J. Phys. Chem. 1996, 100, 3923-3926
3923
IR and Photodecomposition Spectroscopic Study of HOClO and HClO2 in Argon Matrices Klas Johnsson,† Anders Engdahl, and Bengt Nelander* Chemical Center, Thermochemistry, P.O. Box 124, S-221 00 Lund, Sweden ReceiVed: September 27, 1995; In Final Form: December 5, 1995X
HOClO and HClO2 were produced by addition of hydrogen atoms to OClO on the surface of growing argon matrices at 17 K. They were identified using infrared spectroscopy. Their photodecomposition spectra were measured.
Introduction The oxides and oxo acids of chlorine have been the subject of intense studies ever since Rowland and Molina1 pointed out that chlorine atoms from the photolysis of freons may play an important role in the destruction of stratospheric ozone. Theoretical calculations2-5 indicate the existence of three stable isomers of HO2Cl: the peroxide HOOCl, the acid HOClO, and finally HClO2, where the hydrogen atom is bound to chlorine. It has been suggested that some of these compounds can act as reservoir compounds for stratospheric chlorine.3,4 The calculations of Francisco et al.4 predict that HOOCl is the most stable isomer and it has its two lowest electronically excited states at 4.3 and 5.5 eV. The oscillator strengths of the transitions to these states are predicted to be small; the first state which is expected to give a strong UV absorption is found at 7.3 eV. The energy of formation of HOClO is calculated to be 13.5 kcal/mol more endothermic that of HOOCl.4 For this isomer, the transition to an electronically excited state at 3.8 eV is estimated to have a nonzero intensity. The least stable of the three isomers is HClO2, with an energy of formation 57.7 kcal/ mol more endothermic than that of HOOCl.4 The calculations predict that it has a relatively strong UV band at 6.5 eV.4 All three compounds would form from hydrogen atoms and OClO in exothermic reactions, but these reactions would require the participation of a third body to stabilize the reaction product, which would make them very slow under stratospheric conditions. HOOCl and HOClO could form from exothermic reactions between HOO and OClO. We have added hydrogen atoms, obtained by passing argon mixed with molecular hydrogen or water through microwave discharges, to chlorine dioxide on the surface of growing argon matrices at 17 K. A number of bands appear, which are absent in discharge experiments with no chlorine dioxide or when the gas mixtures were co-condensed with the discharge off. Two sets of bands, which differed in their photochemical behavior were identified. By comparing the results from experiments with hydrogen or deuterium atoms with calculated infrared spectra, the two products HOClO and HClO2 were identified. We have also measured the rate of decomposition as function of wavelength for the two compounds (photodecomposition spectrum). Experimental Section Chlorine dioxide was prepared by dripping sulfuric acid (Merck p.a.) on potassium perchlorate (Fluka purum p.a. † Permanent address: Linko ¨ ping Institute of Technology, Department of Physics and measurement technology, S-581 83 Linko¨ping, Sweden. X Abstract published in AdVance ACS Abstracts, February 15, 1996.
0022-3654/96/20100-3923$12.00/0
>99.0% pure). The chloride dioxide was collected at -78 °C. Chlorine was removed from the crude product by pumping for 5 min at -78 °C. Gas mixtures of hydrogen (deuterium) or water and argon were prepared by standard manometric techniques. The hydrogen and deuterium concentrations used correspond to between 1:100 and 1:33 if all hydrogen (deuterium) had been trapped in the matrix. Since the needle valves used to regulate the gas flows at the deposition catalyze the decomposition of chlorine dioxide, it was introduced into the matrix by passing a regulated flow of argon over solid chlorine dioxide, kept at ca -120 °C. The flows of the two gas streams were regulated with Nupro needle valves. The total deposition rate was 10 mmol/h, the deposition time 2 h, and the deposition temperature 17 K. The matrices were formed on the combined CsI-sapphire window of a Displex 208 cooled cryostat. The argon containing the hydrogen atom source was passed through a quartz tube where a discharge was excited using an Evenson cavity and a microwave generator (Opthos MPG 4). The gas flow had a 90° bend immediately before entering the cryostat, to minimize the irradiation of the matrix. The quartz tube was treated with sulfuric acid before each experiment. Infrared spectra between 220 and 4000 cm-1 at 0.5 cm-1 resolution were recorded with a Bruker 113v FTIR spectrometer. Spectra were obtained at 17 K. A 300 W Xe lamp was used for the irradiation experiments. After an initial spectrum had been recorded, the matrix was irradiated for approximately 1 h through a CuSO4 solution, which passes only radiation at wavelengths longer than 350 nm.6 After a new IR spectrum had been recorded, the matrix was irradiated with the full radiation from the Xe lamp for approximately 30 min, using a water filter to remove the nearinfrared radiation from the lamp. Photodecomposition spectra were obtained using the following procedure. First an initial spectrum was recorded. Then the matrix was irradiated for a fixed time at the desired wavelength band using a Oriel 1/8 m monochromator. The new spectrum was ratioed against the previous spectrum. This procedure was repeated for a new wavelength until the complete spectrum had been obtained. The irradiation time was kept short enough to keep the loss of the compounds under investigation small, but large enough to give measurable changes in intensity in its most intense infrared absorption bands. The cryostat was held in a fixed position through the entire measurement procedure. This made it possible to measure absorbance changes as small as 0.001. The photodecomposition spectra given in Figure 3 have been compensated for changes in lamp intensity and grating efficiency. Since the deuterated compounds were formed in higher yields and had sharper absorption © 1996 American Chemical Society
3924 J. Phys. Chem., Vol. 100, No. 10, 1996
Johnsson et al.
TABLE 1: Comparison between Measured and Calculated Vibrational Frequencies (cm-1) and Band Intensities of the H/DOClO and the H/DClO2 Isomers calcd ref 4 (km/mol) molecule
assign
HOClO
ν(ClO) ν(HO) ν(OCl) δ(HOCl) δ(OClO) τ νas(ClO2) νs(ClO2) ν(ClH) δ(OClO) νs(HOCl) δas(HOCl) ν(ClO) ν(DO) ν(OCl) δ(DOCl) δ(OClO) τ νas(ClO2) νs(ClO2) ν(ClD) δ(OClO) δs(DOCl) δas(DOCl)
HClO2
DOClO
DClO2
a
exptl (rel intensities)a this work 973.9c (0.6), 965.0d 3527.1 (1.0) 591.5c (0.2), 589.9d 1176.9 (0.2) 1132.0c (1.0), 1123.8 969.4c (0.4), 963.2d 2116 (0.2)
974.7c (1.0), 966.2d 2605.4 (0.9) 594.5c (0.3), 592.7d 864.5 (0.2) 1115.9c (1.0), 1104.8d 969.3c (0.4), 962.4d 1550.4c (0.1), 1547.5d
ref 5 (km/mol) MP2/ECPb 1140 (171) 3765 (70) 552 (215) 1115 (41) 289 (6) 407 (131) 1186 (214) 1018 (84) 2231 (72) 412 (16) 1036 (24) 1020 (107) 1129 (164) 2741 (42) 550 (215) 824 (33) 266 (7) 328 (80) 1149 (313) 1019 (90) 1605 (38) 408 (15) 779 (3) 764 (18)
MP2/6-311G(2d)
CCSD(T)/TZ2P
1172 (85) 3654 (58) 584 (149) 1206 (88) 422 (153) 302 (2) 1227 (224) 1100 (69) 2243 (226) 424 (10) 1065 (36) 1101 (56)
935 (68) 3755 (67) 540 (86) 1186 (43) 393 (111) 249 (1) 1093 (75) 1022 (0.1) 2168 (123) 415 (13) 905 (60) 1009 (127)
ref 2 (D2 Å-2 amu-1) MP2/6-31G* 1171 (2.0) 3649 (2.0) 582 (3.5) 1206 (2.3) 303 (0.05) 423 (3.5)
The relative intensities are the sum of the contribution from all sites and chlorine isotopes. b See ref 17. c Observed 35Cl. d Observed 37Cl.
Figure 1. Infrared absorption bands of HOClO (lower curves) and DOClO (upper curves) in argon matrices. (a and b) the (H)OCl(O) and (HO)ClO stretches, (c) the DOCl(O) bend, (d) the HOCl(O) bend, and (e, f) the DO and HO stretches, respectively.
bands, the photodecomposition spectra were obtained from D2 experiments. The DO stretch at 2605.4 cm-1 was used to monitor the DOClO concentration, and the asymmetric stretch at 1115.9 cm-1 was used for the DClO2 concentration. Caution: Potassium chlorate and especially chlorine dioxide are explosiVe and must be handled with care. Results Five experiments with H2, four with D2, and one with an H2-D2 mixture as sources of hydrogen atoms were carried out. In addition to the bands of chlorine dioxide and traces of water and carbon dioxide, the initially recorded spectra had a number of bands, which were observed only when the discharge was active. The new bands could be separated into two sets from their photochemical behavior. The first set was eliminated by irradiation with wavelengths longer than 350 nm, while the
Figure 2. Infrared absorption bands of HClO2 (lower curves) and DClO2 (upper curves) in argon matrices. (a, b) the symmetric and antisymmetric ClO stretches and (c, d) the DCl and HCl stretches, respectively.
second set was eliminated by irradiation with the full output of the Xe lamp. The bands assigned to HOClO (DOClO) and to HClO2 (DClO2) are given in Table 1 and they are depicted in Figures 1 and 2. They were shown to be due to two separate compounds, by concentration dependency and photodecomposition kinetic studies. The assignment to the particular HO2Cl isomers will be discussed below. In addition to these bands, a few bands for which the concentration and kinetic studies showed that they were not due to the two compounds above were observed, and they are collected in Table 2. In H2 experiments we noted a band at 1388.6 cm-1 due to HOO7 and bands at 1240.0 cm-1 and in one experiment also at 3586.4 cm-1 due to HOCl.8 The corresponding deuterated compounds were observed in the D2 experiments. The discharge always produced some HCl or DCl depending on the hydrogen atom source. Traces of ClOClO with bands at 994.4 and 986.0 cm-1 9
HOClO and HClO2 Argon Matrices
J. Phys. Chem., Vol. 100, No. 10, 1996 3925
TABLE 2: Observed Bands Directly after the Deposition in the Hydrogen and Deuterium Experiments. Bands due to HOClO/DOClO and HClO2/DClO2 Are Given in Table 1 molecule
assignment
wavenumber (cm-1)
Aa
Bb
DOCl (OClO)n OClO‚H2O OClO ClOClO′ ? OClO‚H2O OClO
δ(DOCl) aggregate νs νs ν(ClO′)
D H/D H/D H/D H/D H/D H/D
C C C C W C
HOCl ? ? HOO ? ? ? ?
δ(HOCl)
910.9 925 933.8-938.7c 941.1, 947.1, 948.5 986.0-994.4c 1020.0 1078.8-1090.6c 1089.0-1100.8,c 1094.6-1106.4,c 1096.2-1108.2c 1240.0 1316.6 1357.7 1388.6 1462.4 1473.0 2402 3353
νas νas
ν(OO)
H/D H H H H H/D H/D D H
C C
C W
a Observed in the hydrogen (H) and/or in the deuterium (D) experients. b Bands that vanished under irradiation through a CuSO4 solution are marked with C. Bands that vanished under irradiation through a water filter are marked with W. Bands which do not change are not marked. c Observed 37Cl-35Cl pair. Bands from the most populated site are in italics.
Figure 3. The photodecomposition spectra of DClO2 and DOClO.
were observed in most experiments. Irradiation of the matrix transformed OClO into ClOO as has been reported earlier.10 When the OClO concentration was large, we observed the formation of Cl2O4 with bands at 646.3, 1040.6, 1277.6, and 1289.2 cm-1 11 after the first irradiation. In the initial spectra of these experiments, the broad OClO aggregate band at 925 cm-1 was observed. One experiment was carried out with water as the hydrogen atom source. Both the bands assigned to HOClO and HClO2 above were observed. In addition we had strong bands due to HOO (1100.9, 1388.6, and 3412.7 cm-1 7 and to ClO3 (562.7, 899.4, and 1072.1 cm-1 12). The photodecomposition spectra of DOClO and DClO2 are shown in Figure 3. Discussion HOClO, DOClO. Four bands were observed at 3527.1, 1176.9, 973.9, and 591.5 cm-1. The 973.9 and 591.5 cm-1 bands have 37Cl satellites at 965.0 and 589.9 cm-1, respectively. These bands appeared after deposition and were eliminated by the first photolysis, using wavelengths longer than 350 nm. Their intensities had constant ratios both when different initial concentrations were used and in photodecomposition kinetic experiments. A similar set of bands was observed in the D2 experiments at 2605.4, 974.7 (37Cl at 966.2), 864.5, and 594.5 (37Cl at 592.7) cm-1. In the mixed experiment (H2 + D2) both sets were observed, but no new bands were seen. The new compound therefore contains one hydrogen. The presence of
a band at 3527.1 cm-1, which shifts to 2605.4 cm-1 with deuterium, shows that the new compound has an OH bond. The band at 1176.9 cm-1 which shifts to 864.5 cm-1 with deuterium can then be assigned to the corresponding HOX bend from a comparison with HOCl. The HOCl bend is found at 1240.0 cm-1 and the DOCl bend at 910.9 cm-1.8 The 973.9 cm-1 band has a large 35Cl to 37Cl shift; its position is near the ClO stretch of the terminal ClO in XOClO (X ) Cl, Br, or I; refs 9 and 13) indicates that the new compound has the structure HOClO. The position of the 591.5 cm-1 band supports this assignment since its position is close to what one expects for the (H)OCl(O) stretch. The suggested assignment implies that the hydrogen atom binds to an oxygen atom of OClO without any further rearrangement, similar to what was found for halogen atoms.13 The positions of the assigned bands are in reasonable agreement with the calculated band positions of HOClO given by McGrath,2 Francisco et al.,4 and Nieminen.5 HClO2, DClO2. In addition to the bands assigned to HOClO we observe a set of peaks which is due to a single compound according to concentration dependency and photodecomposition kinetic studies. These peaks vanish together when the matrix is irradiated with the full output from the Xe lamp but are unaffected by radiation with wavelengths longer than 350 nm. We observe a corresponding set of peaks in the D experiments. These peaks are given in Table 1. The observation of a band at 2116 cm-1 which shifts to 1550.4 cm-1 with deuterium indicates the presence of a weak HX bond in the new compound. Both bands are rather broad, but the D band has a shoulder shifted from the main band by an amount expected for a 35Cl-37Cl shift. If the hydrogen atom simply adds to the chlorine atom of OClO the new HCl bond is likely to be weak and might have a HCl stretching fundamental in the appropriate position. Both the H and D compounds have several bands in the 1100 and 960 cm-1 regions (Figure 2). Each of the stronger peaks is accompanied by a weaker peak in the position where the 37Cl satellite of a Cl-O stretch is expected. The positions of the bands are close to where the antisymmetric and symmetric Cl-O stretches of the Y-shaped XClO2 (X ) Cl, Br, or I) compounds and of OClO are found.13 The question remains if all peaks are due to the antisymmetric and symmetric ClO stretches from molecules in different trapping sites or if there is a near coincidence of the stretches with for instance a bending fundamental. The calculated band positions (refs 2, 4, and 5) suggest this possibility. We believe that the splittings are due to different trapping sites, however, since the relative intensities of the bands appear to be influenced by the hydrogen concentration. We note in particular that more D2 is trapped in the matrix upon deposition than H2, we therefore expect that some molecules may have small vibration frequency shifts due to the presence of a D2 in the same trapping site. Addition of Atoms of OClO. We have previously shown that halogen atoms (Cl, Br, and I) exclusively add to an oxygen atom of OClO.13 This is surprising, since the ClOClO isomer is calculated to be 9.2 kcal/mol less stable than ClClO2, which would result if the attacking chlorine atom added to the chlorine atom of OClO.14 Oxygen atoms add to the chlorine of OClO forming ClO3,15 which is calculated to be 10 kcal/mol more stable than the O addition product OClOO.16 Hydrogen atoms seem to add to the chlorine and oxygen of OClO with comparable probability, in spite of the fact that the HClO2 isomer is calculated to be 44.3 kcal/mol less stable than the HOClO isomer.4 It is clear that the reaction paths are not controlled by the energetics of the overall reaction. We note that hydrogen atoms, which are spherically symmetrical, do not discriminate between the two possible reaction paths, while
3926 J. Phys. Chem., Vol. 100, No. 10, 1996 halogen atoms and oxygen atoms, which have nonspherical ground states, choose exclusively the oxygen or the chlorine reaction site, respectively. Photodestruction Spectra. The measured photodecomposition spectra are given in Figure 3. They give the absorption spectrum of the transition to the lowest electronically excited state for each of the two molecules, weighted with the quantum yield for decomposition. Francisco et al.4 do not report if their calculated excited states are bound or not, so we do not know which are the lowest repulsive states. We note that HOClO is calculated to have a transition with a significant intensity at 330 nm, not too far from the measured peak of the photodecomposition spectrum of HOClO at 400 nm. It is interesting to compare with the photoisomerization spectra of XOClO13 where for X ) Cl the peak is found at 450 nm with X ) Br at 525 nm and X ) I at 575 nm. For HClO2 the first calculated electronic excitation is close to 190 nm and its photodecomposition spectrum has a peak at 290 nm. For the XClO2 compounds, the peaks in the photodecomposition spectra are found at 300 nm for chlorine, at 325 nm for bromine, and at 350 nm for iodine. The ordering of the peaks is the same as for the XOClO compounds. In our opinion the agreement between the calculated first transitions of HOClO and HClO2 and the peaks in the photodecomposition spectra is quite reasonable and suggests that the calculated excited states are repulsive. Photodecomposition Products. In the case of the halogen atom additions to OClO, the initially formed isomers XOClO rearranged to XClO2 when they were irradiated in the visiblenear-UV region of the spectrum.9,13 One possibility could be that excited XOClO dissociates into a spherically symmetric 2P 1/2 halogen atom and OClO and that this halogen atom now can bind to the chlorine of OClO, forming XClO2. When ClO3 is irradiated, it rearranges to OClOO or to ClOOO. In the case of HOClO and HClO2 we have not been able to detect the decomposition products. They do not rearrange to the other isomer. The most likely possibility, HCl and oxygen, may not
Johnsson et al. be easy to detect, since there is always some HCl and oxygen present in the matrix directly after deposition, possibly as a result of decomposition of intially formed HOClO or HClO2 by reaction with metastable argon atoms from the discharge. O2 and HCl form a weak complex, and the matrix cage this complex inherits from its parent compound may have a shape which does not allow it to take up its most stable conformation, thereby broadening its HCl band so much that it becomes hard to detect. Acknowledgment. This work was supported by the Swedish Natural Science Research Council and the Swedish Environmental Protection Agency. The authors thank Dr. J. Nieminen for permission to use his calculations prior to publication. References and Notes (1) Molina, M. J.; Rowland, F. S. Nature 1974, 249, 810. (2) McGrath, M. P.; Clemitshaw, K. C.; Rowland, F. S.; Hehre, W. J. J. Phys. Chem. 1990, 94, 6126. (3) Lee, T. J.; Rendell, A. P. J. Phys. Chem. 1993, 97, 6999. (4) Francisco, J. S.; Sander, S. P.; Lee, T. J.; Rendell, A. P. J. Phys. Chem. 1994, 98, 5644. (5) Nieminen, J. Personal communication. (6) Calvert, J. G.; Pitts, J. N. Photochemistry; J. Wiley: New York, 1966; p 728. (7) Jacox, M. E.; Milligan, D. E. J. Mol. Spectrosc. 1972, 42, 495. (8) Schwager, I.; Arkell, A. J. Am. Chem. Soc. 1967, 89, 6006. (9) Jacobs, J.; Kronberg, M.; Mu¨ller, H. S.; Wilner, H. J. Am. Chem. Soc. 1994, 116, 1106. (10) Arkell, A.; Schwager, I. J. Am. Chem. Soc. 1967, 89, 5999. (11) Christe, K. O.; Schack, C. J.; Curtis, E. C. Inorg. Chem. 1971, 10, 1589. (12) Grothe, H.; Willner, H. Angew. Chem., Int. Ed. Engl. 1994, 33, 1482. (13) Johnsson, K.; Engdahl, A.; Ko¨lm, J.; Nieminen, J.; Nelander, B. J. Phys. Chem. 1995, 99, 3902. (14) Lee, T. J.; Rohlfing, C. M.; Rice, J. E. J. Chem. Phys. 1992, 97, 6593. (15) Johnsson, K. Manuscript in preparation. (16) Rathmann, T.; Schindler, R. N. Chem. Phys. Lett. 1992, 190, 539. (17) Pacios, L. F.; Christiansen, P. A. J. Chem. Phys. 1985, 82, 2664. Wallace, N. M.; Blaudeau, J. P.; Pitzer, R. M. Int. J. Quantum Chem. 1991, 40, 789.
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