Proton transfer through the water gossamer

Abstract

The diffusion of protons through water is understood within the framework of the Grotthuss mechanism, which requires that they undergo structural diffusion in a stepwise manner throughout the water network. Despite long study, this picture oversimplifies and neglects the complexity of the supramolecular structure of water. We use first-principles simulations and demonstrate that the currently accepted picture of proton diffusion is in need of revision. We show that proton and hydroxide diffusion occurs through periods of intense activity involving concerted proton hopping followed by periods of rest. The picture that emerges is that proton transfer is a multiscale and multidynamical process involving a broader distribution of pathways and timescales than currently assumed. To rationalize these phenomena, we look at the 3D water network as a distribution of closed directed rings, which reveals the presence of medium-range directional correlations in the liquid. One of the natural consequences of this feature is that both the hydronium and hydroxide ion are decorated with proton wires. These wires serve as conduits for long proton jumps over several hydrogen bonds.T he mechanism by which protons move through water is at the heart of acid-base chemistry reactions. Understanding the reaction coordinates of this process has been one of the most challenging problems in physical chemistry due to the sheer complexity of water's hydrogen bond network (1-4). Developing a molecular basis for these phenomena is of great relevance in energy conversion applications such as in the design of efficient fuel cells (5). Over 200 y ago, von Grotthuss proposed a mechanism by which water would undergo electrolytic decomposition (6). He imagined that proton conduction involved the collective shuttling of hydrogen atoms along water wires. The early 20th century found many of the great scientists of the time developing conceptual models to understand the properties of water and its constituent ions (7,8). Detailed insights into the mechanisms of proton transfer (PT) came much later from a combination of both ab initio molecular dynamics (AIMD) simulations (3, 9-13) and force-field approaches based on the empirical valence bond formalism (14-16). The current textbook picture of the Grotthuss mechanism that has resulted from these studies involves a stepwise hopping of the proton from one water molecule to the next (1,17,18). This process occurs on a timescale of 1-2 ps. For a successful transfer, the model requires solvent reorganization around the proton-receiving species to develop a coordination pattern like that of the species it will convert to, a process known as presolvation. In all of these characterizations of the Grotthuss mechanism, the role of the connectivity of the water network was not brought to the forefront (3, 19).Sometimes PT has also been thought to take on coherent character involving jumps of several protons simultaneously. In this spirit, Eigen (20) suggested that the proton could delocalize over extended hydrogen...

Nuclear quantum effects and hydrogen bond fluctuations in water

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How amide hydrogens exchange in native proteins

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