Ion-molecule collision theory

Su T and Bowers MT (1979) Classical ion-molecule collision theory. In Bowers MT (ed) Gas Phase Ion Chemistry, Vol. 1, pp. 83-118. New York Academic Press. 

A comparison of theoretical collision rate constants can be found in Table 8.1, and further discussion of classical ion-molecule collision theory can be found in reference 23. 

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. 

R. C. Bhattacharjee and W. Forst, Statistical Theory of Energy Transfer in Ion-Molecule Collisions, paper presented at Seventh International Mass Spectrometry Conference, Florence, 1976. 

It has been mentioned that phase space theory, i.e. assuming a loose transition state, has been able to explain the translational energy releases in the decomposition of certain ion—molecule collision complexes [485] and in some unimolecular decompositions measured by PIPECO (see Sect. 8.2). There is a larger number of translational energy releases from PIPECO and a body of data as to translational energy releases in source reactions of positive ions formed by El [162, 310] (Sect. 8.3.1) with which the predictions of phase space theory are in poor agreement. The predicted energy releases are too low. 

Ion-molecule collisions have been of interest for a long time, starting with Langevin s treatment of the simplest possible case in 1905. The recent interest in elementary reactions has inevitably led to experimental studies of ion-molecule collisions and these have raised questions of theory. For a number of years we have carried out theoretical investigations on collisions of ions with dipolar molecules. Although the collisions considered are relatively simple, they contain the essential features of many real systems. In fact, such collisions are prototypes for many of the ion-molecule collisions of small molecules occurring in experimental work of current interest. In this paper two important aspects of ion-molecule collisions are considered collision cross section and collision time. 

H. Nakamura, Semiclassical approach to charge-transfer processes in ion-molecule collisions, State-Selected and State-to-State Ion-Molecule Reaction Dynamics. Part 2 Theory, Advances in Chemical Physics LXXXII (M. Baer and C. Y. Ng, eds.), Wiley, New York, 1992, p. 243. 

Experiments conducted in the recent years on the kinematics of ion-molecule collisions have revealed that sometimes, particularly at relative kinetic energies higher than 10 eV (see e.g. [183]), these reactions proceed by a direct mechanism rather than via a long-lived complex. This means that in reaction Ar" -j--> ArH+ + H, for example, Ar+ collides with only one hydrogen atom in the H2 molecule. The second functions as a spectator and is not involved in the collision. The theory of such reactions is at its initial stage and compared to the statistical theory it contains additional parameters which are difficult to calculate because this would require invoking short-range forces (see Sect. VII.21). 

Bass L, Su T, Bowers MT. (1978) Theory of ion-molecule collisions. Effect of anisotropy in polarizability on collision rate constant. Int. J. Mass Spec. Ion Proc. 28 389-399. 

The ratio kq/kf can now be identified as ki9/k9a. We can calculate an upper limit for kl9 from ion-molecule reaction rate theory by assuming that this reaction occurs with a collision efficiency. This leads to k9il < 1 X 109 sec. -1 (7). 

Because T -> V energy transfer does not lead to complex formation and complexes are only formed by unoriented collisions, the Cl" + CH3C1 -4 Cl"—CH3C1 association rate constant calculated from the trajectories is less than that given by an ion-molecule capture model. This is shown in Table 8, where the trajectory association rate constant is compared with the predictions of various capture models.9 The microcanonical variational transition state theory (pCVTST) rate constants calculated for PES1, with the transitional modes treated as harmonic oscillators (ho) are nearly the same as the statistical adiabatic channel model (SACM),13 pCVTST,40 and trajectory capture14 rate constants based on the ion-di-pole/ion-induced dipole potential,... 

The collision theory of reaction rates states that molecules, atoms or ions must collide effectively in order to react. For an effective collision to occur, the reacting species must have (1) at least a minimum amount of energy in order to break old bonds and make new ones, and (2) the proper orientation toward each other. 

Why do factors such as temperature and concentration increase or decrease the rate of a reaction To answer this question, chemists must first answer another question What causes a reaction to occur One obvious answer is that a reaction occurs when two reactant particles collide with one another. This answer is the basis for collision theory In order for a reaction to occur, reacting particles (atoms, molecules, or ions) must collide with one another. 

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