Dynamic coordination

The Hamilton s equations for three-body system described by the general dynamical coordinates qp are... 

In Chapter 5, we discuss in a simple way static (crystalline field) and dynamic (coordinate configuration model) effects on the optically active centers and how they affect their spectra (the peak position, and the shape and intensity of optical bands). We also introduce nonradiative depopulation mechanisms (multiphonon emission and energy transfer) in order to understand the ability of a particular center to emit light in other words, the competition between the mechanisms of radiative de-excitation and nonradiative de-excitation. 

The chromium complexes are proved to be silanediyl complexes, as shown by the silicon-transition metal bond lengths (Table 5) and by the extreme low field shift of the 29Si NMR signals (124.9 and 121.2) at 22 °C for R = H and CH2NMe2, respectively (Table 6). The 29Si NMR shifts of these complexes are temperature-dependent due to the hindered rotation of the phenyl ring and dynamic coordination of the nitrogens to the Si atom. 

Yoshida and coworkers described the control of free-radical reactivity during reduction by dynamic coordination pyridylethyl-substituted tin hydrides (123, 124) appear to selectively reduce alkyl iodides and bromides in preference to chlorides, as illustrated in equations 102 and 103805. Dumartin and his associates reported the immobilization of substrates required for the synthesis of 17a-(iodovinyl)estradiol through hydrostannylation with a polymer-supported tin hydride (equation 104)803. Baba and coworkers described the use of Bu2SnIH in the synthesis of nitrogen heterocycles (e.g. 125) (equation 105)804. 

Bis(o-dimethylaminomethylphenyl)dichlorosilane reveals in solution an intramolecular dynamic coordination of one dimethylaminomethyl group to silicon with pentacoordi-nation122,123 (equation 69). The corresponding monochloro derivative shows in solution an equilibrium between a neutral hexacoordinated modification (which is also observed in the solid state) and an ionic pentacoordinated modification124 (equation 70). 

This radical change in outlook builds on the richness of constitutional diversity and the benefits of variability. It stresses the virtues of instructed mixtures [3, 31], such as was revealed in the self-selection processes occurring in the side-by-side self-assembly of double helical metal complexes (helicates), whereby only the correctly paired double helicates were produced from a mixture of ligands and metal ions in dynamic coordination equilibrium [31, 37c]. It is this work that first led us in the early 1990s to envisage a dynamic chemistry bringing into play the constitution of chemical species. 

CDC also encompasses dynamic coordination chemistry [35, 38, 40], whereby the coordination of metal ions induces the preferential formation of specific ligand molecules and/or induces reversible changes in them. Such processes may be traced back to early work on coordination reactions of imine-based macrocyclic ligands, when now revisited in the light of constitutional dynamics [52],... 

A more recent example of dynamic coordination chemistry controlled by anion recognition was demonstrated with the series of ligands 2, which formed linear binuclear triple helicates with Co2+ anions (Fig. 4) [16,17]. The asymmetric ligand 2a, functionalized at one end with an amide hydrogen-bond donor group, may form two types of helicate structures head-to-head-to-head (HHH) and head-to-head-to-tail (HHT) isomers. With the weak hydrogen-bond acceptor C104 as counteranion, a 3 1 mixture of HHH HHT isomers was observed. However, with the... 

The H NMR spectra of 3 and 5 are temperature dependent TTie analysis of these spectra argues for a mutual dynamic coordination of the terminal nitrogen atoms to the silicon center at room temperature [3] at lower temperature a rigid hexacoordinated complex is formed (Scheme 2). The values of AG for this transition are of the same order of magnitude as that for similar processes [4]. 

Activation by Dynamic Coordination. As a method for the activation of tetraor-ganometals toward electron transfer, dynamic coordination has recently received significant research interest because it does not utilize orbital interactions. The utility of this concept is demonstrated by the following example The oxidation potential of (3-oxobu-tyl)tributylstannane is less positive than that of tetrabutylstannane (Table 7) [142]. 

The dynamic coordination is also effective for the activation of a-heteroatom-sub-stituted tetraorganosilanes [143]. The oxidation potentials of the 2-pyridylethyl (2-Pye)-substituted compounds are less positive than those of the corresponding parent compounds. The decrease of the oxidation potential can be explained in terms of the coordination of the pyridyl group to silicon in the cation radical intermediate (Table 8). 

The fast component is clearly related to electronic polarization, Pfast = Pd, while the slow component, connected to nuclear motions of the solvent molecules, is often called the orientational polarization (Pslow = Pot), or inertial component (PsioW = Pin)- This simplified model has been developed and applied by many authors we shall recall here Marcus (see the papers already quoted), who first had the idea of using Psiow as a dynamical coordinate. For description of solvent dynamical coordinates in discrete solvent models see Warshel (1982) and other papers quoted in Section 9. 

For polyatomic complex systems the use of a single dynamical coordinate 5, as in Marcus theory of ET reactions and in the following generalizations described above, may not be sufficient. The extension of these formalisms to many coordinates have been exploited by several groups. In the already quoted work on the extension of VTST to reactions in solution, Truhlar et al. (1993), generalize Lee and Hynes formalism defining a solvent coordinate yi for each internal coordinate of the solute q ... 

By comparing Hynes s and Rivail s models some common points and some differences are evident. Both studies use an HC1-H20-H20 unit as solute which is treated as a supermolecule in a QM description. Both of them introduce a one-dimensional adiabatic representation of the proton transfer potential but, while Rivail assumes solvent equilibration along the whole proton transfer path, Hynes uses a fixed, but appropriate, solvent configuration. The definition of appropriate configuration is reached using some concepts that are not present in Rivail s treatment, i.e. the crossing of two YB structures ( reactant and product ) in the space of solvent dynamical coordinates. 

The maintenance of cellular homeostasis implies the dynamic coordination of cellular processes to finely compensate for subtle variations of the external and internal environments (e.g., pH, osmolarity, nutrients, and oxygen supply) but also to monitor and regulate intracellular signalling and compartmentalization. Cellular stress occurs when a threat to homeostasis is detected by highly reactive and efficient protective mechanisms. Depending on the intensity (dose) and duration (exposure) of the stress, these defenses can either cope without any observable change in cellular homeostasis or be overcome, resulting in a detectable shift from basal metabolic or cellular functions. 

Most dynamic effects result from details or problems, elaborated or detected during design. Tools can extract the corresponding information from master documents, describing the product of the design process, in order to support dynamic coordination. 

In section 2, the formalism behind the separation of nuclear and electronic systems is sketched. The theory leading to an approach complementing the BO scheme used hitherto is introduced. The driving idea is to eliminate the use of the dynamical coordinates in defining reference frames. A rigged BO scheme is obtained where a one-to-one mapping between chemical species and electronic... 

Note that the electronic wave functions in the R-BO scheme have the parameter set aoi only as labels to remind us of the existence of an attractor related to the electronic wave function. It is the attractor (trapping potential) which acts on the nuclear dynamics. The aoi s are not dynamic coordinates in themselves. Thus, eq.(8) simply says that the set of all electronic wave functions in the R-BO scheme are orthogonal. The electronic Hamiltonian being diagonal for all R means that eq.(4) is consistent with eq.(8) and (11). 

A body-fixed frame can easily be defined without the introduction of constraints among the dynamical coordinates of the problem. Observe that the system is invariant to the 0(3), the orthogonal rotation group. And, as aoi rotates so does the stationary model electronic state function. The electron-nucleus separation problem is hence solved in a manner differing radically from the standard approach. 

M can be considered to be a fixed reference frame wherein the H2 is described by the usual spherical coordinates , , and . The complete description of the exchange process involves the four dynamical coordinates , , , and z, which can be simplified because presumably the H2 remains perpendicular to the z axis during the rotation and is not displaced appreciably relative to M along . In contrast to r and , the two coordinates z and do not play an important role in the dynamics... 

Their formation can be rationalized in several ways. It may involve insertion of the isocyanide into the M—Et bond of an intermediate monoethyl complex, but it can also be explained by substitution of one of the chloro ligands in the starting material by the metalated aldimine, which results from the reaction of f-BuNC with EtMgCl. NMR data suggest a dynamic coordination behavior for the tethered ether ligand and also for the (imino)propionyl group, whose coordination mode may be either or... 

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