J. Phys. Chem. 1995,99, 3965-3968
3965
The UV and IR Spectra of the ClClO Molecule Klas Johnsson,+Anders Engdahl, and Bengt Nelander" Division of Thermochemistry, University of Lund, Chemical Center, P.O. Box 124, S-221 00 Lund, Sweden Received: August 4, 1994@
The UV and IR absorption spectra of the ClClO molecule in argon matrices have been studied. An estimate of its UV absorption cross section is given. It has a maximum value of 1.3 x cm2/molecule at 260 nm. The intensities of the infrared bands of ClClO and ClOCl have been determined.
Introduction The ClClO molecule was first reported in the work of Rochkind and Pimentel,' who irradiated argon and nitrogen matrix-isolated ClOC1. They found two of the fundamentals of ClC10; the third was not observed since its position was located below the lower wavenumber limit of their spectrometer. This band was found by Chi and Andrews2at 239.4 cm-'. They used both Raman and infrared spectroscopy to detect ClClO, which was produced in argon matrices by photoisomerization and by proton beam irradiation of ClOC1. In this work, they also labeled ClClO with lSO, and from a normal coordinate analysis, they found that the ClClO molecule can be described as a C1 atom weakly bound to a C10 molecule. Recently, Lee3 made a comprehensive theoretical study of the ClClO molecule. Among other things, he investigated its structure, vibrational frequencies, and IR band intensities. He also estimated the isomerization energy between the ClOCl and ClClO molecules. He found ClClO to be the less stable species with an isomerization energy of 14.9 kcal/mol. We have prepared the ClClO molecule by irradiation of ClOCl in argon matrices. The W absorption cross section of the ClClO molecule and the intensities of its infrared bands have been estimated. The results of our measured infrared band intensities are compared with the values calculated by Lees3
Experimental Section ClOCl was prepared by a method similar to that of Rochkind and PimenteL4 Chlorine (Matheson, 99.5%), degassed before use, was passed several times through a tube filled with dry yellow mercury(II) oxide (Merck p.a.) at room temperature.The resulting gas mixture was condensed with C02(s), and the remaining chlorine was removed by pumping on the mixture for 5 min. ClOCl was stored in liquid nitrogen and was degassed before use. Argon (L' Air liquide, 99.9995% pure) was passed through a glass spiral immersed in liquid oxygen before use. The purity of ClOCl was checked by W and IR spectroscopy. A small amount of OClO (less than 1%) was the only detected impurity, and it occurred only in the first fraction of a given batch of ClOC1. The UV absorption cross section of ClOCl at room temperature at 260 nm was found to be 1.86 f 0.05 x cm2/molecule,in good agreement with the values given by L i d (1.83 x cm2/molecule)and by Molina and Molina6 (1.94 x cm*/molecule). Gas mixtures of ClOCl and argon were sprayed on a CsI window cooled by an Air Products CS208 refrigeration system. Permanent address: Linkoping Institute of Technology, Department of Physics and Measurement Technology, S-58 1 83 Linkoping, Sweden. Abstract published in Advance ACS Abstracts, February 15, 1995. @
0022-3654/95/2099-3965$09.00/0
The deposition rate was 10 mmol/h and the deposition temperature 17 K. The deposition time was normally 30 min. 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.7 To avoid ClOCl decomposition in the needle valve, argon was passed over ClOCl kept at a fixed temperature between -132 and -127 "C depending on the desired concentration. An increase of the temperature by 2-3 "C approximately doubled the ClOCl concentration. For a temperature of -130 "C, the matrix contained approximately 1% ClOC1. ClClO was produced in the matrix by the photoisomerization of C10Cl.'~2*8 In all experiments, a 300 W xenon lamp was used for irradiation. The radiation passed through a "filter A ' solution which transmits radiation between 320 and 428 nm.9 The radiation was concentrated on the CsI window of the cryostat with a pair of quartz lenses. Infrared spectra were recorded between 200 and 4000 cm-' with a Bruker 113v FTIR instrument. The resolution used was 0.5 cm-' between 200 and 500 cm-' and 0.1 cm-' between 500 and 4000 cm-'. Spectra were 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. Caution. ClOCl is explosive and must be handled with care!
Results The UV Absorption Cross Section of ClClO. After deposition of ClOCl in an argon matrix at 17 K, the UV and IR absorbance spectra of the matrix were recorded. The observed UV absorbance spectrum is shown in Figure 1 (lower curve). The absorbance of ClOCl, denoted Dclocl, was measured at 260 nm. After irradiation of the matrix, UV and IR absorbances were recorded again. A fraction r of the ClOCl molecules had then been converted to C1C10.'.2~8 7 was determined from the decrease of the infrared band of ClOCl at 638 cm-I. Typically, 7 was 0.15 after 10 min of irradiation and 0.38 after 120 min when a photostationary state had been reached. The recorded W absorbance spectrum of the matrix after irradiation is shown in Figure 1 (upper curve). The value of the absorbance at 260 nm is denoted D,,,. Assuming that the [ClOCl] f [ClClO] is constant (see below), the absorption cross section of ClClO at 260 nm, uclclo, was estimated using the expression ~ c , c ,= o
(r - 1 + ~ s " I n ~ ~ c l o c l ~ ~ c l o c l ~ 7
Figure 2 gives the UV absorption cross section of ClClO, 0 1995 American Chemical Society
Johnsson et al.
3966 J. Phys. Chem., Vol. 99, No. 12, 1995
3.4
a, 0 C
m
n i 0 10
n Q
Figure 1. Recorded UV absorbance from an argon matrix containing approximately 2 9 ClOCl (lower curve). The upper curve shows the same
matrix after irradiation.
1.5
c
0
.ri
c i a O
h
a 3 0 a
L')
1010 0 O L
E
c
&' E 0
1.0
o\ O
*a
0.5
L b
O d i n 0
n + m
>
3
x
0.0 W a v e l e n g t n r,m
Figure 2. Absorption cross section of the ClClO molecule obtained in an argon matrix at 17 K.
corrected for background and ClOCl absorption. We assume that the W absorption cross sections of UCICIO and UCIOCI are perturbed by the matrix in the same way and therefore refrain from applying the Polo-Wilson correction.'O Our value of uclclo is therefore an estimate of the gas phase absorption cross section. aclclo was determined from five experiments with different initial ClOCl concentrations. A value of uclclo of 1.3 x lo-'? cm*/moleculewas obtained. The major sources of error in the estimate of aclclo are the determination of 7 (&lo%,the IR bands of ClOCl are very weak) and the determination of DCIOCI (approximately 110% due to the uncertainty of the base line in the UV spectra (see Figure 1)). The Intensity of the Infrared Bands of ClClO. The infrared spectrum of ClClO in argon matrices has been studied by several In Figure 3, the three fundamentals of ClClO are shown. The strongest bands (the C1-0 stretch, at 961.7 and 953.4 cm-I of CP5C10 and C137C10,respectively) were resolved further into two bands by using 0.1 cm-I resolution (the 35ClC10to 37ClC10isotope shift is -0.7 cm-' (Figure 3)). The main peak of the CI-CI stretch of ClClO was found at 374.1 cm-I. Since the two chlorine atoms of ClClO are inequivalent, this fundamental was split into four peaks with
intensities in the ratio 9:3:3:1 by the 35C1-37C1 isotope shift2 (Figure 3b). The bend was observed at 240 cm-' (Figure 3c). To estimate the infrared band intensities of the ClClO molecule, the concentration of ClOCl per surface unit of the matrix was calculated before irradiation from ( ~ 4 C I O C l= ~clocl/(loge)%,,,
Dclocl is the UV absorbance of ClOCl in the matrix (see Figure l), ucloc~is its UV absorption cross section, C is the ClOCl concentration in the matrix (molecules/cm3), and d is the matrix thickness in centimeters. We assume that the effect of the matrix on absorption cross sections of both UV and IR bands is described by the Polo-Wilson equation.I0 Since we try to estimate the gas phase intensities of the infrared bands, we use the gas phase value, uclocl, in the equation above. The infrared band intensities, A(k)clocl, of ClOCl were estimated using A(~),IOCl
= ~ ~ c l o c l ~ ~ c l , c l ~ J ; , ~dv ~~(ldT)
where the integral is extended over all isotopic components of
The W and IR Spectra of the ClClO Molecule
J. Phys. Chem., Vol. 99, No. 12, I995 3967
where the integral is extended over all isotopic components of the bands. Table 1 summarizes our results on the infrared spectrum of ClClO. The total uncertainties in A(k)clc~oare estimated to be 550%.
950
FIGURE 3 b
a
965
W a v e n u m b e r c mI
0 C
ru
n L
360
Wave n u m b e r
FIGURE
0
cm-
380
cm-'
255
3~
I
225
Wavenumber
Figure 3. Three IR fundamentals of ClClO obtained in an argon matrix at 17 K: (a) the C10 stretching region, (b) the ClCl stretching region, and (c) the bending region.
TABLE 1: IR Data of ClClO and ClOCl ClClO"
fundamental VI
v2 v3
ClOCl"*b
wavenumber (cm-')
band intensity (kdmol)
wavenumber (cm-l)
band intensity (kdmol)
374.2 (336) 240.2 (214) 961.9 (918)
28 (33) 6 (5) 31 (38)
638.9 (624) (288) 677.1 (667)
0.7 (1) (0.1) 0.5 (4)
a The values in the parentheses are calculated by Lee.3 In this work, the intensity of the ClOCl bending band was too weak to be observed.
the bands. The estimated errors of A(~)CIOCI are 530%. The results of the IR band intensity estimate are tabulated in Table 1. The bend of ClOCl was too weak to be measured. After photoisomerization, the infrared and W spectra were remeasured. Assuming that ClOCl was converted to ClClO without loss, the concentration of ClClO per surface unit of the matrix is given by
Discussion The UV absorption cross section and the infrared band intensities of ClClO have been obtained under the assumption that ClOCl photoisomerizes to ClClO without loss. In the gas phase, irradiated ClOCl loses a chlorine atom. The general experience from matrix experiments is that chlorine atoms from photodissociations do not leave the matrix cage where they are formed, unless they have a very large excess energy. The C1C10 pair formed after excitation will therefore either react to ClClO or re-form ClOCl. In fact, when we irradiated ClOCl in argon matrices, a photostationary state was reached after approximately 30 min and further irradiation did not change the ClOCl or the ClClO concentrations. When ClOCl was photolyzed in an oxygen-doped argon matrix, no ClOO was formed,8 supporting the assumption that the chlorine atom cannot move to the matrix. It therefore seems clear that the ClOCl present before the irradiation is partioned between ClOCl and ClClO after irradiation, as was assumed to obtain the ClClO infrared band intensities. Lee's calculated band positions for the symmetric and antisymmetric C10 stretches of ClOCP are in excellent agreement with their measured positions. The calculated intensity of the symmetric stretch is in good agreement with our measured intensity, but the calculated intensity of the antisymmetric C10 stretch is a factor of 8 larger than the measured value. In the gas phase spectra of Rochkind and Pimentel? v3 looks more intense than V I , but in their solid state spectra, V I is more intense. Rochkind and Pimentel suggest that the larger gas phase intensity of v3 as compared with V I is only apparent and due to a more concentrated rotational fine structure of v3. It seems unlikely that the argon matrix perturbs ClOCl so strongly that the intensity ratio of the two C10 stretches changes by 1 order of magnitude. We therefore suggest that the calculated intensity of v3 is too high. For ClClO, both the calculated band positions and the corresponding intensities are in good agreement with our measured values. The shape of the W absorption band of matrix-isolated ClOCl is similar to its gas phase band. In order to estimate the W absorption cross section of ClC10, one has to assume that the effect of the matrix on the W absorption spectra of ClOCl and ClClO is similar and the estimate given above refers to gaseous ClClO. The Polo-Wilson equationlo predicts a 16% increase in the intensity when going from gas to solid argon ( n h = 1.29"), which is less than our experimental uncertainties. If we assume that this equation gives an estimate of the order of magnitude of the matrix perturbation, this is smaller than our experimental uncertainties.
The integrated band intensity A(~)CICIO of each fundamental k of the ClClO molecule (base e) is calculated as
Acknowledgment. This work was supported by the Swedish Natural Science Research Council, h u t and Alice Wallenbergs Stiftelse, Stiftelsen Futura, and NFR during our participation in the EC Environmental Programme, Contract No. EVSVCT9 1-0016.
Measuring the area ~ & g ( ~ d l dv ) (cm-I) of each IR fundamental k of the ClClO molecule and using the expression of (Cd)clclo above, we find that A(k)clclo is given by
References and Notes (1) Rochkind, M. M.; Pimentel, G. C . J . Chem. Phys. 1967, 46, 4481.
Johnsson et al.
3968 J. Phys. Chem., Vol. 99, No. 12, 1995 (2) Chi. F. K.; Andrews, L. J . Phys. Chem. 1973, 77, 3062. (3) Lee, J. L. J . Phys. Chem. 1994. 98, 3697. (4) Rochkind. M. M.; Pimentel. G. C. J . Chem. Phys. 1965. 42. 1361. ( 5 ) Lin, C. L. J . Chem. Eng. Dura 1976, 21, 411. (6) Molina, L. T.; Molina, M. J. J . Phys. Chem. 1978. 82, 2410. (7) Engdahl, A.: Nelander, B. J . Chem. Phys. 1989, 91, 6604; 1990, 92. 6336.
(8) Johnsson, K.; Engdahl. A,: Ouis. P.; Nelander, B. J . Phys. Chem. 1992, 96, 5778. (9) Calvert, J. G.: Pitts, J. N. Photochemist?: J. Wiley: New York, 1966; p 728. (10) Polo, S. R.; Wilson. M. K. J . Chem. Phys. 1955. 23. 2376. (11) Sinnock. A. C.; Smith, B. L. Phys. Lett. 1968, 28A. 22.
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