Reaction between an ion and a molecule

Typical observed values of A are 109—1012 mol 1 dm3 s with very little variation in the magnitude of the A factor for a given reaction in different solvents. 

The reactant molecule will be very much less solvated than the reactant ion, while the activated complex will be solvated to a similar extent to the initial state 

Since the total degree of orientation of the solvent is approximately the same for the initial state and activated complex, the change in orientation is small and close to zero, and approximately constant for a given reaction in a variety of solvents. Hence A factors are not expected to vary much with change of solvent. 

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According to theory (22), the rate constant for reaction between an ion and a molecule can be expressed in terms of the polarizability of the neutral species and of the reduced mass of the reacting pair... 

An analogous treatment for reaction between an ion and a molecule suggests that for reaction of an... 

In principle the use of the entropy of activation as a criterion is straightforward. The electrostatic contribution to this quantity, A5 i, for a reaction between two cations is predicted from simple electrostatic theory to be less than that for a reaction between an ion and a neutral molecule. If the reactions are otherwise similar, the overall entropies of activation can be expected to differ in the same way ... 

Ion-molecule reaction The reaction between an ion and a neutral molecule which leads to the production of an adduct ion. 

For the reaction between an ion and a dipolar molecule, the rate is largely uninfluenced by ionic strength. A relation of the type... 

The radicals, the chlorine atom and the hydrogen atom, are marked with dots for clarity. The unpaired electron or the baton is carried on in the chemical system, from radical to radical. A reaction between an ion and a stable molecule would proceed in a similar fashion a new ion would form and cany on the electrical charge (the baton). 

In summary, an increase in tfeeofthe medium causes the increased reaction rate in the presence of ions with the same sign, and a decreased reaction rate when the ions have opposite signs. The rate of reaction between an ion and a neutral molecule will decrease as the dielectric constant of the solvent increases. The reader may want to consult further Reichardt s text for further theoretical discussions [102],... 

For a reaction between two dipoles having no net charge, the second term disappears, and the solvent effect is given entirely by the last term Eq. (5-93) then equals Eq. (5-88) in Section 5.4.3. For a reaction between an ion and a dipole (or between two charged dipoles) both terms must be included. The simplest case is for the reaction of a monovalent, structureless ion A of charge za = 0) with a neutral molecule B... 

The emphasis in this chapter is on bimolecular reactimis, i.e. reactions of the type A + B. Both the theoretical framework and the experimental techniques differ for reactions between an ion and a neutral molecule and between two neutral molecules, and they are discussed separately, in Sects. 3.3 and 3.4 respectively. In ion + neutral molecule reactions, the intermolecular forces are comparatively Imig range and theory concentrates on the calculation of the so-called capture rate coefficient. 

An understanding of the factors affecting the rate coefficient for reaction between an ion and a neutral atom or molecule centres on the calculation of the capmre rate coefficient. Capture brings the reactants into sufficiently close proximity for chemical interaction to occur and reaction to take place. Intermolecular forces were discussed in Sect. 1.4 for the reaction A + B. The attractive potential varies as Rab > where Rab is the distance between A and B. The effective potential energy, Veff (Rab)< is obtained by adding the energy of orbital motion of A and B, giving ... 

Since methyl dioxolane can amount to as much as ten percent of the dioxane, a termination of this kind could account for the low yield from regenerated ions but this scheme puts a lot of weight on the presence of dioxolane, a product which could, after all, arise by direct rearrangement of oxide to acetaldehyde. Furthermore, the zero order dependence on monomer does not necessarily rule out direct reaction between oxonium ion and monomer for it will be recalled that Meerwein observed that the inner oxonium salt complexed very strongly with a molecule of ether. If reaction were to occur between an ion and a monomer molecule held in this way, the rate could be independent of the monomer concentration. 

Gas-phase ion-molecule reactions often proceed via potential energy surfaces with multiple wells [7a], involving the intermediacy of various species including ion—neutral complexes [8]. A hypothetical example showing a double well potential is shown in Fig. 2. Note that the initial collision complex between an ion and a neutral (exemplified by [A+(BQ] in Fig. 2) is more stable than the separated ion and neutral and this excess energy of association can be used to overcome reaction barriers, thus driving the formation of products. This is one of the main reasons... 

Once again, considerable effort has gone into the theoretical modeling of ion-molecule reactions [7b] in order to establish the reaction efficiency (expressed as the experimental rate divided by the theoretical rate—thus an efficiency of 1 means that every collision between an ion and a neutral will result in the formation of products) of these reactions. 

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