Electron transfer preequilibrium

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... 

The electron-transfer paradigm in Scheme 1 (equation 8) is subject to direct experimental verification. Thus, the deliberate photoactivation of the preequilibrium EDA complex via irradiation of the charge-transfer absorption band (/ vCT) generates the ion-radical pair, in accord with Mulliken theory (equation 98). 

When k eCp/kc <1, the rate-determining step is the forward homogeneous electron transfer reaction, meaning that the system behaves as just analyzed above. When, conversely, k ed /kc 1, the rate-determining step is the follow-up reaction, while the homogeneous electron transfer plays the role of a preequilibrium. The governing kinetic parameter is then... 

If the system were to remain in this situation, no kinetic information concerning the follow-up reaction would be available. In the other limiting case, the catalytic response is governed by the follow-up reaction, while electron transfer acts as a preequilibrium. The response is thus a function of the dimensionless parameter... 

The situation where the radical-substrate coupling is a preequilibrium to the homogeneous electron transfer step, termed the rsdDISP2 mechanism, prevails when ).-d 3> /d)b. Then equation (6.56) becomes... 

If the electron donor is so efficient a reductant as to react with the acceptor with a rate constant equal to the diffusion limit, then not much information can be derived from the experiments, except the knowledge of the diffusion limit itself. The opposite situation, where an endergonic electron transfer is followed by a fast bond-breaking step, is of more interest. There is then competition between the follow-up reaction and the backward electron-transfer step. If the latter is faster than the former, kinetic control is by the bond-breaking step, the electron-transfer step acting as a preequilibrium. Under these conditions, there is no difficulty to conclude from the adherence to the rate law (61) that the overall reaction is stepwise rather than concerted, since, in the concerted case, the rate law would be (62). If, in... 

A change in the rds is suggested over the temperature range examined. A near zero value of is usually associated with an exothermic preequilibrium (Sec. 2.6). Formation of a PCu(I) Fe(III) adduct is suggested within which electron transfer occurs. At lower temperature a higher AFF obtains and formation of the adduct may involve a reorganisational step. 

Inner-sphere electron transfers are characterized by (a) temperature-independent rate constants that are greatly higher and rather poorly correlated by Marcus theory (b) weak dependence on solvent polarity (c) low sensitivity to kinetic salt effects. This type of electron transfer does not produce ion radicals as observable species but deals with the preequilibrium formation of encountered complexes with the charge-transfer (inner-sphere) nature (see also Rosokha Kochi 2001). 

Electron donors (D) and acceptors (A) constitute reactant pairs that are traditionally considered with more specific connotations in mind, such as nucleophiles and electrophiles in bond formation, reductant and oxidant in electron transfer, bases and acids in adduct production, and anion and cations in ion-pair annihilation (12). In the latter case, the preequilibrium formation of contact ion pairs (CIP), that is,... 

Since the overall reaction proceeds under conditions where no Co(CO)4 radicals from Co2(CO)g cleavage can be detected, splitting of the dinuclear species Co2(CO)gL, which is formed in a rapid preequilibrium (equation 4), is responsible for radical formation during the induction period. The important step for the formation of the observed products is then outer-sphere electron transfer see Outer-sphere Reaction) as depicted in equation (6). This requires the Co(CO)3L radical to act as a reducing agent towards Co2(CO)g. Since, from electrochemistry and pulse radiolysis of Co2(CO)g, it can... 

The importance of the work term is shown by the observation of chiral recognition in some electron-transfer reactions. Differences in the rate of reaction of different optical enantiomers of the same complex, with a common chiral complex, are attributed to differences in the preequilibrium constant Xjp rather than electronic effects. ... 

The proposed mechanism involves a preequilibrium of enolate formation (HBO ") followed by a rate-determining electron transfer ... 

It is, therefore, useful to divide the overall observed rate constant, k, into an equilibrium constant, K, for the formation of the electrochemical precursor species and a first-order rate constant, k, (s" ), for electron transfer within the precursor state ( 12.3.3). If the electron-transfer step is rate determining (the so-called preequilibrium model), then ... 

The two work-corrected rate constants, k and kj., should be distinguished carefully. The latter quantity arises from the use of the preequilibrium model and describes the rate constant (s ) for electron transfer at a given electrode potential taken wnth respect to the precursor intermediate for the preequilibrium model. The former quantity is an intermolecular (cm s ) rate constant, which can be utilized in conjunction with either the preequilibrium or collisional-rate formulation. In view of Eqs. (a), (n) and (o) ... 

A distinction, however, is possible from kinetic studies (see Scheme 11). The slow step of the proton transfer mechanism is the proton transfer process (eqs. 14 and 22), thus the rate law should be first order in the radical species and first order in the base. The slow step of the disproportionation mechanism, on the other hand, is the electron transfer process following the solvent coordination pre-equilibrium, thus the rate law should be second order in the radical species and first order in the solvent under the conditions in which Kgq[S] 1 (preequilibrium favouring the 17-electron species). In addition, stronger donor solvents leads to faster disproportionation processes (e.g. MeCN CH2CI2, THF), while the proton transfer process should be much less solvent dependent. 

The mechanism of hydride abstraction from rhenium alkyl complexes of the type [Re(Cp)(NO)(PPh3)(R)] (50) has been examined using electrochemical techniques and provides the first rate data for metal alkyl/Ph3C reactions. Both the a-hydride abstraction in equation (17) and )8-hydride abstraction in equation (18) processes are shown to involve a preequilibrium electron-transfer step shown in equation (19), followed by rate-determining hydrogen-atom transfer between... 

As depicted in Fig. 7, at potentials more positive than E, k2 > k-i and the first electron transfer is rds while for < E, k-i > k2 and the second electron transfer becomes rate-determining. The Tafel slope in the first case is 2RT/F and iRTj2E for the preequilibrium in the second case. 

Focusing on the electron transfer act, theory for electron transfer presumes a preequilibrium factor for complex formation, and the following electron transfer ... 

Thayer has measured rate constants for the demethylation of methylcobalamin by complex halides such as [AuCU] , [TlBr4], and [PtCU] . Fanchiang has shown that methylcobalamin reacts with [AUX4] (X = Cl or Br) with a 2 1 stoichiometry to give aquocobalamin, an oxidized corrin ring, and colloidal gold. Kinetic data have been analyzed in terms of a preequilibrium (36), electron transfer (37), and then reactions (38)-(40). 

He obtained a reduced product with no deuterium incorporated at position-5. The results thus clearly shows that there is a direct hydrogen (or two-electron) transfer to position-5, and that in the preequilibrium complex, the NADH probably does not occupy the area adjacent to positions 1,9, and 10. This result is a strong argument against the Hamilton proposal where this FAD analogue acts as an hydride acceptor. 

Oxidation of phenol by tris(l,10-phenanthroline)osmium(III) is second order in Os(III) and phenol and inverse second order in Os(II) and acidity. A mechanism is inferred in which the phenoxyl radical is produced through a rapid proton-coupled electron transfer (PCET) pre-equilibrium, followed by rate-limiting phenoxyl radical coupling. Application of Marcus theory indicated that the rate of electron transfer from phenoxide to osmium(III) is fast enough to account for the rapid PCET preequilibrium, but it did not rule out the intervention of other pathways such as concerted proton-electron transfer or general-base catalysis DFT studies, at B3LYP/LACVP level, of the oxidation of ethylene by osmium tetroxide, osmyl hydroxide, and osmyl chloride indicated that in the reaction of osmium tetroxide, the [3 4- 2] addition pathway leading to a five-membered metallacycle intermediate is more favourable than the [24-2] addition. The reaction with osmyl hydroxide is less favourable. In the reaction with osmyl chloride, the [24-2] addition pathway is more favourable than the [3 4-2] addition. ... 

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