Charge transfer constants

The charge transfer constant Cp (or group charge transfer) is an electronic substituent constant defined from the dissociation constant of a complex between tetracya-noethylene and an X-substituted parent compound [Hetnarski and O Brien, 1975] ... 

Hetnarski, B. and O Brien, R.D. (1975). The Charge-Transfer Constant. A New Substituent Constant for Structure-Activity Relationships. J.Med.Chem., 18,29-33. 

Electronic parameters, e.g. Hammett a constants, field and resonance parameters, parameters derived from spectroscopic data, charge transfer constants, dipole moments, and quantum-chemical parameters. 

Charge-transfer constants [313] and k [314, 315] were derived from association constants of different analogs with tetracyanoethylene and 1,3,5-trinitrobenzene, respectively. Although charge-transfer interactions are supposed to be important for the interaction of some drugs with their binding sites e.g. in DNA intercalation) [116], they have not attracted much attention in QSAR studies. Values are known for only few substituents but they are definitely interrelated with other electronic parameters. 

Figure 6-4 The relationship of the reversibility factor f, and the heterogeneous charge transfer constant k°. The concentrations of the electroactive species in the electrode surface were evaluated at the peaks of cyclic voltammogram by digital simulation with E° = — 1.529 V, a = 0.78, and D = 1.8 x 10 cmVs at — 30°C. The reversibility factor /, was calculated through Equation (1). (a) 300mV/s, = 590s (b) 50mV/s,...

The choice of such experimental data for comparison with calculations is based on a successful prediction of an anomalous ratio of charge transfer constants in the series of the Na-K-Cs OS cations obtained for the systems of the wM" NbF7 type [5]. It turned out that, for the most stable particles, the correct ratio of the calculated activation charge transfer energies is fulfilled, that is, corresponding to the ratio of the charge transfer constants in the series of OS cations of Na-K-Cs. Therefore, one may assume that the composition of the most thermodynamically stable particles also characterizes the most probable dynamic composition of the electroactive complex in the melt, which imder certain conditions takes a predominant part in the electrochemical charge transfer. 

Viggiano A A, Morris R A and Paulson J F 1994 Effects of f and SFg vibrational energy on the rate constant for charge transfer between and SFg int. J. Mass Spectrom. ion Processes 135 31-7... 

NMR signals of the amino acid ligand that are induced by the ring current of the diamine ligand" ". From the temperature dependence of the stability constants of a number of ternary palladium complexes involving dipeptides and aromatic amines, the arene - arene interaction enthalpies and entropies have been determined" ". It turned out that the interaction is generally enthalpy-driven and counteracted by entropy. Yamauchi et al. hold a charge transfer interaction responsible for this effect. 

Wetting and capillarity can be expressed in terms of dielectric polarisabilities when van der Waals forces dominate the interface interaction (no chemical bond or charge transfer) [37]. For an arbitrary material, polarisabilities can be derived from the dielectric constants (e) using the Clausius-Mossotti expression [38]. Within this approximation, the contact angle can be expressed as ... 

The ortho effect may consist of several components. The normal electronic effect may receive contributions from inductive and resonance factors, just as with tneta and para substituents. There may also be a proximity or field electronic effect that operates directly between the substituent and the reaction site. In addition there may exist a true steric effect, as a result of the space-filling nature of the substituent (itself ultimately an electronic effect). Finally it is possible that non-covalent interactions, such as hydrogen bonding or charge transfer, may take place. The role of the solvent in both the initial state and the transition state may be different in the presence of ortho substitution. Many attempts have been made to separate these several effects. For example. Farthing and Nam defined an ortho substituent constant in the usual way by = log (K/K ) for the ionization of benzoic acids, postulating that includes both electronic and steric components. They assumed that the electronic portion of the ortho effect is identical to the para effect, writing CTe = o-p, and that the steric component is equal to the difference between the total effect and the electronic effect, or cts = cr — cte- They then used a multiple LFER to correlate data for orrAo-substituted reactants. 

Donor solvent Formation constant X(20°C)/1 mol- -AH / kJ moC Charge-transfer band W/nm fmax A VI/cm- 2 ... 

In view of the magnitude of crystal-field effects it is not surprising that the spectra of actinide ions are sensitive to the latter s environment and, in contrast to the lanthanides, may change drastically from one compound to another. Unfortunately, because of the complexity of the spectra and the low symmetry of many of the complexes, spectra are not easily used as a means of deducing stereochemistry except when used as fingerprints for comparison with spectra of previously characterized compounds. However, the dependence on ligand concentration of the positions and intensities, especially of the charge-transfer bands, can profitably be used to estimate stability constants. 

It is apparent (Fig. 1.21) that at potentials removed from the equilibrium potential see equation 1.30) the rate of charge transfer of (a) silver cations from the metal to the solution (anodic reaction), (b) silver aquo cations from the solution to the metal (cathodic reaction) and (c) electrons through the metallic circuit from anode to cathode, are equal, so that any one may be used to evaluate the rates of the others. The rate is most conveniently determined from the rate of transfer of electrons in the metallic circuit (the current 1) by means of an ammeter, and if / is maintained constant it can eilso be used to eveduate the extent. A more precise method of determining the quantity of charge transferred is the coulometer, in which the extent of a single well-defined reaction is determined accurately, e.g. by the quantity of metal electrodeposited, by the volume of gas evolved, etc. The reaction Ag (aq.) -t- e = Ag is utilised in the silver coulometer, and provides one of the most accurate methods of determining the extent of charge transfer. 

The rate of this charge transfer is not necessarely identical with the rate constant of ion com-plexation in homogeneous solution which may be diffusion limited 751... 

It should be noted that the properties of a CTC depend to a considerable degree on the conditions of their preparation. Temperature increase, in particular, favors the accumulation of complete charge transfer states in a CTC. In the case of a CTC obtained in solution, the increase of dielectric constant of the solvent has the same effect. The method of preparation of a CTC also affects the kinetic curves of the accumulation and depletion of complete transfer states arising at protoirradiation. 

The reaction between the photoexcited carbonyl compound and an amine occurs with substantially greater facility than that with most other hydrogen donors. The rate constants for triplet quenching by amines show little dependence on the amine a-C-H bond strength. However, the ability of the amine to release an electron is important.- - This is in keeping with a mechanism of radical generation which involves initial electron (or charge) transfer from the amine to the photoexcited carbonyl compound. Loss of a proton from the resultant complex (exciplex) results in an a-aminoalkyl radical which initiates polymerization. The... 

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