Host electroneutral

Positively charged and electroneutral macroheterocycles as host molecules for anions 97CRV1609. 

Cation Vacancies If the cation of the host structure has a lower charge than the cation that is replacing it, cation vacancies may be introduced for the preservation of electroneutrality. Alternatively, the substitution of an anion by one of lower charge may also achieve this in certain systems. For example, NaCl is able to dissolve a small amount of CaCl2, and the mechanism of solid-solution formation involves the replacement of two Na+ ions by one Ca ion, leaving one vacancy on the Na" sublattice, Nai 2xCa Cl (where x denotes a vacancy). 

Anion Vacancies If the cation of the host structure has a higher charge than the replacing cation, electroneutrality may be maintained by introducing vacancies into the anion sublattice. The best-known examples of anion vacancies occur in the stabilized zirconia, such as calcium- or yttrium-stabilized zirconia. The high-temperature... 

The key argument is rooted in a simple observation concerning any dissociation KL K + L. The individual subunits K and L are in general not electroneutral while they are part of the original host molecule, whereas the corresponding radicals certainly satisfy electroneutrality. A charge neutralization accompanies the transformations K K and L L. This constraint solves our problem. A few examples (Table 12.1) help us understand why this argument is important. 

These examples indicate (1) the net charge of K (i.e., to what extent K departs from exact electroneutrality as long as it is part of its host molecule) and (2) the corresponding response in A (K) energy (i.e., how K feels this departure from electroneutraUty). 

The idea embodied in the concept of charge neutralization is simple molecular subunits that are not individually electroneutral in the host molecule must imperatively restore their correct numbers of electrons when dissociation takes place. 

The generalization of these arguments is straightforward. CNE is a pivotal concept that permits us to relate any K in a molecule, described by A (K), to the corresponding electroneutral K°, described by A (K°). In order to learn how any K embedded in its host molecule differs in energy from the ground-state radical K, it suffices to know once and for all how K° differs from K. ... 

Another important consideration for electrochemical studies is the choice of electrolyte, the ionic compound that is added to maintain electroneutrality and provide a means of charge flow through solution. Because at least one of the oxidation states of the host will be charged, it is possible that ion-ion interactions will play a role in the observed electrochemistry. Indeed, this is useful if the objective is to design a redox-dependent ion receptor, but it can be an interference if the guest is neutral. 

Zeolites have increasingly found applications as catalysts, adsorbents and ion exchangers [1]. Their microporous properties of the inorganic host-guest systems are based primarily on the structure of the tetrahedral framework built from the TO4 tetrahedral and the possible variation of T atoms (Si, Al, P, Zr, Sn, Ti, 621, B, etc.). The guest species such as organic and/or inorganic cations fill the pore space of the framework to achieve electroneutrality. 

In order to explore the thermodynamic properties, and especially the chemical potential ofthe intercalation compounds, a lattice gas model [10] has been adopted under the assumption that intercalated ions are localized at specific sites in the host lattice, with no more than one ion on any site, and that local and global electroneutrality is observed and there is no strong interaction between the electrons and the intercalated ions. It should be noted that, in solid-state chemistry, this model is often referred to as ideal solution approximation when used to describe the thermodynamics of nonstoichiometric compounds. According to this model, the chemical potential of A in A8MO2 in Equation (5.3) can be divided into two terms as... 

Many inorganic solids are capable of undergoing insertion reactions with small ions such as H, Li", and Na". The host solid in these reactions undergoes reduction in order to maintain electroneutrality. Inserted ions may be removed by oxidation of the insertion compounds. The structures of the insertion compounds are closely related to those of the respective hosts, with the inserted cation occupying formerly empty sites of the host. 

The anticrown chemistry term has been coined to describe the way in which many electroneutral hosts were built to recognize anions. That is the incorporation of multiple and convergent Lewis-acid centers into preorganized cyclic molecular scaffolds. The Lewis-acid centers, usually transition metals or elements like B, Si, Sn or Hg, should expose their electron-deficient sites for interaction with the lone electron pairs of the anions. The hydrogen-bonding interaction of an anion with an amide N - H mentioned above has also been widely used in the preparation of neutral receptors for anionic guests [8]. 

Berger. M. Schmidtehen. F.P. Electroneutral artificial hosts for oxoanions active in strong donor solvents. J. Am. Chem. Soc. 1996, 118 (37). 8947-8948. 

Figure 2.1 Poly(vinylferrocene) in the oxidized state acts as a host for (CIO4) anions which are inserted to maintain electroneutrality.

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