Thermodynamic template

The most general definition of a template is as a structure-directing agent. In surfactant solutions the final templated polymers can be either discrete nanoparticles or mesostructured bulk materials as a consequence of polymerization, respectively, in the non-continuous or continuous domains of the template. Thermodynamically stable media, such as microemulsions, equilibrium vesicles, or lyotropic mesophases are especially useful as templates because of their structural definition and reproducible morphologies. The mesostructure of a thermodynamically stable template is defined by composition and temperature, but this same feature makes the structure unstable to changes in temperature, pH, or concentration. The aim of template synthesis is to transfer the self-organized template structure into a mechanically and chemically stable, durable, and processable material. 

In contrast, the thermodynamic template effect involves a particular template species (usually a transition-metal ion) binding to a ligand that is complementary to itself, within an equilibrating mixture of products that are formed without the involvement of the template. The binding of the template thermodynamically stabilises the most complementary product (usually a macrocyclic compound). An excellent example is the preparation of phthalocyanine (2.8). Treatment of 1,2-dicyanobenzene with either boron trichloride or uranyl chloride results in two different-sized macrocycles (2.6 and 2.7, respectively) (Scheme 2.4). Macrocycles 2.6 and 2.7 are themselves only stable when the template is still present. On the removal of the template, the normal phthalocyanine (2.8) is formed, which is highly stable and forms many coordination complexes with a range of transition-metal ions. This is also a very attractive synthetic procedure for the preparation of unsymmetrical phthalocyanines. 

The amide formation reaction (highlighted by the circle) leads to the production of a hydrogen-bonded dimer (ZZ) of the reaction product Z with the template Z. The dimer is in thermodynamic equilibrium with free template in the reaction medium. 

An important part of the optimization process is the stabilization of the monomer-template assemblies by thermodynamic considerations (Fig. 6-11). The enthalpic and entropic contributions to the association will determine how the association will respond to changes in the polymerization temperature [18]. The change in free volume of interaction will determine how the association will respond to changes in polymerization pressure [82]. Finally, the solvent s interaction with the monomer-template assemblies relative to the free species indicates how well it will stabilize the monomer-template assemblies in solution [16]. Here each system must be optimized individually. Another option is simply to increase the concentration of the monomer or the template. In the former case, a problem is that the crosslinking as well as the potentially nonselective binding will increase simultaneously. In the... 

Metal template reactions, 1, 416, 433 equilibrium kinetic, 1, 434 thermodynamic, 1, 434 Metal tolerance amino acid complexes, 2, 964 plants, 2, 963 Metal toxicity... 

Thermodynamic control (Figure 1, right) is based on adsorption of substances until quasi-equilibrium stage. In this case, the surface ratio of the adsorbed species is defined by the ratio of products of their concentration and binding constant. This deposition is much less influenced by poorly controllable fluctuations of external conditions and provides much better reproducibility. The total coverage can be almost 100%. Because of these reasons, the thermodynamic control is advantageous for preparation of mixed nanostructured monolayers for electrochemical applications including a formation of spreader-bar structures for their application as molecular templates for synthesis of nanoparticles. 

Competitive PCR (cPCR) has emerged as the best strategy for controlling the sam-ple-to-sample variability of PCR. In cPCR different templates of similar lengths and with the same primer binding sequences are coamplified in the same tube. This ensures identical thermodynamics and amplification efficiency for both templates. The amount of one of the templates must be known and, after amplification, products of both templates must be distinguishable and separately quantifiable. 

Fig. 6-11. Stabilization of monomer template assemblies by thermodynamic considerations.

Two possible roles for the metal ion in a template reaction have been delineated (Thompson Busch, 1964). First, the metal ion may sequester the cyclic product from an equilibrium mixture such as, for example, between products and reactants. In this manner the formation of the macrocycle is promoted as its metal complex. The metal ion is thus instrumental in shifting the position of an equilibrium - such a process has been termed a thermodynamic template effect. Secondly, the metal ion may direct the steric course of a condensation such that formation of the required cyclic product is facilitated. This process has been called the kinetic template effect. 

Due to recent developments in synthesis, the preparation of nanocrystalline polymorphs, which are usually unstable as bulk phases, has been achieved for several materials such as ZrC>2, Ti02 and various perovskites. The appearance of these exotic materials does not necessarily mean that they are thermodynamically stable, since the kinetics (templates and surfactants) are probably more important for the processes than the thermodynamics. Adsorption of water may also play an important role as in the case of alumina, but in the data given in Figure 6.19 the effect of water has been accounted for [25]. 

The ability of a chemical to act as a template is frequently attributed to a combination of thermodynamic and kinetic factors. As has been defined by Busch [3] a thermodynamic template binds more strongly to one of the products present in an equilibrium (i.e. a mixture under thermodynamic control) shifting the reaction towards the formation of this specific product which is then obtained in higher yields. In contrast, kinetic templates operate under irreversible conditions by stabilising the transition state leading to the final product. 

Scheme 11 Hydrogen-bonding templated preparation of catenane 18 under thermodynamic control...

The threading-followed-by-capping method has been recently employed by Stoddart to prepare a [2]rotaxane under thermodynamic control [60]. In this approach, the dibenzylammonium ion 28 - which is terminated by an aldehyde function - is mixed with the dibenzo[24]crown-8 ether (20) to form a threaded species. Upon addition of a bulky amine, the aldehyde-terminated template can be converted into an imine in a reversible reaction establishing a dynamic equilibrium (see 29 and 30 in Scheme 17). 

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