" or initiated by visible light with a Ru complex as sensitizer and a Co complex as the intermediate electron carrier.- - For the 1-bromo derivative, photohomolytic CBr bond dissociation is proposed. "... 

All of the atomic species which may be produced by photon decomposition are present in plasma as well as the ionized states. The number of possible reactions is therefore also increased. As an example, die plasma decomposition of silane, SiH4, leads to the formation of the species, SiH3, SiHa, H, SiH, SiH3+ and H2+. Recombination reactions may occur between the ionized states and electrons to produce dissociated molecules either direcdy, or tlrrough the intermediate formation of excited state molecules. 

There are alternatives to the addition-elimination mechanism for nucleophilic substitution of acyl chlorides. Certain acyl chlorides are known to react with alcohols by a dissociative mechanism in which acylium ions are intermediates. This mechanism is observed with aroyl halides having electron-releasing substituents. Other acyl halides show reactivity indicative of mixed or borderline mechanisms. The existence of the SnI-like dissociative mechanism reflects the relative stability of acylium ions. 

The Dissociation Constant of Nitric Acid. The largest value of K in Table 9 is that for the (HS04) ion. In Fig. 36 there is a gap of more than 0.2 electron-volt below the level of the (H30)1 ion. As is well known, several acids exist which in aqueous solution fall iu the intermediate region between the very weak acids and the recognized strong acids the proton levels of these acids will fall in this gap. The values of K for these acids obtained by different methods seldom show close agreement. Results obtained by various methods were compared in 1946 by Redlich,1 who discussed the difficulties encountered. 

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