Bimodal proton transfer in acid-base reactions in water

Abstract

We investigate one of the fundamental reactions in solutions, the neutralization of an acid by a base. We use a photoacid, 8-hydroxy-1,3,6-trisulfonate-pyrene (HPTS; pyranine), which upon photoexcitation reacts with acetate under transfer of a deuteron (solvent: deuterated water). We analyze in detail the resulting bimodal reaction dynamics between the photoacid and the base, the first report on which was recently published. We have ascribed the bimodal proton-transfer dynamics to contributions from preformed hydrogen bonding complexes and from initially uncomplexed acid and base. We report on the observation of an additional (6 ps)(-1) contribution to the reaction rate constant. As before, we analyze the slower part of the reaction within the framework of the diffusion model and the fastest part by a static, sub-150 fs reaction rate. Adding the second static term considerably improves the overall modeling of the experimental results. It also allows to connect experimentally the diffusion controlled bimolecular reaction models as defined by Eigen-Weller and by Collins-Kimball. Our findings are in agreement with a three-stage mechanism for liquid phase intermolecular proton transfer: mutual diffusion of acid and base to form a "loose" encounter complex, followed by reorganization of the solvent shells and by "tightening" of the acid-base encounter complex. These rearrangements last a few picoseconds and enable a prompt proton transfer along the reaction coordinate, which occurs faster than our time resolution of 150 fs. Alternative models for the explanation of the slower "on-contact" reaction time of the loose encounter complex in terms of proton transmission through a von Grotthuss mechanism are also discussed.

Solvent‐Dependent Photoacidity State of Pyranine Monitored by Transient Mid‐Infrared Spectroscopy

“…However, since a transition from photoacid to photobase due to proton transfer is accompanied by a change in the electronic charge distribution, the vibrational transitions due to modes with CÀO stretching content will change in the frequency position and absorption cross-section. [50,51] This is most pronounced when comparing the IR spectra of the groundstate HPTS photoacid and photobase in the frequency range between 1450-1600 cm À1 .…”

Early events associated with the excited state proton transfer in 2-(2′-pyridyl)benzimidazole

“…Proton transfer proceeds in 250 ps, for example, in encounter pairs formed by diffusion of uncomplexed photoacid and base molecules. 17,18 Proton transfer in acetonitrile-water mixture from strong photoacids 6-hydroxyquinolinium and 6-hydroxy-1-methylquinolinium is associated with a rather long time constant of approximately 1 ns, the process being diffusion controlled. 19 Long time constants of approximately 100 ps have been observed in the perylene quinones, hypocrellin, and calphostin C as a result conformational change and chain dynamics associated with the proton transfer a͒ Author to whom correspondence should be addressed.…”

Deprotonation Dynamics and Stokes Shift of Pyranine (HPTS)

“…Nibbering and co-workers have pointed out that visible pumpprobe spectra can be difficult to interpret because of overlapping transitions and the electronic states that are observed can be highly sensitive to solvent reorganization, which may interfere with observation of the proton-transfer process. 9,22 The goal of this paper is to analyze the visible pump-probe spectra of HPTS in a global manner and definitely decouple the solvation (Stokes shift) and proton-transfer dynamics. The important question is whether the ∼3 ps dynamics observed in the strictly electronic spectroscopy is just part of the Stokes shift or is it in fact the initial deprotonation step.…”

“…9,[20][21][22] Proton-transfer dynamics were monitored in H 2 O and D 2 O by probing the fingerprint region of the IR spectrum, which appear to change vibrational frequency upon HPTS deprotonation. In such experiments, it is likely that an electronic Stokes shift would not be evident.…”