J. Phys. Chem. 1993, 97, 10312-10318
10312
Infrared Spectra of HCI Complexed/Ionized in Amorphous Hydrates and at Ice Surfaces in the 15-90 K Range Lance Delzeit, Brad Rowland, and J. Paul Devlin' Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078 Received: May 7, 1993; In Final Form: July 15, 1993'
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 a t 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 H X at temperatures above -60 K.
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Introduction The spectroscopy of icy substances with a history of exposure to varying levels of HCl has been a subject of recent studies devised to provide insight to surface chemistry within polar stratospheric clouds. Since the cloud temperatures are typically near 200 K,particular interest has focused on ice-HC1interactions occurring above 150 K,l though limited data have been reported for 110 K.2 It is clear from these reports that HCl reacts readily with an icy surface and that, at the higher temperatures, the HCl permeates thin films of ice to produce (ionic) amorphous mixtures of HCl hydrates (referred to as amorphous hydrates throughout this paper). A recent trajectory computation of the uptake of HCl by a crystalline ice surface? which did not provide for ionization of the acid, reached interesting conclusions about the ice-HC1 association/complexation but does not realistically represent the high-temperature regime. The possibility that at a much lower temperature the molecular complex is the stabilized form of the HC1-ice interaction is anticipated from experimental (- 12 K) matrix-isolation spectroscopic results, which show that HC1 acts as a proton donor to form a 1:l molecular H-bonded complex with HzO that, though sensitive to the matrix medium, can be recognized by intense bands near 2600 cm-1 (Le., downshifted by 150-200 cm-l from pure condensed-phase HC1stretching-modefrequencie~).~,5 This H-bondingshift has been modeled in ab initio calculations,which predict a 6-fold enhancement of the band intensity of the HC1 stretch mode.6
* Abstract published in Advance ACS Abstracts, September 15, 1993.
A great deal of related information has also been reported for the complexation and ionization of nitric acid exposed to varying amounts of H20.'-10 Like HCl, HNO3 is not ionized by contact with a single water molecule, but matrix-solvation data have been interpreted as indicating that HNO3 interaction with three or more HzO molecules results in ionization9 (though gas-phase mass-spectrometricresults suggest that four water molecules are required).ll It is also known, from spectroscopy of -60 K amorphous d e p o s i t s of nitric acid with water, for water ratios ranging from 1 to greater than 3, that ionization is promoted by multiple acid-water interaction^.^ Specifically, the extent of ionization increases in a regular manner from a few percent in a 1:l deposit to nearly 100% in a 5:l sample, with the extent of ionization quite insensitive to temperature from 60 K to 160 K. Similar results have been found12 for the amorphous hydrates of perchloric acid, with incomplete but progressively more extensive ionization noted within 60 K films having water ratios that increase from 1 to 3. The extent of ionization of these oxyacids, to form anions having distinct and easily recognizable vibrational spectra, is semiquantitativelyobvious. Similar data have not been reported for the complete amorphous hydrate series of HCl (or HBr), with only the spectra of the amorphous hydrates having water ratios of 6 and 4 available,137 though the spectra of a series of crystalline hydrates have been p~b1ished.l~ Since HCl lacks a polyatomic anion, the analysis of the data for the HCl hydrates must focus on the spectra of the (hydrated) hydronium ion and the shifted stretching-modeband of the complexed HC1, with complexation with either wate+ or a halide ion15known to produce comparable band positions. The theoretical prediction6
0022-3654/93/209710312$04.00/0 0 1993 American Chemical Society
IR Spectra of HCl in Amorphous Hydrates that, for the water complex, the latter mode should give rise to a strongly enhanced absorption near 2600 cm-I makes that band a likely useful probe of complexed molecular HCl within any HCl-HzO system. In several recent papers1620we have described infrared data obtainedfor ice surfacesand molecular adsorbateson ice surfaces. Data from two complementary experimental methods have been combined with insightsfrom dynamicalsimulations2' to construct a realistic view of the surface of ice and microporous amorphous ice16J8uMand of the nature and strengths of the interaction of several different small molecules with the ice surface groups.17Jg The experimental methods, which are basic to the present study as well, can be characterized as (1) large-cluster, or nanocrystal (specifically, 30-nm diameter for ice clusters),l* gas-phase static-cell infrared spectroscopy and (2) thin-film microporous ice infrared spectroscopy. The former technique provides information for crystalline cubic ice surfacesat temperatures above 80 K the latter allows the detailed complementary study of the vibrational spectra of the surface and small-molecule adsorbates of low-density microporous ice in the 12-120 K range. (In this paper, large ice clusters that are clearly submicroscopic in size, as judged from an absence of Mie scattering in the near infrared, are referred to as nanocrystals.) Samples prepared within either of these two methods have an unusual percentage of the molecules at surface sites, permitting the measurement of detailed vibrational spectra of the surfaces/ adsorbates with high levels of signal-to-noise. These spectra, combined with the insights provided by simulation methods,19-21 have resulted in a clearer recognition that the surfaces of the amorphous ice micropores are rich with dangling groups, either 0-H bonds or oxygen lone-pair electrons, and that the dangling groups usually belong to H20 molecules that are either 2- or 3-coordinated to other water molecules, while the crystalline ice clustersurfacesare limited to dangling groups from 3-coordinated surface H20 molecules.18 In either case, the shifting of the vibrational frequencies of the dangling 0-H groups and/or of the small molecule adsorbates is a useful probe of the smallmolecule-to-surface interactions. The present study represents an attempt to use the versatile (ice) surface spectroscopy techniques to determine: (a) is there a temperature below which the conversion of ice to the (amorphous) ionic hydrates of HCl is prohibited by energetics/mobility, and (b), at sufficiently low temperature, does the ice surface appear "dry" to an HC1 molecule so that the primary interaction is the hydrogen-bonded complex as described in the theoretical study of Kroes and Clary' and resembling the 1:l complex with to make satisfactory water in an NZ m a t r i ~ ? However, ~ arguments using the surface spectroscopy data, the published literature on the crystalline hydrates of H C P must first be supplementedby data for both amorphous and crystalline hydrates of HCl and HBr. This preliminary study is necessary in part because of a deficiency of amorphous-phase data and, in part, because of a past failure to recognize the inadvertent inclusion of the amorphous 1:l hydrate spectra in the published series" of infrared spectra of the crystalline HCl and HBr hydrates (vide infra),
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Experimental Section
The HCl (HBr) amorphous hydrate thin films were prepared by low-temperature (15 or 85 K) depositions using premixtures of HC1 (HBr) (g) with water vapor of the desired water ratio. Most of the samples were deposited at 85 K on a compressed cadmium telluride infrared-transparent substrate but in a few instances the temperature was lowered to 15 K to assure that the deposits were, in fact, amorphous. The series of crystalline HCl (HBr) hydrates was prepared by using the method described by Sheppard et. al.,l3 which dependson thevolatility of the hydrogen halides to cause the sequential formation of the crystalline hydrates
The Journal of Physical Chemistry, Vol. 97, No. 40, 1993 10313 during warming of a 1:1 deposit from 85 to 21 5 K within a closed infrared cell. During this warming phase, the evolution of HCl from the sample as a function of temperature was monitored by using a Hastings thermocouple pressure gauge. The amorphous 1:l deposit was observed to crystallize to the monohydrate at -150 K. Loss of HX(g) from the sample to the closed cell chamber at 190and 2 10K signaled the subsequent conversion to the higher hydrates. The data for HCl adsorbed on nanocrystals of cubic ice were obtained by using a liquid nitrogen cooled static infrared cluster cell that has been described previously.22 This precooled (85 K) cell has two entry ports for sample addition: One was used to load the cell with 100/1 gaseous mixtures of helium/H20 to pressures of -300 Torr, leading to the instantaneous formation of the ice nanocrystals. The second port, fitted with a Teflon tube perforated to allow distributed release of gas along the 15cm length of thecell, was used for the ensuing addition of aliquots of a 1000/ 1 mixture of helium and HCl. This second gas mixture was added in increments of N 10 Torr with the effect of each increment monitored by spectroscopic observation of the ice clusters. In a typical sequence, the cell pressure was increased beyond 300 Torr and the HCl was made available to the ice clusters with an abundance ranging from 30 to 120 ppm or from 0.5 to 2.0% of the total water content of the cluster cell. The spectra of HC1 and HBr interacting with microporous amorphous ice at temperatures in the 15-100 K range were obtained by using thin films of the low-density microporous ice. After formation of ice films of 1.5-pm thickness on a CsI substrate held at 15 K by a helium closed-cyclecooler, an overlayer of amorphous HCl was added to a thickness of -0.05 pm. The temperature dependence of the HC1-ice interaction was then determined by the measurement of the infrared spectrum for each sample, held for 10 min at each of a series of progressively higher temperatures, using increments of 10 K. The infrared spectra were recorded by using Bio-Rad FTS 40 or FTS 20 spectrometers at nominal resolutions of 2 and 4 cm-1 for the thin-film and large-cluster samples, respectively.
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Results and Discussion The analysis of the data for HCl (HBr) adsorbed on either of the high-surface-area forms of ice, i.e., the microporous ice and crystalline ice in the form of large clusters, is greatly facilitated by reference to relatively complete infrared spectroscopic data for the amorphous and crystalline hydrates of HCl as well as aspects of the spectra of related HBr hydrates. For that reason new results for the hydrates will be examined first. Spectra of HCl Amorphous Hydrate Films. Reports of the infrared spectra of low-temperature thin-film samples of the amorphous hydrates of HCl are few in number and appear to be limited to the 4:l and 6:l ratios of water to HCl.'s7 The spectra for these two hydrates, reproduced from the literature, are presented in Figure 1, along with new results for -2: 1 (curve c), 1:l (curve b), and
