Oscilatory behaviour observed in the kinetics of reversible proton transfer reactions in aqueous solutions.

Photoacids undergo ultra-fast reversible proton transfer reactions to and from liquid aqueous and non-aqueous solutions. The oscillation period depends on the excited-state acidity of the photoacid and at least one competing reversible proton acceptor site. This result is proton dissociation profiles still exhibiting at very long times a t^-3/2 dependence over time with population oscillations super imposed on it. We have recently observed oscilatory behaviour in the long time tail when there is a two- site competition for the proton when both sites exhibit reversible proton binding following the initial proton dissociation event into the solution. Over even longer times, which depend on the proton retention time at the competing protonation site, the proton is ultimately released into the bulk solution. Our kinetic model captures all details of this complex kinetic system and we carry out full simulations of the coupled diffusion multi-reactive kinetic system which after best-fitting the multi-reaction parameters gives synthetic reaction profiles indistinguishable from the measured ones.