Stability of Ions

FIGURE 2.13 Mass spectra of (a) 2-bromobutane and (b) terf-butyl bromide. 

It should be noted that the high-energy M-Br ions formed in the mass spectrometer no doubt rearrange to some common ion before leaving the source. Nonetheless, the activation energy for the fragmentation, which determines the rate of the reaction and thus the intensities of the peaks, is dependent on the structure of the initially formed ion, no matter how fleeting its existence. 

It is significant that this series of compounds behaves in an analogous manner under solvolytic conditions. That is, tert-butyl bromide reacts rapidly under S l conditions first to form the tri-methyl carbonium ion, then the product fcrf-butanol (Equation 2.22), while 1-bromobutane reacts slowly under these conditions, and the chief product is not the alcohol but a mixture of butenes (Equation 2.24). 

To carry this analogy a little further, allylic and benzyUc halides fragment faster than the saturated counterparts. The carbonium ions formed in both types of reaction are stabilized by resonance with neighboring jt-electron systems (Equation 2.25). Thus, the carbon-halogen bonds in the unsaturated compounds are more susceptible to both solvolytic and El-induced cleavage. 

The correlations presented here cannot of course be considered quantitative since the reaction processes are quite different. Reactions in solution depend on such factors as solvation and equilibria, whereas mass spectral reactions are strictly unimolecular decompositions of activated molecular ions. However, as a qualitative approach, these comparisons can be useful in the evaluation of spectra of related compounds. Differences in the intensities of various fragment peaks have been used to distinguish between numerous sets of isomeric compounds. 

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The design of the mass spectrometer may influence its use in a particular kind of measurement. The study of electronic state-specific ions and their reactions has mainly been carried out using the GIB method. Metastable ions (ions produced by the ionization process but decomposing on the way to detection) can be observed in many of Type (1) mass spectrometers and metastable ions aid our understanding of the ionization process and stability of ions. Sequential reactions and kinetic studies of ion-molecule reactions are difficult with the simpler mass spectrometers of Type 1 and so more complex hybrid mass spectrometers have to be used. The ions observed in micro- or milliseconds after the ionization process may or may not be the same as ion observed seconds after the ionization process, which is a limitation in the use of Type 1 mass spectrometers. 

Section 3.2 includes an extensive discussion on the formation of odd-electron bonds, ion pairing, and the distonic stabilization of ion-radicals at the expense of separation between their spins and charges. Section 3.3 deals with ion-radicals from the class of even spin-charge distribution. This class occnrred more frequently in scientific works of past decades. However, the reader will find newly developed manifestations of the principle of the released electron, concerning spread conjugation and the fates of ion-radical precursors with increased dimensionality. 

Spin-Charge Separation (Distonic Stabilization of Ion-Radicals)... 

It is important to differentiate between the effects of a nonnucleophilic salt such as Mg(C104)2 on one hand, and a weak nucleophilic salt such as Et4NOAc on the other. The effect of nonnucleophilic salts on photo-oxygenation via electron transfer can be understood as the stabilization of ion-radicals by coulombic interaction, resulting in the suppression of a back electron transfer between ion-radicals. The weak nucleophilic salts cause unusual effects. The addition of the anionic nucleophile to the cation-radical and complexation of the weak nucleophilic salt with the ion-radicals bring about these effects. 

The ion trap is a device that utilizes ion path stability of ions for separating them by their m/z [53]. The quadrupole ion trap and the related quadrupole mass filter tvere invented by Paul and Steinwedel [57]. A quadrupole ion trap (QITor 3D-IT) mass spectrometer operates with a three-dimensional quadrupole field. The QIT is formed by three electrodes a ring electrode with a donut shape placed symmetrically between two end cap electrodes (Fig. 1.20). 

Decreasing order of extent of H-bonding in water and order of stability of ions by solvation. 

This chapter is not concerned with the thermodynamic stability of ions with respect to their formation. Rather, it is concerned with whether or not a given ion is capable of existing in aqueous solution without reacting with the solvent. Hydrolysis reactions of ions are dealt with in Chapter 3. The only reactions discussed in this section are those in which either water is oxidized to dioxygen or reduced to dihydrogen. The Nernst equation is introduced and used to outline the criteria of ionic stability. The bases of construction and interpretation of Latimer and volt-equivalent (Frost) diagrams are described. 

Figure 5.1 A diagram showing the theoretical and practical limits of stability of ions in solutions of pH between 0 and 14...

The abundance of an ion depends on (i) its stability, (ii) the stability of neutral particles formed contemporaneously with the ion, and (iii) the energy of the bond(s) cleaved during formation of the ion. The stability of ions and radicals is determined by the usual structural features long known in organic chemistry for example, tertiary ions and radicals are thermodynamically favored over secondary and primary ones and conjugation with a double bond, an aromatic system, or a pair of --electrons of a hetero atom increases the stability of ions and radicals. 

The same physical interactions described above for micelle surfaces prove to be operative in vesicles, and depending on the arrangement of donors or acceptor among the four possible aggregates shown in Fig. 6, either accelerated electron exchange or stabilization of ion radical pairs can be observed (276). 

The relative involvement of the cyclic (thiiranium or phenylselenenium) and open species in the addition reactions has been investigated by NMR. These studies have revealed dramatic substituent effects on the relative equilibrium stabilities of ions 170-172... 

Ion radicals of conjugated acyclic or aromatic hydrocarbons (butadiene or naphthalene) are typical examples of the species with a released unpaired electron. They are named ir-elec-tron ion radicals and have a spin distribution along the whole molecular contour. An important feature of such species is that all the structural components are coplanar or almost coplanar. In this case, spin density appears to be uniformly or symmetrically distributed over the molecular framework. Spin-density distribution has a decisive effect on the thermodynamic stability of ion radicals. In general, the stability of ion radicals increases with an enhancement in delocalization and steric shielding of the reaction centers bearing the maximal spin density. 

Attractive interactions between ion radicals and their environment may lower the energy of the corresponding molecular orbital populated with an unpaired electron. This enhances the stability of ion radicals and favors the stepwise pathway of the reaction e.g.,... 

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