Cryogenic HC1-ice samples, chosen to maximize the possibility that the primary H20-HCl interactions will include molecular complexation of HC1 with H20, have been studied by infrared spectroscopy. A thorough review/extension of the spectroscopy of HCl (HBr) amorphous and crystalline hydrate films has revealed the need for a significant reassignment of the published crystalline hydrate infrared spectra. From this reassignment, and new data for the amorphous hydrates, the band position for the stretching mode of HCl (-2550 cm-l), DCl ( -1820 cm-I), and HBr (-2220 cm-l) complexed with HzO within the 1 : 1 amorphous hydrate mixture has been established. This band, together with the spectra of the ionic components of the amorphous hydrate mixtures, has then been used as a probe of the interaction of HC1 with the extensive ice surfaces present in samples of gas-phase ice nanocrystals (85 K) and microporous amorphous ice samples prepared at 15 K. This molecular complex band is observed as the dominant spectral feature that emerges as samples of microporous ice, coated with a thin film of HCl, are warmed through the 15-60 K range. However, the major infrared bands that develop upon warming the HCl/amorphous ice system above 60 K, or as ice nanocrystals are exposed to HC1 at 85 K, are those of the ionic amorphous hydrate mixtures. The results indicate that the limited molecular mobility and activation energy available at temperatures below -50 K result in the kinetic stabilization of the molecular complex of HCl H-bonded to the ice surface oxygen sites, while at temperatures above 60 K, HCl, in the presence of ice, ionizes as it forms amorphous hydrate surface layers, ultimately of a 1:l composition. This study reveals a qualitatively different ionization behavior of the hydrogen halides within the amorphous hydrate mixture than has been observed for the nitric and perchloric oxyacids (for which ionization is quite limited for the 1:l composition even into the stable liquid phase): a difference that presumably reflects the very strong hydrogen bonding of H3O+ to multiple neighbor chloride and bromide ions. The identification of the stretching-mode bands of the molecular H(D)X--HzO complex as a useful probe of the extent of ionization within noncrystalline hydrogen halidewater systems is an important byproduct of this study, a study that establishes the strong tendency of ice to form an amorphous ionic hydrate mixture when exposed to HX at temperatures above -60 K.

The use of a new method of preparing micron-thick deposits of nanocrystals of ice for Fourier transform infrared sampling, with the nanocrystals supported on a vertical infrared window, has greatly improved the signal-to-noise levels of the spectra available for large ice clusters. High quality spectra of modes of the surface molecules are reported, even for regions that underlie the intense bands of the bulk ice modes. These experimental features are most clearly displayed through the use of difference spectra. For example, the difference between spectra obtained for nanocrystals, before and after an annealing cycle that significantly increases the average cluster size, reflects the decrease in number of surface groups and the corresponding increase in number of interior molecules. Similarly, differences between spectra of bare and adsorbate-covered nanocrystals, obtained at the same temperature for the same ice sample, show the significant shifts of ‘‘surface-localized’’ ice modes caused by the adsorbate molecules. These difference spectra, and similar spectra for amorphous ice, are rich with information about the (three) distinct types of ice surface water molecules and their interactions with small adsorbate molecules. The extraction of that information has been initiated by comparison of the experimental difference spectra from two sizes of D2O cubic ice nanocrystals with simulated difference spectra for a relaxed cubic ice surface compared to bulk cubic ice. From these comparisons specific experimental features have been assigned to modes of the three categories of surface D2O(HDO) molecules: (a) three-coordinated molecules with dangling-D—2725 (2713) cm−1; (b) three-coordinated D2O molecules with dangling-O—2645 (∼2600) cm−1; (c) relaxed four-coordinated molecules—∼2580 (∼2550) cm−1. Also, information has been obtained on the approximate positions (cm−1) of other modes of surface molecules: (a) D-bonded part of dangling-D(H) molecules; ∼2350; (b) dangling-O molecules; ∼2500; (c) four-coordinated molecules; 2300–2500. The computations also indicate that, of the various modes of the surface molecules, only the higher frequency modes of the dangling-D and dangling-O are strongly localized; and only the dangling-D mode is localized on individual surface molecules.

Infrared absorption bands that have previously been assigned to vibrations of the dangling bonds (db) of water molecules at the surfaces of crystalline ice clusters and the micropores of amorphous ice have been investigated to determine their usefulness in probing molecular-level events at icy surfaces. This study has established that the db bands are sensitive to adsorption of gases at the cluster or micropore surfaces, and that reversible adsorption/desorption at the db sites is readily monitored spectroscopically. Consequently, energetics for the interactions with adsorbents such as H2 and N2 are potentially measurable. It has also been demonstrated, for both clusters and the micropores, that surface HOD molecules give unique db band positions, and that the intensities of the db bands are indicative of a strong preference of the surface HOD molecules to engage in deuterium bonding to the subsurface molecules. The unique positions of the HOD db bands also signals a potential for using isotopic-exchange data to monitor point-defect activity at icy surfaces.

Probing icy surfaces with the danglingOHmode absorption: Large ice clusters and microporous amorphous ice J. Chem. Phys. 95, 1378 (1991); 10.1063/1.461119 Spectra of dangling OH bonds in amorphous ice: Assignment to 2 and 3coordinated surface molecules J. Chem. Phys. 94, 4091 (1991); 10.1063/1.460638The OH stretching region infrared spectra of low density amorphous solid water and polycrystalline ice Ih

Ab initio calculations are carried out on the H2O...N2, H2O...H2, and H2O...CO complexes. Infrared spectra of the complexes are investigated, with an emphasis on the effect of weak bonding on the frequencies and the infrared intensities of the monomers. Connections are explored between the computational results and the experimentally measured infrared spectra of ice surfaces covered by H2, N2, and CO adsorbate. Additional issues addressed include the influence of the counterpoise correction on the equilibrium geometry of the complexes, and the analysis of the different contributions (exchange, dispersion, electrostatic) to the weak bonding, and to the frequency shifts.