Rotational Brownian motion of the proton crane operating in a photoinduced long-distance intramolecular proton transfer

Abstract

The photoinduced proton transfer from the OH group to the endocyclic N atom in 7-hydroxy-8-(N-morpholinomethyl)quinoline (HMMQ) has been studied as a function of temperature at constant viscosity and as a function of viscosity at constant temperature for solutions in alkanols and in alkanenitriles. The overall rate constant, k, is determined by the rotational motion of the side group. There is an energy barrier on the reaction path, which does not depend on the solvent. The motion along the reaction path is described as the Brownian motion of a particle suffering hydrodynamic friction and moving under the influence of an intramolecular Coulomb potential and disappearing at a sink. The influence of dielectric friction is neglected. The friction coefficient is considered to be time-independent. The resulting Smoluchowski equation is solved numerically, using initial and boundary conditions imposed by intramolecular hydrogen bonds required for proton transfer. A good agreement between theory and experiment is obtained. A fractional power dependence of k on viscosity is obtained, which is not the same for alkanols and nitriles as solvents. This difference is attributed to a back transfer of the proton to its original site in the case of nitriles. In the case of alkanols, the back transfer is blocked by hydrogen bonding of the original site with the solvent, after the initial deprotonation of the OH group.