Ion-molecule collision complexes

These results, obtained on a FT-ICR mass spectrometer, led to the proposal that these reactions proceed through long-lived ion-molecule collision complexes which can undergo secondary reactions within the complex. The mechanism, sketched in Scheme 41, predicts the formation of products originating from attack of F on the neutral by proton transfer, SN2 or elimination reactions. 

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. 

J. V. Dugan, J. H. Rice, and J. L. Magee, Evidence for long-lived ion-molecule collision complexes from numerical studies, Chem. Phys. Letters 3, 323-326 (1969). 

For ion-molecule collision complexes this has been experimentally implemented by cooling the complex through a supersonic expansion (Levy, 1981, 1984). Even further, one can reactivate these stable complexes either by collisions or by IR multiphoton absorption, as discussed in Chapter 7. The non-covalent bonding in the complex is shown for example by the inequivalence of the two Cl atoms in a CICH3 Cl complex, that dissociates very preferentially to the attacking Cl isotopomer. 

All mass spectrometers must function under high vacuum (low pressure). This is necessary to allow ions to reach the detector without undergoing collisions with other gaseous molecules. Indeed, collisions would produce a deviation of the trajectory and the ion would lose its charge against the walls of the instrument. On the other hand, ion-molecule collisions could produce unwanted reactions and hence increase the complexity of the spectrum. Nevertheless, we will see later that useful techniques use controlled collisions in specific regions of a spectrometer. 

Wherever an ion approaches a neutral (molecule or atom), which does not have a permanent dipole, its Coulomb field induces a dipole within this neutral which results in an attractive force. This leads to the formation of an ion-neutral collision complex when the impact parameter is below a critical value, and, as has been shown by Gioumousis and Stevenson, the rate constant for formation of such complexes is independent of temperature, and has the value... 

Vibrational degrees of freedom have been entirely neglected in our studies, and the increased lifetime of the collisions arises from internal rotation of the dipole moment. As we have noted above, vibrations should be easily excited in ion-molecule collisions that involve multiple reflections. Inclusion of vibrations and additional internal rotational degrees of freedom (for complexes such as CH3CN+ -f CHgCN) increases the parameter and the lifetime. ... 

While isolated laboratory experiments involving reactions which may be important in flames provide information under controlled conditions, these conditions may be somewhat removed from those which are found in flames. Clearly our reaction rate determinations should be extended to higher temperatures, which may cause the association reactions seen to have lower rates, due to dissociation of the ion/molecule reaction complexes formed. Also, studies at higher pressures should be performed, where there is increased opportunity for collisional stabilization of the collision complexes. As data from work in our laboratories and those of others accumulate, they can be used to refine computational models such as those already reported (10), in order to more fully test the proposed ionic soot formation mechanism. 

The efficiency with which the translational (kinetic) energy of the ion/molecule collision is transformed into internal energy in the collisional activation step is a highly complex problem but it is possible to understand some practically important aspects in a relatively simple fashion. The objective is to increase Ej to a value sufficiently greater than Ej , that fragmentation proceeds at a rate sufficient to provide a useful )deld of the product... 

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). 

It has been suggested (41) that the reaction efficiency of an Ion-molecule collision will Increase with an increase in the lifetime of the Ion-molecule complex there Is ev dence j that the ordering of efficiencies of reactions of CF2H, CF., and CF2CI is to a large degree, determined by the relative lifetimes or the collision complexes. The selectivity of reaction site in deuterium labelled n-butane and propane of these three ions, as shown in Table V throws some light on this matter. 

A significant recent experimental advance is the introduction of tandem mass spectrometers for studying ion-molecule reactions. Examining various isotope effects as a function of translational energy can provide detailed information about reaction mechanisms. Tandem experiments can also observe many of the possible reaction channels for a given collision complex. Such information provides valuable clues to the chemical and physical nature of the intermediates in ion-neutral interactions. 

Phenomenological evidence for the participation of ionic precursors in radiolytic product formation and the applicability of mass spectral information on fragmentation patterns and ion-molecule reactions to radiolysis conditions are reviewed. Specific application of the methods in the ethylene system indicates the formation of the primary ions, C2H4+, C2i/3+, and C2H2+, with yields of ca. 1.5, 1.0, and 0.8 ions/100 e.v., respectively. The primary ions form intermediate collision complexes with ethylene. Intermediates [C4iZ8 + ] and [CJH7 + ] are stable (<dissociation rate constants <107 sec.-1) and form C6 intermediates which dissociate rate constants <109 sec. l). The transmission coefficient for the third-order ion-molecule reactions appears to be less than 0.02, and such inefficient steps are held responsible for the absence of ionic polymerization. 

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