Temperature-dependent solvent polarity effects on adiabatic proton transfer rate constants and kinetic isotope effects

Temperature-dependent solvent polarity effects are presented for proton transfer (PT) reactions in a polar environment in the proton adiabatic regime, in which proton motion is quantum mechanical, but is not tunneling. In this adiabatic perspective, the reaction activation free energy barrier DeltaG(double dagger) is in the solvent coordinate, with contributions due to proton and hydrogen bond vibration zero point energy, and is strongly dependent on the polarity of the solvent. In particular, the consequences of the feature that solvent polarity decreases with increasing temperature is explored within this nontraditional PT picture, via a continuum dielectric solvent model. For an acid ionization PT reaction, the activation free energy barrier increases with increasing temperature due to the solvent polarity's temperature dependence (although the so-called intrinsic barrier and reorganization energy for the thermodynamically symmetric reaction varies in the opposite direction). The resulting PT rate constant k has a reduced effective activation energy in an Arrhenius plot due to the temperature-dependent solvent polarity effect; this differs from the activation free energy via an activation entropy effect. The Arrhenius behavior of the kinetic isotope effect k(H)/k(D) is, however, largely unchanged by the solvent polarity's temperature dependence due to a cancellation of effects between isotopes. Some further consequences of the solvent polarity's temperature dependence for adiabatic PT systems are discussed.