Probing the mechanisms of proton transfer in liquid water

Abstract

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 … [↵][1]1To whom correspondence should be addressed. E-mail: pkusalik{at}ucalgary.ca. [1]: #xref-corresp-1-1