Observation of a Zundel-like transition state during proton transfer in aqueous hydroxide solutions

Abstract

It is generally accepted that the anomalous diffusion of the aqueous hydroxide ion results from its ability to accept a proton from a neighboring water molecule; yet, many questions exist concerning the mechanism for this process. What is the solvation structure of the hydroxide ion? In what way do water hydrogen bond dynamics influence the transfer of a proton to the ion? We present the results of femtosecond pump-probe and 2D infrared experiments that probe the O-H stretching vibration of a solution of dilute HOD dissolved in NaOD/D2O. Upon the addition of NaOD, measured pump-probe transients and 2D IR spectra show a new feature that decays with a 110-fs time scale. The calculation of 2D IR spectra from an empirical valence bond molecular dynamics simulation of a single NaOH molecule in a bath of H2O indicates that this fast feature is due to an overtone transition of Zundel-like H3O 2 ؊ states, wherein a proton is significantly shared between a water molecule and the hydroxide ion. Given the frequency of vibration of shared protons, the observations indicate the shared proton state persists for 2-3 vibrational periods before the proton localizes on a hydroxide. Calculations based on the EVB-MD model argue that the collective electric field in the proton transfer direction is the appropriate coordinate to describe the creation and relaxation of these Zundel-like transition states.Grotthuss mechanism ͉ 2D infrared spectroscopy P roton transfer in water is the process at the heart of biological redox chemistry and energy conversion processes such as photosynthesis and cellular respiration (1). Although its extraordinary speed and efficiency has generally been attributed to the rearrangement of O-H bonds that effectively move the charge rather than a specific proton, the water structure around the charge defect and the mechanism of translocation remain challenging to study experimentally. Researchers commonly discuss the solvated excess proton in terms of 2 limiting structures, the Eigen cation (2), a triply solvated hydronium ion (H 3 O ϩ ⅐3H 2 O), and the Zundel ion (3), a proton equally shared between 2 water molecules (H 5 O 2 ϩ or H 2 O⅐H⅐OH 2 ϩ ). However, the difference in energy between these 2 configurations is small, on the order of the energy of a hydrogen bond (2-3 kcal/mol) (4), which allows these structures to interconvert on femtosecond to picosecond time scales (5, 6). Thus, to meaningfully address the structure of the excess proton, one must also consider its dynamics. Similar questions related to this view of aqueous proton transfer (PT) exist for the hydroxide ion, whereby proton transfer occurs from water to OH Ϫ . What are the ion's limiting structures, and in what manner do changes in the water structure influence the ion's structural diffusion?Initial proposals for transport of the hydroxide ion described the process as the mirror image of that observed in acids (7) and postulated the existence of a stable H 3 O 2 Ϫ state (8). Ab initio simulations (9-11) suggest a different scenario in whi...

On the recombination of hydronium and hydroxide ions in water

“…Earlier experiments on the recombination of H 3 O þ and OH − in water were limited by time resolution and could only examine the slow diffusive step of the recombination process (11,12). Our results suggest the formation of transient species during the recombination occurring on the time scale of approximately 0.5 ps that could provide a signal for experimental detection (32,33). Conclusion and Perspectives.…”

Communication: A concerted mechanism between proton transfer of Zundel anion and displacement of counter cation

“…[6][7][8][9] It is also well known that the proton transfer in the aqueous Zundel anion is strongly affected by the structural rearrangements of the surrounding water molecules. [10][11][12][13] However, much less is known about what happens when H 3 O 2 − is neutralized by a counter cation. [14][15][16][17][18] This communication reports a theoretical analysis of the influence of a series of alkali metal cations M + (M = Li, Na, and K) on the structure and dynamics of the gasphase low-barrier hydrogen-bonded species M + (H 3 O 2 − ), as a first step to understand the influence of counter cations on proton transfer in basic media.…”

Proton transfer through the water gossamer

“…First, the motions on the femtosecond timescale do not exclusively involve the proton rattling between two waters as this is the same timescale over which protons can move in a correlated fashion over several water molecules (32,33). Second, an analysis of the mean square displacement of the proton from our simulations suggests that the effective diffusion constant of the proton, although perhaps fortuitously close to the experimental value, can originate from a mixture of processes involving fairly long-lived trapped protons and concerted proton hopping events over several waters (SI Appendix).…”