Mechanism of ion - molecule reactions

The power of these techniques in elucidating the detailed mechanisms of ion-molecule reactions is well demonstrated by this example. Undoubtedly, we can expect to see increasing use of the ultrafast pump-probe technique in the field of ion-molecule reaction dynamics. 

Many organic anions have been found to undergo hydrogen/deuterium exchange in the presence of deuterated reagents, such as p,0 and CH,OD. Such exchange reactions have proved to be a valuable method of probing mechanisms of ion/molecule reactions and structures of ions in the gas phase. 

Professor Henry Eyring has been a leader in the field of reaction kinetics for almost forty years. His work has not only established a pattern for the expert in the field but it has changed the way scientists generally think about kinetics his influence is ubiquitous. The work described here is a probing into the mechanism of ion-molecule reactions. We feel we owe much to the Eyring tradition and are honored to have this paper in a volume dedicated to him. 

This study deals with the effect of Si/C substitution on the mechanism of ion-molecule reactions for series of compounds containing C-N-C, C-N-Si, and Si-N-Si linkages. Reactions of nucleogenic free methyl cations with diisobutylamine, isobutylaminotrimethylsilane, and hexamethyldisilazane were studied for this purpose. 

One of the chief reasons for the recent extensive work in this field has been the recognition that ion-molecule reactions are highly relevant to radiation chemistry. The possibility that certain simple reactions, such as the formation of H3+, participate in the mechanism of product formation was appreciated much earlier 14), but wider applicability of this concept required that the generality of such reactions be demonstrated by an independent, unequivocal method. Mass spectrometry has been the predominant means of investigating ion-molecule reactions. The direct identification of reactant and product ions is appealing, at least in part, because of the conceptual simplicity of this approach. However, the neutral products of ion-molecule reactions cannot be determined directly and must be inferred. Gross chemical measurements can serve as an auxiliary technique since they allow identification of un-... 

A major complication in applying radiation chemical techniques to ion-molecule reaction studies is the formation of nonionic initial species by high energy radiation. Another difficulty arises from the neutralization of ions, which may also result in the formation of free radicals and stable products. The chemical effects arising from the formation of ions and their reactions with molecules are therefore superimposed on those of the neutral species resulting from excitation and neutralization. To derive information of ion-molecule reactions, it is necessary to identify unequivocally products typical of such reactions. Progress beyond a speculative rationalization of results is possible only when concrete evidence that ionic species participate in the mechanism of product formation can be presented. This evidence is the first subject of this discussion. 

The conclusions on the occurrence of ion-molecule reaction in the radiolysis of ethylene are not seriously affected by the uncertainties in the neutralization mechanism. It must be assumed that neutralization results in the complex species which constitute the ionic polymer, — i.e., the fraction of the ethylene disappearance which cannot be accounted for by the lower molecular weight products containing up to six carbon atoms. 

The photoproduction of CH3+ in Titan s atmosphere initiates a sequence of ion-molecule reactions. The mechanism is as follows ... 

Finally, it should be recognized that the double well potential model is simple and qualitative, although it can explain the results obtained for several gas-phase ion/molecule reactions. However, there are also many examples of ion/molecule reactions which proceed through a multistep mechanism as will be shown further on in this review. Of course, the corresponding potential energy surface is in that case much more complicated than the one in Fig. 4. At any rate, for a clear presentation of the many reactions of organic anions studied over the last decade, the discussion below will be focused both on several types of reactions and on various classes of anions. 

MS/MS applications are plentiful, for example in elucidation of structure, determination of fragmentation mechanisms, determination of elementary compositions, applications to high-selectivity and high-sensitivity analysis, observation of ion-molecule reactions and thermochemical data determination (kinetic method). We will examine them in detail through a few examples. 

Ion cyclotron resonance (i.c.r.) spectrometry is a specialised technique of growing importance. It enables the detailed study of ion-molecule reactions (and their mechanisms) in the gas phase, giving both absolute and relative thermochemical quantities and rate constants. 

We designed a novel three-compartment source (wide-range radiolysis source) for our research mass spectrometer, which was first used to study the radiolysis of methane. The present technique, employing flow, low pressure, localized ionization, and electric fields appears to be a straightforward approach to the problem, and we hoped that this technique would resolve some of the above discrepancies. Our objectives were to (a) determine the percent abundance of the various reactive primary species—ionic and neutral (b) ascertain the percent abundance of stable products under conditions that would minimize subsequent reactions of reactive stable products (c) calculate G values for these products (d) measure the relative contribution of ion-molecule reactions to the formation of stable products (e) obtain the threshold energies and yield curves for such products to assign their precursors and (f) postulate, from the above information and pressure studies, a mechanism for the production of the radiolytic products from methane. 

In the previous section, the theoretical interpretations of ion—molecule reactions have been developed on the basis of the complex mechanism, i.e. on the assumption that reactions proceed via an intermediate complex... 

Olson (10) where a series of ion/molecule reactions were combined with acetylene oxidation reactions and a computational model developed. This model gave ion profiles reasonably similar to those actually observed in sooting flames, and also predicted concentrations of ions sufficiently high that they might be considered as soot nucleation sites. Aspects of an ionic mechanism... 

Z. Herman, J. Kerstetter, T. Rose, and R. Wolfgang, Crossed-beam studies of ion-molecule reaction mechanisms. Disc, Faraday Soc, 44, 123-136 (1967). 

A. Good, D. A. Durden, and P. Kebarle, Mechanism and rate constants of ion-molecule reactions leading to formation of H (H20) in moist oxygen and air, J. Chem. Phys. 52, 222-229 (1970). 

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