Ion-molecule potential

In a classical picture of Penning ionization,24 the molecules approach along a trajectory on the initial A + B (real) potential V0(r). Ionization occurs at a specified (but random) value of the internuclear distance, r/( and the products then complete their trajectories on an ion-molecule potential V+(r) for A + B +. Neglecting the momentum of the ejected electron, deflection functions can be computed according to whether the ionization occurs on the incoming or out going part of the V0(r) trajectory. These are... 

At the simplest level, it is assumed that a necessary and sufficient condition for reaction is that the colliding reactants must surmount the centrifugal barrier in the effective radial potential. A model for the ion-molecule potential must then be assumed when that chosen is the ion-induced-dipole potential, this approach yields, of course, the familiar Langevin model. ... 

A greater A reduces F but raises Fj, with the difference versus Equation 2.33 increasing at higher m/M up to 20% and 40%, respectively. Hence diffusion is more isotropic with realistic ion-molecule potentials than simple (Table 2.2), though F > Fx even for A = 1.35. The most sophisticated approach is varying A as a function of Tfp, which is parameterized for some d>. 

The decrease in reactivity with increasing temperature is due to the fact that many low-energy ion-molecule reactions proceed tln-ough a double-well potential with the following mechanism [82] ... 

B. H. Lengsfield and D. R. Yarkony, Nonadiabatic Interactions Between Potential Energy Surfaces Theory and Applications, in State-Selected and State to State Ion-Molecule Reaction Dynamics Part 2 Theory, M. Baer and C.-Y. Ng, eds., John Wiley Sons, Inc., New York, 1992, Vol, 82, pp. 1-71. 

Let us now consider the same species of molecule situated in a particular solvent and dissociated into a pair of ions. The potential-energy curve will be similar but will have a much shallower minimum, as in Fig. 86, because in a medium of high dielectric constant the electrostatic attraction is much weaker. Let the dissociation energy in solution be denoted by D, in contrast to the larger Dvac, the value in a vacuum. 

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 ... 

This study has made no substantial improvement in the original theory of distorted waves. By evaluating the vibrational transition probability explicitly for the inverse (12-6-4) power potential, however, we were able to study some interesting aspects of the ion-molecule collisions. We summarize them here. 

However, a maximum is reached at about 15 volts, and a decrease is observed at higher voltages. With increasing voltage, the kinetic energy of the primary ions is also increased. The increase in primary ion current apparently is counterbalanced and finally exceeded by the decrease in the cross-section of the ion molecule reaction. In addition, discrimination of CH5 + ions which are formed via a complex becomes more effective. At a potential of 20 volts between trap and chamber, most of the primary ions have kinetic energies around 20 e.v., and a secondary CH5+ ion... 

Some of the problems encountered in the mass spectrometric study of ion-molecule reactions are illustrated in a review of the H2-He system (25). If the spectrometer ion source is used as a reaction chamber, a mixture of H2 and He are subjected to electron impact ionization, and both H2+ and He+ are potential reactant ions. The initial problem is iden-... 

Intramolecular Isotope Effects. The data in Figure 2 clearly illustrate the failure of the experimental results in following the predicted velocity dependence of the Langevin cross-section. The remark has been frequently made that in the reactions of complex ions with molecules, hydrocarbon systems etc., experimental cross-sections correlate better with an E l than E 112 dependence on reactant ion kinetic energy (14, 24). This energy dependence of reaction presents a fundamental problem with respect to the nature of the ion-molecule interaction potential. So far no theory has been proposed which quantitatively predicts the E l dependence, and under these circumstances interpreting the experiment in these terms is questionable. 

A necessary condition for ion-molecule reactions that has not been considered thus far is that of continuity between reactant and product potential energy surfaces. Many reactions of ions and molecules take place with / a transition from one potential energy surface to another. If no suitable crossings between the respective surfaces exist, then obviously orbiting ion-molecule collisions cannot produce chemical reac-... 

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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