Prototropic charge migration in water. Part 2.—Interpretation of nuclear magnetic resonance and conductivity data in terms of model mechanisms

Abstract

The mechanism of prototropic charge migration (p.c.m.) in water is investigated theoretically, employing topological concepts as well as continuum theory. A system for classification of p.c.m. models is presented. Two models are explored in detail: a one-step model and a concerted model, both of which include non-Markovian features. The two main sources of experimental data on p.c.m. are (1) nuclear spin relaxation, which monitors proton dynamics, and (2) electric conductivity measurements, which probe charge transport. The complementary nature of the information from the two methods makes it possible to test the consistency of postulated p.c.m. models. However, a number of interpretational problems must first be solved. Thus we consider the questions of n.m.r.-inactive proton exchanges and water molecule–prototropic charge re-encounters, both of which are crucial for a correct interpretation of n.m.r. data. The interpretation of conductivity data hinges on the separation of the prototropic and the diffusive contributions. We argue that these contributions are not independent: there is also a significant contribution from cross correlations. The origin of these cross correlations is the non-equilibrium polarization of the aqueous solvent. Several other consequences for the p.c.m. process of non-equilibrium polarization are discussed. A concerted p.c.m. mechanism requires hydrogen-bonded chains of the appropriate polarity. A topological model is presented and the length distribution of such chains is calculated in terms of this model.