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)(+).

Reaction rates for the conversion of ice nanocrystals within 3-D arrays, to the hemi- and monohydrates (and deuterates) of ammonia, have been determined for temperatures in the range 100 to 128 K. The loss of ice and the growth of the hydrate product, as a function of time, temperature, and the activity of ammonia at the surface of the particles, has been monitored using transmission FT-IR spectroscopy. Though this study has focused on the ammonia−ice system, the results may provide general insights to the low-temperature formation of hydrates from ice particles. The ammonia hydrate formation follows a nucleation stage that occurs only after saturation of the ice surface with ammonia molecules; the propagation of the reaction depends on ammonia diffusion, not within the ice but through a hydrate crust that quickly envelops the particles. Apparently, adsorbed ammonia molecules do not achieve a free energy consistent with the nucleation of a new (hydrate) phase until saturation of the low free energy ice surface sites is complete. After nucleation occurs, diffusion of ammonia through the hydrate crust may be rate controlling, the determining parameters being the chemical activity of the ammonia adsorbed on the particle (hydrate) surface and the thickness of the hydrate crust. A diffusion coefficient for ammonia in the amorphous “hemihydrate” has been determined as 2.8 × 10-19 cm2/s at 102 K with E a = 15 kcal/mol, while the coefficient found for the crystalline hemihydrate was 1.1 × 10-17 at 107 K with E a estimated as 12 kcal/mol.

The Fourier transform infrared (FT-IR) spectra of the bending modes of the three categories of molecules at the surface of ice nanocrystals have been determined for both H2O and D2O samples and the response of the bending mode of the dangling hydrogen surface molecules to association with adsorbates of varying acceptor strengths has been examined. From these combined data for water molecules in a wide range of environments at the ice surface, a clear picture of the dependence of the bending mode frequency on the extent of H-bonding is revealed for ice-related conditions. These frequencies have been examined in terms of the linear equation developed by Falk that relates the water decoupled bend mode frequency to H-bond strength through the average of the symmetric and asymmetric stretch mode frequencies. The published value of 1735 cm−1, for the decoupled bend mode frequency of bulk H2O ice, is consistent with this presentation of the new data, but a new value, higher by at least 40 cm−1 than the accepted value of ∼1220 cm−1, is indicated for D2O ice. A value of 1265 cm−1 is offered as the best estimate of the bend mode frequency of D2O ice.

The relationship between the degree of ionization and the environment of a strong acid is of basic scientific interest. Often this relationship reduces to the interdependence of iodacid hydration and proton transfer. Despite the presence of pure water, the surface of crystalline ice, particularly at cryogenic temperatures, is one of limited (controlled?) availability of water of hydration. Here, the detailed nature of the ice surface and the states of strong acids adsorbed to ice at cryogenic temperatures are examined. These subjects are of special current interest since the ability to model the complex chemistry that occurs on the surfaces of water-rich particles in the atmosphere, particularly in the stratosphere over the polar regions, requires a valid concept of the acid-ice interface. Our combined spectroscopic and simulation studies have identified the surface of free-standing ice particles as badly disordered, with a range of water-ring sizes and an increased level of H-bond saturation relative to an ordered ice surface. FT-IR results are reported for the interaction of the surface of such ice particles with submonolayer amounts of adsorbed DCI, DBr, and HNO, and for multilayer exposure to DCI. The DCI and DBr adsorbed states demonstrate behavior familiar from observations on strongly bound molecular adsorbates. Two methods have been devised for exposure of the nanocrystals to HNO,. One gives an ionic state initially, while the initial state of the other approach is molecular. In both instances, the system is observed to evolve, with time/warming, towards a common mixed molecular-ionic adsorbed state. ~ ~

FT-IR spectroscopy of ice nanocrystals, within micrometer thick assemblies, allows observation of the complete stretch-mode vibrational spectrum of the ice surface bilayer with a high level of signal-to-noise. For these high surface area samples, the influence of submonolayer quantities of DCl on the D2O ice surface is clearly observable. The effects for DCl are analogous to those for strongly H-bonding covalent adsorbates in the following respects:  (a) the extent of shifting of the ice surface-mode vibrational frequencies, (b) the induction of order at the ice surface followed by relaxation of the subsurface to oxygen-ordered interior ice, (c) the partial reversibility of the DCl adsorption upon exposure to a second strong adsorbate, and (d) the observation of the internal vibrational mode of the adsorbed DCl. These results identify covalently adsorbed DCl as the thermodynamically stable form for submonolayer exposures at 125 K.