Equilibrium complexes, ground-state

Nickel(ii) and cobalt(ii) complexes continue to be the most widely studied first-series transition metal complexes. The well resolved NMR spectra arise from the very rapid electron-spin relaxation which occurs as a result of modulation of the zero-field splitting of these ions. In the case of 4-coordinate nickel(ii), only tetrahedral complexes (ground state Ti) are of interest since the square-planar complexes are invariably diamagnetic. Many complexes, however, undergo a square-planar-tetrahedral dynamic equilibrium which can be studied by standard band-shape fitting methods (Section B.l). 

If it is assumed that the ground-state complex is nonfluorescent and the complexation follows a single equilibrium... 

The ability to detect discrete rovibronic spectral features attributed to transitions of two distinct conformers of the ground-state Rg XY complexes and to monitor changing populations as the expansion conditions are manipulated offered an opportunity to evaluate the concept of a thermodynamic equilibrium between the conformers within a supersonic expansion. Since continued changes in the relative intensities of the T-shaped and linear features was observed up to at least Z = 41 [41], the populations of the conformers of the He - lCl and He Br2 complexes are not kinetically trapped within a narrow region close to the nozzle orifice. We implemented a simple thermodynamic model that uses the ratios of the peak intensities of the conformer bands with changing temperature in the expansion to obtain experimental estimates of the relative binding energies of these complexes [39, 41]. 

The model [39] was developed using three assumptions the conformers are in thermodynamic equilibrium, the peak intensities of the T-shaped and linear features are proportional to the populations of the T-shaped and linear ground-state conformers, and the internal energy of the complexes is adequately represented by the monomer rotational temperature. By using these assumptions, the temperature dependence of the ratio of the intensities of the features were equated to the ratio of the quantum mechanical partition functions for the T-shaped and linear conformers (Eq. (7) of Ref. [39]). The ratio of the He l Cl T-shaped linear intensity ratios were observed to decay single exponentially. Fits of the decays yielded an approximate ground-state binding... 

Consider the enzyme-catalyzed and noncatalyzed transformation of the ground state substrate to its transition state structure. We can view this in terms of a thermodynamic cycle, as depicted in Figure 2.4. In the absence of enzyme, the substrate is transformed to its transition state with rate constant /cM..M and equilibrium dissociation constant Ks. Alternatively, the substrate can combine with enzyme to form the ES complex with dissociation constant Ks. The ES complex is then transformed into ESt with rate constant kt , and dissociation constant The thermodynamic cycle is completed by the branch in which the free transition state molecule, 5 binds to the enzyme to form ESX, with dissociation constant KTX. Because the overall free energy associated with transition from S to ES" is independent of the path used to reach the final state, it can be shown that KTX/KS is equal to k, Jkail (Wolfenden,... 

Although an electrophilic reagent should and probably does form -complexes with the aromatic substrate, it is not useful to consider such complexes as intermediates. They represent points on a possible path, but only one of several possible paths, to the a-bonded transition state. If there is a complete equilibrium between the ground state, the 7r-complex, and the transition state, then the particular path taken by the... 

If K0 is the molar equilibrium constant for the formation of the complex in the ground state... 

The ground state complex formation between A + CC14 which may only be of contact type is indicated from changes in columns (4) and (5) for low and high concentrations of anthracene. A Benesi-Hildebrand plot for A + CC14 in cyclohexane gives the value of K, the equilibrium constant, for complex formation1 ... 

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