Encounter complex substitution

Equations (2.10) and (2.12) are identical except for the substitution of the equilibrium dissociation constant Ks in Equation (2.10) by the kinetic constant Ku in Equation (2.12). This substitution is necessary because in the steady state treatment, rapid equilibrium assumptions no longer holds. A detailed description of the meaning of Ku, in terms of specific rate constants can be found in the texts by Copeland (2000) and Fersht (1999) and elsewhere. For our purposes it suffices to say that while Ku is not a true equilibrium constant, it can nevertheless be viewed as a measure of the relative affinity of the ES encounter complex under steady state conditions. Thus in all of the equations presented in this chapter we must substitute Ku for Ks when dealing with steady state measurements of enzyme reactions. 

Electrophilic aromatic substitution is generally considered to proceed via a 71-complex (or an encounter complex) between the electrophile E+ and the... 

Additionally, ER groups can also be bonded in bridging configurations in multimetallic compounds (Table 9). These complexes are most often encountered via substitution reactions. Reactions employing the substitution methodology will be discussed in Section 3.08.7. 

Xas = 0.7 M for formation of an encounter complex between azide ion and substrate, which then undergoes unassisted ionization to form a triple ion intermediate k[ = ki, Scheme 2.4) or, with a smaller association constant and a small compensating rate increase from a formally bimolecular substitution reaction k[ > k. Scheme 2.4). 

One interesting proposal86 is that the encounter pair is a radical pair N02 ArH formed by an electron transfer (SET), which would explain why the electrophile, once in the encounter complex, can acquire the selectivity that the free N02+ lacked (it is not proposed that a radical pair is present in all aromatic substitutions only in those that do not obey the selectivity relationship). The radical pair subsequently collapses to the arenium ion. There is evidence87 both for and against this proposal.88... 

Ohashi et al. [136,139] have proposed a scheme describing the formation of ortho adducts and substitution products from anisole and the three dimethoxyben-zenes with acrylonitrile, methacrylonitrile, and crotonitrile (Scheme 39). Here, the ortho cycloadduct is supposed to be formed directly from an encounter complex or exciplex, whereas the substitution product arises via formation of an ion pair from the complex, followed by protonation of the radical anion and radical... 

Electrophilic aromatic substitution represents one of the most important applications of the transformation of an arene via a CT (precursor) complex. The process is considered to proceed via an (encounter) -complex between the electrophile (E+) and the aromatic substrate (ArH), which collapses in a single rate-limiting step to the Wheland intermediate or a-... 

It has been concluded that encounter complexes (or exciplexes) and ion-pairs are the key intermediates in these reactions. Addition of acrylonitrile to N- unsubstituted imidazoles gives the N- substitution product, while the N- substituted compounds undergo cycloaddition (80AHC(27)24l). 

The general model for an ion-molecule substitution process in the gas phase involves formation of a reactant encounter complex (AB ), conversion over an energy barrier e o to a product encounter complex (CD ), and dissociation to products (Fig. 30b). 

Anthraquinonecarboxamide in its Sj state will photoadd two molecules of water. The photoreactions of diphenylhomobenzoquinones in the presence of amine donors have been studied. 1-Bromo substituted diphenylhomobenzoqui-none irradiated in the presence of triethylamine induces opening of the cyclopropane ring with formation of 2-diphenylmethyl-5-methyl-l,4-benzoquinone, and with dimethylaniline, a mixture of an aminated bicyclic dione and bis(p-dimethyl-aminophenyl)methane is produced. Irradiation of boron difluoride complexes derived from 1,3-diketones can lead to a number of different types of process, in particular exciplex formation and slow cycloaddition. Evidence has now been advanced to suggest that excitation of these complexes leads directly to an exciplex without the participation of an encounter complex, and that this solvolyses to free ions. 

Since c = - G, substituting the simplifying X = ab into Equation (8) gives Equation (10) which is the Marcus equation (for reaction within the encounter complex). 

S213 As discussed in Section 21.6, the Eigen-Wilkins mechanism for substitution in octahedral complexes suggests the formation of an encounter complex [V(H20)6] Cl , in the first, fast step ... 

The Eigen- Wilkins mechanism applies to ligand substitution in an octahedral complex. An encounter complex is first formed between substrate and entering ligand in a preequilibrium step, and this is followed by loss of the leaving ligand in the rate-determining step. 

BHandHLYP/aug-cc-pVDZ calculations have been used to model the 5n2 reactions between C10 and BrO and Me-, Ft-, and Pr-Cl in the gas phase. ° The results indicate the 5ivf2 reactions occur in the usual manner from the encounter complex, to the transition state, to the product complex. However, the product complexes are long-lived and react further with Cl or C10 in an 5N2-induced elimination of HCl giving the aldehyde, and an 5N2-induced substitution reaction with the 5ivf2-induced elimination reaction predominating. The C10 reactions are faster than the BrO reactions and the reaction rates decrease and the transition states become more product-like and looser from Me-Cl to Et-Cl to Pr-Cl. Reasons for these effects are presented. Transition state stiuctures and energy surfaces for all the reactions are given. 

Brpnsted correlations for proton transfer and methyl transfer between pairs of mimicked 4-substituted pyridines have been simulated by means of AMI molecular-orbital calculations.The relationship to the Marcus equation is also considered. It is concluded that the Brpnsted coefficient fi provides an approximate measure of the position of the transition structure along the reaction coordinate between reactant and product encounter complexes. 

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