Elementary reactions in polynuclear

ELEMENTARY REACTIONS IN POLYNUCLEAR IONS AND AQUEOUS-MINERAL INTERFACES A NEW GEOLOGY... 

These difficulties created an incentive to focus theoretical investigations on polynuclear ions, which were structurally well characterized, and, though innovative experimental studies using NMR to follow the dynamics of oxygen-isotope exchange processes were beginning to yield unprecedented details of elementary reactions in aqueous solutions (36). 

Precatalytic Reactions and Xpre. The catalyst precursor must transform under reaction conditions into intermediates to obtain an active system. This transformation may involve, in a small number of cases, only a single elementary step, for example, the dissociation of a ligand from a transition-metal complex. However, a series of elementary reaction steps are usually required to convert the catalyst precursor. Useful examples include (1) the degradation of a polynuclear precursor to mononuclear intermediates, (2) the modification of a precursor with a ligand L which is used to control selectivity, and (3) the transformation of finely divided metal. The characteristic time scale for the precatalytic reaction will be denoted tpre, and the instantaneous reaction rate will be denoted Ppre- Precatalytic phenomena and the associated induction periods have been directly monitored in a number of in situ spectroscopic studies using a variety of mononuclear, dinuclear, polynuclear, and metallic precursors (11). 

EVANS provides a summary of the reactions of organometallics with oxide surfaces that lead to well-defined surface species including mononuclear and polynuclear complexes and monometallic and bimetallic particles. These surface reactions are described by the same principles encountered in molecular chemistry the reaction classes include nucleophilic attack at the ligands, electrophilic attack at the metal-carbon bond, oxidative addition, Lewis base adduct formation, redox reactions, etc. The synthesis of well-defined reactive sites on surfaces by these organometallic routes will facilitate the study of elementary steps in surface chemistry. 

Figure 13.27. Surface models for crystal growth (a) mononuclear growth, (b) polynuclear growth, and (c) screw dislocation growth. Along the step a kink site is shown. Adsorbed ions diffuse along the surface and become preferentially incorporated into the crystal lattice at kink sites. As growth proceeds, the surface step winds up in a surface spiral. Often the growth reaction observed occurs in the sequence c, a, b. (From Nielsen, 1964.) (d) Salient features and elementary processes at surfaces.

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