Reaction between ions and dipoles

A rather different process involving an assymetric reactant is provided by attachment of an electron to dipolar oxidants in hydrocarbon solvents 

Using this potential energy in eqn. (44) gives the steady-state rate coefficient as approximately [260] 

The rotational relaxation times of these nitrocompounds have not been measured. Comparison with the studies of perylene by Klein and Haar [253] suggests that most of these nitrocompounds have rotational times 10—20 ps in cyclohexane. For rotational effects to modify chemical reaction rates, significant reaction must occur during 10ps. This requires that electron oxidant separations should be (6 x 10-7x 10-11)J/2 2 nm. Admittedly, with the electron—dipole interaction, both the rotational relaxation and translational diffusion will be enhanced, but to approximately comparable degrees. If electrons and oxidant have to be separated by 2 nm, this requires a concentration of 0.1 mol dm-3 of the nitrocompound. With rate coefficients 5 x 1012 dm3 mol-1 s 1, this implies solvated electron decay times of a few picoseconds. Certainly, rotational effects could be important on chemical reaction rates, but extremely fast resolution would be required and only mode-locked lasers currently provide 10 ps resolution. Alternatively, careful selection of a much more viscous solvent could enable reactions to show both translational and rotational diffusion sufficiently to allow the use of more conventional techniques. 

Bakale et al. [261] have studied the reaction of a wide range of dipolar reactants with the solvated electron in cyclohexane at 293 K and found that the effective reaction radius increases approximately proportionately to the dipole moment. They suggested that the correct reaction radius was such that U(r) = — k T in eqn. (120), i.e. about 0.8, whereas from eqn. 

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Reactions between ions and dipoles were discussed in Chap. 5, Sect. 

Debye and Huckel (J) have derived an expression for the work function of an ion in an ion atmosphere in solution. They and others (J, S, 4) have applied this function to various phenomena in liquid media. The authors (2) have previously deduced, in a similar way, the field around a dipole and have combined it with Onsagers (5) theory of polar liquids to obtain an equation that explains the electrostatic effects on the rates of reaction between ions and dipolar molecules (2). The equation has been applied (2,6,7,8) to the rates of several ion-dipolar molecular reactions. 

Although it was realized more than a decade ago that reactions between ions and molecules with a permanent dipole moment may be more rapid at low T than reactions between ions and non-polar molecules, this effect was not taken into account in early models. Important reactions in diffuse clouds whose rates will be affected at low T Include (Marquette et ai. 19856 see also Rowe this volume)... 

Because the collisions between ions and molecules in the gas phase are governed by physical (ion-dipole, ion-induced dipole) rather than chemical forces, it is possible to calculate rather accurately the collision rate constant (6, 7). We then express the efficiency of the reaction as the fraction of collisions which lead to products. 

The polarity of the solvent will influence different types of reactions in different ways, depending upon whether they involve ions, dipoles or polarisable molecules. At the simplest level, we can analyse the effects of the solvent in terms of the different degrees of solvation of species in the initial state and the transition state. For example, in the reaction between pyridine and methyl iodide (Equation 3.24) the reactants are separate neutral molecules, the products are separate fully formed ions, but the transition structure is a single molecular entity with an appreciable degree of polarity. 

A collision between atoms, molecules, or ions is not like one between two hard billiard balls. Whether or not chemical species collide depends on the distance at which they can interact with one another. For instance, the gas-phase ion-molecule reaction CH4+ + CH4 CH5+ + CH3 can occur with a fairly long-range contact. This is because the interactions between ions and induced dipoles are effective over a relatively long distance. By contrast, the reacting species in the gas reaction CH3 + CH3 C2H are both neutral. They interact appreciably only through very short-range forces between induced dipoles, so they must approach one another very closely before we could say that they collide. ... 

Activities of electrons and protons capable of participating in chemical reactions are determined only by conditions under which the rest of water components interact between themselves on the way to equilibrium. Main forms of such interaction are association, i.e., attaching of one component to the other with the formation of more complex compounds, and dissociation, i.e., destruction of these complex compoxmds. A series of sequential associations with the formation of larger super-molecular compoxmds is called complexation. In this process participate mostly ions and dipoles of H O, much more seldom neutral molecxiles with the covalent bond. 

In a number of recent publicationsN - solvent effects in relation to substitution kinetics have been treated under the headings of reactions between ions, ions and dipoles and between dipoles. All the current theories depend on the application of transition-state theory to the solution kinetics. However, there is little doubt that Identical expressions can be rationalised without the acceptance of transition state theory. The... 

The possibility of a barrier which inhibits a reaction in spite of the attractive ion-dipole potential suggests that one should make even crude attempts to guess the properties of the potential hypersurface for ion reactions. Even a simple model for the long range behavior of the potential between neutrals (the harpoon model ) appears promising as a means to understand alkali beam reactions (11). The possibility of resonance interaction either to aid or hinder reactions of ions with neutrals has been suggested (8). The effect of possible resonance interaction on cross-sections of ion-molecule reactions has been calculated (25). The resonance interaction would be relatively unimportant for Reaction 2 because the ionization potential for O (13.61 e.v.) is so different from that for N2 (15.56 e.v.). A case in which this resonance interaction should be strong and attractive is Reaction 3 ... 

Pulsed source techniques have been used to study thermal energy ion-molecule reactions. For most of the proton and H atom transfer reactions studied k thermal) /k 10.5 volts /cm.) is approximately unity in apparent agreement with predictions from the simple ion-induced dipole model. However, the rate constants calculated on this basis are considerably higher than the experimental rate constants indicating reaction channels other than the atom transfer process. Thus, in some cases at least, the relationship of k thermal) to k 10.5 volts/cm.) may be determined by the variation of the relative importance of the atom transfer process with ion energy rather than by the interaction potential between the ion and the neutral. For most of the condensation ion-molecule reactions studied k thermal) is considerably greater than k 10.5 volts/cm.). 

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