J. Phys. Chem. 1992,96,6185-6188 is a clear underestimate, the F, for GeS is less than for GeS2. Schnockel and Koppe pointed out that the difference between triple and double bonds was less for silicon than for carbon compounds and suggested that this difference would decrease with Ge and Sn compounds.10 The present work supports this trend. The G e 4 and Ge-S stretching force constants are larger for the triatomic species (classical double bonds) than for the diatomic molecules (classical triple bonds). This all indica- that u bonding is weak with heavier elements and that terminology developed for carbon is not generally applicable to the heavier membcrs of the group 14 family. COddOM
The new molecules SGeS, SGeO, and OGeO were produced from the reaction of elemental germanium, sulfur, and oxygen in an argon discharge and trapped in solid argon for infrared absorption studies. Germanium, sulfur, and oxygen isotopic shifts substantiated assignment of the 649-662- and 1048-1061-cm-l bands to the antisymmetric stretching vibrations of SGeS and OGeO, respectively, and predicted linear structure for these molecules. Isotopic shifts also identified the mixed sulfideoxide SGeO in the 981-990-cm-' region. Force constants for the triatomic oxides and sulfides are larger than for the diatomic molecules in the case of germanium, which is a reversal of the case with carbon. This further underscores the trend of diminishing difference between molecules where double and triple bonds are expected based on the precedent set by carbon and shows that u bonding is less important in the heavier group 14 diatomics.
Acknowledgment. We gratefully acknowledge financial support from NSF Grant CHE 88-20764. Regbtry No. GeS, 12025-32-0; GeS2, 12025-34-2; SGeO, 14184804-6; Ge02, 1310-53-8; Ar, 7440-37-1.
6185
References rad Notes Schnockel, H. Angew. Chem., Inr. Ed. Engl. 1978, 17, 616. (2) Schnockel, H. 2.Anorg. Allg. Chem. 1980,460, 37. (3) Anderson, J. S.; Ogden, J. S . J. Chem. Phys. 1969,51, 4189. (4) Hastie, J. W.; Hauge, R. H.; Margrave, J. L. Inorg. Chim. Acra 1969, 3, 601. ( 5 ) Schnockel, H. Angew. Chem. 1978,90, 638. (6) McCluskey, M.; Andrews, A. J. Phys. Chem. 1991, 95, 3545. (7) Boz, A.; Ogden, J. S.;Orgee, L. J . Phys. Chem. 1974, 78, 1763. (8) Ogden, J. S.;Ricks, M. J. J . Chem. Phys. 1970, 52, 352. (9) Withnall, R.; Andrews, L. J. Phys. Chem. 1990, 94, 2351. (IO) Schnockel, H.; Koppe, R. J . Am. Chem. Soc. 1989, 111,4583. ( I 1) Shapiro, C. V.; Gibbs, G. C.; Laubengayer, A. V. Phys. Reu. 1932, 40, 354. (12) Barrow, R. F. Proc. Phys. Soc., London 1941,53, 116. (13) Drummond. G.; Barrow, R. F. Proc. Phys. Soc., London 1937,49, 543. (14) Meyer, B.; Jones, Y.;Smith, J. J. J. Mol. Spectrosc. 1971,37, 100. (15) Meyer, B.; Smith, J. J.; Spitzer, K. J . Chem. Phys. 1970,53, 3616. (16) Hoeft, J.; Lovas, F. J.; Tiemann, E.; Tischer, R.; Torring, T. 2. Naturforsch. 1969, 240, 1217. (17) Stieda, W. U.; Tiemann, E.; Torring, T.; Hoeft, J. 2.Narurforsch. 1976,310, 374. (18) Uehara, H.; Horiai, K.; Susoka, K.; Nakagawa, K. Chem. Phys. L a . 1989, 160, 149. (19) Marino, Ch. P.; Guerin, J. P.; Nixon, E. R. J . Mol. Specfrosc. 1974, 51, 160. (20) Koppe, R.; Schnockel, H. J. Mol. Srrucr. 1990, 238,429. (21) Mielke, Z.; Brabon, G. D.; Andrews, L. J. Phys. Chem. 1991, 95, 75. The 1:2:1 isotopic triplet at 917.9, 913.9, and 909.7 cm-I with 50% sulfur-34 confirms the identification of SiS2 (22) Brabon, G. D.; Mielke, Z.; Andrews, L. J . Phys. Chem. 1991, 95, 79. (23) Smith, G. R.; Guillory, W. A. J . Chem. Phys. 1972,56, 1423. (24) Andrews, A,; Spiker, R. C., Jr. J . Phys. Chem. 1972, 76, 3208. (25) Allavena, M.; Rysnik, R.; White, D.; Calder, V.; Mann, D. E. J. Chem. Phys. 1969, 50, 3399. (26) Henberg, G. Infrared and Raman Spectra of Polyaromic Molecules; Van Nostrand: Princeton, NJ, 1945. (27) Andrews, L.; McClusky, M. J. Mol. Spectrosc., in press. (1)
Molecular Interactions with Icy Surfaces: Infrared Spectra of CO Absorbed in Microporous Amorphous Ice J. Paul Devlin Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078 (Received: January 8, 1992)
Spectroscopic effects produced by small mijlecules absorbed within the micropores of amorphous ice have been examined for ethylene oxide, CF,, and carbon monoxide in an effort to characterize the interaction of CO with the ice surface. The absorption of CO at temperatures above 28 K produces a large shift (44 cm-') in the position of the infrared bands of the dangling OH bonds at the pore surfaces and results in two distinct stretching-modebands for the carbon monoxide, with one of the bands having an unusually high-frequency position (2152 cm-I). The influence of the absorbed small ether, ethylene oxide, was predictable as the dangling-bond bands of the ice "disappear", presumably because of an H-bond interaction with the absorbed ether. Because of this blocking of the dangling groups, the absorption of carbon monoxide into amorphous ice containing preabsorbed ethylene oxide (17%) occurs without interaction with the dangling bonds. Since the band at 2152 cm-' is very weak in this case, it is assigned to CO bonded to the dangling OH groups. This assignment is supported by the observation that coabsorption with the hydrophobic gas CF4,which has only a minor influence on the spectrum of the dangling OH groups and which is expected to cause preferential association of coabsorbed CO with the dangling OH groups, enhances the relative intensity of the 2152-cm-' band.
introductioa
Several aspects of the surface of ice have been revealed in recent theoretical and experimental studies of ice clusters and microporous amorphous ice.'-' In particular, through a combination of simulation' and spectroscopic results? it has been established that dangling OH group are abundant at the surfaces of both crystalline and amorphous ice. Further, these nonbonded groups are present throughout films of vapor deposited amorphous ice, presumably on the walls of the pores of what has been charac-
terized, through gas adsorption studies, as a permeable microporous substance." Two narrow infrared absorption bands in the non-H-bonded region of the ice spectra have been assigned to dangling O H groups of surface H 2 0 molecules, either 2- or 3-coordinated with other water molecules.' Exposure of the microporous ice to small molecules results in their ready uptake, provided the molecules are mobile at the temperature of the ice substrate.' The relative strength of the interactions of the small molecules with the dangling OH bonds,
0022-3654/92/2096-6185$03.00/0Q 1992 American Chemical Society
6186 The Journal of Physical Chemistry, Vol. 96, No. 15, 1992
and the extent to which the pore surfaces are covered by the absorbent, are apparent from the emergence of distinct new infrared bands that are characteristic of the surface groups associated with particular absorbent molecules. Further, the stretching modes of homonuclear diatomic molecules, such as H2 and N2, are activated through interaction with the dangling groups and provide a second indication of the nature and strength of the surface interactions.8 Most recently, measurements of the induced infrared spectra and estimates of moleculesurface interaction strengths, using simulated surface structures and theoretical modeling of the vibrational spectra, have expanded the understanding of the interaction of H2 with microporous ice in the 12-40 K range.g One outcome of this recent study has been clear evidence that the absorbed H2 molecules are present in at least three subsets: (1) electrostatically associated with a dangling OH group, (2) predominantly electrostatically bound to a single surface oxygen atom, and (3) bound, through van der Waals interactions, to several surface water molecules. This raises the possiblity that, in general, small molecules, when absorbed in microporous ice, will divide into recognizable subsets. In this paper we examine a case where this is clearly so. Spectroscopic measurements by Sandford et al. of codeposits of CO and H 2 0 at 10 K showed the presence of CO molecules in two different types of sites.1° However, because the samples were prepared by deposition, it was presumed that at least one type of site was interstitial in nature. However, the infrared spectra for absorbed CO, as determined in the present study, can only be interpreted in terms of at least two subsets of molecules associated with surface states. A fundamental distinction between those subsets will be revealed using data for samples with coadsorbed molecules that either preferentially block the dangling OH groups (ethylene oxide) or avoid the surface OH groups and associate strongly with the surface oxygens (CF,). To interpret these data, attention must first be focused on spectroscopic results for separately absorbed CO, ethylene oxide, and CF4 within the microporous ice.
Experimental Section Microporous amorphous ice samples were prepared by deposition of the vapors of Cambridge Isotope Laboratories “100%” deuterium oxide that had been subjected to several freezepumpthaw cycles to remove dissolved gases. Deposition times of 15 min were used to prepare ice films having a thickness of -3 pm. D 2 0 was used in preference to H 2 0 to take advantage of the exceptionally clear infrared window at the frequency of the dangling OD groups. The CsI substrate temperature was adjusted within the 12-120 K range using the cooling power of an Air Products’ CS-202 closed-cycle helium refrigerator and a resistance heater. Sample temperatures were monitored using a Lakeshore diode sensor mounted in a brass block in contact with the CsI window. Samples containing absorbed CO, CF,, and CO/CF, were prepared by depositing the gas as a thin layer on an existing film of amorphous ice followed by warming to a temperature at which the gas molecules become mobile and penetrate the porous ice. Such penetration was signaled by a marked shift of the dangling OH infrared bands to values characteristic of interaction with the particular gas. Saturation of the ice with CO occurs at a ratio of water to absorbed CO of -15:l. Results and Discussion A. Absorbed individual Gases. The influence of several different absorbed gases (H2, Ar, N2, and CH4) on the frequency of the surface OH (OD) groups of microporous amorphous ice have been reported previ~usly.~ However, since no results have been reported for absorbed ethylene oxide, CF,, or CO, spectroscopic results for each of these gases, separately absorbed in microporous amorphous ice, will be analyzed before considering the coabsorbed gas experiments. Amorphous ice samples with absorbed molecules are prepared by exposure of a film of pure amorphous ice to the absorbent gas at temperatures sufficiently high that the mobile gas molecules penetrate and become absorbed on the walls of the ice micropores.
Devlin
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Figure 1. (A) Comparison of the dangling-bond band intensity for (a) pure D20amorphous ice with bands of amorphous ice which contains approximately (b) 10%and (c) 15% ethylene oxide. (B) Variation of the dangling-bond band position and intensity with tempeature for amorphous ice with a CF4 overlayer at (a) 50 K, (b) 60 K, (c) 75 K, and (d) 120 K.
For hydrogen gas the appropriate temperature is the minimum sampling temperature of 12 K,8 but, for other gases, significantly higher temperatures are required. For example, absorption of N2 is very slow below -18 K, while CO does not diffuse into the micropores below -26 K and much higher temperatures, near 55 K, are required for absorption of CF,. Even higher temperatures must be used for absorption of ethylene oxide and, since the pore structure collapses extensively above -70 K,2 it is not possible to absorb ethylene oxide on microporous ice that has an abundance of dangling OD groups. On the other hand, samples prepared by the codeposition of appropriate amounts of small gas molecules with water vapor have infrared spectra that are usually indistinguishable from those prepared by absorption into a preformed ice film. The structure of a codeposition seems to be dictated by the condensing water molecules, with the small gas molecules assuming typical absorbed positions within the microporous ice structure that evolves. With this in mind, samples with ethylene oxide absorbed in microporous ice have been prepared by cocondensation with water vapor at 12 K, while CF4and CO samples have been obtained by absorption from an overlayer into microporous ice at 28 and 58 K, respectively. 1 . Absorbed Ethylene Oxide. Since ethylene oxide is a small ether molecule with a capability of forming weak but definite hydrogen bonds to the dangling OD groups, it was anticipated that the ether molecules would associate with the dangling groups and shift the associated vibrational-mode frequencies into the hydrogen-bonded region of the spectrum, Le., to positions hidden by the intense ice bands. That this occurs is apparent from Figure 1A in which the intensity of the db bands of pure amorphous ice, with no absorbed gas, is seen to be approximately 5 times greater than for a sample prepared by deposition with 10% ethylene oxide. The significanceof this result for the present study is that, through bonding with the ethylene oxide molecules, the dangling groups are blocked from association with other (coabsorbed) gases. 2. Absorbed CF,. Spectra obtained for 12 K amorphous ice samples with a postdeposited thin film of CF4 are interpretable in terms of a film of pure microporous amorphous ice coated with a film of pure glassy CF,. This sample configuration apparently remains unchanged during warming to - 5 5 K at which temperature the CF, molecules become mobile as evidenced by (1) crystallization of the CF4 film and (2) shifting of the ice surface bands as the CF4 diffuses into the micropores. The second effect is shown clearly in Figure 1B. The band of curve a, which was obtained at 50 K, is essentially unshifted from the three-coordinate dangling-OD-group band position of pure D 2 0 amorphous ice (2728 cm-I) but a pronounced shift of this band to lower frequency
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The Journal of Physical Chemistry, Vol. 96, No. IS, 1992 6187
Molecular Interactions with Icy Surfaces
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Figure 2. Effect of an overlayer of carbon monoxide on microporous amorphous ice on the D20dangling-bond infrared bands: spectra were measured after approximately 15 min at (a) 25 K, (b) 28 K, (c) 30 K, and (d) 35 K.
is apparent for the sample at 60 K (curve b). At 75 K (curve c) the band has stabilized at a value of 2720 cm-' which represents an 8 - a ~shift ' from the pure ice value. Since the rwo-coordinate dangling OD groups disappear upon annealing at temperatures above -50 K,' the 2748-cm-' band, which is apparent in Figure lA, is missing from this series of spectra. Although the influence of absorbed CF4on the dangling bonds of the ice is apparent from the 8-cm-' shift of Figure lB, this shift is surprisingly small. It is less than the 10-cm-' shift produced by interaction with absorbed H l and only 20% as large as the shift caused by absorbed CO (vide infra). However, CF4 is held much more strongly by the surface than is H2, as is apparent from the failure of the surface to release the CF4 even at 120 K. Much of the ice porosity is lost during annealing, as has been demonstrated previ0usly,2~~ but the band position of the dangling groups that remain (curve d of Figure 1B) signals the presence of CF4 on the surface. The failure of absorbed CF4 to produce the large surfaceOD band shift observed for other small molecules is taken as tentative evidence that CF4 is adsorbed at positions within the micropores other than the dangling OD groups with the observed band shift caused by indirect interactions with the dangling OD groups. This view is consistent with the known hydrophobic nature of CF4. 3. Absorbed CO. Because of its importance to astrophysical and, perhaps, atmosphericchemistry, the interaction of CO with the surface of ice is of particular interest and is a central concern of this paper. The behavior of a thin film of CO deposited on a film of amorphous ice can be deduced from Figure 2 which shows the response of the ice dangling OD bands to the presence of CO as a function of sample temperature. At temperatures below 25 K the bands have the position and appearance typical of a deposit of pure microporous amorphous ice (curve a). However, at 28 K (curve b) a new band emerges near 2684 cm-' and becomes totally dominant for samples held a few minutes at 30 K (curve c). This result shows quite clearly that CO diffusion into the amorphous ice requires a temperature nearly 10 K above the minimum value for the similar diffusion by nitrogen molecules. Further, the large shift of the dangling OD bands, by -44 cm-l, which is twice that produced by N2,is indicative of a surprisingly strong interaction of CO with the dangling OD groups, although the shift is remarkedly similar in magnitude to that induced in the OH stretching mode of methanol through complexation with CO in an argon matrix." It has been noted that the results obtained for codeposits of small molecules with water vapor are very similar to spectra obtained
2140
2120 2160 Wavenumbers
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Figure 3. (A) Comparison of (a) the stretching-mode infrared absorption for CO absorbed in pure D20amorphous ice at 42 K with that for CO absorbed in ice with (b) coabsorbed CF4 and 58 K and with (c) preabsorbed ethylene oxide (- 17%) at 58 K. (B) Temperature dependence of the stretching-mode bands of CO coabsorbed with CF, at (a) 50 K for 40 min, (b) 56 K for 20 min, (c) 56 K for 100 min, and (d) 58 K for 1000 min. Coabsorption of CO and CF4 into ice was by diffusion from a thin overlayer of an equal mixture of the two gases, and the absorbance scale of the figure refers to that exceptionally dilute case.
for molecules absorbed into amorphous ice. In particular, codeposits of CO and water vapor at 30 K produced samples with dangling OD bands indistinguishable from the band of curve d of Figure 2. Further, the infrared absorption by the stretching mode of the absorbed CO, as presented in curve a of Figure 3A, is very similar to that previously reported for a codeposit of a 20: 1 mixture of water and CO.'O The top curve of Figure 3A, obtained for the sample of Figure 2 after warming to 42 K, shows a doublet with the intensity near 2152 cm-'approximately half that at 2136 cm-I. The two bands of similar appearance, observed in the 20: 1 codeposit, have been discussed in some detail by Sandford et al. However, that discussion lacked benefit of the knowledge that both bands are produced by CO molecules absorbed in the ice micropores. Further insight to the nature of these absorption bands can be gained by considering the response to the presence of coabsorbed CF4 and ethylene oxide. B. Coabsoibed Gases. The data presented for CO establish that (a) two CO stretching-mode bands, at 2152 and 2136 cm-', are caused by CO molecules absorbed in the microporous ice and (b) a subset of absorbed CO molecules interact strongly with the dangling OD groups, but it is not obvious which of the two CO bands is caused by the CO "complexed" with the OD groups. In terms of the known vibrational frequencies of the CO molecule in different environments, including the gad2and crystalline phases as well as "isolated" in a variety of matrices'O and encaged in a structure I clathrate hydrate,13the position of the 2136-cm-' band can be viewed as normal while the frequency of the 21 52-cm-l band is exceptionally high. However, a similar band position (2155 cm-I) has been reported for CO adsorbed on the surface of crystalline NaCl, for which the bonding is primarily to single surface Na+ ions,14 and the matrix complexation of CO with several proton donors, including H20, has clearly demonstrated that a shift of the CO mode into the 2150-2170-cm-' range occurs when CO functions as a proton acceptor.15J6 Such data strongly suggest that the 2152-cm-' band of CO adsorbed in amorphous ice results from an interaction with the electron deficient part of the surface, i.e., the hydrogen of the dangling OH group. This expectation can be confirmed by examining the spectra for CO coadsorbed with ethylene oxide and CF,. The spectra of Figure 1A have demonstrated that absorbed ethylene oxide molecules block the dangling OD groups of microporous ice. Consequently, CO adsorbed into amorphous ice prepared from a 5:l mixture of water and ethylene oxide should be blocked from the dangling OD groups and forced to interact
6188 The Journal of Physical Chemistry, Vol. 96, No. 15, 1992
with other sites on the micropore surfaces. The result of such an experiment, presented as the spectrum of curve c of Figure 3A, shows that the band near 2152 cm-’ is nearly eliminated. This further indicates that the 2152-cm-I band is caused by CO molecules which interact directly with the dangling OD groups. This assignment is confirmed by the influence of coadsorbed CF, on the relative intensities of the two CO bands. It was argued earlier that the evidence that CF4 interacts strongly with the ice micropore walls, while perturbing the surface-OD-group vibrational modes only slightly, suggests that the CF, preferentially associates with other sites on the pore surfaces. By contrast, carbon monoxide has been shown to bond with sufficient strength to cause a 44-cm-’ shift of the bands of the dangling OD groups (Figure 2). It follows that the coabsorption of CF, and CO should cause a partitioning of the absorbents that reflects the tendency of the CF, to avoid the dangling OD groups. When an amorphous ice sample was coated with a thin film of 80% CF, and 20% CO and warmed to 58 K, the small molecules were coadsorbed into the ice sample and the absorbed CO produced the spectrum of curve b of Figure 3A. This spectrum shows a marked enhancement of the relative intensity of the 2152-cm-’ band (as well as a further that reflects a selective association of the blue shift to 2155 an-’) CO molecules with the dangling OD groups as the absorbed CF, apparently blocks CO access to other parts of the micropore surfaces. A clearer view of the role of the CF4 can be gauged from the series of spectra for the CO stretching-moderegion presented in Figure 3B. The spectrum of curve a was obtained for the ice sample, coated with the thin film of CO and CF,, after warming to 50 K. At this temperature, the CO molecules can diffuse into the micropores while the CF, molecules are relatively immobile. As a result, the ratio of the two bands in the CO region is very similar to that for pure CO absorbed in amorphous ice (curve a of Figure 3A). However, upon warming to 56 K,the CF, becomes mobile (as described previously), diffuses into the microporous ice and competes successfully with the CO for the surface sites, with the exception of the dangling OH bonds. Consequently, the CO band at 2155 cm-l grows disproportionately as the CO becomes selectively associated with the dangling OH groups.
Conclusions Carbon monoxide absorbs in microporous amorphous ice at temperatures above -28 K, occupying at least two distinct types of micropore surface sites. Among the several small “non-hydrogen bonding” molecules that have been investigated, the strength of the adsorption of CO at the dangling OD bonds is
Devlin exceptional as indicated by a downshift in the dangling-bond band position of 44 cm-I. The response of the relative intensities of two CO stretching-mode bands, to CO competition for surface sites with coabsorbed ethylene oxide and CF,, supports the view that the higher frequency CO band (2152 cm-l) is produced by the carbon monoxide molecules interacting directly with the dangling OD bonds of the pore surfaces. The second CO band (2136 cm-l) is then tentatively identified with CO molecules interacting with the oxygen atoms of the pore-surface water molecules. The relative intensities of the two CO bands, for the case of CO absorbed in pure amorphous ice at 42 K, suggest that the number of absorption sites in the two classes (dangling OD and oxygen) exist in an approximate ratio of 1 to 2. Such a ratio is consistent with results that simulate the absorption of H2 on amorphous ice cluster^.^
Acknowledgment. The funding of this research by the National Science Foundation under grant CHEM-9023277 is gratefully acknowledged. Frequent discussions with Dr. Victoria Buch and Dr. Marek Wojcik about the interactions of small molecules with icy surfaces are also acknowledged. Registry No. H20, 7732-18-5; CF4, 75-73-0; CO, 630-08-0; ethylene oxide, 75-21-8.
References and Notes ( I ) Zhang, Q.; Buch, V. J. Chem. Phys. 1990,92, 1512, 5004. (2) Rowland, B.; Devlin, J. P. J. Chem. Phys. 1991, 94, 812. (3) Rowland, B.; Fisher, M.; Devlin, J. P. J . Chem. Phys. 1991,95, 1378. (4) Mayer, E.; Pletzer, K. B. Nature 1986, 319, 298. (5) Schmitt, B.; Ocampo, J.; Klinger, J. J. Phys. (Fr.) 1987, Collque C1, SUPPI.to NO. 3, 48, ‘21-519. (6) Laufer, D.; Kaochavi, E.; Bar-Nun, A. Phys. Reo. 1987, 836, 9219. (7) Buch, V.; Devlin, J. P. J. Chem. Phys. 1991, 94, 4091. (8) Rowland, B.; Devlin, J. P. Proc. Inr. Symp. Chem. Phys. Ice, Sapporo, Jpn. Sepr. 1-6, 1991; Maeno, N., Hondoh, T., Eds.; Hokkaido University Press: Sapporo, 1992; p 183. (9) Hixson, H.; Devlin, S.M.; Wojcik, M.; Devlin, J. P.; Buch, V. J. Chem. Phys., in press. (IO) Sandford, S.A.; Allamandola, L. J.; Tielens, A. G. G. M.; Valero, G. J. Astronom. J. 1988, 329, 498. (1 1) Murto, J.; Ovaska, M. Spectrochim. Acta 1983, 39A, 149. (12) Herzberg, G. Spectra of Diatomic Molecules; Van Nostrand: Princeton, NJ, 1950. (13) Devlin, J. P. Spectra and Structure of Clathrate Hydrates Preuared by Vacuum Cryogenic Techniques. Paper No. 9, Internatidnal Workshop on the Structures of Ice and the Clathrate Hydrates, Otaru, Japan, Sept 8-10, 1991. (14) Ewing, G. E. I n t . Reu. Phys. Chem. 1991, 10, 391. (15) Andrews, L.; Arlinghaus, R. T.; Johnson, G. L. J . Chem. Phys. 1983, 78, 6347. (16) Andrews, L.; Davis, S. R. J. Chem. Phys. 1985,83, 4983.
