Donor equilibrium constants, determination

Since the nature of the hydride chemical shifts, particularly in transition metal hydride complexes, is not simple [32], there is no reliable correlation between Sh and the enthalpy of dihydrogen bonding. Nevertheless, the chemical shifts of hydride resonances and their changes with temperature and the concentration of proton-donor components, for example, can be used to obtain the energy parameters for dihydrogen bonding in solution. As earlier, the enthalpy (A/f°) and entropy (AS°) values can be obtained on the basis of equilibrium constants determined at different temperatures. Let us demonstrate some examples of such determinations. 

In equilibrium constant determinations, we need extinction coefficients to evaluate equilibrium concentrations for free and bonded species. The absorption peaks have to be resolved. This is in general attainable by using an inert diluent as third component besides proton-donor and proton-acceptor molecules. Let us now consider difficulties emerging from infrared studies. 

Analogous investigations were performed by Madeja et al. [Ma 76] with the mixed complex consisting of the hydrogen-bond containing nickel(II) macrocyclic complex (I) and thiocyanate ion. The equilibrium constants determined in solutions prepared with various donor solvents are listed in Table 4.8. By comparison with Table 4.7, it can be seen that, with the exception of acetonitrile, the stability of the thiocyanate complex is higher than that of the iodide complex in every solvent this... 

Comproportionation equilibrium constants for Equation 9.3 between dications and neutral molecules of carotenoids were determined from the SEEPR measurements. It was confirmed that the oxidation of the carotenoids produced n-radical cations (Equations 9.1 and 9.3), dications (Equation 9.2), cations (Equation 9.4), and neutral ir-radicals (Equations 9.5 and 9.6) upon reduction of the cations. It was found that carotenoids with strong electron acceptor substituents like canthaxanthin exhibit large values of Kcom, on the order of 103, while carotenoids with electron donor substituents like (J-carotene exhibit Kcom, on the order of 1. Thus, upon oxidation 96% radical cations are formed for canthaxanthin, while 99.7% dications are formed for P-carotene. This is the reason that strong EPR signals in solution are observed during the electrochemical oxidation of canthaxanthin. 

Insertion of the equilibrium constants will result in equations that can be solved for [h ] as a function of px2, which, in turn, can be used to determine the values of the other defects as a function of pXl (Section 7.10.2). The diagrams for donor and acceptor doping of MX in which electronic defects dominate over Schottky equilibrium (Fig. 8.2a and 8.2c) can be compared to that for undoped material (Fig. 7.11), redrawn here (Fig. 8.2b). 

The magnitude of the equilibrium constant, K, is a measure of the strength of interaction between the two molecules. The equilibrium constant has been determined for a variety of complexes (3). The rate of formation of this complex is usually much higher than the rate of polymerization. The polymerization proceeds by adding donor-acceptor units to the growing chain. 

Similar results have been obtained [23] in determining spectroscopic equilibrium constants and enthalpy of complexing 1,3,5-trinitrobenzene (as an acceptor) with aromatic hydrocarbons (as electron donors). Based on the results, it has been also inferred that donor force of alkyl-benzenes increases with increasing number of methyl substituents. 

A series of seven substituted BIPS were treated in toluene and ethyl acetate with phenol and with 4-nitrophenol as proton donors. The changes in absorption spectra indicated the presence of H-bonded complexes with the colored form. The equilibrium constants and thermodynamic parameters for their formation were determined. The complex formation slowed the thermal fade rate significantly.126... 

The thermodynamics and kinetics of NCS-, CN-,871 Cl-, Br-, NCS- and I- 872 substitutions at [Rh(TPPS)(H2 O)] have been reported the reactions involved are shown in equation (169), and the parameters determined are summarized in Table 60. Spectrophotometric titrations showed two inflection points as OH- is added to [Rh(TPPS)(H20)2]3-, and the consecutive pKx values (7.01 and 9.80 at 20 °C) correspond to the pKk values for fac- and mer-[RhCl3(H20)3], suggesting that the TPPS6- anion and 3 Cl- ligands are comparable electron donors toward the Rh center.872 The trends in the equilibrium constants (Table 60) imply that Rhni is a soft (class B) add in these complexes the NCS- ion is presumed to be sulfur bonded, although no direct evidence is presented to support this assumption. 

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