Ionization true associative

IV. OTHER SPONTANEOUS IONIZATION MECHANISMS A. True Associative Ionization... 

In contrast to the associative process arising in Pgl systems when the ejected electron carries away so much energy that the collision partners cannot separate, we deal in this section with ionization processes that can only occur because energy is gained by formation of a bond between the collision partners after ionization. Such true associative ionization (AI) systems are characterized by the energy conditions... 

Figure 34. Two possible mechanisms for true associative ionization (a) adiabatic path and ionization by dynamic coupling (b) diabatic path and ionization by electronic coupling.

It can be seen that the added complexity of ion association is likely to make any simple model of ion-ion interactions very difficult to apply without a number of ad hoc assumptions concerning ionic radii. This is particularly true for ionic strengths in excess of 0.01 M or for low-dielectric-constant media. However, a further difficulty is raised by the problem of the nature of an ion pair. If we consider the simple case of univalent ions A+ B forming an ion pair, it is possible to picture the pair as varying in character from one in which the charges remain separated by the sum of the ionic radii of A+ + B to a molecule in which A and B form a covalent bond, not necessarily even polar in character. Nor is it necessarily true that a given species will behave the same in different solvents. If there is a tendency to covalent bond formation, then it is quite possible that the polarity of the A—B bond will depend on the dielectric constant of the solvent. Covalently bound molecules which ionize are considered as weak electrolytes, and they are not treated by the methods of Bjerrum, which are meant for strong electrolytes. The differences may not always be clear, but the important interactions for the weak electrolyte are with the solvent, and these we shall consider next. 

It was mentioned earlier in this chapter that acid amides and nitrocompounds form conducting solutions in liquid ammonia and hydrazine the ionization in these cases is undoubtedly accompanied by, and is associated with, compound formation between solute and solvent. The same is true of triphenylmethyl chloride which is a fair electrolytic conductor when dissolved in liquid sulfur dioxide it also conducts to some extent in nitromethane, nitrobenzene and acetone solutions. In chloroform and benzene, however, there is no compound formation and no conductance. The electrolytic conduction of triphenylmethyl chloride in fused aluminum chloride, which is itself a poor conductor, appears to be due to the reaction... 

Desolvation. As discussed in Section 3.2, solute molecules or ions can be solvated, which means that one or more solvent molecules are associated with the solute. This is especially true for ions or ionized groups in aqueous solutions the solvation (in this case hydration) then is due to ion-dipole interactions, which are fairly strong, particularly for positive groups (cations). Before an ion can be incorporated into a crystal lattice, desolvation has to occur. This implies a temporary increase in free energy, causing an activation barrier for crystallization. This will slow down the crystallization rate. 

Nucleophilicity and Ionization Potentials. Previously, the suggestion that the differences in the nucleophilic ranking between the n and the N+ scale may stem from a relatively large amount of electron transfer present in the transition state described by the N+ scale was made. If this statement is true, then some correlation between the N+ scale and the solution ionization potentials of the nucleophiles should exist. Because these data are not available, we have calculated (5) the energy associated with K3 using the thermodynamic cycle shown in Scheme I. 

The value is actually a pH at which the concentrations of ionized and unionized forms of the analyte are equal. The actual pH of the mixed eluent containing organic is higher than that of pure buffer. Thus, the analyte retention will be dependent on these higher pH values. However, the retention factors are associated with the pH of the buffer before addition of organic. This leads to a shift of pH dependence of the analyte retention to lower values. Therefore, the pK value calculated from this dependence will be lower than the true value. 

The continuous parts of the spectra ("continua") of atomic and molecular systems were traditionally thought of as incoherent sinks that result in "rate-like" processes and irreversible decay. While this view may sometimes be true for confinua whose coupling matrix element varies relatively slowly with energy ("flat" continua), experiments of fhe last two decades have demonstrated coherent behavior in many laser-mediated processes associated with continua. The "dressing" of confinua by light was shown to cause coherent phenomena, such as induced transparency, nonexponential decay and recurrences, and "above-threshold" ionization and dissociation processes, involving optical transitions within continua. 

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