Weakly bound encounter complex

Consider reaction 25.22. The first step in the Eigen-Wilkins mechanism is the diffusing together of MLg and Y to form a weakly bound encounter complex (equilibrium 25.23). 

Whereas optimization of the geometry of a single molecule is relatively straightforward, the same procedure can encounter difficulties for a molecular complex, particularly if weakly bound. The first problem here is that the force constants for motions of one molecule relative to the other are quite a bit smaller than those for stretches or bends wholly within one molecule. One tactic to circumvent this difference is to perform a geometry optimization using frozen subunits. That is, the internal geometry of each partner can be taken as fixed, and the optimization carried out over the intermolecular parameters. The final result is thus not fully optimized but the earlier restraint can then be released and the geometry now optimized over all parameters, intramolecular as well as intermolecular. 

Reaction (4) implies a weak associative interaction, erroneously defined previously as of outer-sphere type (76). Encounter complexes have heen described and characterized in the nitrosylation/denitrosylation reactions of aromatic compounds and were proposed to he inner-sphere adducts containing weakly bound NO (77). They were considered as immediate precursors of transition states for intramolecular electron transfer, as in reaction (5). The equifihrium constants for reactions (6—7) should be high. The ligand interchange within the adduct-complex, reaction (6), is rate-controlled by the cleavage of the Fe —H2O bond, coupled to a fast NO -coordination. Therefore, for the kinetic analysis, processes (6—7) could be collected into a single kinetic constant fe h20-... 

For the purpose of this work, we will call a complex of two or more interacting atoms/molecules a supermolecule. Supermolecules may exist for a short time only, e.g., the duration of a fly-by encounter ( 10-13 s). Alternatively, supermolecules may be bound by the weak van der Waals forces and thus exist for times of the order of the mean free time between collisions ( 10-10 s), or longer. In any case, it is clear that, in general, supermolecules possess a spectrum of their own, in excess of the sum of the spectra of the individual (non-interacting) molecules that make up the supermolecule. These spectra are the collision-induced spectra, the subject matter of this monograph. 

Here, electrophoresis often employs principles used in chromatography - interaction of the separands with another phase, which is called the pseudo-stationary phase. The pseudostationary phase is a substance that is added to the separation system in the column, and can interact with the species to be separated. The substance can be neutral, then it does not have its own electrophoretic movement or it can be charged, then it can move in the column with certain mobility. In both cases, the analytes with their own electrophoretic movements encounter on their way the molecules of the pseudostationary phase and interact with them by forming a temporary complex or by association. During the time when the separands are bound to the pseudostationary phase its mobility is different however, when it is free it moves with its own mobility. As the rate constants of the interaction are mostly very high, in analogy with weak electrolytes, the analyte then moves with a certain mean mobility that lies somewhere between the bound and free mobilities. The mean mobility is in this way dependent on the interaction (complex-ation) constant. Many substances can used for this purpose, such as 2-hydroxyisobutyric acid, which forms complexes with many ions, especially with lanthanides, and enables their electrophoretic separation when added to the separation systems. 

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