Bimolecular Reactions in the Gas Phase

The advent of methods for determining proton affinities by studying bimolecular reactions in the gas phase has provided a wide range of interesting thermochemical data. 

There are only a few elementary bimolecular reactions in the gas phase, e.g. the reaction... 

However, as Dewar has pointed out [1], since a proper substrate of an enzyme fits tightly into its active site, this precludes the presence of water molecules in the active site, except when the water takes part in the reaction. Consequently, any subsequent reaction takes place in the absence of solvent and may be compared with bimolecular reactions in the gas phase. The specificity and high rates of enzymatic processes are readily understood on the basis of this analogy. 

Three-body collisions play a major role in many atmospheric and combustion reactions. A much cited explanation for third-body-assisted bimolecular reactions in the gas phase is the following sequence of two binary collisions. 

In the early 1920s these ideas were developed in some detail. In particular, attention was focused on calculating the value of the second-order rate constant for a bimolecular reaction in the gas phase. It is important to recognize that experimentally such a rate constant will reflect in some way an average over many billions of individual reactions. It was therefore essential to develop a model of behaviour at the molecular level that could be averaged in a suitable way in order to make comparisons with experiment. Effectively, this restricted the theoretical development to gases since the behaviour of particles in a gas was far better understood than the corresponding behaviour in a solution. 

Collision theory for a bimolecular reaction in the gas phase treats the individual reactant species as hard spheres and introduces a threshold energy for the reaction. The expression derived for the temperature dependence of the bimolecular second-order rate constant is of the same form as that for the Arrhenius equation. The theoretical A-factor is related to the rate at which reactant species collide and is calculated to be of the order of 10 dm mol s , although experimental values can be smaller than this by several orders of magnitude. 

Part 5 is dedicated entirely to modem reaction dynamics involving bimolecular reactions in the gas phase. It should be noted that, during the last decade or so, a huge development has taken place in our understanding of chemical reactions in other environments, such as in clusters or at surfaces Part 6 is dedicated to these studies. 

A value much more negative than this implies the loss of much freedom of motion on formation of the complex, and corresponds to a small value of P, the steric factor in the collision theory. On the other hand, less negative or even positive values of A 5 occasionally occur, though rarely if ever for bimolecular reactions in the gas phase. They imply that the complex is very loosely bound, or, in solution, that the complex is less tightly solvated than are the reactant species. 

Bimolecular reactions in the gas phase frequently involve equilibrium to form a complex, followed by an isomerization over an energy barrier via a transition structure to the product or new complex that dissociates to products ... 

Transition state theory applies to all mono-, bi-, trimole-cular processes, in the gas phase as well as in liquid phase (and equally to heterogeneous processes), in contrast to elementary collision theory, which is limited to bimolecular reactions in the gas phase. 

In gases an elementary act of the bimolecular reaction occurs by the collision of two particle-reactants if an excessive energy of colliding particles exceeds the activation barrier and the configuration of the formed pair is convenient for the reaction. In liquid (solution) the bimolecular act occurs in somewhat different way. At first particle-reactants diffuse in solution and get into the same cage, neighboring for some time, they collide in it and undergo transformation in one of the collisions if the same conditions are fulfilled as those for the bimolecular reaction in the gas phase. The situation is somewhat more difficult when molecular complexes or electrostatic interactions appear between molecules in solution, which will be considered in the next... 

Similar conclusions are drawn when this problem is considered in the framework of the transition state theory. The rate constant of the bimolecular reaction in the gas phase... 

---