ISSN 00360244, Russian Journal of Physical Chemistry A, 2015, Vol. 89, No. 1, pp. 28–37. © Pleiades Publishing, Ltd., 2015. Original Russian Text © N.E. Strokova, S.V. Savilov, I.I. Morozov, T.V. Yagodovskaya, V.V. Lunin, 2015, published in Zhurnal Fizicheskoi Khimii, 2015, Vol. 89, No. 1, pp. 33–42.
CHEMICAL KINETICS AND CATALYSIS
Laboratory Simulations of the Interaction between Ozone and Chloroacetic Acids in the Conditions Close to Stratospheric N. E. Strokovaa, S. V. Savilova, I. I. Morozovb, T. V. Yagodovskayaa, and V. V. Lunina a
b
Department of Chemistry, Moscow State University, Moscow, 119991 Russia Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, 117977 Russia email:
[email protected] Received February 7, 2014
Abstract—The interaction between ozone and mono, di, and trichloroacetic acids are studied using a flow vacuum gas discharge setup in a regime close to stratospheric conditions (in the temperature range of 77 to 250 K, at pressures of 10–3 to 0 Torr, and in the presence of ice). The interaction between ozone and trichlo roacetic acid starts at 77 K, while interaction with monochloroacetic acid begins when the temperature is raised to 200 K. The reactions are assumed to proceed via different mechanisms: chlorine oxides of different composition are the reaction products, as is shown using lowtemperature IR spectroscopy. Preliminary adsorption of the acids on a surface of ice raises the temperature of interaction to 190 K. Keywords: ozone, chloroacetic acids, chemistry of the stratosphere. DOI: 10.1134/S0036024415010264
INTRODUCTION
during the production and processing of vinyl chloride [2] and the disinfection of drinking water; it can also be formed by combustion due to forest fires and the burning of waste [3]. Monochloroacetic acid is an intermediate in the synthesis of dyes. Atmospheric air flows can carry chloroacetic acids far from the sources of their formation. Along with their behavior in aque ous solutions and on the surfaces of particles in strato spheric clouds and aerosols, the genesis of their prod ucts of degradation is thus among our greatest ecolog ical problems. Laboratory simulations of processes in the upper stratosphere are complicated experimental tasks, as they require us to consider a number of factors. The most important of these are the multicomponent and heterophase characters of the investigated systems, which determine the concentration gradient of surface reactants; low temperatures and pressures; and the chains of consecutive redox reactions [1, 4]. By con sidering these factors, we can monitor the mechanism and effects of external conditions and impurities on the course of one process or another. This work continues our efforts in [5, 6] and is devoted to investigating the stratospheric transforma tion of chloroacetic acids. Such processes are hetero geneous due to polar clouds (sometimes referred to as nacreous clouds) in the stratosphere. They are mainly composed of ice crystallites that include such crystal lization sites as soot or oxide particles. It was shown earlier that HCl and HBr begin to interact with ozone upon the dissociation of halide adsorbed on surfaces of ice, followed by the diffusion of protons into the bulk phase. The halogen anions then react with polar ozone
Many studies have dealt with the problem of ozone layer degradation as one sign of the anthropogenic damage to the environment, upsetting the ecological systems and biosphere of the Earth. Nowadays, the question of why we should lower the concentration of stratospheric ozone is of the greatest importance: Is it a selfregulating natural process or a consequence of scientific and technological progress? As a rule, the reaction cycles of ozone degradation in the stratosphere are photochemical and the reac tions are associated with the presence of active radicals of halogens, nitrogen oxides, and HO *2 , of which the first has the greatest effect [1]. The problem of their emergence in the lower stratosphere continues to be important. Chloroacetic acids belong to a wide class of environmentally abundant organochlorine substances and are very active chemical compounds that nega tively affect the environment and living organisms [2, 3]. At present, one natural source of trichloroacetic acid is the biological activity of microorganisms in the natural hypersaline lakes of the Caspian district and South Africa [2]. It should be emphasized that the emission of chlorinecontaining compounds from all of the world’s hypersaline lakes could exceed those of similar compounds into the atmosphere from world industry. There are also anthropogenic sources of trichloroacetic acid. Chloroacetic acids were used as herbicides in agriculture several decades ago, resulting in their accumulation in the environment. In modern industry, trichloroacetic acid can enter the atmosphere 28
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molecules to form chlorine or bromine oxides in the low oxidation state; these are transformed into higher oxides due to the interaction with ozone. The mecha nism of interaction should obviously change for reac tions between organic nondissociating compounds and ozone. This work is devoted to studying this mechanism. EXPERIMENTAL The following compounds were used in this work: oxygen from a cylinder (special purity grade, for the synthesis of ozone); bidistilled water (resistance, >6 MΩ); and commercially available (Alfa Aesar) monochloroacetic (99% purity), dichloroacetic (98% purity), and trichloroacetic acids (99% purity). The interaction of the reactants was studied in situ on the surfaces of golden mirrors in an optical vacuum cryogenic cell of a unique design using lowtempera ture FTIR spectroscopy on an FSM 1201 scanning spectrometer (OOO Infraspek, Russia) at wavelengths of 400 to 4000 cm–1 (resolution, 1 cm–1) and temper atures of 77 to 250 K. The spectrometer was finetuned by recording the spectrum of a polystyrene film (thick ness, 0.025 mm) that served as a reference. Experiments to simulate stratospheric processes were conducted following the procedure described in [5, 6] using a flow vacuum gas discharge setup, com bined with an optical cell mounted in a compartment of the IR spectrometer. Ozone was synthesized in a gas discharge tube via the recombination of discharge 1 + excited oxygen radicals (О2 ( Σg ), О(1D), О(1S)О2 (1Δg)) at graphite electrode voltages of 1.4–1.5 kV and a current strength of 180–200 mA for 5–10 min. Other reactants were supplied to the system through low pressure evaporation from a roundbottomed flask (evaporator). Prior to experiments on the laboratory simulation of atmospheric processes, the setup and the vacuum cryogenic cell were evacuated to a residual pressure of 6 × 10–4 Torr. The IR spectrum was recorded and was used as the background. The reactants were then con secutively sputtered through the vacuum valve of the evaporator onto the mirrors of the liquid nitrogen– cooled cell, and the excess of the sputtered reactants was pumped out. The spectra of the initial substances (Н2О, О3, CCl3COOH, CHCl2COOH, and CH2ClCOOH) were preliminarily recorded to achieve the best comparison and identification of absorption bands. To model the heterogeneous processes of the lower stratosphere, water and an appropriate reactant con secutively sputtered onto the cell mirrors were brought into contact with ozone after preliminary pumping to a residual pressure of 6 × 10–3 Torr. The IR spectrum was recorded after each reactant was deposited. Liquid nitrogen from the moveable part of the cell was then evaporated, raising the temperature of the condensate RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
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on the mirrors. The rise in temperature was recorded using a digital thermometer equipped with a copper– constantan thermocouple. The process was conducted in a dynamic vacuum, so the gaseous products that formed were condensed in a lowtemperature trap upstream of a forevacuum pump. The products were heated to 285 K while monitoring the changes in the absorption spectra of the system. RESULTS AND DISCUSSION Using the above procedure, two types of experi ments were conducted to achieve a better description of the system: one on the direct interaction between ozone and chlorinesubstituted acids with no ice, and one after their preliminary adsorption on the ice film formed by sputtering water vapor on the golden mir rors of the cryogenic optical cell. The spectra showing the interaction between the chloroacetic acids and ozone upon direct contact of the reactants are presented in Figs. 1–3, while those recorded after preliminary adsorption of the acid on the ice film are shown in Figs. 4 and 5. The absorption bands in the vibrational spectra of the initial chloro acetic acids are attributed to their sources in Table 1. The progress of the reaction was monitored using the emergence of new absorption bands corresponding to the vibrations of oxygen–halogen bonds. The pat terns of the processes were studied in the temperature range of 77 to 250 K. In the case of tri and dichloroacetic acids, the interaction with ozone started at 77 K, as was indi cated by the emergence in the IR spectrum of absorp tion bands corresponding to νas and νsClOCl, Cl2O3, and ClO2 [8, 9] (Figs. 1 and 2, Table 2). The spectra of acid–ozone systems continued to change over time (~15 min) at a constant temperature of 77 K, their overall intensity increased, and the bands correspond ing to the C=O and C–O–H bonds shifted. These changes in the spectrum over time were related to kinetic hindrances, i.e., diffusion and the distribution of ozone over the condensed acid surface. Spectrum intensity increased with temperature. The intensity of the band at 1037 cm–1 (νasO3) rose sharply at 135 K, indicating that unreacted ozone transitioned to the gas phase. The bands correspond ing to OH stretching vibrations (3100–3500 cm–1) also changed with temperature and shifted to shorter wavelengths, indicating the formation of intermolecu lar hydrogen bonds [10]. The overall intensity of the spectrum increases at temperatures higher than 230 K in trichloroacetic acid–ozone and dichloroacetic acid–ozone systems, due possibly to the thermal correlation of the force constant of vibrations [10]. At 240 K, the IR spectrum of the investigated system is completely equalized in terms of intensity as a result of condensed phase disordering. Vol. 89
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Fig. 1. IR spectrum for the interaction between trichloroacetic acid and ozone: (a) (1) CCl3COOH and (2–6) CCl3COOH + O3; T = (1–4) 77, (5) 145, and (6) 172 K; τ = (2) 0, (3) 2, and (4) 4 min. (b) (1–5) CCl3COOH + O3; T = (1) 200, (2) 213, (3) 220, (4) 225, and (5) 233 K. τ is the length of interaction between ozone and the investigated substance.
With monochloroacetic acid, absorption bands appear at 1040, 1315, and 1456 cm–1 (Fig. 3a). These correspond to antisymmetric stretching
vibrations of ozone and the bending vibrations of C–O–H and O ⎯ O in peroxide compounds. The extension of the band at 3000–3500 cm–1 can be
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Fig. 2. IR spectrum for the CCl2HCOOH + O3 system: (a) (1) CCl2HCOOH and (2–5) CCl2HCOOH + O3; T = (1–4) 77 and (5) 100; τ = (2) 0, (3) 2, and (4) 4 min. (b) (1–5) CCl2HCOOH + O3; T = (1) 200, (2) 210, (3) 225, (4) 235, and (5) 245 K. RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
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Fig. 3. IR spectrum for the interaction between monochloroacetic acid and ozone: (a) (1) CCl3COOH and (2–6) CCl3COOH + O3; T = (1, 2) 77, (3) 147, (4) 195, (5) 200, and (6) 220 K; τ = (2) 0–10 min. (b) CCl3COOH + O3; T = (1) 220, (2) 225, (3) 228, (4) 234, and (5) 240 K.
attributed to the formation of O–H bonds. Spec trum intensity increased in 5 min due to the redis tribution of ozone over the condensed acid surface. No new absorption bands appeared. As in the previ
ous cases, ozone evolution to the gas phase was detected spectrally as the temperature rose to 147 K. The spectrum of the system remained virtually the same up to 210 K.
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Fig. 4. IR spectrum for the H2O + CCl3COOH + O3 system: (a) (1) ice, (2) ice + CCl3COOH, and (3–6) ice + CCl3COOH + O3. T = (1–4) 77, (5) 130, and (6) 150 K; τ = (3) 0 and (4) 3–10 min. (b) Ice + CCl3COOH + O3. T = (1) 150, (2) 190, (3) 210, (4) 220, (5) 230, (6) 240, (7) 245, and (8) 250 K. RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
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Fig. 5. IR spectrum for the H2O + CCl2HCOOH + O3 system: (a) ice + CCl2HCOOH + O3; T = (1) 77, (2) 190, (3) 200, (4) 220, and (5) 240 K; (b) ice + CClH2COOH + O3; T = (1) 100, (2) 225, (3) 235, (4) 240, (5) 245, and (6) 250 K. RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
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Table 1. Attributing absorption bands (cm–1) in the IR spectra of mono, di, and trichloroacetic acids νexp CClH2COOH
νexp CCl2HCOOH
νexp CCl3COOH
Attributed to [7]
672 803 1228 1315 1439 1508 1736 2523 2620 2737 2952 3099
694 826 1214 1313 1420 1508 1745 2483 2572 2684 3016 3433
697 818 1090 1265 1418 1510 1700 – – – 2965 3289
νs C–Cl νas C–Cl ν C–O δCOH δCH fanning δCH νC=O νCH νsCH νasCH νOH dimer νO–H monomer
Table 2. Attributing absorption bands (cm–1) of chlorine oxides νexp CClH2COOH + O3
νexp CCl2HCOOH + O3
νexp CCl3COOH + O3
Attributed to [8, 9]
736 888
740 779
740 760 778 959 1317
νsCl2O2 νasCl2O Cl2O3 νasClO2 δOOH
953
The overall intensity of absorption increased after CCl3COOH was brought in contact with ozone (Fig. 1a). New bands at 740, 759.6, 778, and 959 cm–1, corresponding to Cl2O, Cl2O3, and ClO2, respectively, appeared in the spectrum. The band at 1264 cm–1 cor responding to δ(COH) in CCl3COOH shifted to 1268 cm–1, while the band formed at 1317 cm–1 corre sponded to the bending vibrations of the OOH angle. The formation of (CO)–OOH groups is indicated by the emergence of the absorption band at 1754 cm–1 in the spectrum [7]. The overall intensity of the spectrum continued to grow over time (Fig. 1a), and a band cor responding to the asymmetric stretching vibrations of ozone appeared at 1040 cm–1. The intensities of the bands at 817, 1090, and 1267 cm–1 attributed to vibrations in CCl3COOH molecules fell as the temperature rose to 200 K (Fig. 1b). The intensities of the bands at 640, 740, 760, 778, 952, and 1318 cm–1 and in the regions of C–H and O–H stretching vibrations (2600–3350 cm–1) grew, indicating the consumption of the acid in the reaction. The intensity of the band at 2684 cm–1 increased as the temperature rose to 213 K. A substantial change in the absorption spectrum was detected at 220 K: new bands appeared at 860, 1140, 2256, 3364, and 3484 cm–1, corresponding to RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
the formation of chlorine oxides and O–H bonds in formic acid, and the overall intensity of the bands at 1650–3330 cm–1 increased. The vibration frequencies corresponding to the Cl–O bonds in ClO2 (953 cm–1) and Cl2O3 (band at 780 cm–1) gradually disappeared, due possibly to their evolution into the gas phase or further oxidative transformation. The absorption bands of pure dichloroacetic acid are attributed to their sources in Table 1. New absorp tion bands (669 cm–1 δ ClHCl–, 740 and 779 cm–1 νas and νsClOCl, 1038 cm–1 νasO3) appeared in the spec trum after CCl2HCOOH came into contact with ozone (Fig. 2a). The increased intensity of the absorp tion bands at 1723 and 1750 cm–1 indicates the forma tion of (CO)–OOH groups [7]. As was mentioned above, an increase and then a reduction in the absorp tion band at 1038 cm–1 corresponding to νasО3 were observed as the temperature rose to 140 K. The spec trum of the system remained virtually the same up to 200 K. As was mentioned above, no absorption bands cor responding to the O–Cl bond were detected after CClH2COOH came into contact with ozone. New absorption bands at 736 and 888 cm–1 corresponding to the symmetric and asymmetric stretching vibrations Vol. 89
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of the Cl–O bonds in Cl2O appeared only when the temperature rose to 220 K [8, 9] (Fig. 3a). Upon reaching a temperature of 225 K, followed by heating to 240 K, there was an abrupt widening of the band at 800 cm–1 and its intensity increased. Broad new bands appeared at 2238, 3077, and 3469 cm–1 (Fig. 3b), due possibly to the formation of a great many hydrogen bonds. The O–Cl bonds are usually detected in the range of 600–900 cm–1. The widening of this band, its greater intensity at 800 cm–1, and the shift of the maximum from 799 to 755 cm–1 indicate an increase in the number of O–Cl bonds. The effect the ice support has on the interaction between ozone and chloroacetic acids was studied in our next series of experiments (Figs. 4, 5). Distilled water was sputtered onto the surface of a mirror, and acid was deposited on the water’s surface. This led to the emergence of new bands corresponding to vibra tions in the molecules of the acid; their maxima were shifted slightly due to the formation of hydrogen bonds with the surface of the ice. Intermolecular hydrogen bonds formed upon adsorption, and the bands are attributed to their sources in Table 3. Since the investigated acids partially dissociate upon interaction with water to form CClxH(3–x)COO– acid residue, we would expect a reduction in the reac tivity of tri and dichloroacetic acids due to an increase in the energy of the C–C bond. Unlike the previous example, no abrupt change in the spectrum is observed when chloroacetic acids adsorbed on ice come into contact with ozone. The new peaks that appear at 1441 cm–1 and the twin peaks at 1037 and 1050 cm–1 can be attributed to the stretching vibra tions of bonds in ozone. The intensity of the absorp tion bands corresponding to the C–O and C=O bonds also increases. No significant changes in the spectrum are observed up to 190 K. The chloroacetic acids inter act effectively with ozone only at temperatures higher than 190 K, when bands attributable to the vibrations of bonds in chlorine oxides of different composition (presumably Cl2O, ClO, and ClO2) appear. With CCl3COOH at 190 K, many fine reflections appear in the region of 730–1000 cm–1 (737, 800, 824, 836, and 854 cm–1) corresponding to vibrations of the O–Cl bonds in chlorine oxides. A new absorption maximum appears at 1387 cm–1, while the bands at 1037 and 1441 cm–1 disappear. The next changes appear at 210 K (Fig. 4b). The intensity of the broad band at 730–1000 cm–1 dimin ishes, the intensity of the maximum at 1382 cm–1 increases, and the intensity of the maximum at 1300 cm–1 declines. An abrupt change in the spectrum is observed at 230 K: the intensity in the region of 730– 1000 cm–1 falls against the background of the overall intensity of the spectrum, and all fine reflections dis appear. The peaks at 1400–1800 cm–1 attributed to bending vibrations of water molecules are more pro nounced, and the reflection at 1720 cm–1 is barely vis ible. At 250 K, the intensity of the band at 730–
Table 3. Shifts in the absorption bands of CCl3COOH after adsorption on ice νCCl3COOH
νCCl3COOH ads
Attributed to [7]
697 818 1090 1265 1418 1510 1700 – – 2965 3289
684 803 1226 1285
νs C–Cl νas C–Cl νC–O δCOH δCH fanning δCH νC=O νsCH νasCH νOH dimer νO–H monomer
1431 1728 2504 2600 3160 3330
1000 cm–1 diminishes and broad maxima appear at 600–738 cm–1 (with satellites at 673 and 723 cm–1) and at 818 cm–1. For dichloroacetic acid, the main changes in the spectrum from the reaction with ozone begin to appear at 190 K as reflections at 1369 cm–1. At 220 K, the intensity of the spectrum falls sharply and the absorption bands shift to 1271 and 1361 cm–1. At 240 K, the overall intensity of the spectrum increases due to the thermal dependence of the amplitude of bond vibrations. Our experiment with monochloroacetic acid was similar. The main changes in the spectrum of the inter action between ozone and monochloroacetic acid first appear in the temperature range of 220–240 K (Fig. 5b). A tendency toward lower intensity of the bands at 1800, 2530, and 2710 cm–1 is observed below 235 K, due possibly to the partial desorption of the ice film from the mirror surface. As the temperature rose to 245 K, a new band appeared at 660 cm–1; the band at 1137 cm–1 almost disappeared; the intensity of the broad bands at 1300, 2200, and 2710 cm–1 grew, the band at 1800 cm–1 shifted to 1730 cm–1; and the broad bands in the range of 3000–3500 cm–1 became equal in intensity and merged into one. CONCLUSIONS The emergence in the IR spectrum of the absorp tion bands attributable to νas and νsClOCl in Cl2O3 and ClO2 oxides marked the beginning of the reaction between chloroacetic acids and ozone. With tri and dichloroderivatives, interaction began at 77 K; with monochloroacetic acid, it began only at 200 K, due possibly to the redistribution of electron density from the carbon atom of the chloromethyl group, depend ing on the number of electronwithdrawing substitu ents. For the adsorption of chloroacetic acids on ice, which can occur in nacreous clouds, we observed pre
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liminary dissociation with the formation of CClxH(3 ⎯ x)COO– acid residue, reducing reactivity due to an increase in the C–C bond energy. The tem perature at which interaction began thus rose to 190 K. ACKNOWLEDGMENTS The authors are grateful to students P.A. Il’in and D.A. Sokolova of the Faculty of Chemistry, Moscow State University, for their help in our experiments. This work was supported by the Russian Foundation for Basic Research; the Division of Chemistry and Materials Science of the Russian Academy of Sci ences, program OKhNM1; and the RF Ministry of Education and Science, grant no. GK 02.740.11.5176. REFERENCES 1. V. V. Lunin, M. T. Popovich, and S. N. Tkachenko, Physical Chemistry of Ozone (Mosk. Gos. Univ., Mos cow, 1998) [in Russian]. 2. M. R. Flid and Yu. A. Treger, Chemistry and Technology of Vinyl Chloride (Kalvis, Moscow, 2008) [in Russian].
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3. G. Asplund, A. Grimvall, and S. Jonsson, Chemo sphere 28, 1467 (1994). 4. A. M. Middlebrook and M. A. Tolbert, Stratospheric Ozone Depletion (University Science Books, Sausalito, CA, 2000). 5. T. A. Vysokikh, T. V. Yagodovskaya, S. V. Savilov, and V. V. Lunin, Russ. J. Phys. Chem. A 82, 686 (2008). 6. T. A. Vysokikh, T. V. Yagodovskaya, S. V. Savilov, and V. V. Lunin, Russ. J. Phys. Chem. A 81, 878 (2007). 7. Infrared and Raman Characteristic Group Frequencies: Tables and Charts, Ed. by G. Socrates, 3rg ed. (Wiley, 2001). 8. K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, 5th ed. (Wiley Inter science, 1997). 9. T. Shimanouchi, Tables of Molecules. Vibration Fre quences Consolidated (National Bureau of Standards, 1972), Vol. 1, p. 1. 10. Application of Spectroscopy in Chemistry, Ed. by V. West (Inostr. Liter., Moscow, 1959) [in Russian].
Translated by E. Yablonskaya
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