Stabilization kinetics

Most radicals are transient species. They (e.%. 1-10) decay by self-reaction with rates at or close to the diffusion-controlled limit (Section 1.4). This situation also pertains in conventional radical polymerization. Certain radicals, however, have thermodynamic stability, kinetic stability (persistence) or both that is conferred by appropriate substitution. Some well-known examples of stable radicals are diphenylpicrylhydrazyl (DPPH), nitroxides such as 2,2,6,6-tetramethylpiperidin-A -oxyl (TEMPO), triphenylniethyl radical (13) and galvinoxyl (14). Some examples of carbon-centered radicals which are persistent but which do not have intrinsic thermodynamic stability are shown in Section 1.4.3.2. These radicals (DPPH, TEMPO, 13, 14) are comparatively stable in isolation as solids or in solution and either do not react or react very slowly with compounds usually thought of as substrates for radical reactions. They may, nonetheless, react with less stable radicals at close to diffusion controlled rates. In polymer synthesis these species find use as inhibitors (to stabilize monomers against polymerization or to quench radical reactions - Section 5,3.1) and as reversible termination agents (in living radical polymerization - Section 9.3). 

Enzyme Kinetics and Stability Kinetic studies, carried out mostly with hydrolases, have shovm that enzymes in organic solvents follow conventional models [12a, 22]. 

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 conclusion, Gdm complexes currently used as MRI contrast agents can be considered as safe drugs, due to their high thermodynamic stability, kinetic inertness as well as to an efficient excretion from the body. Nevertheless, in case of renal impairment, the stability issues become much more important and macrocyclic chelates, which have considerably higher kinetic inertness, should be used instead of linear complexes. We should... 

The main difference between the two mechanisms as they relate to crystal size (discussed in Sec. 2.6) is that the cluster mechanism is three dimensional while the ion-by-ion one is mainly two dimensional. Crystal size in the former is limited largely by the amount of reactant per nucleus The more nuclei, the smaller the final crystal size, since the same concentration of reactants is divided over more nuclei. Temperature affects this by stabilizing (kinetically) smaller nuclei as temperature is lowered, thus increasing the number of nuclei at lower temperature. 

In a nutshell, it may be concluded that DTA, DSC and TGA have been used mainly to determine the thermal properties of explosives like melting points, thermal stability, kinetics of thermal decomposition and temperatures of initiation and ignition etc. Further, the properties which can be calculated quantitatively from the experimentally obtained values are reaction rates, activation energies and heats of explosion. DTA data of some explosives are given [46] in Table 3.6. 

In contrast to their instability free in solution, flavin semiquinones bound to proteins are generally (although there are exceptions) quite stable and can be generated in nearly quantitative yields. This stability results from thermodynamic considerations in which the oxidized/semiquinone oxidation-reduction couple is usually more positive and well separated from the semiquinone/hydroquinone couple. In addition to thermodynamic stabilization, kinetic stabilization of protein-bound semiquinones has also been observed and will be discussed in more detail in subsequent portions of this article. 

Note, however, that this thermodynamic condition alone is not sufficient to guarantee instability of the intermediates. One of the energy barriers may be so high that the faradaic potential region is shifted beyond the standard potential of the first electron transfer, i.e. the intermediate Y is stabilized kinetically. Similar reasoning applies to a non-linear mechanism. 

Carbon dioxide is the drying agent of choice if the goal is to stabilize kinetically constrained structure, and materials prepared by this low-temperature route are referred to by some people as carbogeh In general, carbogels are also different from aerogels in surface functionality, in particular hydrophilicily. 

In colloid science, the terms thermodynamically stable and metastable mean that a system is in a state of equilibrium corresponding to a local minimum of free energy (Ref. [978]). If several states of energy are accessible, the lowest is referred to as the stable state and the others are referred to as metastable states unstable states are not at a local minimum. Most colloidal systems are metastable or unstable with respect to the separate bulk phases. See also Colloid Stability, Kinetic Stability. 

In addition to the features relevant to energy-transfer processes and minimization of nonradia-tive deactivation discussed above, the Lnm environment in a lanthanide-containing luminescent probe must also fulfill several other requirements high thermodynamic stability, kinetic inertness, and a saturated coordination sphere. Furthermore, in case of bio-analyses, the luminescent probe has to comply with biochemical aspects as well, especially if the probe is to be... 

In many systems there is a marked tendency for p-peroxo dimer formation (Scheme 67) but the 1 1 adducts may be stabilized by the use of polar solvents and by bulky groups in the ligand which hinder the dimerization step. They may also be stabilized kinetically by use of low temperatures, however bases with the ability to hydrogen bond will accelerate dimerization653 and a basicity is also important. Detailed discussions of the factors influencing stability are available.644,646... 

The term stable is commonly used in a loose operational sense. If a compound persists under certain conditions (specified or unspecified) it is called stable. This is confusing, because the real questions have to do with inherent thermodynamic stability, kinetic control of reactivity, the nature of the available reactants (e.g., 02, H20) and, finally, the thermodynamic stability of the reaction products. 

Thermal and Photochemical Stability. Kinetically stabilized silaaromatic species shown in Scheme 53 showed some difference in their stability in solntion (Schemes 57... 

ANDREAS BORGSCHULTE GROUP LEADER STABILITY KINETICS ... 

As an empty surface state may be associated with Xj (see also further), reaction (23) electronically amounts to the filling of such a state by Y. A slightly modified form of this mechanism has been proposed by Gerischer and LUbke [51] to account for light-intensity-dependent stabilization kinetics. 

Remembering that Xj actually represents a positively charged surface intermediate, Eq (46) may be considered to represent the neutralization of this positive charge by reaction with H2O. It is reasonable to assume that further electrochemical reaction will, for electrostatic reasons, rather occur between the neutral entity X)-OH and the positively charged intermediate Xj than between two positive Xj intermediates (note that the stabilization kinetics yield no information on chemical... 

Gadolinium-containing chelate Structure Charge Thermodynamic stability Kinetic stability (Dissociation half-life at pH 1.0) Amount of excess chelate (mg/ml)... 

Silenes (compounds with an Si=C double bond) are known to be extremely reactive compounds. Till now, only a few silenes, stabilized kinetically by bulky substituents, could be isolated under normal conditions and were characterized by X-ray structural analyses [1]. An effective stabilization of Si=C systems is also achieved by coordination of a base, such as amines, THF, or fluoride ions, to the electrophilic silene silicon atom [2], and recendy we succeeded in synthesizing thermally unexpectedly stable intramolecularly donor-stabilized silenes [3]. In this paper, the synthesis and, in particular, the behavior of the l-[2,6-bis(dimethylaminomethyl)phenyl]silenes 2a-d are described. 

---