Exothermic ion-molecule reactions

In addition to the effects discussed above, two further possible sources of discrimination peculiar to ion-molecule reactions must be considered. First, although it is known that most primary ions are formed without kinetic energy, such may not be the case for ions produced by ion molecule reactions. Secondary ions formed in exothermic ion-molecule reactions could retain a considerable fraction of the exo-thermicity as kinetic energy and diffuse from the sampling region at a rate considerably greater than predicted from the ambient temperature. The limited evidence to date (40) indicates that the kinetic energy of the product ions is small, but this may not be true for all types of reactions. 

Consideration of work by Buchel nikova (4) on the dissociative attachment of electrons to HC1 leads to the conclusion that k4 is given approximately by k4 10-10 e 20 mtRT cm.3 molecule-1 sec.-1 Since K 4 2 X 10-3 el8 miRT at 2000°K., 4 10 13 cm.3 molecule-1 sec.-1 This is considerably smaller than rate constants for other exothermal ion-molecule reactions, which probably reflects the importance of participation of molecular vibrational energy in such reactions. Remember, however, that the uncertainty in 4 is probably at least an order of magnitude. 

This ion is not reactive with H2 but can react with a number of other heavy atoms assumed to be present in initial stages of the cloud. In particular, reaction with oxygen atoms leads eventually to the production of the molecular ion H30+ by a chain of exothermic ion-molecule reactions studied in the laboratory ... 

If a reactant gas is introduced into the collision cell, ion-molecule collisions can lead to the observation of gas-phase reactions. Tandem-in-time instruments facilitate the observation of ion-molecule reactions. Reaction times can be extended over appropriate time periods, typically as long as several seconds. It is also possible to vary easily the reactant ion energy. The evolution of the reaction can be followed as a function of time, and equilibrium can be observed. This allows the determination of kinetic and thermodynamic parameters, and has allowed for example the determination of basicity and acidity scales in the gas phase. In tandem-in-space instruments, the time allowed for reaction will be short and can be varied over only a limited range. Moreover, it is difficult to achieve the very low collision energies that promote exothermic ion-molecule reactions. Nevertheless, product ion spectra arising from ion-molecule reactions can be recorded. These spectra can be an alternative to CID to characterize ions. 

Such reactions also seem to be more often fast than slow. One of the slowest exothermic ion-molecule reactions (again barring cases where fast charge-transfer competes) is... 

The effect of relative kinetic energy of reactants on exothermic ion—molecule reactions was investigated in the l-ObOs with conventional single-source mass spectrometers by changing the repeller-field strength E, or the source temperature T. For example, Gutbier [150] and Stevenson and Schissler [151] found that the phenomenological cross-sections Q for the reactions... 

In this review we discuss five techniques involving Fourier transform mass spectrometry (FTMS) for determining qualitative and quantitative metal ion-ligand bond energies. These include (i) exothermic ion-molecule reactions, (ii) equilibrium measurements, (iii) competitive collision-induced dissociation, (iv) endothermic ion-molecule reactions, and (v) photodissociation. A key advantage of the FTMS methodology is its ion and neutral manipulation capabilities which permit the formation and study of a limitless number of interesting metal-ion systems. 

Similar results to those just described, with the reaction cross-section independent of, or declining slowly with, v, have been found in experiments at low collision energies on a number of exothermic ion-molecule reactions. These include reactions where a neutral molecular reagent is vibrationally excited, such as that between He+ and Nifv), as well as those like H2 + Hj, NH3 +... 

J. C. Light and S. Chan, Isotopic distributions in exothermic ion-molecule reactions. A simple model, J. Chem. Phys. 51, 1008-1015 (1969). 

J. L. Franklin and M. A. Haney, Translational energies of products of exothermic ion-molecule reactions, J. Phys. Chem. 73, 2857-2863 (1969). 

E. E. Nikitin, Statistical theory of exothermic ion-molecule reactions, Theor. Exp. Chem. 1,275-280(1965). 

Most exothermic ion-molecule reactions involving transfer of heavy particles virtually require no activation energy, a phenomenon that needs serious theoretical support. Only a few of the studied hundreds of ion-molecule reactions involving transfer of heavy particles have an activation energy. For instance, reaction 0 -J- X2 -> N0+ -f- N -f- 105 kJ has been found to proceed with an activation energy of 20 kJ [160]. 

At room temperature exothermic ion-molecule reactions sometimes exhibit a rate coefficient order of magnitude smaiier than the capture prediction. For these reactions a rapid increase of the reactivity with decreasing temperature is sometimes noticed. Low temperature measurements for such reactions are thus absoiuteiy required for interstellar chemistry modeiing. Moreover their kinetic behavior can reveal interesting details about the reaction dynamic since the whole potential energy surface of the reactant is clearly involved in this case. 

The existence of a transient reaction complex Is often invoked to explain the increase of readivity with decreasing temperature, which is found in numerous siow exothermic ion-molecule reactions (Ferguson, 1972). With the CRESU experiment such behavior has been studied for the reactions 02 +CH4 - CH302 +H and N2 +02 — 02 +N2 down to respectively 20 and 8K (Rowe et al, 1984a Barlow et al, 1986 ... 

It is still generally assumed that the rate coefficients for exothermic ion-molecule reactions are almost independent of temperature however, experimental studies at low temperatures often reveal more and more exceptions. One typical example is the reaction... 

There are a number of theoretical prescriptions for determining rate coefficients of exothermic ion-molecule reactions. The simplest originates from concepts developed more than a century ago by Langevin and applies to a reaction between a point ion and a spherical (non-polar) molecule [20]. The dominant long-range attractive force in this case is the ion-induced dipole interaction, which takes the form... 

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