Negative ion-neutral reactions

This chapter is concerned principally with two-body reactions between negative ions and neutral molecules at low energies in the gas phase. Introductory material on the formation and disappearance of negative ions is also included. Emphasis is placed on results arising from the use of mass spectrometer ion sources or double mass spectrometers as the experimental technique. Additional information on negative-ion-neutral reactions may be found in Chapters 7, 8, 11, 15, and 16 of this book. 

This chapter is largely restricted to the study of negative-ion-neutral reactions using the mass spectrometer ion source and double mass spectrometer techniques, and only these methods are discussed in detail in this section. 

The remainder of this chapter is devoted to a discussion of some of the results obtained from studies of negative-ion-neutral reactions. No attempt is made to include extensive discussion either of associative detachment processes (see Chapter 8) or of those many negative-ion reactions which have been studied only at relatively high energies, i.e., above a few hundred eV. Two-body rate coefficients here are invariably given in units of cm sec and cross sections in units of cm. Energies refer to kinetic energies of the reactant ion in the laboratory system, unless otherwise stated. 

Several negative-ion-neutral reactions have been studied, principally in an attempt to determine limits for the electron affinities of important diatomic and triatomic molecules. In the more widely used method, one looks for charge transfer between a low-energy (preferably thermal), negative ion X and the neutral molecule Y. Alternatively, the kinetic energy threshold for an endothermic charge transfer reaction between X and Y is measured. 

Several reports of negative-ion-neutral reactions in sulfur-containing compounds have recently appeared. The reaction... 

Several reported negative-ion-neutral reactions have not been discussed in this chapter. Included in this group are many associative detachment, collisional detachment, and collisional dissociation reactions. 

Using SIFT apparatuses, the rate coefficients and products ions for a large number of positive ion-neutral and negative ion-neutral reactions have been studied in several laboratories around the world. They include the bimolecular reactions of doubly... 

In the subsequent discussion we assume that (kN + kR) and (kNi + kR1) are temperature independent. Certainly the highly exothermic neutralization reactions are essentially temperature independent. They also probably predominate in the sum, and there is some experimental evidence to support this (23). If the electron or negative ion-radical reactions are temperature dependent, they are probably too small to contribute to the sum. Only if they were temperature independent would they contribute significantly, and the sum would then be temperature independent. Since these sums are assumed to be temperature independent, we will designate them by fcD = ( n + r) and fcL = (kNi + km)-... 

Apart from the proton transfer reactions discussed in Section II, phosphorus species undergo a range of other ion-molecule reactions in the gas phase. The types of instruments which have been used to study ion-molecule reactions of phosphorus species include ion cyclotron resonance (ICR) mass spectrometers and the related FT-ICR instruments, flowing afterglow (FA) instruments and their related selected-ion flow tubes (SIFT) and also more conventional instruments This section is divided into four topics (A) positive ion-molecule reactions (B) negative ion-molecule reactions (C) neutralization-reionization reactions and (D) phosphorus-carbon bond formation reactions. 

Negative ions for use as reactants in ion-neutral reactions are usually prepared by dissociative electron attachment, three-body attachment, or ion-pair production. Other techniques available include radiative attachment and surface reactions. Finally, some negative ions can be formed most efficiently by ion-neutral reactions themselves. The following discussion of these processes is intended only to be illustrative, and references are made principally to review articles. 

As for CIDNP, the polarization pattern is multiplet (E/A or A/E) for each radical if Ag is smaller than the hyperfme coupling constants. In the case where Ag is large compared with the hyperfmes, net polarization (one radical A and the other E or vice versa) is observed. A set of mles similar to those for CIDNP have been developed for both multiplet and net RPM in CIDEP (equation (B1.16.8) and equation (B1.16.9)) [36]. In both expressions, p is postitive for triplet precursors and negative for singlet precursors. J is always negative for neutral RPs, but there is evidence for positive J values in radical ion reactions [37]. In equation (B 1.16.8),... 

Hereby the initial positive ion becomes a neutral particle and the initial neutral particle becomes a new species of positive ion. The initial dilute solution must have contained sufficient negative ions to neutralize the positive charges these negative ions take no part in the reaction. As far as the ionic fields are concerned, the field of one species of positive molecular ion is replaced by that of another. 

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