Molecule activated complex

Transition state theory of surface reactions was developed independently by Temldn and Laidler, Glasstone and Eyring shortly after the more general treatment of Eyring and Polanyi appeared. It was supposed that each molecule occupies one elementary space and there is a random distribution of molecules. Activated complexes occupy several adjacent elementary sites (s) and there are several possible positions of activated complexes (g). For a reaction of A and B there is a possibility to have several equivalent positions of the activated complexes (here g=4)... 

The observation that in the activated complex the reaction centre has lost its hydrophobic character, can have important consequences. The retro Diels-Alder reaction, for instance, will also benefit from the breakdown of the hydrophobic hydration shell during the activation process. The initial state of this reaction has a nonpolar character. Due to the principle of microscopic reversibility, the activated complex of the retro Diels-Alder reaction is identical to that of the bimoleciilar Diels-Alder reaction which means this complex has a negligible nonpolar character near the reaction centre. O nsequently, also in the activation process of the retro Diels-Alder reaction a significant breakdown of hydrophobic hydration takes placed Note that for this process the volume of activation is small, which implies that the number of water molecules involved in hydration of the reacting system does not change significantly in the activation process. 

In the case of the retro Diels-Alder reaction, the nature of the activated complex plays a key role. In the activation process of this transformation, the reaction centre undergoes changes, mainly in the electron distributions, that cause a lowering of the chemical potential of the surrounding water molecules. Most likely, the latter is a consequence of an increased interaction between the reaction centre and the water molecules. Since the enforced hydrophobic effect is entropic in origin, this implies that the orientational constraints of the water molecules in the hydrophobic hydration shell are relieved in the activation process. Hence, it almost seems as if in the activated complex, the hydrocarbon part of the reaction centre is involved in hydrogen bonding interactions. Note that the... 

Here we have the formation of the activated complex from five molecules of nitric acid, previously free, with a high negative entropy change. The concentration of molecular aggregates needed might increase with a fall in temperature in agreement with the characteristics of the reaction already described. It should be noticed that nitration in nitromethane shows the more common type of temperature-dependence (fig. 3.1). 

If the activated complex is considered to be in equilibrium with its component molecules, the attainment of the transition state (T.S.) can be treated as being analogous to a bimolecular reaction ... 

The exact nature of the activated complex is difficult to determine. For this reaction, the activated complex might be a pseudomolecule made up of CO and N02 molecules in close contact. The path of the reaction might be more or less as follows ... 

The dotted lines stand for partial bonds in the activated complex. The N—O bond in the N02 molecule has been partially broken. A new bond between carbon and oxygen has started to form. 

The last factor is, again, the rate that molecules can pass over the energy barrier—the activated complex for precipitation. Again there is a rate constant, kp, that is determined by temperature and the height of the energy barrier to precipitation. 

The influence of barriers on thermodynamic properties must have importance in determining the rates of various chemical reactions. It seems certain that the activated complex for many reactions will involve the possibility of restricted rotation and that the thermodynamic properties of the complex will therefore be in part determined by the magnitude of the barriers. Whereas at the moment there is no direct way of determining such barriers, any general principles obtained for stable molecules should ultimately be applicable to the activated state. One might then hope to be able to estimate the barriers and the reaction rates a priori. 

The following assumptions are made (i) the activated complexes are in equilibrium with the reactants, (ii) the energy of a molecule is not altered when an activated complex is substituted for a nearest neighbour, and (iii) the products do not affect the course of reaction, except to define a boundary in surface processes. The various cases can be recognized from the magnitude of the pre-exponential term and calculated values [515] are summarized in Table 7. Low values of A indicate a tight surface complex whereas higher values are associated with a looser or mobile complex. 

Activation energy values for the recombination of the products of carbonate decompositions are generally low and so it is expected that values of E will be close to the dissociation enthalpy. Such correlations are not always readily discerned, however, since there is ambiguity in what is to be regarded as a mole of activated complex . If the reaction is shown experimentally to be readily reversible, the assumption may be made that Et = ntAH and the value of nt may be an indication of the number of reactant molecules participating in activated complex formation. Kinetic parameters for dissociation reactions of a number of carbonates have been shown to be consistent with the predictions of the Polanyi—Wigner equation [eqn. (19)]. 

FIGURE 13.29 This sequence of images shows the reactant molecules in solution as they meet, then either move apart, or acquire enough energy by impacts from the solvent molecules to form an activated complex, which may go on to form products. 

FIGURE 13.30 A reaction profile for an exothermic reaction. In the activated complex theory of reaction rates, it is supposed that the potential energy (the energy due to position) increases as the reactant molecules approach each other and reaches a maximum as they form an activated complex. It then decreases as the atoms rearrange into the bonding pattern characteristic of the products and these products separate. Only molecules with enough energy can cross the activation barrier and react to form products. 

In transition state theory, a reaction takes place only if two molecules acquire enough energy, perhaps from the surrounding solvent, to form an activated complex and cross an energy barrier. 

In this book the discussion has been restricted to the structure of the normal states of molecules, with little reference to the great part of chemistry dealing with the mechanisms and rates of chemical reactions. It seems probable that the concept of resonance can be applied very effectively in this field. The activated complexes which represent intermediate stages in chemical reactions are, almost without exception, unstable molecules which resonate among several valence-bond structures. Thus, according to the theory of Lewis, Olson, and Polanyi, Walden inversion occurs in the hydrolysis of an alkyl halide by the following mechanism ... 

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