Transition states, 3 center-2 electron

Both these processes can be considered to occur in several distinct stages as follows (i) formation of precursor state where the reacting centers are geometrically positioned for electron transfer, (ii) activation of nuclear reaction coordinates to form the transition state, (iii) electron tunneling, (iv) nuclear deactivation to form a successor state, and (v) dissociation of successor state to form the eventual products. At least for weak-overlap reactions, step (iii) will occur sufficiently rapidly (< 10 16s) so that the nuclear coordinates remain essentially fixed. The "elementary electron-transfer step associated with the unimolecular rate constant kel [eqn. (10)] comprises stages (ii)—(iv). 

Chiral metal centers are usually stable in the solid state at ambient temperature, but behave differently in solution where the epimerization process can even occur at room temperature (—293 K) with half-life (ri/2) less than 24 h (Table 3.1) [104]. Brunner et al. have carried out temperature-dependent kinetic experiments for the epimerization reaction and Cl/I exchange on Ru(II) complexes containing cyclopentadienyl and phosphine ligands (5r , Sc)-/(Rru, 5c)-[CpRu(Chairphos)Q] ((S)-Chairphos = (S)-l,3-bis(diphenylphosphanyl)butane) and c/s-/ira s-[CpRu(Dppm-Me)Q] (Dppm-Me = l,l-bis(diphenylphosphanyl) ethane) [104]. Mechanistic studies on epimerization concluded that the chelate ring size and consequently the bond angle between donor atoms and metal ion (P-Ru-P) in the transition state (16-electron species) determine the rate of... 

Two modified sigma constants have been formulated for situations in which the substituent enters into resonance with the reaction center in an electron-demanding transition state (cr+) or for an electron-rich transition state (cr ). cr constants give better correlations in reactions involving phenols, anilines, and pyridines and in nucleophilic substitutions. Values of some modified sigma constants are given in Table 9.4. 

It is notable that pyridine is activated relative to benzene and quinoline is activated relative to naphthalene, but that the reactivities of anthracene, acridine, and phenazine decrease in that order. A small activation of pyridine and quinoline is reasonable on the basis of quantum-mechanical predictions of atom localization encrgies, " whereas the unexpected decrease in reactivity from anthracene to phenazine can be best interpreted on the basis of a model for the transition state of methylation suggested by Szwarc and Binks." The coulombic repulsion between the ir-electrons of the aromatic nucleus and the p-electron of the radical should be smaller if the radical approaches the aromatic system along the nodal plane rather than perpendicular to it. This approach to a nitrogen center would be very unfavorable, however, since the lone pair of electrons of the nitrogen lies in the nodal plane and since the methyl radical is... 

Figure 10-2 presents the Hammett plots for these two reactions. Each correlation is successful, in that a straight line results. The scatter in these plots is reasonably typical of these correlations, where 10-15 percent deviations are not uncommon. The slope for log k versus cr is +0.91. As one can see, this step accelerates with electron-withdrawing substituents, which are the X s with the positive values of a. A positive reaction constant means that the reaction center (here the carbonyl carbon) is more negative in the transition state. This finding supports the suggestion that the RCS is the addition of the semicarbazide to the carbonyl carbon. 

Y is a strong interacting pi electron acceptor. In addition to carbonium and oxocarbonium ion centers, examples of Y groups from Table XIII include Nj and several electrophilic substitution transition states (cf. sets 10,21 and 23) of the type... 

Y is a strongly pi electron donor group. As previously noted in the results section, examples of Y from Table VI include centers of high pi electron charge density at carbon, sulfur, nitrogen, and oxygen. Also included in Table VI are examples of nucleophilic substitution transition states (cf. reactions 21 and 22) of the type... 

It is interesting to note in the latter connection that nucleophilic substitution transition states in which there apparently is not strong delocalization of pi electron density into the substituent tend to fall into the <7 type (cf. reactions 18 and 19 of Table VIII). In set 18, there are two ortho nitro groups which apparently take up much of the pi charge (thus it is unavailable to X), whereas in set 19, the positive piperidinium center may cause (perhaps with assistance from the NH hydrogen bonding permitted by the aprotic solvent) the... 

In this figure, the activation energies of N2 dissociation are compared for the different reaction centers the (111) surface structure ofan fee crystal and a stepped surface. Activation energies with respect to the energy of the gas-phase molecule are related to the adsorption energies of the N atoms. As often found for bond activating surface reactions, a value of a close to 1 is obtained. It implies that the electronic interactions between the surface and the reactant in the transition state and product state are similar. The bond strength of the chemical bond... 

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