Bimolecular ion-molecule reactions

Smith M A 1994 Ion-molecule reaction dynamics at very low temperatures Unimolecular and Bimolecular Ion-Molecule Reaction Dynamics ed C-Y Ng, T Baer and I Powis (New York Wiley)... 

Bimolecular ion/molecule reactions of dienes and polyenes have been extensively studied for several reasons. Some of them have been mentioned implicitly in the previous sections, that is, in order to structurally characterize the gaseous cations derived from these compounds. In this section, bimolecular reactivity of cationic dienes, in particular, with various neutral partners will be discussed, and some anion/molecule reactions will be mentioned also (cf Section IV). In addition, the reactions of neutral dienes with several ionic partners will also be discussed. Of this latter category, however, the vast chemistry of reactions of neutral dienes with metal cations and metal-centred cations will not be treated here. Several reviews on this topic have been published in the last decade178. 

Under ordinary mass spcctrometric conditions only unimolecular reactions of excited ions occur, but at higher ionization chamber pressures bimolecular ion molecule reactions are observed in which both the parent ions and their unimolecular dissociation product ions are reactants. Since it requires a time of 10 5 sec. to analyze and collect the ions after their formation all of the ions in the complete mass spectrum of the parent molecule are possible reactants. However, in radiation chemistry we are concerned with the ion distribution at the time between molecular collisions which is much shorter than 10 5 sec. For example, in the gas phase at 1 atm. the time between collisions is 10 10 sec. and in considering the ion molecule reactions that can occur one must know the amount of unimolecular decomposition within that time. By utilizing the quasi-equilibrium theory of mass spectra6 it is possible to calculate the ion distribution at any time. This has been done for propane at a time of 10 10 sec.,24 and although the parent ion is increased by a factor of 2 the relative ratios of the other ions are about the same as in the mass spectrum observed in 10 r> sec. Thus for gas phase radiolysis the observed mass spectrum is a fair first approximation to the ion distribution. In... 

The field of ion-molecule reactions has profited immensely from the introduction of new techniques in mass spectrometry such as Fourier Transform Ion-Cyclotron Resonance Mass Spectrometry (FT-ICR) and Selected-Ion Flow Tube methods (SIFT). Several accounts of the progress in the field of ion-molecule reactions involving silicon-containing molecules have been published. For review articles the reader is referred elsewhere1,3,4,40-51 A detailed compilation of the kinetic data for bimolecular ion-molecule reactions of positive silicon ions has been published by Anicich52. 

R. C. Dunbar, in Unimolecular and Bimolecular Ion-Molecule Reaction Dynamics (Eds. Cheuk-Yiu Ng, T. Baer and I. Powis), Wiley, Chichester, 1994, pp. 279-309. 

In this review we focus both on major developments in new mass spectrometric techniques and on novel chemical applications of existing mass spectrometric techniques that have been reported since 1990. Emphasis is given to the application of these techniques to the study of bimolecular ion/molecule reactions, radiative association, and dissociative recombination of positive ions. Particular attention is given to the emerging field of interstellar metal-ion chemistry and recent studies of fullerene-ion chemistry and the influence of charge state on this, and related, chemistry. Mass spectrometric studies of the photochemistry of interstellar ions are briefly considered as is interstellar negative-ion chemistry. We conclude with a brief description of the use of mass spectrometry to examine interstellar material that has made the long journey to our solar system. 

The laboratory study of IS ion chemistry has its origins in measurements of bimolecular ion/molecule reaction kinetics. Experimental conditions that are found in most of the mass spectrometers employed in the measurement of bimolecular ion/molecule reactions are far from those found in the cold low-pressure environments of space. Nevertheless, because of the pressure independence of bimolecular reactions and the absence of significant activation energies in most bimolecular ion/molecule reactions, MS measurements performed here on earth do have relevance for the chemistry in space. The substantial database available in the early 1990s on the kinetics of bimolecular ion/molecule reactions important in IS chemistry [8-10] was obtained almost entirely using ion cyclotron resonance (ICR) and flow-tube (FT) mass spectrometry techniques. Both techniques are well established and continue to be used extensively for ion/ molecule reaction measurements generally. 

The displaced electron is generally assumed to be the electron with the lowest ionization energy. In order of probability, this will be a nonbonding electron followed by a 7t bond electron and then a a bond electron. Thus El yields, in the first instance, a molecular ion which is a radical cation with an unpaired electron. In principle, any remaining energy will then be dissipated by bond cleavages that result in the formation of the most stable cation with a paired electron (even-electron ion). These even-electron ions may be formed by homolytic or heterolytic cleavages. This whole process happens very rapidly (<10-8s) and is the reason for the close similarity of El spectra produced across all different instruments. It is important to remember that mass spectral reactions in the El source are unimolecular. This is because the pressure in the El source is too low for bimolecular (ion-molecule) reactions to occur. 

The volume of activation for the hydrolysis of acetamide in dilute acid is —9-4 i 0-5 cm mole (Osborn et al., 1961) which is almost identical with that for the acid-catalyzed and base-catalyzed hydrolysis of methyl and ethyl acetates and for the other simple bimolecular ion-molecule reactions listed in Tables 2 and 6. It seems likely, therefore. 

Ng, C.-Y, Baer, T and Powis, I. (1994) Unimolecular and Bimolecular Ion-Molecule Reaction Dynamics, John Wiley Sons, Chichester. 

The ion chemistry of the olefins and diolefins plays an important role in the gas phase polymerization process (7-9,28,36-38). These monomers can be induced to oligomerize rmd/or polymerize through bimolecular ion-molecule reactions in the gas phase (28). I e reactions can be initiated by an appropriate cation or radical cation which can transfer the charge to the selected monomer. 

Table 11 gives a classification of bimolecular ion-molecule reactions. 

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