Reactions with Gaseous Ions

The reactions of PH3 with ions in the gas phase generally were investigated by mass spec-trometric methods. The experimental rate constants were occasionally compared to maximum rate constants which were calculated by using the average dipole orientation (ADO) theory [1 to 3]. 

---

The relatively long timescales of the ionization, isolation, thermalization, reaction, and detection sequences associated with low-pressure FTICR experiments are generally thought to preclude the use of this technique as a means of examining the unimolecular dissociation of conventional metastable ions occurring on the microsecond to millisecond timescale. Nonetheless, as just demonstrated (Section IIIC), intermediates with this order of magnitude of lifetime are routinely formed in the bimolecular reactions of gaseous ions with neutral molecules at low pressures in the FTICR cell, as in Equation (13). 

Although a quantitative model for the gas-phase interactions of charged species with neutral molecules was derived more than 60 years ago (Langevin, 1905) it was not until 1952 that Tal roze reported the first experimental study on the reactions of gaseous ions occurring in the high pressure ion source of a mass spectrometer. 

After much testing, chemists developed a compound that stabilizes paper diethyl-zinc [Zn(C2H5)2]. Diethylzinc is volatile so it can be sprayed onto books. It reacts with water to form zinc oxide (ZnO) and gaseous ethane (C2Hg). (a) Write an equation for this reaction, (b) ZnO is an amphoteric oxide. What is its reaction with H+ ions ... 

In chemical ionization, gaseous analyte molecules are ionized by ion-molecule reactions with reagent ions, formed by electron ionization from the appropriate reagent gas. The Cl ion source is similar to the El source, but is operated at a higher pressure (0.1-2 Torr). The chemical ionization process is illustrated for a proton transfer reaction, which is the most common... 

Many compounds, including acids, can act as gas phase bases in their reactions with gaseous H30 (H20) ion clusters. However, the method is particularly suitable for the detection of compounds with high gas phase basicities. Nitrogen-containing bases tend to be particularly reactive in the APCI ion source. As a result, APCI provides an excellent method for detecting and determining concentrations of gas phase alkaloids. It should be noted, however, that this method is not suitable for the detection of any alkaloids which may be associated with the particulate phase of ETS. 

Bimolecular reactions noay take place wheai gaseous ions collide with neutrals at thermal energy rather than at multi- or eveai kiloelectronvolt energy. Proton transfer is one of the prominent representatives of an ion-molecule reaction in the gas phase. It is enployed for the determination of GBs and PAs as described below. More sophisticated studies are enabled by reactions of gaseous ions with more complex reagents in the gas phase and the immediate mass spectral analysis of the reaction products [3]. 

The methods for the determination of GBs and PAs (Chap. 2.12) make use of their relation to (Eq. 9.23) and the shift of upon change of [AH] or B, respectively [187,188]. Basically, the value of GB or PA is bracketed by measuring eq with a series of several reference bases ranging from lower to higher GB than the unknown. There are two methods we should address in brief, a detailed treatment of the topic being beyond the scope of the present book, however. The kinetic method makes use of the dissociation of proton-bound heterodimers, and the ther-mokinetic method determines the equilibrium constant of the acid-base reaction of gaseous ions. In general, proton transfer plays a crucial role in the formation of protonated molecules, e.g., in positive-ion chemical ionization mass spectromehy (Chap. 7). 

Gioumousis G and Stevenson D P 1958 Reactions of gaseous moiecuie ions with gaseous moiecuies. V. Theory J. Chem. Phys. 29 294-9... 

The rates of these reactions bodr in the gas phase and on the condensed phase are usually increased as the temperature of die process is increased, but a substantially greater effect on the rate cati often be achieved when the reactants are adsorbed on die surface of a solid, or if intense beams of radiation of suitable wavelength and particles, such as electrons and gaseous ions with sufficient kinetic energies, can be used to bring about molecular decomposition. It follows drat the development of lasers and plasmas has considerably increased die scope and utility of drese thermochemical processes. These topics will be considered in the later chapters. 

When bubbler systems are used for collection, the gaseous species generally undergoes hydration or reaction with water to form anions or cations. For example, when SOj and NH3 are absorbed in bubblers they form HSO3 and NH4, and the analytical techniques for measurement actually detect these ions. Table 13-1 gives examples of gases which may be sampled with bubbler systems. 

The process is represented as a series of steps consisting of the sublimation of the metal, dissociation of the halogen, removal of the electron from the metal and placing it on the halogen, then combining the gaseous ions to form a crystal lattice. These steps lead from reactants to product, and we know the energies associated with them, but the reaction very likely does not literally follow these steps. Reaction schemes in which metal complexes function as catalysts are formulated in terms of known types of reactions, and in some cases the intermediates have been studied independently of the catalytic process. Also, the solvent may play a role in the structure and reactions of intermediates. In this chapter we will describe some of the most important catalytic processes in which coordination chemistry plays such a vital role. 

Figure 3. Positive ion spectrum showing the reaction of V+ with gaseous Sg. Spectrum (a) shows the reaction products when the V+ ions are collisionally cooled before reaction with Sg. Spectra (b) and (c) show the reaction products when the isolated V+ ions are increasingly accelerated by a rf pulse.

Figure 6. The reaction of bare Eu+ ions with gaseous Ph3P.

The reaction of bare metal ions with sulfur has received some attention. The reaction of metal ions with elemental sulfur (S8) was first studied by Freiser and his co-worker (110) in the study of Fe+ with S8. This reaction produced the ions [FeSJ+, where x = 1-10, which under collision-induced dissociation experiments were observed to undergo sequential loss of S2 units. It has been suggested that Fe(S2)5 + would result in the unreasonable oxidation state of Fe(XI) (9). The S2 unit is similar to the 02 molecule with two unpaired electrons and could act as a neutral ligand (not an anion S2 or dianion) and the oxidation state of Fe in the ion Fe(S2)5 + would be (I). Some similar ions have also been produced by the reaction of Fe+ with ethene sulfide, QH4S. The results for Fe+ have now been confirmed in a study of all the transition metals cations (except Tc+) reacting with gaseous sulfur (50, 150). The study shows that [MS4]+ was a common first major ion (some reac-... 

---