Chapter 28
Mechanisms for the Reaction of CH S with NO Downloaded via UNIV OF CALIFORNIA SANTA BARBARA on July 10, 2018 at 08:18:52 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
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Shiro Hatakeyama Division of Atmospheric Environment, The National Institute for Environmental Studies, P.O. Tsukuba, Ibaraki 305, Japan Product analyses for the reaction of CH S with N O were carried out in order to elucidate the mechanism for that reaction in air. SO O was observed by means of FT-IR spectroscopy when NO O was used as a reactant. This is a clear evidence for the formation of CH SO and NO as products of the above reaction. Dependence of the yield of SO on the initial concentration of O and NO2 was observed, which indicates that the secondary reactions of C H S O with O or N O are important in the atmospheric oxidation of reduced organic sulfur compounds. Oxidation of biogenic reduced sulfur compounds such as thiols (RSH), sulfides (RSR), and disulfides (RSSR) is important from a point of view of atmospheric chemistry. Many studies concerning kinetics and mechanisms of those compounds with 0 ( P ) , O H , and NO3 are reported. In those studies the reaction of CH3S radical with 0 or NO was pointed out to be most significant in the atmosphere. However, the mechanism for the reaction of CH S with Ob or N 0 is still unclear by now. In our previous study (i) we reported that CH3S reacts with 0 to give about 90% of S 0 without forming C H radicals. The same conclusion was obtained by Balla and Heicklen (2). In both the studies it was confirmed that CH3S + 0 does not give CH3 radicals. As for the reaction of CH3S with N 0 little information is available concerning the reaction products and the reaction mechanism. CH3SN0 was reported as a product of CH3S + N 0 recombination reaction (2,4). However, its contribution is reported to be minor (2,5). Balla et al. (£) suggested following reaction pathways in addition to the recombination path. 3
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CH3S + N 0 - > CH S + HONO
(1)
CH3S + N 0 - > CH3SO + NO
(2)
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They also reported (£) the rate constants for the reactions of CH3S + NO, CH3S + N 0 , and CH3S + 0 and estimated the rate-constant ratio to be k(NO)/k(0 )>2 x 106 and k(N0 )/k(O )>5 x 10* On the basis of these rateconstant ratio, photooxidation mechanism for reduced sulfur compounds must be re-examined, because most experimental studies used much N 0 . 2
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0097-6156/89/0393-0459$06.00/0 o 1989 American Chemical Society
Saltzman and Cooper; Biogenic Sulfur in the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
460
BIOGENIC SULFUR IN THE ENVIRONMENT
In this study we investigated the reactivity of CH3S radical with 0 and N O utilizing the photolysis of CH3SNO as a source of CH S radical. N 0 0 was also used to check if the oxygen atom of N 0 transferred to a product S 0 molecule. 2
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Experimental The photochemical reactor used for the photolysis of CH3SNO was an 11-L cylindrical quartz vessel (120 mm i.d., 1000 mm in length) equipped with multireflection mirrors for long-path Fourier transform infrared spectroscopy (Block Engineering Co., JASCO International Inc., FTS-496S). Tne light source for photolysis was six yellow fluorescent lamps (National FL20S Y-F, 500 < A 9.5 %, Nippon Sanso). Effects of the different concentration of oxygen or NO? were checked. Typical reaction conditions were [CH SNO]o~o mTorr, [NO] ~1 mTorr, [ ° 2 l p ~ I" mTorr, [O?jo~ 0-760 torr under total pressure of 1 atm with N . Photolysis was performed for about 2 h and the change in the concentration of reactants and products were monitored by F T - I K (path length: 51.6 m, resolution: 1 cm* , scan times: 64). A l l the experiments were carried out at room temperature. m a x
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Results and Discussion At first, photolysis of CH3SNO in the presence of N 0 0 was performed in air in order to confirm that tne reaction of CH S with N 0 gives S 0 possibly via CH3SO + NO formation. Photoirradiation of the mixture ([CH3SNO] = 7.5 mTorr, [N0 ] = 2.0 mTorr, [ N 0 0 ] = 8.9 mTorr under 1 atm of air) gave a mixture spectrum as shown in Figure 1, B. After subtraction of S 0 a signal at 1341.5 cm* was clearly seen (Figure 1, C). This signal agrees well with that of S 0 0 (1341.1 cm- ) reported by Polo and Wilson (8). Since the light emitted by the yellow fluorescent lamps does not photolyze N 0 , the observed S 0 0 is not a product of the reaction of 0 atom released from N 0 0 . The observed yield ratio of S 0 0 / S 0 was 0.36 (IR absorptivity of S 0 0 was assumed to be the same as that of SO?) and it was consistent with the 1:1 formation of S O ^ ) and S 0 as expected from the following mechanism when the initial concentration ratio of N 0 0 to N 0 was taken into account. 1 8
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CH3SNO + hi/ - > CH S + NO
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CH S + N 0 0 - > C H S 0 + NO, CH3SO + N 0 1 8
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(4)
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CH S 0 + 0 - > - > S0 0 + CH 0 3
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(5)
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CH3SO + 0 — > — > S 0 + C H 0 (6) On the basis of these results we can conclude that the reaction of C H S with N 0 gives SO? in air. This is the first clear evidence for the formation of SO? from C H 3 S + N 0 reactions in air. Between two proposed reaction paths, (1) and (2), CH3SO + NO forming path is the most plausible primary reaction, since substantial amount of N 0 was observed as a product and neither CH S nor HONO was detected. 2
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Saltzman and Cooper; Biogenic Sulfur in the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
28. HATAKEYAMA
Mechanism for the Reaction of CHjS with N0
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Next, the effects of the different concentration of oxygen or N 0 were checked. Table I shows the yield of major products; formaldehyde, S 0 , and dimethyl disulfide (DMDS). The yield of D M D S decreased as the 0 concentration increased. S 0 and formaldehyde were produced in almost the same yield. Thus, these two compounds are produced from the same precursor without forming CHj radical. If CH3 radical is formed in air, CH3OH must be formed and the yield of CH?0 must oe smaller than that of S0 . The fact that neither CHiONO nor CH3ONO2 was formed also indicates that CH3 radical is not formed in this reaction system. Since the production of the compound which is tentatively assigned to C H S N 0 (2,4) was also detected in this reaction system by FT-IR spectroscopy, the recombination of CH3S with N 0 should be an additional path for this reaction. However, the sum of the concentration of nitrogenous compounds, i.e., [CH3SNO] + [N0 ] + [NO], after 2 h of irradiation with about 5 0 % consumption of CH3SNO during this reaction time is more than 9 5 % of initial [CH SNO] + [NO?JQ + [NO] . Thus, the fraction of adduct formation path is minor as pointed out by Niki et al. (2). The increase of the yield of S 0 with the increase of 0 also supports the contention that the secondary reaction of CH3SO radical with 0 can be the source of S 0 as depicted by Equations 5 and 6. By analogy with the mechanism for the formation of SO? from CH SO? + 0 (2), the following equation is proposed here to explain tne formation of S 0 from CH3SO. 2
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CH3SO + 2 0 ---> C H 0 + S 0 + H 0 . 2
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(7)
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Dependence of the yield of S 0 on the concentration of N 0 was also observed (Table I). It suggests that the reaction of CH3SO with N 0 can be an additional reaction channel to form S0 . Recently, Lovejoy et al. (2) reported the reaction of HSO with N 0 and suggested the mechanism to be the formation of H S 0 + NO. By analogy with this reaction following mechanism is proposed here for the reaction of CH SO + N 0 . 2
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CH3SO + N 0 — > C H S 0 + NO
(8)
C H S 0 + 0 — > C H 0 + S 0 + O H (2).
(9)
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There is another reaction path for CH3SO radicals. As shown in Table I, DMDS was produced in a high yield, particularly in the absence of 0 . This is rather a curious thing, because the DMDS is usually thought to be formed by the self reaction of CH S radicals (CH S + CH S — > CH3SSCH3). However, in the present reaction condition this self reaction cannot compete with the very rapid reaction CH3S + NO? (rate constant = 1.1 x 10" cm^iolecule-V (6)) in the presence of much N 0 . Thus, there must be another reaction to form DMDS. One possible reaction is 2
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CH3SO + CH3SNO ~ > CH3SSCH3 + N 0
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(10)
From the results of the present study it became clear that the reaction of C H S with N 0 in the presence of 0 gives S 0 and C H 0 . It was also suggested that the reactions of CH3SO and C H S 0 radicals with 0 play a very important role in the formation of SO? from the atmospheric oxidation of reduced sulfur compounds. More detailed studies on the reactions of CH3S, CH3SO, and C H S 0 are needed. 3
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Saltzman and Cooper; Biogenic Sulfur in the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
462
BIOGENIC SULFUR IN THE ENVIRONMENT
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Figure l.(A) Standard spectrum of SO2; (B) Spectrum of the products of CH3SNO photolysis in air with N 0 0 ; (C) Spectrum of S 0 0 obtained by subtraction of SO2 from (B). 1 8
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Table I. Initial conditions and product yields for CH3SNO photolysis at room temperature under 1 atm of total pressure [CH SNO] 3
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Product Yield (%)
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mTorr
DMDS
9.24 7.16 9.49 9.49 7.60
-0 76 152 380 760
1.55 1.06 1.52 1.92 1.86
0.75 0.48 0.97 1.38 0.82
71 52 54 42 20
-0 7 19 18 41
4 9 19 24 36
8.05 9.49 8.76 7.38 5.83
152 152 152 152 152
1.01 1.52 4.73 6.79 7.87
0.65 0.97 1.45 1.94 4.36
67 54 18 13 -
23 19 22 30 45
20 19 25 37 56
HCHO
Saltzman and Cooper; Biogenic Sulfur in the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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28. HATAKEYAMA
Mechanism for the Reaction of CHjS with N0
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Literature Cited 1. Hatakeyama, S.; Akimoto, H . J. Phys. Chem., 1983, 87, 2387. 2. Balla, R. J.; Heicklen, J. J. Photochem., 1985, 29, 297. 3. Niki, H.; Maker, P. D.; Savage, C. M . ; Breitenbach, L . P. J. Phys. Chem., 1983,87,7. 4. MacLeod, H.; Aschman, S. M.; Atkinson, R.; Tuazon, E. C.; Sweetman, J. A.; Winer, A. M.; Pitts, J. N., Jr. J. Geophys. Res., 1986, 91, 5338. 5. Balla, R. J.; Heicklen, J. J. Phys. Chem., 1984, 88, 6314. 6. Balla, R. J.; Nelson, H . H.; McDonald, J. R. Chem. Phys., 1986, 109, 101. 7. Philippe, R. J. J. Mol. Spectrosc. 1961, 6, 492. 8. Polo, S. R.; Wilson, M . K. J. Chem. Phys., 1954, 22, 900. 9. Lovejoy, E. R.; Wang, N. S.; Howard, C. J. J. Phys. Chem., 1987, 91, 5749. RECEIVED July 13, 1988
Saltzman and Cooper; Biogenic Sulfur in the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
