Regime, solvent-controlled

In the normal region, the crossover between the nonadiabatic dynamic solvent control regime and the adiabatic regime is smooth, because in both cases the reaction rate is determined by the diffusive delivery of the reactive wave packet to the transition state. However, the situation is radically different in the inverted region, where the reaction is always nonadiabatic... 

Figure 9.14. Survival probability S(t) of an irreversible ET reaction [Pi(t) in our notation] in the solvent-controlled regime (k -> co) for biexponential solvent relaxation model with the normalized correlation function A(t) The parameter AG is the activation... 

Let us emphasize that the condition C 1 for the solvent control regime is different from the adiabaticity condition, = V /hQ) 2E k T) 1 [reversed Eq. (9.32)]. The latter is determined by the mean-squared velocity of the full reaction coordinate, characterized by 2, while the former involves only the timescale of the slow component, characterized by It is clear from Eq. (9.79) that although is typically 2 , the increasing contribution of fast modes may lead to that nonadiabatic solvent control regime will gradually disappear. 

Figure 9.23. Survival probability S(t) of an irreversible activationless ET reaction [P,(t) in our notation] in the solvent-controlled regime (k -> oo) obtained by Monte Carlo sampling for the Gaussian-correlated process ( ) and the two-component process with C(t)/C(0) = 0.8 tp[ —(t/Tj) ] -H 0.2exp[ —t/(10Tj)] (A) and C(0) = 16 in both cases. Time is in units of t,. Inset the same data replotted on a semilog scale. (Reproduced from [120b] with permission. Copyright (1995) by Elsevier Science.)...

In the limit of large k, Eq. (9.95) reproduces the TST expression (9.85) for /ci2 ill Ills solvent control regime (the integral over velocities is evaluated as or (fljln) AnElk T, if we recall the definitions of... 

As it has appeared in recent years that many hmdamental aspects of elementary chemical reactions in solution can be understood on the basis of the dependence of reaction rate coefficients on solvent density [2, 3, 4 and 5], increasing attention is paid to reaction kinetics in the gas-to-liquid transition range and supercritical fluids under varying pressure. In this way, the essential differences between the regime of binary collisions in the low-pressure gas phase and tliat of a dense enviromnent with typical many-body interactions become apparent. An extremely useful approach in this respect is the investigation of rate coefficients, reaction yields and concentration-time profiles of some typical model reactions over as wide a pressure range as possible, which pemiits the continuous and well controlled variation of the physical properties of the solvent. Among these the most important are density, polarity and viscosity in a contimiiim description or collision frequency. 

The enhanced rate expressions for regimes 3 and 4 have been presented (48) and can be appHed (49,50) when one phase consists of a pure reactant, for example in the saponification of an ester. However, it should be noted that in the more general case where component C in equation 19 is transferred from one inert solvent (A) to another (B), an enhancement of the mass-transfer coefficient in the B-rich phase has the effect of moving the controlling mass-transfer resistance to the A-rich phase, in accordance with equation 17. Resistance in both Hquid phases is taken into account in a detailed model (51) which is apphcable to the reversible reactions involved in metal extraction. This model, which can accommodate the case of interfacial reaction, has been successfully compared with rate data from the Hterature (51). 

In a kinetic regime system, the kinetics of solvent extraction can be described in terms of chemical reactions occurring in the bulk phases or at the interface. The number of possible mechanisms is, in principle, very large, and only the specific chemical composition of the system determines the controlling mechanism. Nevertheless, some generalizations are possible on considerations based... 

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