Preexponential factor solvent effects

Strong interactions are observed between the reacting solute and the medium in charge transfer reactions in polar solvents in such a case, the solvent effects cannot be reduced to a simple modification of the adiabatic potential controlling the reactions, since the solvent nuclear motions may become decisive in the vicinity of the saddle point of the free energy surface (FES) controlling the reaction. Also, an explicit treatment of the medium coordinates may be required to evaluate the rate constant preexponential factor. 

A single value for the rate coefficient for loss of CO from Mn2(CO)10 has been obtained29. At 80 °C, k = 3.9 x 10 6 sec-1. Solvent effects should play only a minor role in this decomposition. Therefore, an assumed value for the preexponential factor of 1014 sec-1 is reasonable. This gives an activation energy of 31.4 kcal.mole-1 which is in accord with the values for Mn(CO)sX. 

Equations [140]-[143] provide a connection between the preexponential factor entering the nonadiabatic ET rate and the spectroscopically measured adiabatic transition dipole mu- It turns out that the Mulliken-Hush matrix element, commonly considered as an approximation valid for m b = 0, enters exactly the rate constant preexponent as long as the non-Condon solvent effects are accurately taken into account. Equation [142] stresses the importance of the orientation of the adiabatic transition dipole relative to the direction of ET set up by the difference diabatic dipole Am. The value of is zero when the vectors mi2 and Am are perpendicular. 

The effect of solvent in an electron transfer is larger than simply through its energetic contribution to A. There is evidence that the dynamics of solvent reorganization, often represented in terms of a solvent longitudinal relaxation time, tl, contribute to the preexponential factor in (3.6.2) (47, 62-65), e.g., Since tl is roughly proportional... 

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