Proton Transport and Related Chemical Processes of Ice

Abstract

Excess protons play a key role in the chemical reactions of ice because of their exceptional mobility, even when the diffusion of atoms and molecules is suppressed in ice at low temperatures. This article reviews the current state of knowledge on the properties of excess protons in ice, with a focus on the involvement of protons in chemical reactions. The mechanism of efficient proton transport in ice, which involves a proton-hopping relay along the hydrogen-bond ice network and the reorientation of water, is discussed and compared with the inefficient transport of hydroxide in ice. Distinctly different properties of protons residing in the ice interior and on the ice surface are emphasized. Recent observations of the spontaneous occurrence of reactions in ice at low temperatures, which include the dissociation of protic acids and the hydrolysis of acidic oxides, are discussed with regard to the kinetic and thermodynamic effects of mobile protons on the promotion of unique chemical processes of ice.

Salt Enrichment and Dynamics in the Interface of Supercooled Aqueous Droplets

“…Protons are another major ionic species in charged droplets. Experiments and computations have detected enhanced proton activity , on ice at 155 K. Molecular dynamics simulations predicted that the migration of surface-hydrated protons is ≈2800 slower than that of bulk protons at 190 K because they can be locally trapped by the undercoordinated water molecules. , Thus, it was deduced that the activity detected in experiments arises from the higher proton concentration. , It has also been found that protons are excluded from the ice bulk because of poor solubility in ice. − The present study and accumulated evidence from the literature on ice supports the hypothesis that protons in cold aqueous droplets will also be expelled to the surface and their migration may slow down due to local trapping. Therefore, the alternating cooling and heating of droplets may also provide a possible method of controlling the degree of protonation in proteins.…”

Frozen Hydrogen Peroxide and Nitrite Solution: The Acceleration of Benzoic Acid Oxidation via the Decreased pH in Ice

“…However, we need to mention that the current level of understanding in thermodynamics in ice, such as an equilibrium constant at a liquidlike region, is very limited, but these issues are beyond the scope of the present study. 51,52 Figure 3b shows the freeze concentration effect at the ice grain boundary. The color in the center box represents the Raman signal intensity at this position (note that red and violet-black indicate high and low intensity, respectively).…”

“…According to the Henderson–Hasselbalch equation, we assumed that the pH of the ice was <1.435 (see Text S2 for a detailed description). However, we need to mention that the current level of understanding in thermodynamics in ice, such as an equilibrium constant at a liquidlike region, is very limited, but these issues are beyond the scope of the present study. , …”

C2H5NO Isomers: From Acetamide to 1,2-Oxazetidine and Beyond

“…Given that species with a CN bond are abundant (27 such have been detected 99 ), one might speculate that acid-induced addition of H 2 O could provide a feasible channel In a recent review article, Lee and Kang discussed the intricacies of proton transport in ice and distinguished between the highly mobile proton in the interior which hops along a chain of water molecules and protons trapped on the surface. 102 They concluded that "spontaneous acid−base reactions may occur under interstellar ice conditions, even without external energy input. Excess protons may be generated by the photolysis of ice particles under ionizing radiation or by the injection of cosmic protons into the ice.…”