Structure and Kinetic Stability

Hydrolysis of cobalt(III) amine complexes occurs by two routes. One route is pH-independent, which is usually measured in acidic conditions and is thus often termed acid hydrolysis or aquation. The second route, base hydrolysis, is usually first order in hydroxide ion and complex concentration, although under certain conditions the reaction may become independent of [OH ] or dependent on the general base (156). 

Cobalt(III) hexaammine is quite inert to hydrolysis. In strongly basic media ([OH ] = 0.1 to 2.1 M) the reaction rate increases and [OH ] apparently reaches a limiting value around 1 M, where the reaction becomes independent of [OH ], 3 x 10 s at 61.8°, (i = 2.0 (157). The mechanism of the reaction involves the SnI(CB) pathway. The limiting rate observed at high pH is thought to refiect a pre-equilibrium ion pair formation between the complex ion and OH , rather than the first-order reaction of the fuly deprotonated complex ion. The rate of 

Cobalt(III) hexakismethylamine is prone to hydrolysis in neutral or slightly basic aqueous solution 158). For the corresponding chloropen-taamine complexes 159), an increase of some 10 was observed in the rate of base hydrolysis of [Co(NH2Me)5Cl] compared to that of [Co(N-HalsCl]. The difference was attributed to steric effects and pointed to a dissociative type of mechanism for the hydrolysis, consistent with an SnI(CB) path. Similar arguments may be employed for the reactivity of the hexaamines. 

the rate constant at that temperture would be 3 x 10 M s , that is, a half-life of around 14 years in 0.5 M NaOH. 

The hydrolysis of [Cofenla] with NaOH in the presence of charcoal has an apparent rate constant of 5 x 10 M s (p, = 1.0) at 25°C 161) that is, the catalyst enhances the reaction some 10 -fold. The catalytic efficiency is not the same for all types of charcoal, however. Like [CoCNHale] , the rate of acid hydrolysis is exceedingly slow. 

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At the same time, many theoretical calculations have been carried out over the last two decades to find new polynitrogens. Computational studies have been able to assist and encourage experimental work to find and identify unknown polynitrogens by providing information about molecular structures and kinetic stabilities on the potential energy surface with respect to N2. Many hypothetical structures of polynitrogens (from N4 to N60) have been predicted by quantum-mechanical calculations, which have led to the... 

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. 

A variety of alkyl amines B, including 1-propylamine, ethylene diamine, 1,3-diaminopropane, and (7 )-l-amino-2-propanol have been used as reactants. The guest exchange kinetic results are reported in Table 13. The presence of more than one reacting [/3-CD-H-A] structure is observed with A = DOPA and penicillamine. The results have been rationalized in terms of specific interactions in the relevant inclusion complexes which determine their structure and relative stability. 

Orbital interaction theory forms a comprehensive model for examining the structures and kinetic and thermodynamic stabilities of molecules. It is not intended to be, nor can it be, a quantitative model. However, it can function effectively in aiding understanding of the fundamental processes in chemistry, and it can be applied in most instances without the use of a computer. The variation known as perturbative molecular orbital (PMO) theory was originally developed from the point of view of weak interactions [4, 5]. However, the interaction of orbitals is more transparently developed, and the relationship to quantitative MO theories is more easily seen by straightforward solution of the Hiickel (independent electron) equations. From this point of view, the theoretical foundations lie in Hartree-Fock theory, described verbally and pictorially in Chapter 2 [57] and more rigorously in Appendix A. 

Srinivasulu, S. and Rao, A.G.A. 1995. Structure and kinetic thermal stability studies of the interaction of monohydric alcohols with lipoxygenase 1 from soybeans (Glycine max). J. Agric. Food Chem. 43 562-567. ... 

Cyclic uptake and release of Ca2+ from the extracellular medium occur during mitosis in Physarum pofycephalum, and correlate with specific structural and kinetic events in the mitotic nuclei.442 The membrane system in the mitotic apparatus in Haemantkus endosperm cells functions in the localized release of Ca2+, so regulating the events of mitosis.443 It is known that calcium exerts effects on the stability of spindle microtubules. An alternative view is that free magnesium concentration acts as the fundamental regulator of the cell cycle.444 Tubulin polymerization depends on the presence of magnesium and the absence of calcium, and control of the Ca2+/Mg2+ ratio is relevant to spindle assembly. 

The fundamental concept of the transition state stabilization was introduced to Linus Pauling in 1948 who said I think that enzymes are molecules that are complementary in structure to the activated complex of the reactions that they catalyze, that is, the molecular configuration that is intermediate between the reacting substances and the product of the reaction . This concept was widely accepted and used for the interpretation of experimental structural and kinetics data on enzyme catalysis, for the design of new substrates and inhibitors and for chemical mimicking of enzyme reactions. Decisive contributions in this area have been made by structural physical methods, X-ray analysis, in particular, and site-directed mutagenesis. 

In the past few decades, almost all of the heavier chalcogen analogues of ketones, i.e. thioketones, selenoketones, and teUuroketones, have been synthesized and characterized. Both thermodynamic and kinetic stabilization methods have been applied to stabilize these unstable double-bond species. In contrast to the doubly bonded systems between carbon and heavier chalcogens, heavier group 14 element analogues of ketones are much more reactive and unstable and hence their structures and properties have not been fully disclosed until recently. ... 

Most protein stability studies have focused their interpretation on either a thermodynamic mechanism or a pure kinetic mechanism, and consequently there is some controversy and confusion over which mechanism is correct. Since the direction of a formulation development effort may depend on which theory is being followed, clarification of the roles of thermodynamic stabilization and kinetic stabilization in given stability problems would provide some practical benefit. This chapter is an attempt to provide such clarification. To this end, the major stresses, or destabilizing effects, that operate during the freeze-drying process are discussed, selected empirical observations regarding pharmaceutical stability in protein systems are presented, and the structure and dynamics in amorphous protein formulations are discussed. 

Very little is known about the influence of grain growth, or crystallization if the membrane is composed of an amorphous alloy, on membrane durability. The as-fabricated permselective metal membrane will be polycrystalline or amorphous, depending on the alloy composition and fabrication method. Amorphous, or metallic glass, structures are far less common than are polycrystalline structures. Both amorphous and polycrystalline structures are quasi-stable, meaning that structures are kinetically stabilized and slow to rearrange to the thermodynamically favored structure. In both cases, this would be a single crystal of the metal. 

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