Kinetic instabilities

Kinetic Instability (Powers as Donors and Acceptors) of Cyclic Conjugate Molecules... 

Scheme 12 The orbital phase conditions for kinetic instability...

Scheme 13 Kinetic instability of antiaromatic molecules and kinetic stability of aromatic molecules...

Lovett, R., Ortoleva, P. Ross, J. (1978). Kinetic instabilities in first-order phase transitions. J. Chem. Phys., 69, 947-55. 

The occurrence of kinetic instabilities as well as oscillatory and even chaotic temporal behavior of a catalytic reaction under steady-state flow conditions can be traced back to the nonlinear character of the differential equations describing the kinetics coupled to transport processes (diffusion and heat conductance). Studies with single crystal surfaces revealed the formation of a large wealth of concentration patterns of the adsorbates on mesoscopic (say pm) length scales which can be studied experimentally by suitable tools and theoretically within the framework of nonlinear dynamics. [31]... 

The kinetic instability of the polyhydrofullerenes is shown by the reaction with 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) in refluxing toluene. Thereby, the Birch-Hiickel products completely convert into Cjq, which shows that the hydrogenation is completely reversible (Scheme 5.2) [42]. 

So far comparisons of isoelectronic systems of types A, B and C are restricted to few examples, becau especially in type B the kinetic instability of the neutral radicals (SEM) becomes so pronounced that the level RED very often caimot be generated. This low persistency of B may be due to the fact that the odd electron is no longer distributed symmetrically over the molecule. In some cases dimeric products have been identified There are, however, tailor made radicals Rsem. -g-55, which exist as highly coloured, destillable compounds Nonetheless, from the... 

Dithiins are the only biomolecules found in nature that are formally nonaromatic living organisms tend to avoid synthesizing antiaromatic compounds because of their thermodynamic and kinetic instability. Ab initio calculations of... 

The first metallofullerenes, La C2 , were discovered by Smalley et al.18,23 after laser vaporization of composite targets made of graphite and lanthanum oxide or chloride. Because of their low-yield synthesis, laborious purification, and often air sensitivity and kinetic instability, studying the physical properties and chemical reactivity of these fascinating compounds was a serious challenge. Fortunately, the high sensitivity of the electrochemical methods was well adapted to study the microgram quantities in which these materials were usually available. The series of M C82... 

Eigenberger, G. (1978). Kinetic instabilities in heterogeneously catalyzed reactions I. Chem. Eng. Sci., 33, 1255-61. 

The mechanism of substitution reactions at saturated silicon centers is well studied, regarding both kinetics and stereochemistry13,14. In contrast, addition reactions to unsaturated silicon centers, such as to disilenes and silenes, are relatively unexplored. The reason is clear suitable substrates for investigations of regio- and stereochemistry and reaction kinetics are not readily available due to inherent kinetic instability of disilenes and silenes. Kinetically stabilized disilenes and silenes are now available, but these are not always convenient for studying the precise mechanism of addition reactions. For example, stable disilenes are usually prepared by the dimerization of silylenes with bulky substituents. Therefore, it is extremely difficult to prepare unsymmetrically substituted disilenes necessary for regio- and/or stereochemical studies. 

An extensive theoretical investigation does not exist for the siloles, but PM3 calculations of formation enthalpies of 2 and its tautomers have indicated that the l//-silole is the most thermodynamically stable species200. The activation barrier for 11 — 2 isomerization was calculated to be 96 kJ moC1, comparable to that for cyclopentadiene2d 116. The (1H + 1H) dimer 1019 is isolated rather than the (2H + 1H) dimer as in the case of phosphole. This directly confirms the thermodynamic stability and the Diels-Alder kinetic instability of 2. The marked difference in the stability of the parent silole and phosphole was explained3 by the relative stabilities of the a bonds in silanes and phosphines (Si > P) and of the ji bonds in silenes and phosphenes (P > Si)117. 

Antiaromatic molecules have, as a rule, very low-lying triplet states, ca. 10 kcal/mol or less above the ground state.89 187 Furthermore, since the a-frame limits the distortivity of the antiaromatic molecule, the triplet excitation is constrained to remain small. Consequently, the instability of antiaromatic species is a kinetic instability associated with availability of low-lying triplet states. A related argument has been used recently by Zilberg and Haas.188... 

Although acetylenic diethers are thermodynamically stable compounds, they show a high kinetic instability that induces polymerization even at low temperatures. ... 

Orgel and Longuet-Higgins analysis can be explained in the following way. Figure 7.8 shows that two n electrons of CB are non-bonding. They make no contribution to molecular stability but still introduce interelectronic repulsions. Thus, they confer thermodynamic instability. Furthermore, Hund s rule requires that T, and both contain a single electron. This adds kinetic instability because diradical structures are extremely reactive.36... 

These orbitals have the correct symmetry for mixing with VP2 and 3. Figure 7.9 shows the interaction of 3 with dx2 y2 in CBFe(CO)3. This two-electron (stabilizing) interaction removes the thermodynamic instability. Furthermore, the single electrons in the parent fragments are paired in the complex, so the kinetic instability is also eliminated. 

Already at the last of these symposia, Pismen (62) in his excellent review of kinetic instabilities was able to give several references and has himself shown how data sampling can contribute its meed of randomness. 

Sheintuch, M., Kinetic Instabilities in Catalytic Oxidation Reactions (a review to appear). 

The question whether the NiBi layer is indeed missing or only too thin to be observed could be answered by carrying out experiments with the artificially prepared Ni-NiBi-NiBi3-Bi specimens. The disappearance of the already existing NiBi intermetallic layer would provide direct evidence for its kinetic instability. Such an attempt was made by M.S. Lee et al209 Unfortunately, the specimens they prepared proved to be not so easy to handle, and the final goal was not achieved. 

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