Guest species

Fig. 26. Clathrate receptor chemistry (a) a chiroselective crystalline host compound (clathrand) (b) a typical guest molecule to be included in the specified configuration and (c) the crystal stmcture of the respective clathrate (A and B denote host and C the guest species) (169).

If we look at the mechanistic and crystallographic aspects of the operation of polycomponent electrodes, we see that the incorporation of electroactive species such as lithium into a crystalline electrode can occur in two basic ways. In the examples discussed above, and in which complete equilibrium is assumed, the introduction of the guest species can either involve a simple change in the composition of an existing phase by solid solution, or it can result in the formation of new phases with different crystal structures from that of the initial host material. When the identity and/or amounts of phases present in the electrode change, the process is described as a reconstitution reaction. That is, the microstructure is reconstituted. 

The second way in which an electroactive species such as lithium can be incorporated into the structure of an electrode is by a topotactic insertion reaction. In this case the guest species is relatively mobile and enters the crystal structure of the host phase so that no significant change in the structural configuration of the host lattice occurs. 

Thus the result is the formation of a single-phase solid solution. The insertion of additional guest species involves only a change in the overall (and thus also the local) composition of the solid solution, rather than the formation of additional phases. 

The primary question is the rate at which the mobile guest species can be added to, or deleted from, the host microstructure. In many situations the critical problem is the transport within a particular phase under the influence of gradients in chemical composition, rather than kinetic phenomena at the electrolyte/electrode interface. In this case, the governing parameter is the chemical diffusion coefficient of the mobile species, which relates to transport in a chemical concentration gradient. 

Transition Metal and Organic Redox-Active Macrocycles Designed to Electrochemically Recognize Charged and Neutral Guest Species Paul D. Beer... 

Intercalation of some guest species, such as alkali metals, can simply be performed via a chemical reaction of a gaseous reactant with graphite. 

The electrochemical intercalation/insertion is not a special property of graphite. It is apparent also with many other host/guest pairs, provided that the host lattice is a thermodynamically or kinetically stable system of interconnected vacant lattice sites for transport and location of guest species. Particularly useful are host lattices of inorganic oxides and sulphides with layer or chain-type structures. Figure 5.30 presents an example of the cathodic insertion of Li+ into the TiS2 host lattice, which is practically important in lithium batteries. 

Figure 1 shows the crystal structure of the 5,10 12,17 19,24 26,3-tetrakis (dimethyl-siladioxa)-l, 8,15,22-tetramethyl[l4]metacyclophane cavitand (7) which has an enforced cavity appropriately sized to include only slim linear guests 12b). This cavitand forms crystals of a 1 1 molecular inclusion complex with CS2, the guest species being almost entirely encapsulated within the host cavity 27). The crystal structure of the complexed... 

In the crystals containing acetic add and water the solvent layers are included in between monolayers of the host, as illustrated in Fig. 8. Molecules of the host now interact simultaneously with two neighboring bands of the guest species. The solvent zone itself consists of cyclic dimers of the acetic acid surrounded by water and oxonium ions. 

The former structure contains an intramolecular H-bond within the host, which stabilizes its planar conformation, and an N-H. ..N link to the guest species. The other structure with acetic acid contains hydrogen-bond stabilized clusters of two hosts and two guests around the crystallographic inversion centers. A distortion of... 

Fig. 11. Stereoview of the 1 1 complex of the dipolar host 6 with acetic acid. Every guest species hydrogen bonds simultaneously to two centrosymmetricaily related and dipolarly interacting host entities (crystal data a = 8.665, b = 8.696, c = 12.077 A, a = 90.82, p = 99.82, y = 105.15°, space group Pi)36)...

Fig. 14. Stereoview of the 1 1 9 benzene clathrate, showing a channel-type arrangement of the benzene guest species 38)...

Notwithstanding the variety of structural patterns that characterize the crystalline inclusions referred to above, it has recently been shown that the clathrate formation can be induced in a more systematic manner. The use of the clathration phenomenon to store selected guest molecules or to separate one type of guest species from another is the subject of the following sections in this account. 

The range of inclusion adducts formed by the organophosphazenes is very broad, the guest species varying from aliphatic and aromatic hydrocarbons to ethers, ketones and alcohols42. Some of the hosts [e.g., tris(o-phenylenedioxy)cyclotriphosphazene 12)] form clathrates not only when recrystallized from organic solvents but also... 

On the basis of previous observations it was anticipated that the clathrate formation would be more selective in controlling guest selectivities if a functionality which could form hydrogen-bonds with guest species and add this strengthening feature to that... 

It is considerably easier to induce an expansion of the periodicity along the c axis, since along this direction the layered structure is stabilized mainly by weaker dispersion forces. Indeed, the structural variation of the guest species caused a significant increase of the c axis from 14.845 A (for the phenol complex) to 15.818 A (for the p-cresol... 

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