Charge transfer dissociation constants

The marked changes in the carbonyl IR bands accompanying the solvent variation from tetrahydrofuran to MeCN coincide with the pronounced differences in colour of the solutions. For example, the charge-transfer salt Q+ Co(CO)F is coloured intensely violet in tetrahydrofuran but imperceptibly orange in MeCN at the same concentration. The quantitative effects of such a solvatochromism are indicated by (a) the shifts in the absorption maxima and (b) the diminution in the absorbances at ACT. The concomitant bathochromic shift and hyperchromic increase in the charge-transfer bands follow the sizeable decrease in solvent polarity from acetonitrile to tetrahydrofuran as evaluated by the dielectric constants D = 37.5 and 7.6, respectively (Reichardt, 1988). The same but even more pronounced trend is apparent in passing from butyronitrile, dichloromethane to diethyl ether with D = 26, 9.1 and 4.3, respectively. The marked variation in ACT with solvent polarity parallels the behaviour of the carbonyl IR bands vide supra), and the solvatochromism is thus readily ascribed to the same displacement of the CIP equilibrium (13) and its associated charge-transfer band. As such, the reversible equilibrium between CIP and SSIP is described by (14), where the dissociation constant Kcip applies to a... 

Alkali, alkaline-earth, and rare-earth metal cations also catalyze electron transfer reactions. Thus, in the pair of Co -tetraphenylporphyrin complex with BQ, no redox reaction takes place, or it takes place too slowly to be determined. The metal cations promote this reaction. For example, in the presence of 80(0104)3, the corresponding rate constant of 2.7 X 10 M s was observed. BQ transforms into benzosemiquinone under these conditions (Fukuzumi and Ohkubo 2000). Zinc perchlorate accelerates the reaction between aromatic amines and quinones (Strizhakova et al. 1985). This reaction results in the formation of charge-transfer complexes [ArNHj Q ]. The complexes dissociate in polar solvents, giving ion-radicals ... 

The demetalation process was followed by absorption spectrophotometry (measurement of the decay, as a function of time and cyanide concentration, of the di-copper(I) complexes characteristic metal-to-ligand-charge-transfer (MLCT) bands in the visible region [111]) which gave access to the kinetic parameters, in particular to the second-order dissociation rate constants CN given in Table 1. 

The CIP dissociation constants evaluated according to Eq. (6) are listed in Table V for some typical charge-transfer salts in t oth polar and nonpolar solvents. The values of Kop measured conductometrically (39) for the related ionic salts PPN+ Co(CO)4, PPN+ V(CO)6", and Na+ BPh4 are included in Table V for comparison. Two features in Table V are particularly noteworthy. First, the magnitudes of Aqp in the nonpolar solvents (THF and CH2C12) are smaller by at least a factor of 100 compared with those in the polar MeCN. Thus, at the concentrations employed in the IR... 

Contact Ion Pair Dissociation Constants of Charge-Transfer Salts in Polar and Nonpolar Solvents ... 

Vitamin K3 (menadione) complexed with polycyclic hydrocarbons (e.g., pyrenes) was investigated by Laskowski using a mixed fusion method [129]. Similar 1 1 complexes exhibited intensification of color upon cooling. These complexes have small association constants [130], and their small, sometimes even positive enthalpies of dissociation are probably due to contact charge transfer in the melt because the colors disappear on solidification of the products, probably due to loss of favorable orientation of the interacting components [1,131]. 

Braun derived an expression for k E) from the 1934 Onsager theory by use of the expression given by Fuoss and Accascina (1959) and Eigen et al. (1964) for the zero-field equilibrium constant for the dissociation of an ion pair. Assuming a field-independent lifetime, Braun determined the field dependence of the charge-transfer state dissociation probability as... 

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