Long-Range Proton Transfer in Aqueous Acid−Base Reactions

Abstract

We study the mechanism of proton transfer (PT) in the aqueous acid-base reaction between the photoacid 8-hydroxy-1,3,6-pyrenetrisulfonic acid (HPTS) and acetate by probing the vibrational resonances of HPTS, acetate, and the hydrated proton with femtosecond mid-infrared laser pulses. We find that PT takes place in a distribution of hydrogen-bound reaction complexes that differ in the number of water molecules separating the acid and the base. The number of intervening water molecules ranges from 0 to 5, which, together with a strongly distance-dependent PT rate, explains the observed highly nonexponential reaction kinetics. The kinetic isotope effect for the reaction is determined to be 1.5, indicating that tunneling does not play a significant role in the transfer of the proton. Rather, the transfer mechanism is best described in terms of the adiabatic PT picture as it has been formulated by Hynes and co-workers [Staib, A.; Borgis, D.; Hynes, J. T. J. Chem. Phys. 1995, 102, 2487. Ando, K.; Hynes, J. T. J. Phys. Chem. B 1997, 101, 10464.], where solvent fluctuations play an essential role in forming the correct hydrogen-bond configuration and solvent polarization to facilitate PT.

Parallel proton transfer pathways in aqueous acid-base reactions

“…13 Recently, the proton transfer between HPTS and different carboxylate bases dissolved in water has been studied by probing the vibrations of the photoacid and an accepting base with femtosecond midinfrared laser pulses. [14][15][16][17][18][19][20][21] These studies showed that the SCK model does not provide an adequate description of the data at all delay times. It was also found that there is not a single well-defined reaction distance at which proton transfer takes place.…”

“…12,[14][15][16][17][18][19][20][21] If the strength and concentration of the base are sufficient, the release of the proton by HPTS * is strongly accelerated. In this acid-base reaction, the diffusion of the reactants through the water medium can play an essential role, as the reactants have to come sufficiently close to let the proton transfer take place.…”

“…Instead, there exists a distribution of hydrogen-bonded reaction complexes that differ in the number of water molecules separating the acid and the base. 14,15,20,21 Additionally, at high base concentrations, most of the proton transfers occur in complexes that are already present in the solution before the HPTS molecule is excited. [14][15][16][17][18][19][20][21] Of course, at low base concentrations, diffusion is important as it is required to generate the hydrogenbonded reaction complexes.…”

Wagging motion of hydrogen-bonded wire in the excited-state multiple proton transfer process of 7-hydroxyquinoline·(NH3)3 cluster

“…Proton transfer is one of the most important reactions in acidbase chemical and biological dynamics caused by the site-specific interaction such as hydrogen bonding [24][25][26][27][28][29]. The skeletal mechanistic picture proposed by Grotthuss is still a guide for our understanding of the proton hopping in hydrogen-bonded systems [30,31].…”

Size effect of water cluster on the excited-state proton transfer in aqueous solvent

“…The well-known Eigen-Weller model has frequently been utilized to depict the ESPT as a two-step process [20,21]: the acidic proton is transferred from the acid to a base molecule after photoexcitation, to form a contact ion pair and subsequently the contact ion pair is separated by diffusion. Recently, the bridging role of water in the proton exchange between an acid and a base in aqueous solution has been demonstrated [22]. However, many details of the proton transfer from photoacid to water remain elusive, for example, the structure and stability of the contact ion pair between the photoacid and the hydrated proton in the Eigen-Weller model.…”