Migration activation enthalpy

Fig. 6 Dependence of migration activation enthalpy on preexponential mobility factor for tilt grain boundaries in Al I ( ) and Al II...

Fig. 12 Migration activation enthalpy H (a) and mobility pre-exponential factor Ao (b) for 38.2° (C>)and 40.5° ( tilt grain boundaries as a function of Ga concentration.

An example of the determination of activation enthalpies is shown in Figs. 11 and 12. A valuable indication for associating the correct minimum with the ionic conductivity is the migration effect of the minimum with the temperature (Fig. 11) and the linear dependence in the cr(T versus 1/T plot (Fig. 12). However, the linearity may be disturbed by phase transitions, crystallization processes, chemical reactions with the electrodes, or the influence of the electronic leads. 

A [1,3]- or [1,5]-H shift is formally required for the rearrangement of 162 to benzene (Scheme 6.40). Quantum-chemical calculations predict that the hydrogen atom migrates in two steps, that is, in consecutive [1,2]-H shifts, with the species 177 being the intermediate, which has to be described either as a diradical [117] or a car-bene [116, 117]. The experimental activation enthalpy for the conversion of 176 into benzene [112] was correctly simulated by the energy of the transition state separat-... 

The activation energy for self-diffusivity of the Ag cations by the interstitialcy mechanisms is the sum of one-half the Frenkel defect formation enthalpy and the activation enthalpy for migration,... 

The 1,3-silyl migrations from C to C occurred with inversion of configuration at silicon and with a rather high activation enthalpy for the 1,3-silyl migration of a-methylallyltrimethylsilane studied by Kwart and Slutsky, AH =47.7 kcalmol-1 and AS = —6.2 calmol-1 K-1297. On the other hand, 1,3-silyl migrations in ketosilanes... 

The mechanism of trimerisation of butadiene by a mixed cobalt(ii) chloride-aluminium triethyl catalyst has been inferred from the natures of the three products characterised. The determination of the enthalpy of dimerisation of aluminium triethyl provides a useful piece of thermochemical data for quantitative discussion of the role and energetics of aluminium triethyl in this type of reaction. Polymerisation of isoprene in the presence of Fe(acac)3-aluminium triethyl-pyridine derivatives mixtures has a negative apparent activation enthalpy, which can be attributed to the instability of the catalytic complex at elevated temperatures. Bis-cyclo-octatetraeneiron(o) is an effective oligomerisation catalyst. The composition of products accessible only by hydrogen migration indicates an oxidative addition-reductive elimination mechanism rather than insertion. 

The activation enthalpy obtained for tracer diffusion could be interpreted as the enthalpy of migration of extrinsic oxygen vacancies induced by impurities with lower valency on niobium sites. 

Alkenes.—Kinetic studies of alkene isomerization, catalysed by transition-metal complexes, are rare. One recent example is isomerization of hex-l-ene, catalysed by Co(N2)(PPh3)3, where the rate is proportional to the square of the catalyst concentration this suggests hydrogen migration in a dinuclear intermediate. The activation enthalpy for this isomerization is 10.1 kcal mol , the activation entropy being - 21 cal deg mol . Isomerization... 

These phenomena are attributed to competitive interactions between point defects and the formation of defect associates. The decrease in activation enthalpy in the dilute range is due to a decrease in association enthalpy as a result of electrostatic interactions between the associated pair and the unassociated acceptor cations. The ensuing decrease in conductivity and increase in activation energy with increasing dopant concentration is ascribed to the development of deep vacancy traps as a result of defect clustering and the formation of microdomains. For associated vacancies the activation energy consists of a migration and an association (dissociation) term Hi = H + H. ... 

These defects correspond to more reactive sites of the passivating oxide. As a consequence, cations are less bound to the oxide matrix and may dissolve faster corresponding to a smaller free activation enthalpy AG. Therefore the passive layer gets thinner at these sites. In addition, the potential drop is redistributed (Figure 7.9). The defect sites of the passive layer are less resistive to ion migration and take over less potential drop. With a total potential difference between the substrate metal and the electrolyte given by the applied electrode potential a larger drop will be located at the oxide-electrolyte and/or the metal-oxide interface which accelerates the electrochemical reactions at these sites. 

However, the activation energy, Ea, will consist of two terms, one representing the enthalpy of migration, AHm, and the other enthalpy of defect formation AH ol. 

As defect clusters tend to disassociate at high temperatures, the aggregation enthalpy, Af/agg, would tend to zero at high temperatures. The high-temperature activation energy would then simply correspond to the migration enethalpy ... 

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