Reaction Moment and Electric-Chemical Mechanism

One of the main aims of investigating electric field effects on chemical transformations is to determine the reaction mechanism and, in simple cases, the dipole moments of the reaction partners. As outlined above, the rate constants provide the key information. 

The reaction dipole moment zfM of a dipolar equilibrium may be obtained from the measurement of continuum properties such as the dielectric permittivity as well as from direct monitoring of concentration shifts produced by an externally applied electric field. In both approaches to reaction properties it is primarily the chemical part of the total polarization that is aimed at. However, the chemical processes are intimately connected with the physical processes of polarization and dipole rotation. In the case of small molecules the orientational relaxations are usually rapid compared to the diffusion limited chemical reactions. When, however, macromolecular structures are involved, the rotational processes of the macromolecular dipoles may control a major part of the chemical relaxations. Two types of processes may be involved if a vectorial perturbation like an external electric field is applied a chemical concentration change and a change in the orientation of the reaction partners. 

It is known that in a random distribution of permanent dipolar or induced dipolar reaction partners the (local) extent of the electric field effect depends on the orientation of the individual dipoles relative to the field direction.Therefore the measured bulk effects always represent orientational averages. In this context it is stressed that the total macroscopic polarization, M, caused by an electric field in a random distribution of particles, is a statistical average that results from the polarizing 

For plane-plate capacitor geometry, which is experimentally most adequate, the field-parallel component M of M is the sum over all field-parallel components nij of the individual moments m. We recall that  

Whereas Eq. (3.1) expresses M in terms of the average contributions of the individual molecular moments m, the continuum approach to M represents the total moment in terms of an overall macroscopic dielectric permittivity 

---