INFRARED-SPECTRA OF HCL COMPLEXED IONIZED IN AMORPHOUS HYDRATES AND AT ICE SURFACES IN THE 15-90-K RANGE

Cryogenic HCl-ice samples, chosen to maximize the possibility that the primary H2O-HCl interactions will include molecular complexation of HCl with H2O, 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 HCI (approximately 2550 cm-1), DCI (approximately 1820 cm-1), and HBr (approximately 2220 CM-1) complexed with H2O 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 HCI 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 HCI, are warmed through the 15-60 K range. However, the major infrared bands that develop upon warming the HCI/amorphous ice system above 60 K, or as ice nanocrystals are exposed to HCI at 8 5 K, are those of the ionic amorphous hydrate mixtures. The results indicate that the limited molecular mobility and activation energy available at temperatures below approximately 50 K result in the kinetic stabilization of the molecular complex of HCI H-bonded to the ice surface oxygen sites, while at temperatures above 60 K, HCI, in the presence of ice, ionizes as it forms amorphous hydrate surface layers, ultimately of a 1:1 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 1:1 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--H2O complex as a useful probe of the extent of ionization within noncrystalline hydrogen halide-water 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 approximately 60 K.