A Theoretical Approach to Ion-Molecule Complexation

In directing their study mainly to complexes or molecules (in particular those containing transition metal ions) that can be considered largely in isolation from the host matrix and for which a localized electron approach is appropriate, chemists have paid less attention to the study of many interesting aspects of the study of concentrated systems. In particular the lattice periodicity of such compounds encourages a band theoretical approach to bonding and structure, and experimental and theoretical study by solid-state physicists has been intense, but the spread of knowledge between dis-... 

Ion-molecule chemistry is often dominated by long-range attractive forces, and thus the dynamics of the capture processes, that is, the formation of a complex, is often the central focus in theoretical studies. There is a long history of the use of classical trajectories to compute the cross sections for the formation of ion-molecule complexes. Often it is assumed that the reaction occurs with the decay of the complex. For example, a recent study of proton transfer between NH3 and NHj used this approach. We will not discuss studies of ion-molecule capture, but rather focus on some of the classical trajectories studies that are being done in which the full dimensionality and all areas of the PES are considered. That is, we will restrict the discussion to work that fits within the context of the neutral chemistry discussed here. 

Ion-molecule radiative association reactions have been studied in the laboratory using an assortment of trapping and beam techniques.30,31,90 Many more radiative association rate coefficients have been deduced from studies of three-body association reactions plus estimates of the collisional and radiative stabilization rates.91 Radiative association rates have been studied theoretically via an assortment of statistical methods.31,90,96 Some theoretical approaches use the RRKM method to determine complex lifetimes others are based on microscopic reversibility between formation and destruction of the complex. The latter methods can be subdivided according to how rigorously they conserve angular momentum without such conservation the method reduces to a thermal approximation—with rigorous conservation, the term phase space is utilized. 

Most of the basic principles needed to approach the luminescence of the lanthanide ions have been described. Luminescence is a fascinating phenomenon, but as it was shown, very intricate because many processes are involved. Therefore, the design of highly luminescent lanthanide compoimds and especially of highly luminescent-sensitized lanthanide complexes is quite unpredictable. From a theoretical point of view, the efficiency of the luminescence can always be associated with one particular feature of the environment of the lanthanide ion. It may be solvent molecules... 

Extensive theoretical and experimental work has previously been reported for supported liquid membrane systems (SLMS) as effective mimics of active transport of ions (Cussler et al., 1989 Kalachev et al., 1992 Thoresen and Fisher, i995 Stockton and Fisher, 1998). This was successfully demonstrated using di-(2-ethyl hexyl)-phosphoric acid as the mobile carrier dissolved in n-dodecane, supported in various inert hydrophobic microporous matrices (e.g., polypropylene), with copper and nickel ions as the transported species. The results showed that a pH differential between the aqueous feed and strip streams, separated by the SLMS, mimics the PMF required for the emulated active transport process that occurred. The model for transport in an SLMS is represented by a five-step resistance-in-series approach, as follows (1) diffusion of the ion through a hydrodynamic boundary layer (2) desolvation of the ion, where it expels the water molecules in its coordination sphere and enters the organic phase via ion exchange with the mobile carrier at the feed/membrane interface (3) diffusion of the ion-carrier complex across the SLMS to the strip/membrane interface (4) solvation of the ion as it enters... 

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