J. Phys. Chem. 1993,97, 9603-9606
9603
A Matrix Isolation Study of the ClOO Radical Klas Johnsson,t Anders Engdahl, and Ben@ Nelander' Chemical Center, Thermochemistry, University of Lund, P.O. Box 124, S-221 00 Lund, Sweden Received: April 14, 1993; In Final Form: June 21, 1993'
The I R and UV spectra of the ClOO radical, in argon matrices, have been measured. ClOO has a lowwavenumber fundamental at 192.4 cm-'. The UV and IR data have been combined and used to estimate the IR intensities of the ClOO bands. The wavelength dependency of the photochemistry of OClO and ClOO in argon matrices has been studied.
Introduction The ClOO radical was postulated to exist by Porter and to account for the rate of formation of C10, when chlorine was flash-photolysed in the presence of oxygen. Its existence and infrared spectrum were established in matrix isolation experiments by Pimentel and Rochkind' and by Arkell and S~hwager.~The study by Arkell and Schwager was particularly thorough and used several methods to prepare the radicaland involved theuseof 160and l8Otoestablish itsstructure. Johnston and co-workers5 used modulation spectroscopy to study the ClOO radical in the gas phase. They could observe both its UV absorption spectrum and the 0-0 stretching band in the infrared spectrum, and they also obtained an estimate of its UV absorption cross section. Benson and Buss6 estimated the enthalpy and free energy of formation of ClOO and suggested that ClOO has a lower free energy of formation than its well-known isomer, OC10. Its fleeting existence is due to the weakness of the C10 bond, for which they calculated a dissociation energy of only 5 kcal/mol. OClO in contrast is protected by large kinetic barriers. In 1987, Molina and Molina7suggested that ClOO forms when ClOOCl is photolysed in the Antarctic stratosphere and subsequently is dissociatedinto a chlorine atom and an oxygen molecule. These reactions form part of an ozone destruction cycle, which is now known to account for approximately 75% of the spring time ozone loss over Antarctica.8 The realization that ClOO is an important species in the stratosphere has lead to a revived interest in its chemistry. Its UV absorption cross section9J0and the equilibrium constant"l for the reaction between chlorine atoms and molecular oxygen have been measured, as well as the formation and destruction kinetics of C100.9-11 Peterson and WerneP have made a large ab initio calculation of its energy of formation, structure, and vibration spectrum. One of the methods that Arkell and Schwager used to prepare ClOO was by the photoisomerizationof OClO in matrices. They found that, in addition to C100, another species with a similar infrared spectrum and with the same composition was formed. This species has been the subject of some spe~ulation,'~ but its real nature has not been established. It seems to be less stable than ClOO to photolysis but appears to survive in matrices indefinitely if the matrix temperature is 17 K or less.14 Arkell and Schwager made some photolysis experiments using filters, which suggested that the relative importance of the two photoisomerization channels of OClO was wavelength dependent. Upon photolysis in the gas phase, OClO normally dissociates into C10 and an oxygen atom.'$ It has been suggested that an additional channel may exist, where OClO first isomerizes to t Permanent address: Linktiping Institute of Technology, Department of Physics and Measurement Technology, S-581 83 Linktiping, Sweden.
Abstract published in Aduance ACS Abstracts, September 1, 1993.
0022-3654/93/2097-9603%04.00/0
C100, which subsequently dissociates into a C1 atom and molecular oxygen.16 The Arkell and Schwager observation was used to conjecture that the isomerization was most effective with radiation of wavelengths close to 365 nm1.17J8Chlorine atoms have been observed when OClO was irradiated with wavelengths around 365 nm; however, the origin of these chlorine atoms is a matter of controversy.19 Considering the very low dissociation energy of the C10 bond of C100, it seemed unlikely to us that the 370-cm-l band of Arkell and Schwager could be the lowest fundamental of C100. We have therefore repeated some of the Arkell and Schwager experiments in a cryostat which allows us to record the infrared spectrum down to 10 cm-1. We have used another cryostat to record infrared and UV absorption spectra on the same sample and in this way obtained estimates of the infrared cross sections of matrix isolated C100. We have also taken advantage of the rapid photodestructionof OClO and its isomers by using a 0.25-m monochromator to limit the wavelength interval of the photolysing radiation. In this way a somewhat more precise measurement of the wavelength dependency of the photoinduced isomerization of OClO has been made.
Experimental Section Chlorine (Matheson, 99.5% pure) was degassed before use. Chlorine dioxide (OClO) was prepared by dripping sulfuric acid (Merckp.a.) ontopotassiumchlorateU(Flukapwump.a., >99.0% pure). The chlorine dioxide was collected at -78 "C, and the remaining chlorine was removed by pumping on the mixture for 5 min. Gas mixtures were prepared by standard manometric techniques and sprayed on a CsI window, cooled by an Air Products CS208 refrigeration system or on the cold mirror of a liquid helium cryostat, which has been described earlier.20.21 The deposition rate was 10 mmol/h and the deposition temperature 17 K. The deposition time was normally 3 h. The deposition system has two separate gas streams which enter the cryostat approximately 10 cm from the cold window. The gas flows are regulated with motor-driven needle valves.22 We noted that the needlevalves catalyzed the decompositionof the chlorine dioxide. We found that we could minimize this problem by allowing the flow of needle valve regulated argon to pass over the chlorine dioxide kept at fiied temperature. The chlorine dioxide temperature was selected in a range between -130 and -110 "C depending on the desired chlorine dioxide concentration. For a temperature of-120 "C thematrixcontainedapproximately 1.5% OC10. ClOO was produced either through the photoisomerization of OClO in argon matrices, through the photolysis of Clz in solid oxygen, or in the reaction between chlorine atoms produced in a microwave discharge and molecular oxygen on the surface of the matrix. In the microwave experiments both argon matrices 0 1993 American Chemical Society
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Johnsson et al.
The Journal of Physical Chemistry, Vol. 97, No. 38, 1993
doped with -2% oxygen and matrices containing approximately equal amounts of argon and oxygen were used. The gas pressure in the microwave discharge was typically 0.5 Torr, and the outgoing microwave effect, 70 W. No oxygen passed through the discharge. Infrared spectra were recorded with a Bruker 113v instrument between 10 and 4000 cm-1 at 0.5-cm-l resolution. The spectral range below 220 cm-1 was only availablewith the helium cryostat. Spectra were normally run at 17 K. UV absorption spectra in the gas phase and in argon matrices were obtained with a Cary 15M spectrometer with home-built electronics for the transmittance measurements and with data collection and storage controlled by a Victor PC. UV spectra were obtained with the compressor-cooled cryostat. Photodestruction spectra were obtained by the following procedure. First an infrared spectrum of the sample was recorded. Then the matrix was irradiated with a 300-W Xe lamp through a monochromator for a fixed time. The time was chosen to be so short that the loss of OClO was a small fraction of the total amount but long enough to give clearly measurable changes at the most efficient wavelengths. The photolysis radiation entered the cryostat through a sapphire window at approximately 45O to the infrared beam of the spectrometer. The cryostat was fixed in the infrared spectrometer throughout the entire experiment. The overlap between the irradiating beam and the measuring beam of the spectrometer was checked prior to the irradiation, by using a CaF2 beam splitter and the mercury lamp source of the spectrometer and allowing red light to pass through the photolysis monochromator. Immediately after the photolysiswas finished, a new infrared spectrum was recorded and ratioed against the spectrum prior to the irradiation. In this way we could measure the changes in the absorbance of OClO and its products with a sensitivity of approximately 0.001 absorbance units. This procedure was then repeated for new wavelengths until the desired photodestruction spectrum had been obtained. The photodestruction spectra have been compensated for the wavelength dependency of the lamp intensity and grating efficiency. Caution: Potassium chlorate and especially chlorine dioxide are explosive and must be handled with care.
Results
Infrared Spectrum of (300.In a series of experiments, we have added chlorine atoms from a microwave discharge to molecular oxygen on the surface of an argon matrix. Chlorine and oxygen (both mixed with argon) entered the cryostat through separate inlets, to insure that no oxygen passed through the discharge. In these experiments, we observed product bands at 1442.5,407.9,and 192.6cm-1. The first two bands wereobserved by Arkell and Schwager4 and assigned by them to the ClOO radical. The 192.6-cm-1 band was outside the range of the spectrometer used by Arkell and Schwager; but it is more intense than the 407.9-cm-l band and its intensity follows that of the 1442.5-cm-1band. We therefore assigned it to C100. In addition, Arkell and Schwager assigned a band at 373 cm-1 to C100. Estimates based upon the OClO photolysis experimentsreported below showed that this band was too weak to be observable in the microwave discharge experiments. We also obtained impurity bands due to HC1, CO, C02, and COC12. We performed one experiment where we photolysed C12 in an oxygen matrix. In this experiment we obtained product bands at 1436 and 203 cm-I. Further, bands due to HCl, C02, and H2O appeared. The positions of the 1436- and 203-cm-1 bands are close to those of the previously observed bands of ClOO in argon matrices, and we feel confident that they are due to ClOO in solid oxygen. When we performed a microwave discharge experiment using only oxygen instead of the usual argon-oxygen mixture, but still using an argon-chlorine mixture in the discharge, we observed the product bands at 1437 and 203 cm-I, supporting
u I
r
ID
190
LL
190
195
0
z a m
! I 0 LG
370
ffi
a
A
370
390
1415
1445
410
147E
210
400
1420
'
230
430
1440
WAVENIJMBER cm-?
Figure 1. A-C: ClOO bands from the reaction of C1 atoms and oxygen molecules (argon matrix). D-F: ClOO and C100' bands from the photoisomerization of OClO (argon matrix). this opinion. Also in this microwavedischarge experiment, bands due to HCl, COS and H2O appeared. Finally we studied the photolysis of OClO in argon matrices in a series of experiments. Here we observed as product bands, in addition to the band at 192.4 cm-l, all the three bands which Arkell and Schwager assigned to ClOO (see above and Figure 1A-C). We also observed bands at 1416.5, 435.1, and 227.1 cm-1 (Figure 1D-F). The first two of these bands were observed by Arkell and Schwager and assigned to a second species with the composition C100, denoted ClOO*. The 227.1-cm-1 band is inside the wavenumber range studied by Arkell and Schwager but in a region where the water vapor absorption makes observations difficult with the type of spectrometer they used. The intensities of the three bands are in the ratio 40:2:1, with the instrumental resolution of 0.5 cm-I. All three bands appear and vanish together during photolysis, and the 227.1-cm-1 band is therefore assigned to ClOO*. In these experimentssmall amounts of HzO and C02 appeared as impurities. In a freshly prepared OClO sample, a trace of C1204was found. It appeared only in the initial experiments using this sample. Table I summarizes the observations of ClOO from typical experiments with the four different preparation methods used in this work. UV Absorption Spectrum and IR Absorbance of Cl00. The UV spectra of argon matrices containing OClO were recorded before photolysis (Figure 2B) and then again after a short photolysiswhen IR spectra indicated that both ClOO and C100* were present in the matrix. Finally, UV spectra were measured when all the ClOO* had been destroyed (Figure 3B). If we assumethat the UV cross section of ClOO does not change between the gas phase9J0 and the argon matrix, we can use its UV absorbance to determine the ClOO concentration per surface unit in the matrix. ln(Zo/Z)/a = Cd ln(Zo/l) is the measured UV absorbance of C100, Q, its UV absorption cross section (cm2/molecule), C, its concentration in the matrix (molecule/cm3), and d, the matrix thickness in cm. The value of Cd obtained in this way can be used to estimate the infrared intensity of the 1442.8-cm-l band of ClOO.24 41442.8) = JIRln(Zo/Z) dv/Cd
A(1442.8) is the infrared intensity of the 1442.8-cm-I band in cm/moleculeif theintegration is carried out over this band. Using the results from the photolysis experiments, we obtained a band intensity of 3 X cm/molecule. The transition dipole moment of the 1442.8-cm-1 band is then obtained as 4.6 X esu cm. Crude estimates of the intensities of the other infrared absorption bands of ClOO were obtained from comparisons of the product
A Matrix Isolation Study of the ClOO Radical
The Journal of Physical Chemistry, Vol. 97, No. 38, 1993 9605
TABLE i: IR Bands of (300hepared by Different Methods photolysis of reaction of C1 atoms and photolysis of reaction of C1 atoms and OClO (argon matrix) oxygen molecules (argon matrix) chlorine (oxygen matrix) oxygen molecules (araon-oxvaenmatrix) wavenumber absorbance wavenumber absorbance wavenumber absorbance wavenumber absorbance cm-1 (detection limit)' cm-' (detection limit)' cm-1 (detection limitp cm-1 (detection limit)' ~~
VI
1442.8
YZ
408.3
v3
192.4
2U3
375.6
0.93 (0.002) 0.010 (0.004) 0.021 (0.002) 0.007 (0.004)
1442.5
1.21 (0.005) 0.012 (0.009) 0.027 (0.002)
407.9 192.6
1436.3
0.64 (0.003)
1436.8
2.20 (0.003)
202.8
(0.019) 0.025 (0.002)
202.8
(0.010) 0.099 (0.003)
(0.013)
(0.019)
(0.010)
* Peak-to-peak noise level.
l-
0
I
a
-
310
360
410
310
360
410
375
-
387
375 WAVELENGTH nm
WAVELENGTH nm
F i p 2 . A Photodestructionspectrumof OClO with a monochromator slit of 21 nm. B: UV spectra of OClO in an argon matrix (upper curve) and in the gas phase (lower curve). C: As in part A but with a monochromator slit of 1 nm. D: Enlarged part of panel B. 1
lA
I
-
I
IC
ii L L
Y
a
220
260
300
220
260
300
, , , Y
Z
dAVELENGTH nm
240
280
320
240
280
320
WAVELENGTH nm
Figure 3. A and C: Photodestruction spectra of ClOO and C100'. B and D: UV spectra of ClOO and C100* in an argon matrix.
TABLE II: IR Data of C l 0 0 fundamental Vl Y2
y3
2v3
wavenumber, cm-1 1442.8 408.3 192.4 375.6
band intensity, cm/molecule 3 x 10-18 6X 4x 1x
1V20 10-20 10-20
of their maximum absorbance and width (fwhm) with the corresponding product of the 1442.8-cm-l band. Table I1 summarizes our results on the infrared spectrum of C100. Photodestruction Spectra of OCIO,(300*, and C100. The low-resolution (21 nm slit) photodestruction spectrum of OClO follows the UV absorption spectrum closely (compare Figure 2A and B); at higher resolution (1 nm), even the fine structure can be observed (compare Figure 2C and D). Our work indicates that ClOO and C100* are produced at different rates depending on the wavelength used to photolyse OC10. The ratio between
ClOO and C100* is always less than 0.5 and decreases with decreasing wavelength to approximately 0.3 at 335 nm. In one experiment we photolysed OClO in an argon matrix doped with oxygen using a long pass filter with a cutoff at 380 nm until all the OClO had been destroyed. In this experiment we obtained ozone in addition to ClOO and ClOO*. Wereplaced the filter with a water cuvette and continued the photolysis. Then the ozonewas destroyed and OClO and ClOO formed. The OClO absorbance after the second irradiation was approximately 15% of its value directly after deposition. The ClOO absorbance rose by approximately 10% during the last irradiation. These observations indicate that a significant fraction of the excited OClO molecules dissociate into C10 and 0 before forming ClOO and ClOO*. The decrease of the 1442.8-cm-l band during irradiation was used toobtain the photodestructionspectrumof C100. Incontrast to the case of OC10, the photodestruction spectrum of ClOO is shifted relative to its UV absorption spectrum (compare Figure 3A and B). The only product obtained from ClOO was OC10. However the u3 band of OClO from the photoisomerization was found at 1101 cm-l, coincident with a weak component of the OClO band observed directly after deposition (Figure 4 of ref 14). The photodestruction spectrum of C100* was observed to be a broad band starting at 300 nm (Figure 3C). Note that the local minimum at 255 nm may well be an artifact of the ClOO absorption of the matrix. Close to the UV absorption maximum of C100, the C100* photolysis may be slowed down by ClOO absorption. This may be the cause of the dip in the photodestruction spectrum of ClOO*. The UV spectrum in Figure 3D was obtained by subtracting the ClOO absorption after ClOO* had been eliminated, from the spectrum of the same matrix, while it still contained ClOO*. This procedure becomes rather unstable near the UV absorption maximum of C100. We are therefore not convinced that the dip in the UV spectrum of C100* is real. We have not been able to identify the photoproducts of ClOO*. We note however that when we photolysed C100* until its IR absorption was eliminated, a broad diffuse band appeared in the 1420--1440-~m-~ region. If we then warmed the matrix for 5 min at 30 K and recooled it to 17 K, a weak IR absorption due to C100* appeared again.14
Discussion The IR spectrum of ClOO has been studied by Arkell and Schwager using Beckman IR-9 (mid-IR) and Perkin-Elmer 621 (far-IR) spectrometers.4 Thelow wavenumber limit of the PerkinElmer spectrometer was 200 cm-1, but since its could not be evacuated,its sensitivityat the low wavenumber end was probably low. We have prepared ClOO by adding C1 atoms to oxygen in argon and mixed argon-oxygen matrices, by the photolysis of Clz in solid oxygen and by photolysis of OClO in argon matrices (Table I). In these experiments we observe the bands assigned to ClOO by Arkell and Schwager. The weak 408- and 376-cm-l
Johnsson et al.
9606 The Journal of Physical Chemistry, Vol. 97, No. 38, 1993
bands were absent in some experiments, but the estimates based on theintensityofthe 1442.8-cm-I bandshowed that their expected intensities were below the noise level of the spectra. In all these experiments we also observed a band at 192.4 cm-I. The 192.4-cm-1 band increased together with the 1442.8-cm-1 band when OClO was photolysed. When these observations are taken together there can be no doubt that the 192.4-cm-1 band is also due to C100. ClOO is a triatomic molecule and therefore has three fundamental vibrations. However the first overtone of the 192.4-cm-1band is expected near 380 cm-*. We therefore assign the 375.6-cm-1 band to the first overtone of the 192.4-cm-I band activated by a Fermi resonance with the 408.3-cm-l band. The new value for the lowest ClOO fundamental, the C10 stretch, seems much more reasonable for a weakly bounded system than the value based on the previous assignment. It may be pointed out that Peterson and Werner calculated the harmonic frequencies of ClOO to be 1506.6, 390.8, and 181.2 cm-I, respectively,'* in surprisingly good agreement with experiment. Table I1 gives the observed band positions of ClOO and the intensity estimates obtained from a comparison between UV and IR spectra (see above). We expect the estimate of the intensity of the 1442.8-cm-1 band to be correct to within *SO%, and the estimates for the other bands should be less than a factor of 2 from their true values. When Arkell and Schwager photolysed OClO in argon and nitrogen matrices, they obtained, in addition to C100, another product with absorption bands at 1415 and 435 cm-1, which they denoted ClOO*. We also observe C100* when we photolyse OClO in argon matrices, and we have been able to show that it has a third band at 227.1 cm-'. Arkell and Schwager suggested that C100* is closely related to C100. Their conclusion is further supported by the observation of bands at 192.4 and 227.1 cm-I for ClOO and ClOO*, respectively. The UVand photodestruction spectra of C100* appear to be slightly broadened versions of the UV spectrum of C100. Since C100* has been stored in the matrix at 17 K for more than 24 h in this work, it cannot be due to an exited state of ClOO.I3 The remaining possibility is that ClOO* is ClOO trapped ina metastable trapping site. Wenote that OClOupon deposition is trapped in at least three different sites, while OClO which forms from irradiated ClOO is trapped only in one site. The ratio between ClOO and C100* resulting from the photolysis of OClO in the matrix would then depend on the transfer of the excess energy from the conversion of OClO to ClOO to the matrix. If the excess energy passes out from the matrix cavity without changing its structure, C100* is formed, while if it is used to
relax the matrix cage, ClOO results. Note that the infrared spectrum of ClOO formed by isomerization of OClO is identical to that of ClOO formed from C1 addition to 02.This strongly suggests that ClOO is formed in a relaxed matrix cage. It is interesting to note that the photodestruction of ClOO in argon matrices is more efficient in the short-wavelength part of the absorption band (compare Figure 3A and B). We do not know if this is due to the presence of two different dissociation channels for C100.
Acknowledgment. This work was supported by the Swedish Natural Science Research Council, h u t and Alice Wallenbergs stiftelse, Stiftelsen Futura and by the NFR during our participation in the EC Environmental Programme, Contract No. EVSV-CT91-0016. References and Notes Porter, G.; Wright, F. J. Z . Electrochem. 1952, 56, 782. Porter, G.; Wright, F. J. Discuss. Faraday SOC.1953, 14, 23. Rwhkind, M. M.; Pimentel, G. C. J. Chem. Phys. 1%7,46,4481. Arkell, A.; Schwager, I. J. Am. Chem. Soc. 1%7,89, 5999. Johnston, H. S.; Moms, E. D., Jr.; van den Bogaerde, J. J . Am. Chem. SOC.1969, 91, 7712. (6) Benson, S. W.; Buss, J. H. J . Chem. Phys. 1957, 27, 1382. (7) Molina, L. T.;Molina, M. J. J . Phys. Chem. 1987, 91, 433. (8) Anderson, J. G.; Toohey, D. W.; Brune, W. H. Science 1991, 251, (1) (2) (3) (4) (5)
39. (9) Mauldin, R. L.; Burkholder, J. B.; Ravishankara, A. R. J. Phys. Chem. 1992, 96, 2582. (10) Baer, S.; Hippler, H.; Rahn, R.; Siefke, M.; Seitzinger, N.; Troe, J. J . Chem. Phys. 1991,95,6463. (11) Nicovich, J. M.; Kreuter, K. D.; Shackelford, C. J.; Wine, P. H. Chem. Phys. Lett. 1991,179, 367. (12) Peterson, K. A.; Werner, H.-J. J . Chem. Phys. 1992, 96, 8948. (13) Gole, J. L. J. Phys. Chem. 1980,84, 1333. (14) Johnsson, K.; Engdahl, A.; Ouis, P.; Nelander, B. J. Phys. Chem. 1992, 96, 5778. (15) Okabe,H.PhotochemistryofSmallMolecules;J. Wiley: NewYork, 1978. (16) Vaida, V.; Solomon, S.;Richard, E. C.; Riihl, E.; Jefferson, A. Nature 1989, 342, 405. (17) Riihl, E.;Jefferson, A.; Vaida, V. J. Phys. Chem. 1990, 94, 2990. (18) Bishenden, E.; Haddock, J.; Donaldson, D. J. J. Phys. Chem. 1991, 95, 2113. (19) Davis, H. F.; Lee, Y. T.J. Pbys. Chem. 1992,96, 5681. (20) Gmelins Handbuch der Anorganischen Chemie 8 Auflage, Chlor Ergfinzungsband Teil B-Lieferung 2, Verlag Chemie: Weinheim/Bergstr. 1969. (21) Fredin, L.; Rosengren, Kj.;Sunner, S . Chem. Scr. 1973, 4, 93. (22) Engdahl, A.; Nelander, B. J. Chem. Phys. 1989,91,6604;1990,92, 6336. (23) Fredin, L. Chem. Scr. 1974, 5, 193. (24) Mulliken, R. S. J . Chem. Phys. 1939, 7, 14.
