Detachment step

The scheme in Fig. 5.5 indicates that the ligand, for example, oxalate, is adsorbed very fast in comparison to the dissolution reaction thus, adsorption equilibrium may be assumed. The surface chelate formed is able to weaken the original Al-oxygen bonds on the surface of the crystal lattice. The detachment of the oxalato-aluminum species is the slow and rate-determining step the initial sites are completely regenerated subsequent to the detachment step provided that the concentrations of the reactants are kept constant, steady state conditions with regard to the oxide surface species are established (Table 5.1). If, furthermore, the system is far from dissolution equilibrium, the back reaction can be neglected, and constant dissolution rates occur. 

Under steady state conditions, i.e., if the original surface sites are regenerated completely after the detachment step (Table 5.1) and if it is assumed that surface protonation equilibria are retained and kept constant by controlling the solution pH, one may write... 

In addition, for the pentanal cluster, all other reaction channels observed start with either Norrish type I, Norrish type II or H detachment reactions, but continue then further, yielding reactions between different pentanal molecular, so-called cross-molecular reactions. Especially, the H detachment step is noteworthy. In the bare pentanal molecule, this pathway was observed in only 1 % of the trajectories. Here, it is involved in most of the trajectory as one of the observed steps of cross-molecular reactions. 

The following chapters will discuss the characteristics of the elementary steps starting with those that form the Entry of catalytic cycles and finishing with those that appear usually in the Detachment step. It should be noted already here that most of the elementary steps functioning as Entry might also appear in the Attachment step (see Table 1.), although in general, combinations of alike steps are less common than other sequences. 

Of the steps listed in Table 1. some are encountered more frequently, while others are less common. Transition metal catalyzed processes usually begin with oxidative addition or coordination-addition as an Entry, which is commonly followed by transmetalation or insertion in the Attachment phase. The final Detachment step is either reductive elimination, or p-hydride elimination, depending on the nature of the intermediate. 

Probably the most common detachment step in late transition metal catalyzed processes is reductive elimination. In this transformation two groups, that are both attached to the same metal centre, will be released and form a covalent bond, with the concomitant formation of a metal whose formal oxidation state, coordination number and electron count are decreased by two. Figure 1-9 presents a general order of the ease of reductive elimination for the most common complexes. 

Linkers. Linkers mainly affect the attachment and detachment steps. Figure 7.7 gives data for the Stille reaction 18 with various chemical linkers. The characteristic carbonyl bands of the squarate moiety at 1780 and 1750 cm, and those of the ester or amide linkage bonds, allow facile monitoring... 

Figure 7.8 shows schematically the various steps in the surface reaction-controlled dissolution of hydrous oxides or hydroxides. A number of protons equal to the valency of the Me center is needed to detach an Me(aq) group into the aqueous solution. Stepwise protonation occurs relatively quickly. The slow, rate-determining step is detachment of the Me(aq) group (Stumm, 1986). With a divalent metal oxide, two neighboring surface groups must become protonated. The rate of detachment (step... 

This method also requires that 0=0. A variation on the method eliminates the detachment step. Here one holds the plate at the point where the bottom of the plate is in the plane of the liquid surface. At this point the surface tension holds up the weight of the meniscus, using the perimeter of the plate (p) ... 

Halicioglu, T., and Srivastava, D., Energetics for bonding and detachment steps in etching of Si by Cl. Surf. Sci. 437, L773-L778 (1999). 

On the other hand, dissolution under steady-state conditions requires, an explained by Furrer and Stumm (1986), that after each metal detachment step the active site will be regenerated (i.e., the fraction of active sites to total surface site ... 

The rate of formation of dissolved iron(II), d[Fe2 + ]diss/df, depends, in addition, on the efficiency of detachment of reduced surface iron ions from the crystal lattice. The detachment step is a key step in the overall dissolution kinetics of slightly soluble minerals, since it is assumed to be the rate-determining step (Stumm and Furrer, 1987). The efficiency of detachment depends primarily on the crystallinity, and thus the stability of the iron(IIJ) hydroxide phase, and also on the coordinative surrounding of the reduced surface metal centers. It has been shown that a combination of a reductant and a ligand that forms stable surface complexes in the dark is especially efficient for the thermal reductive dissolution of hydrous iron(III) oxides (Banwart, et al. 1989). The role of a ligand as an electron donor and as a detacher in the photochemical dissolution of hydrous iron(III) oxides remains to be elucidated. 

There are various models for the potential (i.e. supersaturation) dependence of the heterogeneous nucleation rate. According to the small cluster model developed by Walton (21) and Stoyanov (22), the formation of a cluster can be treated as a sequence of attachment and detachment steps. In equilibrium, the attachment and detachment rates are equal, whereas supersaturation leads to an increase in the attachment rate and growth of the cluster. The result of this theoretical analysis is the following expression for the nucleation rate, Jnuci (15) ... 

Note that the steps are treated as two separate equilibrium attachment/detachment steps. The forward reaction step is seen as the attachment/detachment of Y anion to the inactive site S X, ... 

This method also requires that 0 = 0. A variation on the method eliminates the detachment step. Here one holds the plate at the point where the bottom of the... 

---