15450
J. Phys. Chem. 1996, 100, 15450-15453
Photochemistry of Chlorine Dioxide in Polycrystalline Ice (T ) 140-185 K): Production of Chloryl Chloride, Cl-(OClO) Christopher J. Pursell,* Jay Conyers, and Carilee Denison Department of Chemistry, Trinity UniVersity, 715 Stadium DriVe, San Antonio, Texas 78212 ReceiVed: May 9, 1996; In Final Form: June 11, 1996X
The photochemistry of chlorine dioxide, OClO, in polycrystalline ice has been investigated at T ) 140-185 K using FTIR and UV-vis spectroscopy. Photolysis with ultraviolet light at 360 nm produced chloryl chloride, Cl-(OClO). The unique vibrational bands at 1210, 1040, 520 and 430 cm-1, along with the characteristic UV absorptions at 300 and 235 nm, were used for definitive assignment of the photochemical product. The proposed mechanism for the formation of Cl-(OClO) involves photolysis of OClO to Cl + O2 followed by migration of the Cl to a nonphotolyzed OClO. While clustering of the OClO may play a role in the formation of the product, experimental evidence indicates that the OClO prefers to exists as a monomer solvated by the water. These results have potential implications for stratospheric ozone loss: under the very dilute conditions that would exist on polar stratospheric ice particles, the photochemical conversion of OClO to Cl + O2 would represent a new source of Cl atoms for ozone depletion.
Introduction Our laboratory has been investigating chemical processes in and on amorphous and polycrystalline water ice with special emphasis on reactions that may lead to stratospheric ozone destruction. Previously, we had reported our initial study of the photolysis of chlorine dioxide, OClO, in amorphous ice at T ) 80 K.1 Here we report our results for the photolysis of OClO in polycrystalline ice at T ) 140-185 K. OClO photochemistry is of interest due to its possible role in stratospheric ozone loss. While the dominant gas-phase photolysis pathway, OClO + hν f ClO + O, does not lead to net ozone loss, other possible pathways have been suggested.2,3 One involves the isomerization to the chlorine peroxy radical, ClOO, followed by dissociation to O2 and Cl,
OClO + hν f ClOO ClOO f O2 + Cl
(1)
while another is the direct photodissociation to O2 and Cl.
OClO + hν f O2 + Cl
(2)
Though the ClOO isomer is thermodynamically more stable than OClO, it is kinetically very unstable and will quickly dissociate. If the ClOO is considered an intermediate, these two paths are essentially identical. The production of ozone-destroying Cl atoms by either of these two pathways would represent a potentially new mechanism for ozone loss. Additionally, it has been suggested that the presence of atmospheric aerosols, i.e., polar stratospheric clouds (PSCs), may influence the photolysis and possibly enhance Cl production.3,4 In order to address the possibility that OClO photochemistry on atmospheric aerosols could ultimately lead to ozone loss, we have been examining the photolysis of OClO in amorphous and polycrystalline water ice in the laboratory. Our first report concerned the photolysis (at 360 nm, the peak of the UV absorption) of OClO in amorphous ice at T ) 80 K. Using FTIR spectroscopy along with UV-vis spectroscopy, we * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, August 15, 1996.
S0022-3654(96)01341-X CCC: $12.00
discovered that OClO was quantitatively converted to ClOO, similar to the inert matrix results.5-8 There was no evidence for ClO formation. Though the “apparent” photoisomerization could be explained by the matrix cage effect alone, we suggested that electronic state perturbation by the ice could affect the photochemistry and enhance the formation of ClOO by reaction 1. Alternatively, perturbation causes reaction 2 to be favored followed by the cage effect. In this report, we present our results for the photolysis of OClO at λ ) 360 nm in polycrystalline ice at T ) 140-185 K. The photoproduct has been identified as chloryl chloride, Cl(OClO), a structural isomer of (ClO)2 with the Cl attached offaxis to bent OClO with Cs symmetry. This assignment has been made using the unique vibrational bands at 1210, 1040, 520, and 430 cm-1, along with the characteristic UV absorptions at 300 and 235 nm.9,10 We believe the formation of Cl-(OClO) in ice involves photolysis of OClO to Cl + O2 (reaction 1 or 2) followed by migration of the Cl to a nonphotolyzed OClO. Though clustering of the OClO may play a role in the formation of the product, we have discovered that the OClO prefers to exists as a monomer solvated by the water. These results suggest that, under the very dilute conditions that would exist in the stratosphere, the photochemical conversion of OClO on polar stratospheric ice particles would lead to Cl atoms. This would therefore represent a new source of Cl atoms for ozone depletion. Experimental Section The experimental setup has been described in detail previously.1 Briefly, an optically transparent window (KBr for both infrared and ultraviolet light) was attached to the copper block of a liquid nitrogen-cooled dewar (International Cryogenics) with an indium ring between the window and copper block to ensure good thermal contact. The temperature was measured using a silicon diode embedded in the copper block and was controlled by adjusting the flow of liquid nitrogen and by adjusting the current through a heating wire attached to the block (Lakeshore 321 Temperature Unit). The dewar, which had outer KBr windows attached to the vacuum jacket, sat inside the sample compartment of an FTIR spectrometer (Nicolet Magna © 1996 American Chemical Society
Photochemistry of OClO in Polycrystalline Ice
J. Phys. Chem., Vol. 100, No. 38, 1996 15451
Figure 1. Infrared spectrum of ∼2 µm microporous amorphous ice at 80 K (upper spectrum) and polycrystalline ice at 150 K (lower spectrum). The inset displays the dangling O-H vibration near 3700 cm-1 due to the O-H projecting from the surface, which is absent in the polycrystalline ice.
550) and could be transferred to an UV-vis spectrometer (Hitachi 3000). A standard glass vacuum line with a water-cooled diffusion pump was used for handling the gases. The chlorine dioxide was synthesized from stoichiometric amounts of sodium chlorite and sodium persulfate by simply placing the two salts in water according to the following exothermic reaction,11
Figure 2. Infrared difference spectra of OClO in amorphous ice at 80 K (upper panel) and polycrystalline ice at 150 K (lower panel) after ∼5 min of photolysis at 360 nm. The OClO peaks at 1100 cm-1 have decreased (negative peaks). The new positive peak at 1440 cm-1 is due to ClOO formed in amorphous ice, while the 1210 and 1040 cm-1 peaks are due to Cl-(OClO) formed in polycrystalline ice. The small peaks ∼5-10 cm-1 to the red are due to the naturally occurring 37Cl isotope.
NaClO2(aq) + Na2S2O8(aq) f OClO(g) + Na2SO4(aq)
state, which we observe near T ≈ 140 K. We are able to distinguish amorphous ice from polycrystalline ice based on their different infrared spectra. In particular, the polycrystalline ice has the following unique characteristics: the intense peak near 3200 cm-1 is sharper with a central peak and two shoulders, there is a negative dip near 1000 cm-1, and the peak near 840 cm-1 is sharper and moves to the blue.15 After adjusting the temperature anywhere from T ) 140 to 185 K, the ice films were allowed to stabilize to ensure thermal equilibration and crystallization. A background IR spectrum was collected, and then the ice film was exposed to 360 nm ultraviolet light for 1-5 min. Following photolysis an IR spectrum was collected which represents a difference spectrum. Figure 2 is an example of IR difference spectra after OClO photolysis. The upper panel is for OClO in amorphous ice at T ) 80 K while the lower panel is for OClO in polycrystalline ice at T ) 150 K. The negative peak at 1100 cm-1 is due to the loss of OClO, while the positive peaks are due to the photoproducts. For OClO photolysis in amorphous ice at T ) 80 K, the infrared peak at 1440 cm-1 is due to the chlorine peroxy radical, ClOO,8 as we reported previously.1 The results are reproduced here for comparison purposes. The photolysis of OClO in polycrystalline ice at T ) 140185 K always produces the unique set of infrared peaks at 1210, 1040, 520, and 430 cm-1 (see Figures 2 and 3). These frequencies compare very well to the literature values of Cl(OClO) in a neon matrix (cf. 1216.37, 1044.73/1041.20, 522.50, and 440.43 cm-1) and the gas phase (cf. 1218.2, 1041.5, 522.7, and 440.5 cm-1).9,10 Other possible photoproducts include ClOOCl with infrared peaks at 754, 543, 648, and 418 cm-1, ClOClO with peaks at 994, 696, and 440 cm-1, and Cl2O3 with peaks at 1227/1214/1202 and 1060/1057 cm-1.8,10 (These infrared measurements were for the species trapped in an argon matrix.) Though there is an intense band due to the ice near 840 cm-1 that could obscure some of these peaks, using difference spectra increases are effective sensitivity and allows
The yellow OClO gas was dissolved in the water as it was produced. After three pump-thaw cycles the chlorine dioxide solution was ready for use. As we reported previously, the vapors above the OClO in water solution are nearly pure OClO.1 The chlorine dioxide gas was mixed with pure water vapor in various ratios (0.1-10% OClO vapor to water vapor). After allowing for mixing, the gas mixture was slowly sprayed onto the cold KBr window (T ) 80 K) using a stainless steel injector tube. The gas flow was controlled using a needle value such that the deposition rate was 15 µmol/h, and the total deposition time was ∼30 min. The ice growth was monitored using FTIR spectroscopy, as described previously.1 Typical ice film thicknesses were 1-5 µm. The temperature was then raised to T ) 140-185 K and was stabilized to better than 1 K. After the ice films (with a dilute concentration of OClO) were characterized using FTIR and UV-vis spectroscopy, they were exposed to low-power ultraviolet light (λ ) 360 nm, P ∼ 100 µW). The UV light source was a 75 W Xe arc lamp with a high-throughput tunable monochromator (PTI LPS-220/A1010). After a period of UV light exposure, an IR spectrum was collected followed by a UV-vis spectrum. Results Under our experimental conditions, the ice that forms at T ) 80 K is considered microporous amorphous ice.12,13 Raising the temperature to T > 140 K converts the amorphous ice to polycrystalline ice.14 The IR spectrum of both microporous amorphous and polycrystalline ice is shown in Figure 1. The microporous amorphous ice has a characteristic sharp peak at 3700 cm-1 due to the dangling O-H projecting from the surface (including the pore’s surface); see Figure 1 inset.12,13 As the temperature is raised, the intensity of the dangling O-H peak decreases because the pores collapse. Further heating of the ice causes it to undergo a phase transition to the polycrystalline
15452 J. Phys. Chem., Vol. 100, No. 38, 1996
Figure 3. Infrared difference spectrum (from 600 to 400 cm-1) of OClO in polycrystalline ice at 150 K after ∼5 min of photolysis at 360 nm. The OClO peak at 450 cm-1 has decreased (negative peak). The new positive peaks at 520 and 430 cm-1 are due to Cl-(OClO) formed in polycrystalline ice.
Figure 4. UV-vis difference spectra of OClO in amorphous ice at 80 K (upper panel) and polycrystalline ice at 150 K (lower panel) after ∼5 min of photolysis at 360 nm. The negative structured feature centered at 360 nm is due to the OClO. The new positive peak at 260 nm is due to ClOO formed in amorphous ice, while the 300 and 235 nm peaks are due to Cl-(OClO) formed in polycrystalline ice.
us to observe any significant peaks in that spectral region. Since no other infrared peaks are observed, our infrared results indicate that the only (or at least the major) product from the photolysis of OClO in polycrystalline ice is Cl-(OClO). UV-vis spectroscopy was also utilized in these studies. Before photolysis a spectrum of the ice film (with OClO) was collected from 500 to 200 nm and used as a background or reference spectrum. After photolysis, another spectrum was collected. Subtraction of these two spectra gives difference spectra as shown in Figure 4. The upper panel is for photolysis of OClO in amorphous ice at T ) 80 K while the lower panel is for OClO in polycrystalline ice at T ) 150 K. The negative structure centered at 360 nm is due to the OClO (note the characteristic vibrational structure) while the positive peaks are due to the photoproducts. For OClO photolysis in amorphous ice at T ) 80 K, the positive feature centered at 260 nm is due to ClOO.8 Again, these results are reproduced from our earlier report1 for comparison purposes. The new features that appear
Pursell et al.
Figure 5. Infrared spectrum of 10% OClO in ice starting at 80 K (upper spectrum) and warming to 120 K (middle spectrum) and then 150 K (lower spectrum). The larger peak at 450 cm-1 is due to the bending vibration of OClO monomer, while the 470 cm-1 peak is attributed to OClO clusters. As the temperature is increased, the cluster peak decreases, indicating that the OClO (with greater mobility at the higher temperatures) prefers to exists as a monomer solvated by the water.
from the photolysis of OClO in polycrystalline ice at T ) 140185 K are centered at 300 and 235 nm. These wavelengths are in very close agreement with those for Cl-(OClO) in a neon matrix (cf. 293 and 234 nm) and in the gas phase (cf. 296 and 226 nm).9,10 These UV-vis results indicate that the only (or at least the major) product from the photolysis of OClO in polycrystalline ice is Cl-(OClO). In order to determine the importance of cluster formation, we carried out a series of experiments with varying amounts of OClO. We have discovered infrared features that can be attributed to OClO clustering. Figure 5 is an IR spectrum of 10% OClO in ice starting at T ) 80 K. The larger peak at 450 cm-1 is due to the bending vibration of the OClO monomer,8 while the peak at 470 cm-1 agrees with published results for the bending vibration of the OClO dimer.4 (We have also condensed pure OClO at T ) 80 K, and it also gives a peak at 470 cm-1.) As the temperature is increased, the 470 cm-1 peak decreases relative to the monomer peak. This peak is also not observed for lower concentrations of OClO in water. This suggests that the OClO prefers to exist as a monomer solvated by the water ice rather than to exist as a cluster. It has been suggested to us that the OClO might be concentrated at grain boundaries in the ice and then phase separate upon warming. If this were so, then the 470 cm-1 peak due to the clustering or solid OClO should actually increase in intensity, which is contrary to our observations. Thus, it appears that clustering does not play an important role in the photochemistry. Discussion The photochemistry of OClO in polycrystalline ice from T ) 140-185 K produces chloryl chloride, Cl-(OClO). Both the infrared spectrum (with four unique peaks at 1210, 1040, 520, and 430 cm-1) and the UV-vis spectrum (with two characteristic features at 300 and 235 nm) agree with published results.9,10 There appear to be no other products formed from the photolysis at 360 nm over the temperature range studied. In particular, we find no evidence for ClO formation or products due to ClO formation. Recall that the gas-phase photolysis of OClO is dominated by the production of ClO. We believe the photochemistry of OClO in ice, both amorphous ice at T ) 80 K and polycrystalline ice at T ) 140185 K, can be explained in either of two ways. In both cases
Photochemistry of OClO in Polycrystalline Ice the ice perturbs the excited electronic states of the OClO and controls the outcome of the photochemistry. The first mechanism involves the photolysis of OClO to form ClOO with excess internal energy (reaction 1). In amorphous ice at T ) 80 K, the internally hot ClOO is able to dissipate the excess energy to the ice cage and form the stable isomer of OClO, as we observed. In polycrystalline ice at T ) 140-185 K, the ClOO does not lose its excess energy, and it falls apart into Cl + O2 with excess translational energy. The Cl is able to break through the softer ice cage and migrates through the ice. When the Cl atom encounters a nonphotolyzed OClO, it forms Cl-(OClO), as we observe. Alternatively, the ClOO can be thought of as an intermediate that is stabilized at T ) 80 K but not at T > 140 K. The other possible mechanism involves the direct photolysis of OClO to Cl + O2 (reaction 2). In amorphous ice at T ) 80 K, the Cl and O2 are unable to separate due to the cage effect. They recombine to form the thermodynamically more stable isomer ClOO, as we observe. In polycrystalline ice at T ) 140185 K, the ice cage is weaker and unable to restrict the photoproducts’ mobility. The Cl atom then migrates through the ice, encounters a nonphotolyzed OClO, and combines to form Cl-(OClO), as we observe. We should also mention the possibility of a clustering mechanism. This would negate the need for Cl atom migration, but would still require electronic state perturbation in order to explain the results. However, as we presented above in the Results section, clustering does not appear to be important, and the OClO actually prefers to exist as a monomer solvated by the water. Conclusion The photochemistry of OClO in polycrystalline ice at T ) 140-185 K has been examined. The only product appears to be chloryl chloride, Cl-(OClO), a structural isomer of (ClO)2 with the Cl attached off-axis to bent OClO with Cs symmetry. This assignment has been made using the unique vibrational bands at 1210, 1040, 520, and 430 cm-1, along with the characteristic UV absorptions at 300 and 235 nm.9,10 We believe the formation of Cl-(OClO) in ice involves photolysis of OClO to (i) ClOO which falls apart into Cl + O2 or (ii) directly to Cl + O2, followed by migration of the Cl to a nonphotolyzed OClO. In amorphous ice at T ) 80 K, the ClOO is stabilized or the Cl and O2 recombine to form ClOO by the cage effect. In polycrystalline ice at T ) 140-185 K, the ClOO is not stabilized or the Cl and O2 do not recombine and mobile Cl atoms result. These Cl atoms find nonphotolyzed OClO and combine to form Cl-(OClO). Though clustering of the OClO may play a role in the formation of the product, we have discovered that the OClO prefers to exists as a monomer solvated by the water.
J. Phys. Chem., Vol. 100, No. 38, 1996 15453 Recent studies indicate that OClO does adsorb to ice under stratospheric conditions, though the concentration is very low.16,17 Our results would then suggest that, under the very dilute conditions that would exist in the stratosphere, the photochemical conversion of OClO on polar stratospheric ice particles would lead to Cl atoms. This would therefore represent a new source of Cl atoms for ozone depletion. Acknowledgment. Professor V. Vaida at the University of Colorado is to be thanked for her encouragement and helpful discussions during the course of this work. Additionally, the published work on both ClOO and Cl-(OClO) by Professor H. Willner and his group has been extremely helpful. We appreciate his most thorough work. We thank Professor J. P. Devlin at the Oklahoma State University for a very helpful discussion concerning amorphous and polycrystalline ice. We also acknowledge the helpful comments of a reviewer. C.J.P. thanks his colleagues in the Chemistry Department for their support, especially the encouragement of Professors Bill Kurtin and Nancy Mills. Our studies have been made possible by the generous financial support of Trinity University, the Dreyfus Foundation (Camille and Henry Dreyfus Start-up Grant Program for Undergraduate Institutions), the Research Corporation (Cottrell College Science Award), and the donors of the Petroleum Research Fund (administered by the ACS). References and Notes (1) Pursell, C. J.; Conyers, J.; Alapat, P.; Parveen, R. J. Phys. Chem. 1995, 99, 10433. (2) Vaida, V.; Solomon, S.; Richard, E. C.; Ru¨hl, E.; Jefferson, A. Nature 1989, 342, 405. (3) Vaida, V.; Simon, J. D. Science 1995, 268, 1443. (4) Flesch, R.; Wassermann, B.; Rothmund, B.; Ru¨hl, E. J. Phys. Chem. 1994, 98, 6263. (5) Arkell, A.; Schwager, I. J. Am. Chem. Soc. 1967, 89, 5999. (6) Rochkind, M. M.; Pimentel, G. C. J. Chem. Phys. 1967, 46, 4481. (7) Johnsson, K.; Engdahl, A.; Nelander, B. J. Phys. Chem. 1993, 97, 9603. (8) Mu¨ller, H. S. P.; Willner, H. J. Phys. Chem. 1993, 97, 10589. (9) Mu¨ller, H. S. P.; Willner, H. Inorg. Chem. 1992, 31, 2527. (10) Jacobs, J.; Kronberg, M.; Mu¨ller, H. S. P.; Willner, H. J. Am. Chem. Soc. 1994, 116, 1106. (11) Masschelein, W. J. Chlorine Dioxide: Chemistry and EnVironmental Impact of Oxychlorine Compounds; Ann Arbor Science: Ann Arbor, MI, 1979. (12) Devlin, J. P.; Buch, V. J. Phys. Chem. 1995, 99, 16534, and references therein. (13) Buch, V.; Devlin, J. P. J. Chem. Phys. 1991, 94, 4091. (14) Hallbrucker, A.; Mayer, E.; Johari, G. P. J. Phys. Chem. 1989, 93, 4986. (15) Bertie, J. E.; Labbe´, H. J.; Whalley, E. J. Chem. Phys. 1969, 50, 4501. (16) Graham, J. D.; Roberts, J. T.; Brown, L. A.; Vaida, V. J. Phys. Chem. 1996, 100, 3115. (17) Brown, L. A.; Vaida, V.; Hanson, D. R.; Graham, J. D.; Roberts, J. T. J. Phys. Chem. 1996, 100, 3121.
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