Electrostatic interaction between reactants

Whereas in acetonitrile the rate limiting step is an opening of the solvent shell of a reactant, in benzonitrile the back reaction of (5) between the protonated acridine orange cation (BH ) and the 3-methyl-4-nitrophenolate ion (A ) to form the ion pair is diffusion controlled (although the overall reaction to the neutral molecules is an endothermic process). Because of its lower dielectric constant than acetonitrile, the electrostatic interactions between reactants in benzonitrile outweigh specific solvent effects. In other words, in benzonitrile a rate limiting coupling of proton transfer to the reorientation of solvent dipoles does not occur and the measured rates are very fast. The ion recombination (I) + (II) in benzonitrile has a diffusion controlled specific rate (theoretical) k = 9 -1 -1... 

Thus, Marcus theory ascribes the free energy of activation to three factors electrostatic interactions between reactants in the transition state, bond distortion to the nuclear configuration of the transition state, and rearrangement of the solvent sphere around the transition state complex. 

To understand the nature of the pH dependence of Lab on pH, we consider a simple phenomenological model in which we suppose only electrostatic interactions between reactants and in which only two amino acid residues, close to Qb, are involved in the proton uptake by RCs after the first flash. The two amino acids, designated here as D (i.e.. Asp) and E (i.e., Glu), generate four different protonation states DHEH, D-EH, DH E- and D E. Electrostatic interactions between amino acid residues and quinone acceptors give rise to different pKs and rate constants for the various ionization states of D and E and the charge states of Ae quinones. Instead of one state of the quinone acceptors, for example QaQb> it is necessary to consider four QaQb(DHEH), QaQb(D"EH), (JaQbCDHE ) and QaQb(D E ) One-electron transfer between the quinones involves 12 possible states, with 10 independent equilibrium constants and pKs [17] ... 

Effects of cationic (cetylpyridinium chloride, CPC) and anionic (SDS) micelles on the rate of reaction of chromium(VI) oxidation of formaldehyde have been studied in the presence and absence of picolinic acid. Cationic micelles (CPC) inhibit whereas anionic micelles (SDS) catalyze the reaction rates that could be attributed to electrostatic interactions between reactants (cationic metal ions and catalyst H+) and ionic head groups of ionic micelles. Experimentally determined kinetic data on these metaUomicellar-mediated reactions have been explained by different kinetic models such as pseudophase ion-exchange (PIE) model, Monger s enzyme-kinetic-type model, and Piszkiewicz s cooperativity model (Chapter 3). The rate of oxidation of proline by vanadium(V) with water acting as nucleophile is catalyzed by aqueous micelles. Effects of anionic micelles (SDS) on the rate of A-bromobenzamide-catalyzed oxidation of ethanol, propanol, and n-butanol in acidic medium reveal the presence of premicellar catalysis that has been rationalized in light of the positive cooperativity model. ... 

The concepts of electronegativity, hardness, and polarizability are all interrelated. For the kind of qualitative applications we will make in discussing reactivity, the concept that initial interactions between reacting molecules can be dominated by either partial electron transfer by bond formation (soft reactants) or by electrostatic interaction (hard reactants) is a useftxl generalization. 

The above calculations refer to the formation of encounter pairs from neutral reactants. If the reactants are charged, the electrostatic interaction between them modifies the frequency of the encounters. Debye has shown that the effect of the electrostatic interaction can be usefully discussed in terms of the distance (/) at which this interaction is equal to kT (Debye, 1942). In S.I. units, this distance is given by (9) where qx and qB are the charges on the... 

In the second route described by Dautzenberg et al. [41], NPs are generated via electrostatic interactions between oppositely charged PEs of comparable molar mass and of strong ionic groups at very low concentrations (in the range of 0.01-1 g/L). The resulting product maintains the non-stoichiometric ratio of reactants and is insoluble. 

In first approximation, the Hughes-Ingold approach to predict solvent effects has been shown to be viable for ionic liquids. This approach qualitatively considers electrostatic interactions between ions or dipolar molecules, and states that more polar solvents stabilise charged transition states, leading to increased rates if the charge density of the transition is higher than in the reactants [147, 148],... 

From the sphere-dimer studies, two major conclusions emerge. The first is that the trajectory method can be extended to structured reactants with anisotropic reactivity and anisotropic direct forces and hydrodynamic interactions. The second major conclusion is that complicated electrostatic interactions between species with anisotropic reactivity can "steer" the approaching particles into favorable orientations and enhance the reaction rate. For these model studies, rate enhancements up to 20% have been obtained. The second conclusion is likely to be of considerable relevance to molecular biology. In the third and final series of simulations, the Brownian dynamics trajectory method is applied to a particular biological system. 

Micelles concentrate reactants from the surrounding medium and provide microenvironments favorable to reaction. Rate enhancement arises from electrostatic and hydrophobic interactions between reactants and micelles. Rates may be strongly dependent on the struture of the reactant. For example, the hydrolysis of methyl ort/iobenzoate is catalyzed by micellar sodium dodecylsulfate, whereas the hydrolysis of methyl ortho-formate (which has less hydrophobic character and less affinity for the micelle core) is not ". The more pronounced the hydrophobic nature of the reactant, the more rapid is the micellar catalysis. ... 

Let us again examine the A-B reactive system and its A°-B° promolecule reference, the latter consisting of the free reactant densities brought to their current positions in A-B, for the finite separation between the two subsystems. It should be observed, that this hypothetical state also corresponds to the electrostatic stage of the interaction between reactants, when the electron distributions and internal subsystem geometries are held frozen at finite inter-reactant separations. When the subsequent Hirshfeld partitioning of the known overall ground-state density of A-B is performed, one obtains the uniquely defined, equilibrium subsystems in the reactive system. 

This expression predicts abnormally low pre-exponential factors for reactions between ions of like sign, and abnormally high pre-exponential factors for reactions between ions of unlike sign. It is seen that the influence of the electrostatic interaction between the reactants—unfavorable for ions of like sign, fovorable for ions of unlike sign—makes itself felt exclusively in the pre-exponential factor, the apparent energy of activation being influenced in the opposite direction. 

Some understanding of solvent effects has been provided by comparisons of the same reaction in the gas phase and in solution. Some reactions, however, do not occur at all in the gas phase, and one must then be content with comparing their rates in different solvents. When such comparisons are made it is sometimes found that the solvent does not have much effect on the rate. When, on the other hand, ions are involved as reactants or products, solvents usually have a much greater effect on rates, because of the rather strong electrostatic interactions between ions and solvent molecules. 

This implies that there is enough interaction between reactants in the transition state to make the probability of electron transfer equal to one, although normal bonding forces are assumed to be much weaker than electrostatic ones. There are occasions when the interaction is thought to be so weak that k > 1 and the effect of "nonadiabaticity" on the reaction rate is sometimes used as a rationale for differences between observed and predicted rate constants. 

The term requires some explanation. It refers in essence to electrostatic interactions between charges developed or destroyed in the reaction centers during the course of the reaction and charges in other parts of the reactants, in particular in substituent groups. A simple example is provided by the two dissociation constants of a dicarboxylic acid e.g.. 

---