Kinetic and thermodynamic stability

We can then consider combining Equations 5.29 and 5.30 into one equation (5.31), namely  

This expression, as written, also corresponds to the one we use to define the equilibrium constant, K. It can be shown that there is a simple relationship between these terms (5.32), namely  

This holds provided that no stable intermediate is involved in the reaction. If the rate at which ML dissociates into M and L (defined by the rate constant, kb) is slower than the rate at which M and L assemble into ML (defined by the rate constant kf), then there will always at equilibrium be a larger amount of ML present than of M and L, which translates into a large thermodynamic stability constant K, which equates with the ratio of these two kinetic terms. In effect, for such simple steps, there is a thermodynamic-kinetic link. At this level, however, we shall not dwell on this too deeply. 

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The kinetic stabilities and the donor-acceptor properties of cyclic conjugated molecules [68] have been described (Scheme 12) in the theoretical subsection (Sect. 2.2.2) to be controlled by the phase property. There is a parallelism between the thermodynamic and kinetic stabilities. An aromatic molecule, benzene, is kinetically stable, and an antiaromatic molecule, cyclobutadiene, is kinetically unstable (Scheme 13). 

The effects of cyclic 6n electron conjugation have been found in the optimized geometries of pentazole 17 [102] and hexazine 18 [97], The N=N bond is longer than the isolated double bond in NH=NH. The N-N single bond in the tetrazadiene moiety is shorter than the single bond in NH NH. The bond lengths in 18 are nearly intermediate between those in NH NH and NH=NH. The aromatic character of pentazoles was supported by the effect of electron donating substituents on the thermodynamic and kinetic stabilization [103],... 

C-functionalization of azamacrocycles is in general more difficult to achieve than N-functional-ization. The latter has been developed to an extent that macrocycles carrying side chains with various ligating groups are accessible, and Ni11 complexes can be prepared with high selectivity and predisposition for high thermodynamic and kinetic stability. 

Fig. 6. Schematic representation of the relation between thermodynamic and kinetic stability in the studied equilibria.

It is important to note that, even when the coordination geometry prescribed by the macrocyclic cavity is ideal for the metal ion involved, unusual kinetic and thermodynamic properties may also be observed (relative to the corresponding open-chain ligand complex). For example, very often the macrocyclic complex will exhibit both enhanced thermodynamic and kinetic stabilities (kinetic stability occurs when there is a reluctance for the ligand to dissociate from its metal ion). These increased stabilities are a manifestation of what has been termed the macrocyclic effect - the multi-faceted origins of which will be discussed in detail in subsequent chapters. 

In recent years, the amount of research time devoted to materials chemistry has risen almost exponentially and sulfur-based radicals, such as the charge-transfer salts based upon TTF (tetrathiafulvalene), have played an important role in these developments. These TTF derivatives will not be discussed here but are dealt with elsewhere in this book. Instead we focus on recent developments in the area of group 15/16 free radicals. Up until the latter end of the last century, these radicals posed fundamental questions regarding the structure and bonding in main group chemistry. Now, in many cases, their thermodynamic and kinetic stability allows them to be used in the construction of molecular magnets and conductors. In this overview we will focus on the synthesis and characterisation of these radicals with a particular emphasis on their physical properties. 

The use of visible and UV spectrometry for quantitative analysis by comparing the absorbance of standards and samples at a selected wavelength is one of the most widespread of all analytical techniques. It is also one of the most sensitive. The analysis of mixtures of two or more components is facilitated by the additivity of absorbances. This has been discussed earlier (p. 356). Other applications include measurement of the absorption of complexes as a function of solution conditions or time to establish their composition, and to determine thermodynamic and kinetic stability for analytical purposes or for more fundamental studies. 

Confinement of ion-radicals considerably changes their reactivity. What is more important for practical applications is that the confinement increases the ion-radical stability. For instance, the cation-radicals of polyanilines (emeraldines) sharply enhance their thermodynamic and kinetic stabilities when they are formed encapsulated in cucurbituril (Eelkema et al. 2007). Emeraldines have electric condnctivity as high as 1 X 10 cm (Lee et al. 2006). Encapsulation of emer-... 

The complexation process is characterized by its thermodynamic and kinetic stability and selectivity, i.e. by the amount of energy and the amount of information brought into operation. Thus, conceptually, energy (interaction) and information are at the bottom of the recognition process of one chemical entity by another, and the design of molecular systems capable of forming stable and selective complexes becomes a problem in information storage and readout at the molecular level. 

Nesbitt et al. (1981) performed a detailed analysis of the thermodynamic and kinetic stability of perovskite. Thermodynamic calculations and data for natural groundwaters and hydrothermal... 

The thermodynamic and kinetic stabilization ofnickel(III) macrocyclic complexes by axial coordination has prompted a number of new approaches. Studies with tetraaza macrocyclic ligands with pendant donors acting as potential fifth ligands have had some success (96, 99,100). Oxidation of [Ni"[14]aneN4CH2CH2py]2+ in aqueous solution... 

However, it was difficult to exclude a hypothetical exo form at least as a minor component in solution or in the gas phase. Since the chemical and physical properties of the exo and endo forms should differ sharply, their relative thermodynamic and kinetic stability... 

The Gd3+ chelates used as CAs are administered into the body, and accordingly the requirements for their clinical use are very rigorous. The most important conditions to be met are the lack of toxicity, rapid excretion, high water solubility, a high relaxation effect, high thermodynamic and kinetic stabilities and specific in vivo distribution [2]. 

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