Step-mediated dissociation

The step-mediated indirect channel to hydrogen dissociation... 

Further evidence that it is a step mediated channel responsible for the indirect dissociation on the W(1 0 0) surfaces comes from a comparison with the results for the step mediated dissociation on Pt(5 3 3) (Section 3.2) which exhibit very similar dynamical characteristics. A similar mechanism is likely to be responsible for the indirect dissociation channel on Ni(9 9 7) [89] (Fig. 25)., Y0 decays with Ei over the range 0 < Tq(meV) < 150 on all of the metal surfaces studied where steps are suggested to be responsible for indirect dissociation. S0 is also rather insensitive to 7 s in all cases, and, S (0n) exhibits precursor type dependencies. 

The step mediated indirect dissociation of hydrogen on W(1 00) provides an explanation for the insensitivity of the channel to alloying W(1 00) with an inert diluent on W(1 00)-c(2 x 2)Cu, or decorating the surface with nitrogen on W(1 00)-c(2 x 2)N. Both modifications lead to an expected increase in the activation barrier associated with the direct channel to hydrogen dissociation. The indirect channel for H2 dissociation, however, on both surfaces remains the same as that found on W(1 0 0). This contrasts with the case of N2 dissociation on W(1 0 0) and on the W(1 0 0)-c(2 x 2)Cu alloy surface. The increase in the activation barrier induced by alloying the surface with Cu associated with the direct channel to N2 dissociation is accompanied by the disappearance of the accommodated indirect channel to N2 dissociation observed on W(1 0 0). This difference between N2 and H2 dissociation is accounted for by the mediation of step sites in the indirect dissociation of H2, but dissociation at the W(1 0 0)-c(2 x 2)Cu surface unit cell in the case of N2. 

Low activation energy values are indicative of corrugated surfaces. From the difference in the found energy barriers, it is also concluded that CO adsorption is the rate-determining step for CO dissociative adsorption, followed by the dissociation step. These findings suggest a precursor-mediated mechanism for CO dissociative adsorption. 

Gee AT, Hayden BE (2000) The dynamics of adsorption on Pt(533) step mediated molecular chemisorption and dissociation. J Chem Phys 113 10333... 

FIGURE 20 7 The mecha nism of amide hydrolysis in acid solution Steps 1 through 3 show the for mation of the tetrahedral intermediate Dissociation of the tetrahedral inter mediate is shown in steps 4 through 6... 

Most of the free-radical mechanisms discussed thus far have involved some combination of homolytic bond dissociation, atom abstraction, and addition steps. In this section, we will discuss reactions that include discrete electron-transfer steps. Addition to or removal of one electron fi om a diamagnetic organic molecule generates a radical. Organic reactions that involve electron-transfer steps are often mediated by transition-metal ions. Many transition-metal ions have two or more relatively stable oxidation states differing by one electron. Transition-metal ions therefore firequently participate in electron-transfer processes. 

The proposed mechanism is outlined in Figure 8.5 and is called SnI, standing for substitution nucleophilic unimolecular. The first step, a unimolecular- dissociation of the alkyl halide to for-rn a carbocation as the key inter-mediate, is rate-deter-rnining. An energy diagrffln for the process is shown in Figure 8.6. 

Figure 2.4 Noradrenergic inhibition of Ca " currents and transmitter release in sympathetic neurons and their processes, (a) Inhibition of currents through N-type Ca " channels by external application of noradrenaline (NA) or by over-expression of G-protein P y2 subunits, recorded from the soma and dendrite of a dissociated rat superior cervical sympathetic neuron. Currents were evoked by two successive 10 ms steps from —70 mV to OmV, separated by a prepulse to -1-90 mV. Note that the transient inhibition produced by NA (mediated by the G-protein Go) and the tonic inhibition produced by the G-protein Piy2 subunits were temporarily reversed by the -1-90 mV depolarisation. (Adapted from Fig. 4 in Delmas, P et al. (2000) Nat. Neurosci. 3 670-678. Reproduced with permission), (b) Inhibition of noradrenaline release from neurites of rat superior cervical sympathetic neurons by the 2-adrenoceptor stimulant UK-14,304, recorded amperometrically. Note that pretreatment with Pertussis toxin (PTX), which prevents coupling of the adrenoceptor to Gq, abolished inhibition. (Adapted from Fig. 3 in Koh, D-S and Hille, B (1997) Proc. Natl. Acad. Sci. USA 1506-1511. Reproduced with permission)...

So far, catalytic systems in which the mediator plays the role of both catalyst and electron carrier have been considered. Figure 4.21 shows an example where these two roles are dissociated.21 The catalyst, in the sense of a chemical catalyst, is the Co(II) porphyrin embedded in the Nafion (a trademark of Dupont) film, while the electron are shuttled by the ruthenium hexamine 3 + /2+ couple attached electrostatically to the Nafion backbone. The catalytic reaction now involves two successive steps, as expected for a chemical catalysis process (see Sections 4.2.1 and 4.3.1), calling for the definition of two characteristic currents. One has the same... 

The Ag+ ion is labile. Even with cryptands, which react sluggishly with most labile metal ions, Ag reacts with a rate constant around 10 M s (in dmso). The higher stability of Ag(I) complexes compared with those of the main groups I and II resides in much reduced dissociation rate constants. Dissociation tends to control the stability of most metal cryp-tand complexes. Silver(I) is a useful electron mediator for redox reactions since Ag(I) and Ag(II) are relatively rapid reducers and oxidizers, respectively. Silver(I) thus promotes oxidation by sluggish, if strong, oxidants and catalyses a number of oxidations by S20 in which the rate-determining step is... 

Translation to lattice energy transfer is the dominant aspect of atomic and molecular adsorption, scattering and desorption from surfaces. Dissipation of incident translational energy (principally into the lattice) allows adsorption, i.e., bond formation with the surface, and thermal excitation from the lattice to the translational coordiantes causes desorption and diffusion i.e., bond breaking with the surface. This is also the key ingredient in trapping, the first step in precursor-mediated dissociation of molecules at surfaces. For direct molecular dissociation processes, the implications of Z,X,Y [Pg.158]

The scenario described here, i.e., activated dissociation at terrace sites and precursor-mediated dissociation at step or defect sites, is likely to be a very general one since barriers to dissociation are generally much lower at step sites [353]. There are already many known examples C2H6 dissociation on Ir(lll) [354], CH3OH dissociation on Pt(lll) [355] and neopentane dissociation on Pt(lll) [356], etc. 

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