Activated complexes, definition

When a reaction between two or more molecules has progressed to the point corresponding to the top of the curve, the term transition state is applied to the positions of the nuclei and electrons. The transition state possesses a definite geometry and charge distribution but has no finite existence the system passes through it. The system at this point is called an activated complex. ... 

In order to obtain a definite breakthrough of current across an electrode, a potential in excess of its equilibrium potential must be applied any such excess potential is called an overpotential. If it concerns an ideal polarizable electrode, i.e., an electrode whose surface acts as an ideal catalyst in the electrolytic process, then the overpotential can be considered merely as a diffusion overpotential (nD) and yields (cf., Section 3.1) a real diffusion current. Often, however, the electrode surface is not ideal, which means that the purely chemical reaction concerned has a free enthalpy barrier especially at low current density, where the ion diffusion control of the electrolytic conversion becomes less pronounced, the thermal activation energy (AG°) plays an appreciable role, so that, once the activated complex is reached at the maximum of the enthalpy barrier, only a fraction a (the transfer coefficient) of the electrical energy difference nF(E ml - E ) = nFtjt is used for conversion. 

Thus, the steric factor may be explained with the help of entropy change. When two molecules come together to produce the activated complex, the total translational degrees of freedom are reduced (from 6 to 3) and rotational degrees of freedom also diminish. This is compensated by an increase in vibrational degrees of freedom. But the definite orientation in forming the activated complex necessarily reduced the entropy, i.e. AS is negative. This decrease in entropy is small when reaction takes place between simple atoms. The calculated value of kbT/h corresponds to collision frequency... 

The experimental activation volume is unambiguously determined from the slope of In kvs.p but its interpretation is not necessarily that of the original definition, the difference between the volumes of the activated complex and the reactants. In most cases the activation volume is a linear combination of volume changes in steps 1 and 2. In other cases one may have linear combinations of 3 and 4 volume contributions to AV. This will be treated in detail in the next section. 

The problem of the definition of charge and consideration of the sizes of reactants (the diameters of the reactant ions and the activated complex are assumed equal in the derivation of (2.184)) is most acute with reactions of metalloproteins. Probably the most nsed expression for the effect of ionic strength on such reactions is ... 

It is important to bear in mind that the central issue is always that of defining the nature of the activated complex, and the exercise which has been gone through on the simple classification of mechanism carries us part way to the goal, and then only in certain cases. No matter what definition... 

It is not possible at present to evaluate this function theoretically. The system is too complicated. The complexity of theoretical calculations can be best explained by considering the definition of AG and processes involved in the activation process. Definition of AG is given in Figure 6.1. Free energy of activation for the forward reaction is the free-energy difference between the free energy of the activated state G and the free energy of the initial state Gl... 

Assume that equilibrium is maintained between and the reactants despite a unidirectional decomposition of M+. Then if the activated complex, M+, is regarded as an ordinary molecule, possessing the usual thermodynamic properties, with the exception that motion in one direction, i.e. along the reaction coordinate, leads to decomposition at a definite rate (Glasstone et al., 1941b), it can be shown hy classical statistical methods that the rate of decomposition of is equal to kTjh, a universal frequency factor dependent only on temperature and... 

One of the features of transition state theory is that in principle it permits the calculation of absolute reaction rate constants and therefore the thermodynamic parameters of activation. There have been few successful applications of the theory to actual reactions, however, and agreement with experiment has not always been satisfactory. The source of difficulty is apparent when one realizes that there really is no way of observing any of the properties of the activated complex, for by definition its lifetime is of the order of a molecular vibration, or 10-14 sec. While estimates of the required properties can often be made with some confidence, there remains the uncertainty due to lack of independent information. 

In heterogeneous inorganic biomimics objectives (2)—(4) are resolved more easily than for organic mimics. Many acidic and basic sites on an inorganic carrier (matrix) form a situation in the biomimetic system when a definite quantity of these sites will display the required geometry for substrate-activated complex formation. 

We will have a closer look at the coordinate transformations leading to Eq. (10.13) from Eq. (10.3). Furthermore, we will consider some alternative forms of Eq. (10.13). In Eq. (10.13), the potential of mean force is related to the average force on the reaction coordinate. An alternative definition of the potential of mean force, which we will consider, is related to the average force on the atoms of the activated complex exerted by the solvent molecules. 

It is easy to verify that Wmean is the potential of the mean force exerted by the solvent molecules on the activated complex. From the definition in Eq. (10.42), we find, by taking the natural logarithm on both sides of the expression,... 

This study on the kinetic chlorine isotope effect in ethyl chloride50 was extended to secondary and tertiary alkyl halides pyrolyses51. The isotope effects on isopropyl chloride and terf-butyl chloride pyrolysis were found to be primary and exhibited a definite dependence on temperature. They increased with increasing methyl substitution on the central carbon atom. The pyrolysis results and model calculations implied that all alkyl chlorides involve the same type of activated complex. The C—Cl bond is not completely broken in the activated complex, yet the chlorine participation involves a combination of bending and stretching modes. 

Since the moment of inertia ratio will be nearly unity, the major source of the isotope effect at a given energy is the activated complex ratio. A pure intramolecular statistical secondary effect can scarcely arise, since by definition no net differential effect on the competing reaction coordinates arises due to isotopic substitution. However, in practice, small mechanistic effects mentioned earlier can exist for isotopic substitution which preferentially affects one of the competing bond-rupture sites. 

Polarity of solvents — If applied to solvents, this rather ill-defined term covers their overall -> solvation capability (solvation power) with respect to solutes (i.e., in chemical equilibria reactants and products in reaction rates reactants and activated complex in light absorptions ions or molecules in the ground and excited state), which in turn depends on the action of all possible, nonspecific and specific, intermolecular interactions between solute ions or molecules and solvent molecules, excluding interactions leading to definite chemical alterations of the ions or molecules of the solute. Occasionally, the term solvent polarity is restricted to nonspecific solute/solvent interactions only (i.e., to van der Waals forces). 

---