3902
. I Phys. . Chem. 1995, 99, 3902-3904
The CIOCIO, BrOCIO, and IOClO Molecules and Their Photoisomerization. A Matrix Isolation Study Klas Johnsson,' Anders Engdahl, Jennifer Kiilm,* Janne Nieminen,§ and Bengt Nelander" Division of Thermochemistry, University of Lund, Chemical Center, P.O. Box 124 S-221 00 Lund, Sweden Received: December 2, 1994; In Final Form: January 26, 1995@
The compounds XOClO (X = C1, Br, and I) have been prepared in argon matrices, by addition of X to OC10. Visible radiation transforms them into XC102. The wavelength dependency of the transformations has been studied. ClC102 and BrC102 can be isomerized back to ClOClO and BrOC10, respectively, with near UV radiation.
Introduction
~
The processes leading to the formation of the ozone hole in the Antarctic spring involve reactions between chlorine and bromine oxide radicals.' Careful measurements of the reaction rates for the self reactions of the C102 and Br03 radicals and of the reaction between C10 and Br04 have been published. For the self reactions, results at very low temperatures suggest the possibility that the formation of dimers is involved. The dimer of C10 is believed to play a key role in the formation of the ozone hole; it has therefore been much studied both experimentally5-" and the~retically.l*-'~Three more or less stable isomers of the C10 dimer are k n ~ w n . ' ~ -The ' ~ most stable is the chlorine analogue of hydrogen peroxide. Its structure has been obtained from microwave ~pectroscopy,~ and its UV spectrum has been measured in the gas Its IR spectrum was first observed in an argon matrix by Cheng and Lee,9 and later Jacobs et a1.I0 published a very comprehensive study of its spectrum. Muller and Willner" have prepared a Y-shaped isomer, which can be described as a chlorine molecule with two oxygen atoms bound to one of the chlorine atoms. The least stable isomer has the structure ClOClO. It has been prepared in matrices by Jacobs et al.'O by adding C1 atoms to OCIO, by adding 0 atoms to ClOC1, and by photochemical rearrangement of the Y-shaped ClClO2 isomer. The halogen monoxides are polar molecules with dipole moments of 1.24 D (C10i6) and 1.77 D (BrO"); the formation of the ClOClO and BrOClO forms of the dimers should therefore be kinetically favored over the peroxide forms of the dimers at low temperC10 reaction atures. In keeping with this view, the BrO produces atomic bromine and OC10,4a reaction channel which is easy to explain with a BrOClO intermediate and which is favored at low temperatures.4 Attempts to produce matrixisolated C10 in our laboratory invariably produced high concentrations of the ClOClO dimer but no traces of the peroxide dimer.I8 We have prepared the ClOClO, BrOC10, and IOClO molecules by adding chlorine, bromine, or iodine atoms to OClO
+
' Permanent address: Institute of Physics and Measurement Technology, Linkoping University, S-581 83 Linkoping, Sweden. Permanent address: Alfred-Wegener-Institut fur Polar- und Meeresforschung, Am Handelshafen 12. 27570 Bremerhaven. FRG. e Permanent address: Department of Physical Chemistry, University of Helsinki, P.O. Box 13. FIN-00014 Helsinki, Finland. @Abstractpublished in Advance ACS Ahstracrs, March 15. 1995.
IOClO
I
I
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BrCClO
ClCClO
960
970
980
990
i 1000
Figure 1. The C10' stretch of CIOC10 (bottom curve, absorbance of the main peak 0.33), BrOC10' (middle curve, maximum absorbance O . l l ) , and IOCIO' (top curve, maximum absorbance 0.22) (cm-', absorbance).
in argon matrices. We have also prepared ClOClO together with oxygen by photolysis of the OClO dimer, as described by Muller and Willner.I9 We have found that all three molecules rearrange rapidly to Y-shaped isomers containing C102 and with the extra halogen bound to the chlorine. In the chlorine case this isomerization was observed in ref 10. The relative rates of the photochemical transformations as functions of the wavelength of the radiation have been studied. In all three cases it has a maximum in the visible and it shifts toward longer wavelengths from chlorine to iodine. The Y-shaped chlorine and bromine compounds can be transformed back to the originally formed compounds by irradiation with near UV radiation. The process can be repeated many times, but ultimatively oxygen and a halogen are formed. Only in the chlorine case did we observe the peroxy dimer. In the bromine case, the ClOBrO isomer was formed in a minor reaction
0022-3654/95/2099-3902$09.0010 0 1995 American Chemical Society
Letters
J. Phys. Chem., Vol. 99, No. 12, 1995 3903
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Wavelength, nm Figure 2. The photodestruction rate of ClOClO (dashed line), BrOClO (solid line), and IOClO (dotted line)
channel parallel to the Y-shaped compound, and it was rapidly destroyed by 300 nm radiation.
Experimental Details Matrix-isolated OClO was prepared as described earlier.20 Halogen atoms were prepared by passing argon mixed with the halogen through a microwave discharge. Matrices were prepared on a CsI window at 17 K. IR spectra in the 200-4000 cm-' interval were measured with a Bruker 113v instrument. A 300 W xenon lamp in combination with an Oriel 1/8 m monochromator was used for the photolysis studies. A 20 nm band pass was used for all irradiations. For the measurement of photodestruction spectra, the matrix was irradiated for a fixed time, usually 1 min, then the IR spectrum was recorded and the changes in the intensities of the IR bands were obtained. Since the changes were small compared to the total intensities, no corrections for the decreases in the concentrations of the parent molecules have been applied. Before the determination of the photodestruction spectra, the remaining OClO was transformed into ClOO by irradiation at 350-400 nm for about 1 h. This irradiation had only small effects on the XOClO concentrations in the matrix. Since the W absorption of ClOO starts below 290 nm,20 it has only a limited effect on the photodestruction rate on the short wavelength side of the photodestruction spectra of XC102.
Results and Discussion When we added chlorine atoms to OC10, we observed the same two strong bands at 994.4 and 985.5 cm-' as Jacobs et a1.I0 With bromine instead of chlorine the band pair shifted to 991.1 and 982.4 cm-' and with iodine to 974.9 and 966.6 cm-' (Figure 1). In the chlorine case, isotope studies and calculations show that they are due to the C10 stretch of the terminal oxygen atom, and the two peaks are due to the 35Cl and 37Cl isotopomers.I0 It is therefore not unexpected that they shift little, when the terminal chlorine is substituted with bromine or iodine. It is not clear to us why the halogen atom prefers to bind to an oxygen atom of OClO instead of to the chlorine atom; in spite
u CICIO,
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:
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1010
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~
1030
:
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:
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:
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Figure 3. The symmetric and antisymmetric C10 stretches of ClClO;? (bottom, absorbance of the main peak 0.05, left panel, and 0.13, right panel), BrC102 (middle, maximum absorbance 0.10, left panel, and 0.06, right panel), and IC102 (top, maximum absorbance 0.19, left panel, and 0.07, right panel) (cm-I, absorbance).
of that the Y-shaped isomer is more stable, at least in the chlorine case. We observe small amounts of the Y-shaped isomers directly upon deposition in all three cases. We believe that they are formed from the chain isomers by photoisomerization, perhaps by stray light from the microwave discharge. Experiments with oxygen atoms show that they bind preferentially to the chlorine atoms.Is The c103 which formed was identified using the infrared spectral data of Grothe and Willner.2 All three XOClO compounds are rapidly rearranged by visible radiation; Figure 2 shows the photoisomerization spectrum in the bromine case, it has its maximum at 525 nm. The corresponding spectra in the chlorine and iodine cases are similar but have their maxima at 450 and 550 nm, respectively. Crude estimates suggest that the quantum yields for rearrangement are not smaller than 0.1. We note that the calculations of Jensen and OddershedeI3 predict a weak absorption band of ClOClO at 500 nm.
3904 J. Phys. Chem., Vol. 99, No. 12, I995 1
Letters I
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The chlorine compound rearranged to the Y-shaped isomer, observed by Jacobs et a1.,I0 as evidenced by the appearance of its antisymmetric and symmetric C10 stretches at 1213.2 and 1040.7 cm-’ (35Cl isotopomer). In the bromine and iodine cases, similar peak pairs appeared at 1193.5 and 1019.2 and at 1161.2 and 987.4 cm-’ respectively (Figure 3), showing that the bromine or iodine atom had migrated to the chlorine atom. In the bromine case, two equally intense peaks at 834.4 and 832.5 cm-’ appeared after prolonged irradiation with visible radiation. The size of the split and the equal intensity of the two peaks strongly suggest that they are due to a BrO stretch. Since they are due to a compound formed from BrOClO and appear at a significantly higher wavenumber than the fundamental of free BrO we believe that they are due to the BrO’ stretch of ClOBrO’. The C10 stretches of ClOOCl both appear below the fundamental of free C10, while the C10’ stretch of CIOClO’ is observed at a higher wavenumber.’O Irradiation of the Y-shaped compounds with UV radiation produced the unsymmetric linear isomers in the chlorine and bromine cases. In the iodine case, no IOClO was formed; instead IC1 appeared, showing that in this case the Y-shaped compound was converted to the thermodynamically favored product, IC1 and oxygen. Figure 4 shows the photoisomerization spectrum in the bromine case. The corresponding spectrum in the chlorine case is similar but shifted ca 25 nm toward shorter wavelengths.
Acknowledgment. This work was supported by the Swedish Natural Research Council and the Swedish Environmental Protection Agency. Jennifer Kolm was supported by a stipend from Gottlieb Daimler- and Karl Benz-Stiftung, which is gratefully acknowledged.
References and Notes (1) Wayne, R. P. Chemisty of the Atmospheres, 2nd ed.; Clarendon Press: Oxford, 1991. (2) Trolier, M.; Mauldin, R. L.; Ravishankara, A. R. J . Phys. Chem. 1990, 94, 4896. Nickolaisen, S. L.; Friedl, R. F.; Sander, S. P. J . Phys. Chem. 1994, 98, 155. (3) Mauldin, R. L., 111; Wahner, A.; Ravishankara, A. R. J. Phys. Chem. 1993, 97, 7585, (4) Tumipseed, A. A.; Birks, J. W.; Calvert, J. G. J . Phys. Chem. 1991, 95, 4356. (5) Birk, M.; Friedl, R. R.; Cohen, E. A,; Picket, H. M.; Sander, A. P. J . Chem. Phys. 1989, 91. 6588. (6) Burkholder, J B.; Orlando, J. J.; Howard, C. J. J . Phys. Chem. 1990, 94, 687. (7) DeMore, W. B.; Tschuikow-Roux, E. J . Phys. Chem. 1990, 94, 5856. (8) Molina, L. T.; Molina, M. J. J . Phys. Chem. 1987, 91, 433. (9) Cheng, B.-M.; Lee, Y.-P. J . Chem. Phys. 1990, 90, 5930. (IO) Jacobs, J.; Kronberg, M.; Muller, H. S. P.: Willner, H. J . Am. Chem. Sot. 1994, 116, 1106. (11) Muller, H. S. P.; Willner, H. Inorg. Chem. 1992, 31, 2527. (12) McGrath, M. P.; Clemitshaw, K. C.; Rowland, F. S.; Hehre. W. J.; Geophys. Res. Lett. 1988, 15, 883. (13) Jensen, F.; Oddershede, J. J . Phys. Chem. 1990, 94, 2235. (14) Stanton, J. F.; Rittby, C. M.; Bartlett, R. J.; Toohey, D. W. J . Phys. Chem. 1991, 95, 2107. (15) Lee, T. J . ; McMichael Rohlfing, C.: Rice. J. E. J . Chem. Phys. 1992. 97, 6593. (16) Amano, T.; Hirota, E.; Morino, Y.; Saito. S. J . Mol. Spectrosc. 1969, 30. 275. (17) Amano, T.; Yoshinaga, A.: Hirota, E. J . Mol. Spectrosc. 1972, 44, 594. (18) Johnsson, K. Manuscript in preparation. (19) Muller, H. S. P.; Willner, H. J . Phys. Chem. 1993, 97. 10589. (20) Johnsson, K.; Engdahl, A.; Nelander, B. J . Phys. Chem. 1993, 97, 9603. (21) Grothe, H.; Willner, H. Angew. Chem., Int. Ed. EngI. 1994, 33, 1482. JP9432096
