Pairing concept

An elaboration of the ion-pair concept includes an ion sandwich in which a preassociation occurs between a potential nucleophile and a reactant. Such an ion sandwich might be a kinetic intermediate which accelerates dissociation. Alternatively, if a caibocation were quite unstable, it might always return to reactant unless a nucleophile was properly positioned to capture the caibocation. 

The ion-pair concept thus predicts that SmI reactions can display either complete racemization or partial inversion. The fact that this behavior is generally found is evidence that ion pairs are involved in many SnI reactions. There is much other evidence for the intervention of ion pairs ... 

For over a decade, the topological analysis of the ELF has been extensively used for the analysis of chemical bonding and chemical reactivity. Indeed, the Lewis pair concept can be interpreted using the Pauli Exclusion Principle which introduces an effective repulsion between same spin electrons in the wavefunction. Consequently, bonds and lone pairs correspond to area of space where the electron density generated by valence electrons is associated to a weak Pauli repulsion. Such a property was noticed by Becke and Edgecombe [28] who proposed an expression of ELF based on the laplacian of conditional probability of finding one electron of spin a at t2, knowing that another reference same spin electron is present at ri. Such a function... 

Hyp. III.3 is in reality a mathematical formulation of the original electron-pair idea (3) without any explicit reference or connection to quantum chemistry. However, the electron-pair concept is built into both VB and MO theory through postulates of the types Hyps. III. 1, III.2, and... 

Given the power of the concept of covalency and the deeper electronic implications that were realized by G. N. Lewis s octet-rule and shared-electron-pair concepts, it is natural to wonder whether these advances are limited to s/p-block elements or apply to the entire periodic table. 

The inert pair concept has sometimes been advanced to account for the extra stability of atoms or ions which contain a lone pair of s-electrons (e.g.,... 

Quintanar-Guerrero, D., Allemann, E., Fessi, H. and Doelker, E. (1997). Applications of the ion pair concept to hydrophilic substances with special emphasis on peptides, Pharmaceut. Res., 14, 119-127. 

The ion-pair concept leads to two types of activity coefficients y t for free plus dissociated species and y. These y s are related by 1... 

Acidity and basicity are paired concepts that are very often invoked to explain the catalytic properties of divided metal oxides and zeolites. The concept of acids and bases has been important since ancient times. It has been used to correlate large amounts of data and to predict trends. During the early development of acid-base theory, experimental observations included the sour taste of acids and the bitter taste of bases, color changes in indicators caused by acids and bases, and the reaction of acids with bases to form salts. 

The preference for the +2 over +4 oxidation state increases down the group, the change being due to relativistic effects that make an important contribution to the inert pair. The inert pair concept holds only for the lead ion Pb2+(aq), which could have a 6s2 configuration. In more covalent Pb11 compounds and most Sn11 compounds there are stereochemically active lone pairs. In some MX2 (M = Ge or Sn) compounds Ge and Sn can act as donor ligands. 

The ion pair concept, introduced by Bjerrum [14], was critically reviewed by Szwarc [15] and definitions were given based on the mutual geometry of ions and solvent. The existence of loose and tight ion pairs was suggested by Winstein [16] and Sadek [17] and it is now common to speak about free ions (FI) as well as of solvent-separated ion pairs (SSIP) or contact ion pairs (CIP), having in mind the oversimplified picture ... 

The history of the ion-pair concept begins in the 1880s [50] with the theory of Arrhenius that described the electrolytic dissociation in solution as a function of electrolyte concentration and nature. This theory was completely eclipsed by the success of the D-H treatment of electrolyte activity in the 1920s [33] that also paved... 

A critical review of the history of the ion-pair concept indicates that considerations other than electrostatic were scarcely provided by model makers. Clearly, for chromatography, solvophobic interactions usually neglected by ion-pair model makers are aucial. Chromatographers involved in ion-pair strategies realized that the theoretical description of the ion-pair was critical at the time of the ion-pair chromatography introduction in the late 1970s and subsequently, the electrostatic model of ion association dominated scientific debate. 

Thomlinson [78] was the first chromatographer to point out that the classical electrostatic ion-pair concept did not hold for IPRs that were usually bulky hydrophobic ions he also emphasized that in the interfacial region between the mobile and the stationary phases, the dielectric constant of the medium is far lower than that of the aqueous phase. Chaotropes that break the water structure around them and lipophilic ions that produce cages around their alkyl chains, thereby disturbing the ordinary water structnre, are both amenable to hydrophobic ion-pairing since they are both scarcely hydrated. The practical proof of such ion-pairing mode can be found in References 80 and 81 many examples of such pairing modes are reported in the literature [79-86],... 

Conductometry paved the way for the development of the ion-pair concept [3]. The oldest experimental evidence of ion-pairing was obtained from colligative properties and electrical conductivity measurements. It is generally accepted that electroneutral ion-pairs do not contribute to solution conductivity. Conductometry is now a reliable and well established technique even in low millimolar concentration ranges, but the full description of conductance in the presence of ion-pairing is anon-trivial task. To date the most accepted equation was developed by Fuoss and Hsia [92] and expanded by Fernandez-Prini and Justice [93] ... 

Section 2.5.3 in Chapter 2 expounded upon the hydrophobic ion-pair concept The peculiarities of this association mode, not even likely in the Bjerrum s model, were elucidated. Electrostatic attraction is only part of the story and solvophobic interactions are crucial to rationalize experimental evidence that often runs counter to the pristine electrostatic description of the process. 

Interestingly, the estimates of pairing constants obtained by the fitting of retention equations to experimental data increase with increasing analyte chain length (lipophilicity) [60] thereby supporting the hydrophobic ion-pairing concept. 

Salting-in or salting-out of analytes as a consequence of the addition of the IM that leads to decreased or increased surface tension, respectively, of the eluent. IPC ions are usually salting-in agents because they lower the surface tension of the eluent [40] it was recently demonstrated that these salting effects may be revised based on the ion-pair concept. 

The enduring upgrade of column technology and instrumentation to improve data quality and to increase sample throughput via ultra-fast separations is also described. The relation of IPC to other instrumental methods of analysis, the translation of the ion-pair concept in different analytical techniques, and non-separative aims of IPC, that are topics for which there is a dearth in the scientific literature, are critically discussed. 

Various comparisons of experimental and predicted CIDNP spectra have been reported, and an example is given in Fig. 5. Similar agreement is obtained for any of the above-mentioned theories of Qoss-Kaptein-Oosterhoff > Adrian Kaptein and Fischer ). This may be taken as support for the basic radical pair concept which is inherent in all the formulations. 

For example, if the BN bond were to break during the dehydrohalogenation process amine bases could be chosen that selectively capture a proton while the halogenated boron species remains uncoordinated due to steric hindrance. This concept is related to the recently published frustrated Lewis pair concept used for hydrogen activation by sterically demanding borane and amine species [104]. 

In 1926, Bjerrum [137] used Debye-Hiickel theory to describe ion association and took into account the interaction of ions within a short range. He introduced an ion-pair concept, gave a definition of ion pairs as neutral species formed by electrostatic attraction between oppositely charged ions in solution, and showed how ion-pair formation was dependent on the ions size (radius of ions), solvent (dielectric constant), and temperature. 

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