Discrete stages in the solvation and ionization of hydrogen chloride adsorbed on ice particles

Abstract

Ionization and dissociation reactions play a fundamental role in aqueous chemistry. A basic and well-understood example is the reaction between hydrogen chloride (HCl) and water to form chloride ions (Cl(-)) and hydrated protons (H(3)O(+) or H(5)O(2)(+)). This acid ionization process also occurs in small water clusters and on ice surfaces, and recent attention has focused on the mechanism of this reaction in confined-water media and the extent of solvation needed for it to proceed. In fact, the transformation of HCl adsorbed on ice surfaces from a predominantly molecular form to ionic species during heating from 50 to 140 K has been observed. But the molecular details of this process remain poorly understood. Here we report infrared transmission spectroscopic signatures of distinct stages in the solvation and ionization of HCl adsorbed on ice nanoparticles kept at progressively higher temperatures. By using Monte Carlo and ab initio simulations to interpret the spectra, we are able to identify slightly stretched HCl molecules, strongly stretched molecules on the verge of ionization, contact ion pairs comprising H(3)O(+) and Cl(-), and an ionic surface phase rich in Zundel ions, H(5)O(2)(+).

Large variation of vacancy formation energies in the surface of crystalline ice

“…This finding may explain the variation in and high vapour pressure of, ice samples [19]. Moreover, the vacancies will have a spectrum of trapping energies such that the reaction energy landscape of, for example, a trace gas molecule in the vicinity of one trap could be markedly different from another, similar to what has been observed for ice nanoparticles [20]. More generally, the crystalline ice surface can be expected to be more reactive than previously thought, due to the abundance of vacancy traps and the varied polarity of surface sites.…”

Acid–Base Chemistry at the Ice Surface: Reverse Correlation Between Intrinsic Basicity and Proton‐Transfer Efficiency to Ammonia and Methyl Amines

“…Since hydronium ions are generated by HCl ionization at 140 K in the present study, these ions are efficiently solvated by water molecules, which are quite mobile at this relatively high temperature. [17,20] On the other hand, amine molecules deposited on the surface after cooling it to 60 K will be surrounded by a smaller number of water molecules, for both neutral and protonated forms. Thus, the most probable situation is n = m. For simplicity, Table 1 shows thermodynamic functions calculated for n = 5 and m = 0-3.…”

Protons at Ice Surfaces

“…[15] The temperature of the Ru substrate was varied between 40 and 1500 K by a liquid He cryostat and resistive heating. À6 Torr s] at 140 K. Finally, an H 2 O overlayer was deposited to a thickness of 4-12 BLs at temperatures below 100 K. The HCl gas provides excess protons to the film by spontaneous ionization at 140 K, [16,17] and the underlying D 2 O layer serves as a spacer to remove any effects of the Ru substrate on the upper layer, since D 2 O forms a well-ordered film on Ru(0001) at 140 K without dissociation. [18] It was essential to deposit the upper H 2 O layer at low temperature; otherwise the protons remained afloat on the surface during the deposition instead of being buried within the sandwiched layer, as explained below.…”