Concerted and Sequential Proton Transfer Mechanisms in Water-Separated Acid-Base Encounter Pairs.

The proton transfer mechanisms involved inside aqueous, solvent-separated encounter complexes between phenol and carboxyl moieties are studied using ab initio molecular dynamics and computational time-resolved vibrational spectroscopy. This model framework can be viewed as a ground-state analog of the excited-state proton transfer reactions that have been actively investigated using ultrafast spectroscopy. Three qualitatively distinct proton transfer pathways are observed in the simulations. These can be described as direct concerted, direct sequential, and through bulk transfers. The primary difference between the sequential and concerted mechanism is the involvement of a reaction intermediate in which the proton fluctuates for several picoseconds through the hydrogen bonds connecting donor and acceptor but resides primarily on an intervening water molecule in the encounter complex. These results contribute to our molecular level understanding of the diverse processes involved in proton transfer within water-separated encounter complexes.