Probing the mechanisms of proton transfer in liquid water

Despite more than two centuries of experimental and theoretical investigations aimed at understanding the mechanisms by which hydronium (H₃O⁺) and hydroxide (HO⁻) ions are transported through water (1–6), unique results published in PNAS (7) reveal that these can be complex processes that strongly couple to the structural and dynamic properties of the 3D hydrogen bond (H-bond) network in water. Water is well known as a very versatile and ubiquitous solvent, offering a rich environment for a multitude of complex reactions and processes. Its constituent hydronium and hydroxide ions exist as dynamic charge defects that can be viewed as representing the excess proton and proton hole, respectively. Importantly, these ions are known to exhibit anomalously high diffusion rates relative to other simple ions (e.g., Na⁺ or Cl⁻). The phenomena by which H₃O⁺ and HO⁻ move through water have enormous impacts on areas ranging from aqueous acid-base chemistry, enzymatic proton transfer, as well as proton transfer in biological channels, through fuel cell membranes and on ice surfaces facilitating atmospheric reactions (8). Marx (8) has reviewed recent progress in our understanding of the solvation and transport properties of hydronium and hydroxide ions in aqueous environments. To date, the accepted picture of an excess proton in water is that it can exist either as an “Eigen” or “Zundel” cation (Fig. 1 A and B). The proton transfer (PT) process has then been described as a stepwise hopping of a proton (from a H₃O⁺ ion to a water molecule or from a water molecular to HO⁻) within a time frame of roughly 1–2 ps. For a successful transfer, a fluctuation in the local water structure is required; this solvent reorganization is now referred to as the concept of presolvation. This current picture, known as the Grotthuss mechanism in honor of the mechanism proposed by von Grotthuss over 200 y ago (1), essentially views all protons in the ensemble diffusing with a similar mechanism (see schematic in Fig. 1C). Recent investigations (6, 8) have shed more light on this concept by exploring issues such as thermal hopping, impacts of solvation structure, and nuclear quantum effects.

Fig. 1.

Schematic drawings showing (A) the Eigen cation, a hydronium ion with its hydration, (B) the Zundel cation, and (C) a sequence of proton transfers resulting in the displacement of an excess proton along a chain of H-bonded water molecules. In the picture, the proton transfers PT1, PT2, and PT3 are thought to occur as separate steps in the sequence, whereas the work of Hassanali et al. (7) reveals that the three transfers can occur in a concerted manner by coupling to the structural fluctuations of the H-bond network.

In PNAS, Hassanali et al. (7) report a significant advance in our understanding of the PT process in water. The authors revisit the current view of the Grotthuss mechanism, using very powerful first principles simulation methods, and demonstrate the crucial role played by the connectivity of the H-bond water network in this process. Hassanali et al. specifically show that, rather than undergoing an exclusive stepwise hopping, PT in water occurs over a broader distribution of pathways and timescales than has previously been assumed. In their mechanism, the migration of charge involves bursts of activity along “proton wires,” specific chains of water molecules in the network, and entail the concerted motion of several protons followed by resting periods that are longer than expected. A deeper analysis of the 3D H-bond network of water shows that such a mechanism is facilitated by the local topology of the H-bond network, and indeed couples to the fluxions of the proton wires, specifically collective compressions. The directionality of the H-bonds of the network is a determinant characteristic of possible PT pathways (i.e., these are directed pathways in which chains of H-bonded water molecules are all linked in donor–acceptor configurations as in Fig. 1C). Hassanali et al. (7) find, through a detailed analysis of water ring structures, that proton wires naturally decorate the local environments for both the hydronium and hydroxide ions. The authors demonstrate that the mechanism by which the excess proton can diffuse through water is strongly influenced be its local hydration structure, where they find that its transport through the system frequently involves quasiconcerted PT events over wires involving two to three water molecules. These proton propagation events are interspersed by periods of rest during which the proton is apparently trapped (localized) for an extended period.

Similar to the case of the excess proton, the motion of HO⁻ (or the proton hole) was previously believed to involve exclusively stepwise hopping events accompanied with solvent reorganization, although the structural diffusion mechanism had aspects unique to HO⁻. Different from the prediction that follows from a standard Lewis picture for a hydroxide ion, hydrated HO⁻ was found to be typically “hypercoordinated” in the sense that its oxygen prefers to accept four H-bonds in a roughly square planar configuration (6). This arrangement was believed to represent a “resting” or “inactive” state in which HO⁻ is unlikely to accept a proton. Structural fluctuations in the first solvation shell of HO⁻, where the number of accepted H-bonds is reduced from four to three and its hydrogen donates an H-bond to a neighboring water, was seen as necessary for PT to occur. Hassanali et al. (7) find that similar to H₃O⁺, HO⁻ can diffuse through the network in rather large jumps involving multiple and concerted steps, followed by periods of rest. In contrast to previous studies, resting periods of HO⁻ were observed with various types of local H-bond structures. However, in agreement with previous studies, the periods of rest observed by these authors for HO⁻ appeared to be more sensitive to the details of its hydration structure than in the case of the H₃O⁺.

In conclusion, Hassanali et al. (7) demonstrate that proton transfer in water, associated with either H₃O⁺ or HO⁻, can occur as multiscale processes and with multiple dynamics involving concerted proton hops. By examining the associated 3D H-bond network in water, composed of fluctuating directed pathways, these authors have provided fresh insights into PT mechanisms. These findings have clear implications to PT in various systems (e.g., tuning of protein structure to help facilitate concerted proton hopping), and pose new challenges to both experimental and theoretical studies looking to probe this fundamental aspect of acid-base chemistry.