The Journal of
Physical Chemistry VOLUME 97, NUMBER 11, MARCH 18, 1993
0 Copyright 1993 by the American Chemical Society
LETTERS Surface-Defect Vibrational Modes of Large Ice Clusters Brad Rowland, Mark Fisher, and J. Paul Devlin’ Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078-0447 Received: November 9, 1992; I n Final Form: December 29, 1992
A comparison of FT-IR spectra of ice clusters of -25-nm diameter with spectra of larger clusters permits a closer look a t the surface-defect modes of crystalline ice clusters. In addition to the dangling-OH group infrared absorption at 3694 cm-I, a broader band of comparable intensity is revealed a t 3563 cm-’. This band is assigned to the in-phase stretch of surface molecules having a dangling-OH group on the basis of an apparent analogy between the modes of such molecules and the donor water molecule of the water dimer. Further, a n examination of the water bending-mode region, including comparisons with (a) spectra of larger clusters and (b) spectra of clusters for which the dangling-OH groups are hydrogen bonded to a surface dopant, indicates the presence of two surface-defect-mode absorptions. A band a t 1650 cm-’ is assigned to the bending mode of the surface water molecules containing thedangling-OH groups. A broader band a t 1700 cm-’ has been tentatively identified with the surface water molecules containing the “dangling” oxygen atoms.
Experimental Section
Introduction
Recent spectroscopic results for systems having large ice surface areas have shown that infrared absorption bands of the surfacedefect modes of ice provide useful probes of events at ice surfaces.’ Reports of thesespectra haveemphasized the behavior of bands associated with the stretching vibrations of non-H-bonded or ‘dangling”-OH (or OD) groups. In the case of microporous amorphous ice, $wo such bands have been observed (3720 and 3696 cm 1) and firmly assigned to modes of dangling-OH groups of 2- and 3-coordinated surface water molecules, respectively.? Similarly, a single band at 3694 cm I , in the spectra of large cubic-ice clusters suspended in helium gas, is attributed to 3-coordinated surface molecules.3 In this paper a more extended view of the infrared spectra of crystalline ice clusters is presented with emphasis on the various surface-defect modes. Through a comparison of the spectra of clusters with 25-nm diameter size with spectra of (a) larger ice clusters, (b) thin films of cubic ice, and (c) clusters with the dangling-OH groups capped by ether molecules (ethylene oxide), it is shown that three additional molecular-vibrational surface modes are observed.
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The technique previously described of generating and suspending large ice clusters within a static cold cluster cell has been used throughout the present study.h Crystalline ice clusters 25 nm in diameter form when mixtures of an inert carrier gas containing a small amount of water vapor, with a gas/H?O ratio of -200/1, are loaded rapidly into a precooled cluster cell to pressures of bar at temperatures below 120 K. If either higher temperatures or greater load pressures are used, larger clusters are formed with a correspondingly reduced relative surface area and reduced relative surface-mode band strengths3 As an H-bonding acceptor agent, known to bind the dangling-OH icesurface groups, ethylene oxide was doped into certain sample mixtures to provide reference spectra of ice clusters with the bands of the dangling-OH groups missing. In this study, the spectra of such clusters, oversized and/or coated with ethylene oxide, have been used to subtract non-surface-defect-mode bands from the 25-nm-cluster spectra. The spectral subtractions have been made using adsorbance spectra and the subtract facility of a BIO-RAD FTS-40 spectrometer.
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Results Since i t falls at frequencies well above the “bulk” bands of the large ice clusters, the relatively sharp stretching-mode absorption 0 1993 American Chemical Society
Letters
2486 The Journal of Physical Chemistry, Vol. 97, No. 1 1 , 1993
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Figure 1. Infrared spectra of -25-nm crystalline H20 ice clusters after subtraction of reference spectra for larger ice clusters. The bottom spectrum is for clusters suspended in gaseous helium, while the top three spectra are for clusters formed at the indicated temperatures in gaseous nitrogen.
band of the dangling-OH-group is easily detected. In the original examination of the spectra of large crystalline-ice clusters, no attempt was made to reveal additional surface-defect features in the stretching mode region. However, more of the stretchingmode region of the bare ice surfacecan be revealed by subtraction ofthespectrumoflarger iceclustersfrom that of -25-nmclusters (bottom curve of Figure 1). For a helium gas medium a previously unreported feature, of comparable area but greater than twice as broad as the band of the dangling-OH group a t 3694 cm-I, is apparent near 3563 cm-I. That this band is a surface-defect absorption, rather than an artifact of the subtraction, is confirmed by results obtained using a nitrogen-gas cluster medium as a function of temperature. The top three curves of Figure 1 show, in part, what has already been e~tablished.~ Since N2 at a pressure of - l / 3 bar adsorbs on the ice clusters for sampling temperatures near 80 K, the danglingOH band shifts from 3694 to 3679 cm-’ (top ~ u r v e ) .Warming ~ the cluster-sampling temperature to 95 K, and then to 114 K (middlecurves), causes this band to shift toward the bare-surface value. The right side of Figure 1 shows that the newly recognized feature at 3563 cm-’ responds very similarly, both to adsorbed nitrogen and the temperature variation. In fact, the new band is downshifted by -30cm-I by the adsorbed N2, more than twice the displacement of the frequency of the dangling-OH mode. The spectra of Figure 1 are somewhat misleading, however, since the subtractions by which they were obtained also result in other residual features in the bulk-ice stretching-mode region extending from 3 100to 3450 cm-I, some with an intensity greater than the 3563-cm-I band. However, the direction on the absorbance scale of the other features was found to be reversible by a variation in the cluster-sampling procedures while the new surface-mode band invariably displayed a positive absorbance value. The additional features may well contain useful informationabout thestructureof the iceclusters but are not attributed to surface-mode absorption. While a subtraction of the appropriate spectra is necessary to reveal clearly the new surface-defect stretching-mode band at 3563 cm-I, a new surface feature in the bending-mode region is apparent in the raw cluster spectra. Thus, the top curve of Figure 2 for 25-nmclusters is distinguished by a relatively sharp feature at 1650 cm-I that is absent from the spectrum of a thin film of cubic ice (labeled “film”) and sharply reduced in intensity for
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Infrared spectrum in the bending mode region of -25-nm H20 ice clusters (a) compared to spectra of larger ice clusters coated with ethylene oxide ((a) + EO) and of a thin ice film. The bottom curve (b) is the difference between the top two curves. Figure 2.
larger clusters ‘coated” with ethylene oxide (labeled a + EO). The bottom curve (b), which is the difference between the two top curves, more clearly reveals the form of the 1650-cm-I band and also points to the presence of yet another surface-defectmode band near 1700 cm-I. Data (not shown) for smaller clusters with the dangling-OH groups bound to adsorbed ethylene oxide, as revealed by the absence of a band a t 3694 cm-I, have shown that the 1650-cm-I band, but not the 1700-cm-’ feature, is shifted strongly when the dangling-OH groups are bound by acceptor H-bonding adsorbate molecules. This leads to the assignment of the 1650-cm-I band to the bending mode of the 3-coordinated surface water molecules that have dangling-OH groups. The 1700-cm-’ surface-defect feature can then be tentatively assigned to the bending mode of thesurface molecules having danglingoxygen atoms.’ Sinceboth OH groups of such a molecule are hydrogen-bonded, the bendingmode frequency should be similar to that of the isotopically decoupled bending mode for bulk crystalline ice. The best experimental value for thedecoupled cubic-ice mode is 1735 cm-I, while a value of 1703 cm-l is obtained from Falk’s empirical e q ~ a t i o n . Thus, ~ - ~ a frequency near 1700cm-I is expected for the surface-defect bending mode of the dangling oxygen atoms. Further, by analogy to the small magnitude of the shift of the bending mode of the acceptor molecule that accompanies water dimerization (-4 cm-l),lo the bending mode of dangling oxygen surface defects is expected to be relatively insensitive to the presence of surface adsorbates, a supposition that has yet to be tested directly. The bending-mode data have also been extended to clusters which contain a mixture of isotopomers: -30% HzO,20% D20, and 49% HOD. The result is presented in Figure 3, which shows the cluster spectrum (A), a corresponding thin-film ice spectrum (B), and the difference (C). From curve C it is clear that the two H 2 0 bands appear with the same position and form, as observed for pure H 2 0 clusters (Figure 2). A doublet is also apparent in the HOD bending-mode region, with a weak feature at 1403 cm-1 and a surprisingly strong band near 1470 cm-1. On the basis of the bandwidths, relative positions, and response to capping of thedanglinggroupswith ethyleneoxide, the 1403-cm-I band is assigned to the surface HOD molecules with danglingOH groups, and, tentatively, the 1470-cm-’ band, to thedangling oxygen surface molecules, but the cause of the surprising intensity distribution is unknown. The corresponding bending-modes of surface D 2 0 molecules have been observed at 1212 and 1224 cm I, respectively.
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Letters
The Journal of Physical Chemistry, Vol. 97, No. 11, 1993 2487
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Figure 3. Infrared spectrum in the bending mode region of -25-nm ice clusters(A) compared with that o f a thin filmofice (B) with bothsamples containing -49% HOD and 30% H20. The bottom curve (C) is the difference between the top two curves.
Discussion It has been noted previously that the ice-surface water molecules with dangling-OH groups behave, in some respects, in a manner similar to the donor molecule of the water dimer.3 For example, as for the dimer," HOD surface molecules strongly favor an orientation with the OH group dangling while participating in a deuterium bond to a neighbor acceptor water molecule. With that in mind, it is natural to look to the vibrational frequencies of the donor molecule of the dimer for possible leads in a first tentative assignment of the observed surface features. Figure 4, which includes drawings that emphasize the similarities between the cluster surface defects and dimer donor molecules, gives a comparison between observed surface-defectmode frequencies of the three water isotopomers and the accepted assignments for the dimer donor molecule.1° The pattern of observed frequencies, which have been attributed to the surface water molecules with dangling-OH groups, is very similar to that of the dimer and suggests that this tentative assignment has some merit. In particular, the band at 3563 cm-I, which has been shown in Figure 1 to be a useful probe of ice-adsorbate interactions, can be attributed to the in-phase stretch of such surface molecules. It is interesting that the position of this new surface-defect stretching-mode band is, within experimental uncertainty, coincident with an unassigned maximum in the infrared spectrum of the hexamer of water,'* suggesting a common origin. However, until a better representation of the modes of the surface molecules is available (e.g., from simulated spectra of cubic-ice clusters), each of the assignments suggested in Figure 4 should be regarded with skepticism. It is interesting that the bands of the surface-defect modes divide into two groups on the basis of their widths, since the
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bands having the frequencies included in Figure 4 (i.e., assigned to the surface molecules with dangling-OH groups) are approximately half the width of the other features. This may indicate that proton disorder has a reduced influence on the vibrational frequencies of modes localized in the dangling-OH groups compared to modes of surface defects that involve primarily fully H-bonded OH-groups.
Acknowledgment. Support of this research through NSFGrant Chem-9023277 and ACS-PRF No. 25449-AC6 is gratefully acknowledged. The authors also appreciate the helpful comments of V. Buch of the Hebrew University.
References and Notes Rowland, B.; Devlin, J . P. J . Chem. Phys. 1991, 94, 812. Buch, V.; Devlin, J. P. J . Chem. Phys. 1991, 94, 4091. Rowland, B.; Fisher, M.; Devlin, J . P.J. Chem. Phys. 1991,95,1378. Hixson, H . G.; Wojcik, M . J.; Devlin, M. S.;Devlin, J. P.; Buch, V. J . Chem. Phys. 1992, 97. 753. (5) Callen, B. W.; Griffiths, K.; Kasza, R. V.; Jensen, M . B.; Thiel. P. A,: Norton. P. R. J . Chem. Phvs. 1992. 97. 3760. ' (6) Ewing, G. E.; Sheng, D. T. J . Phys.'Chem. 1988.92.4063. Fleyfel, F.; Devlin, J. P. J . Phys. Chem. 1989, 93, 7292. (7) The importance of such surface groups has been highlighted in dynamical simulation studies of amorphous ice clusters by Buch. See, for example: ref 4. (8) Bertie, J . E.; Devlin, J . P. J . Phys. Chem. 1984, 88, 380. (9) Falk, M . Spectrochim. Acta A 1984, 40, 43. (IO) Fredin, L.; Nelander, B.; Ribbegard, G. J . Chem. Phys. 1977. 66. 4065. ( I I ) Engdahl, A.; Nelander, B. J . Chem. Phys. 1987, 86, 1819.
(12) Page, R. H.; Vernon, M. F.; Shen, Y. R.; Lee, Y. T. Chem. Phys. Lett. 1987, 141, I .
