Reversible and Irreversible Trapping

The effect of the presence of both irreversible and reversible traps on hydrogen permeation was later considered by lino. The combination of the two types of trap in the diffusion equation can be represented by 

A more generalized form of the input boundary condition would take into account the entry and exit kinetics (see Section III.3), as noted previously. Unfortunately, the limitations of the concentra- 

For low trap coverages (n- 1), the diffusivity can again be obtained from the time lag  

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A. Salleo, F. Endicott, R. A. Street, Reversible and irreversible trapping at room temperature in... 

Salleo, A., Endicott, F. and Street, R.A., Reversible and irreversible trapping at room temperature in poly(thiophene) thin-fihn transistors. A/ / /. Phys. Lett., 86, 2005. 

The trap theory is supported by experimental results for iron-titanium alloys in which the density of reversible and irreversible traps was varied [67]. Increasing the density of reversible traps, for example, was found to decrease the susceptibility to intergranular cracking, as described above. Reversible traps in the form of PdAl precipitates likewise appear to play an important role in suppressing intergranular cracking of a palladium-modified... 

The lattice hydrogen and the trapped hydrogen at reversible sites are assumed to be in local equilibrium. In addition, the coverage of both reversible and irreversible traps is assumed to be low because of the short charging times used in the pulse technique. The diffusion equation (87) can then be written as... 

The occurrence of trapping is well established for iron and steels [65, 70], but there is a considerable amount of evidence that it also occurs in other metals and alloys [54, 66,71-73]. Table 6 shows reversible and irreversible hydrogen traps in steels [64, 65, 67, 74]. The boundary between reversible... 

Cysteamine (HSCH2CH2NH2) has been found to add to Michael acceptors, such as thujone, in dimethylsulfoxide (DMSO). By contrast, no reaction occurred in nonpolar solvents. NMR spectroscopy was employed to identify good Michael acceptors and sort them into reversible and irreversible thiol sinks with a view of developing a cellular assay for thiol-trapping agents. In another paper, calculated transition-state energies of the reaction of Michael acceptors with MeSH have been used as model system to asses thiol toxicity in aqueous media. ... 

Fig. 31 Overall interaction energy between two DNA-coated colloids, (a) Sketch of the interacting surfaces of two spheres of radius R0 separated by d. The maximum length of hybridized strands is 2L. (b) Total interaction energy as a function of d. It is the sum of the attractive I/DNA from the binding of accessible DNA strands, the repulsive I/rep from electrostatics and/or polymer steric effect, and the van der Waals attraction t/vdw. (c) For weak, short-range I/rep, particles which are unbound at high temperatures are irreversibly trapped in the van der Waals well after DNA hybridization at low temperatures, (d) For strong, medium-range I/rep, DNA binding produces a secondary minimum of reversible aggregation. Reproduced with permission from [138]...

When molecules adsorb to a flat substrate, their conformation is modified due to the geometric confinement between the two interfaces and the direct interaction to the substrate. This state can be far from equilibrium if the adsorption process has been fast and irreversible. In this case, the molecules do not have time to sample the whole assembly of thermodynamic states and get trapped kinetically at contact sites. The reversibility is difficult to achieve because of the great size of the molecules and strong adhesion which might exceeds kBT by far. In order to approach an equilibrium state, the sample has to be pre-... 

Figure 41. Two charge-hopping mechanisms. The donor injects an electron (or hole) into the bridge which consists of discrete redox units, (a) The bridge units are nearly degenerate. Consequently, the injected electron (or hole) moves randomly and reversibly up and down the bridge, finally becoming irreversibly trapped by the acceptor, (b) The bridge units constitute an ordered redox cascade the electron or hole moves essentially irreversibly along the bridge towards the acceptor.

An interesting tandem cyelization is used in the synthesis of (-)-a-pipitzol94. The mechanism proceeds via addition of a tin radical to the alkyne, subsequent stereoselective cyelization of the vinyl radical, and addition of the alkyl radical to the nitrile. The addition of tin radicals to alkenes or alkynes is reversible with an unfavorable equilibrium, but in this case the vinyl radical is trapped selectively and irreversibly by cyelization95. 

Each tubulin subunit binds two molecules of GTP. One GTP-bIndIng site, located in a-tubulin, binds GTP irreversibly and does not hydrolyze it. The second site, located on p-tubulln, binds GTP reversibly and hydrolyzes it to GDP. Thus, tubulin Is a GTPase like bacterial FtsZ protein. In the atomic structure of the tubulin subunit, the GTP bound to a-tubulln is trapped at the interface between the a- and p-tubulin monomers and is thus nonexchangeable. The second GTP lies at the surface of the p-tubulin monomer this GTP is freely exchangeable with GDP (Figure 20-3a). As discussed later, the... 

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