Reactant ions

Unstable species such as O, FI and N atoms, molecular radicals and vibrationally excited diatomics can be injected by passmg the appropriate gas tluough a microwave discharge. In a SIFT, the chemistry is usually straightforward since there is only one reactant ion and one neutral present in the flow tube. 

Chaise-inversion reaction. An ion/neutral reaction wherein the charge on the reactant ion is reversed in sign. 

CoIIisional excitation. An ion/neutral process wherein the (slow) reactant ion s internal energy increases at the expense of the translational energy of either (or both) of the reacting species. The scattering angle can be large. 

Rates and equilibria within these cation-anion recombination reactions are not correlated. Ritchie considers that extensive desolvation of the reactant ions... 

Hisatsune and co-workers [290—299] have made extensive kinetic studies of the decomposition of various ions in alkali halide discs. Widths and frequencies of IR absorption bands are an indication of the extent to which a reactant ion forms a solid solution with the matrix halide. Sodium acetate was much less soluble in KBr than in KI but the activation energy for acetate breakdown in the latter matrix was the larger [297]. Shifts in frequency, indicating changes in symmetry, have been reported for oxalate [294] and formate [300] ions dispersed in KBr. 

Some of the problems encountered in the mass spectrometric study of ion-molecule reactions are illustrated in a review of the H2-He system (25). If the spectrometer ion source is used as a reaction chamber, a mixture of H2 and He are subjected to electron impact ionization, and both H2+ and He+ are potential reactant ions. The initial problem is iden-... 

After identifying the reactant ion, reaction cross-sections were measured as a function of average reactant ion kinetic energy. Q experimental is measured for given values of (eEl) 1/2 in the spectrometer, and experimental values of k... 

Intramolecular Isotope Effects. The data in Figure 2 clearly illustrate the failure of the experimental results in following the predicted velocity dependence of the Langevin cross-section. The remark has been frequently made that in the reactions of complex ions with molecules, hydrocarbon systems etc., experimental cross-sections correlate better with an E l than E 112 dependence on reactant ion kinetic energy (14, 24). This energy dependence of reaction presents a fundamental problem with respect to the nature of the ion-molecule interaction potential. So far no theory has been proposed which quantitatively predicts the E l dependence, and under these circumstances interpreting the experiment in these terms is questionable. 

Intramolecular isotope effect studies on the systems HD+ + He, HD+ + Ne, Ar+ + HD, and Kr + + HD (12) suggest that the E l dependence of reaction cross-section at higher reactant ion kinetic energy may be fortuitous. In these experiments the velocity dependence of the ratio of XH f /XD + cross-sections was determined. The experimental results are presented in summary in Figures 5 and 6. The G-S model makes no predictions concerning these competitive processes. The masses of the respective ions and reduced masses of the respective complex reacting systems are identical for both H and D product ions. Consequently, the intramolecular isotope effect study illuminates those... 

Figure 7. COH+/COD+ ratio as a function of reactant ion average translational energy in a CO-HD mixture...

This reaction was considered the only reaction channel because it is the only known channel which is exothermic with ground state CH4+ ions. Reactions yielding C2H5+ and C2H4 + have been observed and are the least endothermic of the possible reactions of CH4+ with CH4. However, ionization efficiency curves establish CH3 + rather than CH4 + as the reactant ion. Reaction 14 ... 

More direct observations of the kinetic energy dependence of cross-sections should be possible using external ionization techniques where the reactant ion can be chosen by initial mass analysis and, in principle, its energy more readily controlled. Several studies using external ionization techniques, both with (2, 10, 45) and without (20, 21, 27, 41) preliminary mass selection of the reactant ion, have been reported. However, apparently with these techniques it is not possible to obtain well-defined primary ion beams at energies below 0.5-1 e.v. a region of critical importance both experimentally and theoretically. 

In 1960 Tal roze and Frankevich (39) first described a pulsed mode of operation of an internal ionization source which permits the study of ion-molecule reactions at energies approaching thermal energies. In this technique a short pulse of electrons is admitted to a field-free ion source to produce the reactant ions by electron impact. A known and variable time later, a second voltage pulse is applied to withdraw the ions from the ion source for mass analysis. In the interval between the two pulses the ions react under essentially thermal conditions, and from variation of the relevant ion currents with the reaction time the thermal rate constants can be estimated. 

Reactions involving a transfer of a proton or a hydrogen atom are an extremely common type of ion-molecule reaction and are particularly suited for study by the pulsed source technique. The secondary ion will usually occur at an m/e ratio where it is not obscured by abundant primary ions, and the product and reactant ions frequently will differ only slightly in mass, thus minimizing discrimination effects. 

In brief, the method consists of introducing small amounts (partial pressures of 10 3-10 4 torr) of the substance to be investigated into the ionization chamber of a mass spectrometer which contains a high pressure (1 torr) of methane, the reactant gas. Ionization is effected by electron impact, and because the methane is present in such an overwhelming preponderance, all but a negligibly small amount of the initial ionization occurs in the methane. The methane ions then undergo ion-molecule reactions to produce a set of ions which serve as reactant ions in the chemical ionization process. The important reactant ions formed from... 

Thus the reactant ions for chemical ionization formed in the methane plasma consists of approximately equal amounts of a strong gaseous Bronsted acid (CH5+) and ions which can act either as Lewis acids or Bronsted acids (C2H5+ + C3H5+). These reactant ions will effect the chemical ionization with an added substance by proton transfer or hydride ion transfer, both of which may be accompanied by fragmentation of the ion initially formed. 

Thus we think of the chemical ionization of paraffins as involving a randomly located electrophilic attack of the reactant ion on the paraffin molecule, which is then followed by an essentially localized reaction. The reactions can involve either the C-H electrons or the C-C electrons. In the former case an H- ion is abstracted (Reactions 6 and 7, for example), and in the latter a kind of alkyl ion displacement (Reactions 8 and 9) occurs. However, the H abstraction reaction produces an ion oi m/e = MW — 1 regardless of the carbon atom from which the abstraction occurs, but the alkyl ion displacement reaction will give fragment alkyl ions of different m /e values. Thus the much larger intensity of the MW — 1 alkyl ion is explained. From the relative intensities of the MW — 1 ion (about 32%) and the sum of the intensities of the smaller fragment ions (about 68%), we must conclude that the attacking ion effects C-C bond fission about twice as often as C-H fission. 

Note that in contrast with normal paraffins (Equation 10), beta fission at the branch point is exothermic, and the reaction is energetically allowed. A similar exothermic reaction can be written using C2H5 + as the reactant ion, and two other reactions equivalent to Reaction 12 can be written—namely, those with the charge in the initially formed C2i+ ion on the other two branches of the molecule. One of these other reactions will also produce a Ci4 + ion, but the other will produce a Ci5 + ion. 

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