Transfer of Translational Energy in Charge Exchange

Table III. Transfer of Translational Energy in Charge-Exchange Processes"...

If a charge exchange process, A + + B- A -f- B +, occurs when the distance between the two particles is large, we expect that no transfer of translational energy takes place in the reaction and that the same selection rules govern the ionization as in spectroscopic transitions. This means that if the molecule B is in a singlet state before the ionization, the ion B + will be formed in a doublet state after ionization of one electron without rearrangements of any other electrons, at least for small molecules. 

It is therefore important to avoid transfer of translational energy during the charge-exchange process. This can be done in a tandem mass spectrometer of perpendicular type, and therefore this type will be the only one that will be considered in detail in this chapter. 

The selection rules obviously break down if the charge exchange takes place at a very small distance between the colliding particles. In this case, however, transfer of translational energy will also probably take place. As the study of charge-exchange mass spectra in a tandem mass spectrometer of perpendicular type discriminates against such processes, the breakdown of the selection rules is probably without importance for our purposes. 

The question to what extent this goal can be attained has been discussed repeatedly. " Measurements performed in tandem machines with smaller discrimination or in tandem machines of longitudinal type gave results that indicated a considerable transfer of translational energy during the charge-exchange process. The problem was reviewed earlier and therefore only one example will be presented here which illustrates the difference between different machines. 

Typical ion/molecule reactions between anions and neutral molecules [7, 9,152-155] can be classified as displacement (Scheme 2.1, Eq. (2.5)), proton transfer (Scheme 2.1, Eq. (2.6)), charge exchange (Scheme 2.1, Eq. (2.7)) and association (Scheme 2.1, Eq. (2.8)). Among these, the displacement reaction has been studied extensively in the gas phase [156,157], and the prototypical example is an anionic Sn2 reaction studied by Brauman [156]. In addition, interactions between a neutral molecule and an electron involving electron capture [158] and dissociative electron capture [159], are also important types of ion/molecule reactions in the gas phase. A molecule M vhth a positive electron affinity can form a stable anion M by capturing a thermal electron. In the case of dissociative electron capture, capture of an electron by a compound MX leads to a repulsive state of MX, which dissociates to form M and X vhth excess internal and/or translational energy. 

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