Interplay of Ion–Water and Water–Water Interactions within the Hydration Shells of Nitrate and Carbonate Directly Probed with 2D IR Spectroscopy

Abstract

The long-range influence of ions in solution on the water hydrogen-bond (H-bond) network remains a topic of vigorous debate. Recent spectroscopic and theoretical studies have, for the most part, reached the consensus that weakly coordinating ions only affect water molecules in the first hydration shell. Here, we apply ultrafast broadband two-dimensional infrared (2D IR) spectroscopy to aqueous nitrate and carbonate in neat H2O to study the solvation structure and dynamics of ions on opposite ends of the Hofmeister series. By exciting both the water OH stretches and ion stretches and probing the associated cross-peaks between them, we are afforded a comprehensive view into the complex nature of ion hydration. We show in aqueous nitrate that weak ion-water H-bonding leads to water-water interactions in the ion solvation shells dominating the dynamics. In contrast, the carbonate CO stretches show significant mixing with the water OH stretches due to strong ion-water H-bonding such that the water and ion modes are intimately correlated. Further, the excitonic nature of vibrations in neat H2O, which spans multiple water molecules, is an important factor in describing ion hydration. We attribute these complex dynamics to the likely presence of intermediate-range effects influenced by waters beyond the first solvation shell.

Kinetic Isotope Effects and Hydrogen Tunnelling in PCET Oxidations of Ascorbate: New Insights into Aqueous Chemistry?

“…These studies gave some new and surprising insights about KIE-s in the PCET reactions. Moreover, the studies could be in support of the recently proposed [24][25][26][27] involvement of the vibrational water dynamics [24][25][26][27][28][29][30][31][32][33] in activation processes in aqueous chemistry. Proton-coupled electron transfer, as generally accepted today, is broadly defined as any process which includes a transfer of an electron and a proton.…”

“…The OH stretch is not only a local vibration but the coherent vibrational motion involving the stretch-bend coupling that can comprise up to six waters. Furthermore, aside from the intermolecular motions mentioned, it was The explanation [23] of the observed intriguing modulation of the hydrogen tunnelling (and KIE-s) in the reaction in diluted water solutions takes into account the insights obtained from ultrafast 2D IR spectroscopy of water [24][25][26][27][28][29][30][31][32][33]. Here, several points should be noticed.…”

“…The study of KIE-s and hydrogen tunnelling phenomena in the above PCET reaction (Scheme 2) performed in fairly diluted water solutions containing low concentrations of various (inert) partially hydrophobic solutes [23] concentrates on the question of the recent proposal of involvement of the collective excitonic vibrational dynamics in aqueous chemistry [24][25][26][27]. The investigation starts from the expectation that these fairly diluted water solutions probably can be considered as a good representation of neat water solutions, since the vibrational excitonic dynamics is critical in neat water [23,25,26].…”

“…The study of KIE-s and hydrogen tunnelling phenomena in the above PCET reaction (Scheme 2) performed in fairly diluted water solutions containing low concentrations of various (inert) partially hydrophobic solutes [23] concentrates on the question of the recent proposal of involvement of the collective excitonic vibrational dynamics in aqueous chemistry [24][25][26][27]. The investigation starts from the expectation that these fairly diluted water solutions probably can be considered as a good representation of neat water solutions, since the vibrational excitonic dynamics is critical in neat water [23,25,26]. Furthermore, in the fairly diluted solutions the expectation is that there is no influence of the changes of driving force ∆G • , or DAD, the parameters that, according to theory [35,54,73] can influence the magnitude of KIE-s observed in more concentrated water solutions [23]; the only relevant system property change that can be at work in the cases will be related to the changes of water dynamics.…”

Nature of hydration shells of a polyoxy‐anion with a large cationic centre: The case of iodate ion in water

“…Hydration of ions in aqueous media has been a major area of scientific studies [1][2][3][4] in chemical, biological, and many other branches of science. The structure and dynamics of water around ions have been investigated extensively by means of experimental, [5][6][7][8][9][10][11] theoretical, [12][13][14][15][16][17][18][19][20] and also combination of both the methods. [21][22][23][24][25] The polyoxy-anions form a major class of ions which have great relevance to various atmospheric and biological systems.…”