Gas-phase ionic species

The first step is ionization that converts analyte molecules or atoms into gas-phase ionic species. This step requires the removal or addition of an... 

Ionization of the analyte is the first crucial and challenging step in the analysis of any class of compounds by mass spectrometry. The key to a successful mass spectrometric experiment lies to a large extent in the approach to converting a neutral compound to a gas-phase ionic species. A wide variety of ionization techniques have become available over the years, but none has universal appeal. In some techniques, ionization is performed by ejection or capture of an electron by an analyte to produce a radical cation [M+ ] or anion [M ], respectively. In others, a proton is added or subtracted to yield [M - - H]+ or [M — H] ions, respectively. The adduction with alkali metal cations (e.g., Na+ and K+) and anions (e.g., Cl ) is also observed in some methods. The choice of a particular method is dictated largely by the nature of the sample under investigation and the type of information desired. Table 2.1 lists some of the methods currently in vogue. Some methods are applicable to the atomic species, whereas others are suitable for molecular species. Also, some methods require sample molecules to be present in the ion source as gas-phase species, whereas others can accommodate condensed-phase samples. The methods that are applicable to molecular species are the subject of the present chapter those applicable to atomic species are described in Chapter 7. 

These rules contain several key concepts which are readily transferable to other charged systems such as protonated species (i.e., [M + H]+ ions). Before discussing these and other key concepts, several important tools used to help unravel gas-phase ionic fragmentation mechanisms are described. 

Fundamental studies of gas-phase ionic processes are also of interest in other areas, including combustion, the chemistry of the ionosphere, interstellar chemistry and chemical vapour deposition. Another important aspect of gas-phase studies is that they probe the intrinsic reactivity of ionic species in the absence of counter ions and solvent. Indeed, in cases where sufficient data are available, comparisons between solution- and gas-phase studies provide insights into solvent and counter ion effects. ... 

In a similar manner, on-line monitoring of Baylis-Hillman reactions co-catalyzed by ionic liquids was applied to gently fish from solution to the gas phase supramolecular species responsible for the co-catalytic role of ionic liquids in the reaction (Scheme 5.23) [83]. Several supramolecular species formed by coordination of reagents and products were trapped, identified, and characterized via MS analysis and MS/MS dissociations. Via competitive experiments, it was also reported that the efficiency order of different co-catalysts was BMI.CF3C02>BMI.BF4>BMI.PF6, which was the opposite to that observed by Afonso [84] in the liquid phase. Based on the interception of these unprecedented supramolecular species, it was proposed... 

Fuel cell electrodes are porous gas diffusion electrodes, which are usually described in modeling using homogenization [144]. This means that the pore electrolyte and the electrode material share the same geometrical domain and the electric potential in the electronic and ionic conductors are present in the same geometrical domain. Also the concentration variables for the species in the gas phase, the species dissolved in the electrolyte, and the constituents of the electrolyte may be present in the same geometrical domain defined by the gas diffusion electrodes. The electrochemical reactions that occur at the interface between the pore electrolyte and the electrode are introduced as sources or sinks in the material and current balances. To calculate the chemical composition in the electrolyte and in the gas phase in every point in space in a geometrical domain, material balances for each of the species in the solution as well as a conservation of mass for the whole have to be defined. The conservation of mass for the whole solution may eliminate one of the species material balances, which for a dilute solution usually is the solvent s material balance. The constitutive relations in the electrolyte may be the... 

Gas phase ionic interaction energies are large, typically on the order of 50-200 kcal moF. The interaction of water with charged species is nearly as large as a result solution-phase ionic interactions are weak, seldom more than five kcal mol ... 

Electronic structure methods are aimed at solving the Schrodinger equation for a single or a few molecules, infinitely removed from all other molecules. Physically this corresponds to the situation occurring in the gas phase under low pressure (vacuum). Experimentally, however, the majority of chemical reactions are carried out in solution. Biologically relevant processes also occur in solution, aqueous systems with rather specific pH and ionic conditions. Most reactions are both qualitatively and quantitatively different under gas and solution phase conditions, especially those involving ions or polar species. Molecular properties are also sensitive to the environment. 

Section 3 deals with reactions in which at least one of the reactants is an inorganic compound. Many of the processes considered also involve organic compounds, but autocatalytic oxidations and flames, polymerisation and reactions of metals themselves and of certain unstable ionic species, e.g. the solvated electron, are discussed in later sections. Where appropriate, the effects of low and high energy radiation are considered, as are gas and condensed phase systems but not fully heterogeneous processes or solid reactions. Rate parameters of individual elementary steps, as well as of overall reactions, are given if available. 

Electrospray has been successful for numerous azo dyes that are not ionic salts. Several anthraquinone dyes have been analysed by LC-ESI-MS [552]. Electrospray achieves the best sensitivity for compounds that are precharged in solution (e.g. ionic species or compounds that can be (de)protonated by pH adjustment). Consequently, LC-ESI-MS has focused on ionic dyes such as sulfonated azo dyes which have eluded analysis by particle-beam or thermospray LC-MS [594,617,618]. Techniques like LC-PB-MS and GC-MS, based on gas-phase ionisation, are not suitable for nonvolatile components such as sulfonated azo dyes. LC-TSP-MS on... 

Remarkably high reactivity of cationic alkyl complexes of Group 4 metals with 1-alkenes has been observed in gas-phase reactions [129]. Typical ionic species such as TiCl2Me+ react with ethylene, and the insertion followed by H2 elimination gives rise to a cationic allyl complex TiCl2C3H5, which does not react further with ethylene. 

Reactions in solution proceed in a similar manner, by elementary steps, to those in the gas phase. Many of the concepts, such as reaction coordinates and energy barriers, are the same. The two theories for elementary reactions have also been extended to liquid-phase reactions. The TST naturally extends to the liquid phase, since the transition state is treated as a thermodynamic entity. Features not present in gas-phase reactions, such as solvent effects and activity coefficients of ionic species in polar media, are treated as for stable species. Molecules in a liquid are in an almost constant state of collision so that the collision-based rate theories require modification to be used quantitatively. The energy distributions in the jostling motion in a liquid are similar to those in gas-phase collisions, but any reaction trajectory is modified by interaction with neighboring molecules. Furthermore, the frequency with which reaction partners approach each other is governed by diffusion rather than by random collisions, and, once together, multiple encounters between a reactant pair occur in this molecular traffic jam. This can modify the rate constants for individual reaction steps significantly. Thus, several aspects of reaction in a condensed phase differ from those in the gas phase ... 

The experimental approach discussed in this article is, in contrast, particularly amenable to investigating solvent contributions to the interfacial properties 131. Species, which electrolyte solutions are composed of, are dosed in controlled amounts from the gas phase, in ultrahigh vacuum, onto clean metal substrates. Sticking is ensured, where necessary, by cooling the sample to sufficiently low temperature. Again surface-sensitive techniques can be used, to characterize microscopically the interaction of solvent molecules and ionic species with the solid surface. Even without further consideration such information is certainly most valuable. The ultimate goal in these studies, however, is to actually mimic structural elements of the interfacial region and to be able to assess the extent to which this may be achieved. 

Despite these arguments and the conceptual attractiveness of the procedure which is sketched in Fig. 1 convincing evidence for the relevance of a particular gas phase adsorption experiment can only be obtained by direct comparison to electrochemical data The electrode potential and the work function change are two measurable quantities which are particularly useful for such a comparison. In both measurements the variation of the electrostatic potential across the interface can be obtained and compared by properly referencing these two values 171. Together with the ionic excess charge in the double layer, which in the UHV experiment would be expressed in terms of coverage of the ionic species, the macroscopic electrical properties of the interracial capacitor can thus be characterized in both environments. 

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