Transition states and activated complexes

Explain why the names activated complex and transition state are suitable for describing the highest energy point on a reaction s route from reactant to product. 

As in the case of the teacher, it was also impossible to associate the model of chemical kinetics expressed by the textbook with any one of the historical models. This was because its authors seemed to have developed a completely different model in which they merged characteristics of several distinct historical models treated as if they constituted a coherent whole. For instance, when the authors said that there is a species called an activated complex , they had added elements of the transition state theory to the explanation. However, activated complex and transition state are different concepts, derived from different theoretical backgrounds. Such an absence of... 

The terms activated complex and transition state are often used as synonyms however, we shall preserve a distinction. 

Activation Parameters. Thermal processes are commonly used to break labile initiator bonds in order to form radicals. The amount of thermal energy necessary varies with the environment, but absolute temperature, T, is usually the dominant factor. The energy barrier, the minimum amount of energy that must be suppHed, is called the activation energy, E. A third important factor, known as the frequency factor, is a measure of bond motion freedom (translational, rotational, and vibrational) in the activated complex or transition state. The relationships of yi, E and T to the initiator decomposition rate (kJ) are expressed by the Arrhenius first-order rate equation (eq. 16) where R is the gas constant, and and E are known as the activation parameters. 

The case of m = Q corresponds to classical Arrhenius theory m = 1/2 is derived from the collision theory of bimolecular gas-phase reactions and m = corresponds to activated complex or transition state theory. None of these theories is sufficiently well developed to predict reaction rates from first principles, and it is practically impossible to choose between them based on experimental measurements. The relatively small variation in rate constant due to the pre-exponential temperature dependence T is overwhelmed by the exponential dependence exp(—Tarf/T). For many reactions, a plot of In(fe) versus will be approximately linear, and the slope of this line can be used to calculate E. Plots of rt(k/T" ) versus 7 for the same reactions will also be approximately linear as well, which shows the futility of determining m by this approach. 

In qualitative terms, the reaction proceeds via an activated complex, the transition state, located at the top of the energy barrier between reactants and products. Reacting molecules are activated to the transition state by collisions with surrounding molecules. Crossing the barrier is only possible in the forward direction. The reaction event is described by a single parameter, called the reaction coordinate, which is usually a vibration. The reaction can thus be visualized as a journey over a potential energy surface (a mountain landscape) where the transition state lies at the saddle point (the col of a mountain pass). 

The idea that an activated complex or transition state controls the progress of a chemical reaction between the reactant state and the product state goes back to the study of the inversion of sucrose by S. Arrhenius, who found that the temperature dependence of the rate of reaction could be expressed as k = A exp (—AE /RT), a form now referred to as the Arrhenius equation. In the Arrhenius equation k is the forward rate constant, AE is an energy parameter, and A is a constant specific to the particular reaction under study. Arrhenius postulated thermal equilibrium between inert and active molecules and reasoned that only active molecules (i.e. those of energy Eo + AE ) could react. For the full development of the theory which is only sketched here, the reader is referred to the classic work by Glasstone, Laidler and Eyring cited at the end of this chapter. It was Eyring who carried out many of the... 

The top of the activation energy barrier on a potential energy diagram represents the transition state, or change-over point, of the reaction. The chemical species that exists at the transition state is referred to as the activated complex. The activated complex is a transitional species that is neither product nor reactant. It has partial bonds and is highly unstable. There is a subtle difference between the transition state and the activated complex. The transition state refers to the top of the hill on a potential energy diagram. The chemical species that exists at this transition point is called the activated complex. 

The Eyring activated-complex (or transition-state) treatment relates the observed rate constant k to multiplied by the frequency factor k TIh, where k is the Boltzmann constant, T is the absolute temperature, and h is Planck s constant ... 

Absolute Reaction Rate Theory of Eyring Activated Complex- or Transition State- Theory. See "Absolute Rate Theory in Vol 1 of Encycl, p A4-R and in Ref 96, p 134... 

When a collision with the proper orientation and sufficent activation energy occurs, an intermediate state exists before the products arc formed piis intermediate state, also called an activated complex or transition state, as neither the reactant or product, but rather a highly unstable combination of bbth as represented in Figure 1.13 for the decomposition of HI. 

Figure 1.14 shows the potential energy diagram for the decomposition of HI. As can be seen, in-order to reach the activated complex or transition state the proper orientaion (and JsufFicent collision energy must be achieved. Once these requirements/areyiCnieved the reaction continues on to completion and the products are fc... 

A possible example for case a is given in Fig. 16. a-Complexes and transition states are very similar and the activation process consists in a translation of the olefinic bond that leads one of the unsaturated carbon atoms nearer to hydrogen. Activation energies of the transition states are supposed to be very similar starting with either conformer la or IIa. 

The configuration of maximum PE is called the activated complex the transition state or the critical configuration and the unit (X——Y——Z) must attain this configuration before reaction can take place. Possessing the critical energy is not sufficient the fundamental requirement is attainment of this critical configuration. 

The intermediate nuclear configurations between reactants and products are all referred to as transition states for the reaction. The collection of atoms at the saddle point form a supermolecule , referred to as an activated complex, and their state is equivalent to one particular transition state for the reaction. This transition state obviously has a special status among all the transition states, and when one just refers to the transition state of a chemical reaction, it is tacitly assumed that one refers to the activated complex. The symbol f is used to represent activated complexes.2... 

Collision theory, as its name might suggest, focuses on the collisions between particles. The collisions must be frequent, and the colliding particles must have sufficient energy to form an activated complex. The transition-state theory focuses on the behavior of the activated complex. According to the transition-state theory, there are three main factors that determine if a reaction will occur ... 

It is important to also consider the reverse cycle of the reaction shown in Eq. (9-2). This reverse cycle can generate H2 with a very low activation energy. We describe this reverse cycle in detail, presenting the structures, electron states and energy diagrams of the complexes and transition states in the reaction of Eq. (9-2) in the next section, Section 2.2.3. 

We are often interested in the activated complex or transition state of a reaction—that is, the halfway point beyond which the system becomes more likely to progress to the products than return to the reactants. Similarly important are the reaction intermediates, species formed during the course of the reaction, which exist for a significant time interval, but which are ultimately consumed. Reaction intermediates often may be detected physically or chemically. Of the many methods for studying reaction mechanisms, the most important is the determination of the rate law, the quantitative relationship between the reaction speed at a fixed temperature and the concentrations of the reagents. The rate law will often indicate the species that participate in the ratedetermining step of the reaction. 

We can envision the reaction progress as shown in Fig. 15.11. The arrangement of atoms found at the top of the potential energy hill, or barrier, is called the activated complex, or transition state. The conversion of BrNO to NO and Br2 is exothermic, as indicated by the fact that the products have lower energy than the reactant. However, AE has no effect on the rate of the reaction. Rather, the rate depends on the size of the activation energy a. 

According to the theory of absolute reaction rates [4-9], the rate is the product of a universal frequency factor and the concentration of the activated complex or transition state, M (the system in the transient state of highest potential energy), crossing the energy barrier in the direction toward the products. The activated complex, in turn, is postulated to be in equilibrium with the reactants. Say, for a single-step reaction A + B — P ... 

The repulsion of c from ah (owing to the transfer of electrons i and 3) increases as c approaches ab Simultaneously the state a—he becomes more probable and resonance between the states a—he and ah—c somewhat decreases the energy of repulsion. Thus, although it is necessary to expend energy as translational energy in order to bring atom c up to the molecule ah there may ultimately be attained a state in which h is equally joined to both a and c. This active complex or transitional state may be represented as... 

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