J. Phys. Chem. 1982, 86, 2605-2609
Debye relaxation parameters (eq I) are A = 146 X 10-17, cm-' s2, f R = 42 MHz. Only an increase in the background absorption B is noticeable with respect to the data at the same concentration (Table 11). A and f R are within experimental error of the NaC104 data at the
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same concentration.
B = 38 X
Acknowledgment. The authors are grateful to the National Science Foundation for support through Grant No. CHE-8108467.
Mechanism of the O('P) -t H2S Reaction. Abstraction or Addition?+ Donald L. Singleton,' George ParaSkOVOpOUlO8,' and Robert S. Irwin Divlslon of Chembby, Natbnal Research Councll of Canade, Ottawa, Ontario, Canada K1A OR9 (Received: October 29, 1981; In Final Form: February 16, 1982)
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The question of whether the reaction of O(3P)with H2S proceeds by abstraction, 0 + H2S OH + SH, or by addition, 0 + H2S [H,SO]* products, was investigated by quantitative gas chromatographic analysis of N2 and C02produced in mercury-photosensitizedmixtures of N20, H2S, and CO. Reactant pressures were chosen so that less than 5% of the 0 atoms formed from N2O would react with CO, but a large fraction of any OH radicals formed from reaction of 0 with Ha would react with CO to form COP The COzyields were compared with those calculated from an expression derived by steady-state treatment of a simple mechanism. The C02 yields indicated that 52% of the 0 H2S reaction proceeds by abstraction, although it could be as much as 100% depending on the extent of H2S2formation by SH radical recombination and the rate constant for the reaction of 0 atoms with H2S2.
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Introduction The reaction of oxygen atoms, O(3P),with organic sulfides in the gas phase has been shown to proceed at room temperature by addition of oxygen to the sulfur atom followed by fragmentation of the energy-rich a d d ~ c t . l - ~ Even with the thiols, which have a fairly weakly bound hydrogen, D(RS-H) = 90 kcal/mol, the addition route dominates, with the abstraction path 0 RSH RS + OH contributing less than about 10% to the total reacti~n.~ As the simplest member of the series H2S, RSH, RSR, hydrogen sulfide appears ambiguous in its mechanism of reaction with oxygen atoms. Earlier kinetic and product analysis experiments were interpreted in terms of abstraction, reaction la, but Slagle, Graham, and Gutman2 0 + H2S --c OH SH (14
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suggested that the data could also be interpreted in terms of an addition mechanism, reaction lb, analogous to the
0 + H2S --c HSO
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(1b)
reaction of oxygen atoms with organic sulfides and thiols. However, the thermochemistry of reaction l b is uncertain, and it may be too endothermic to make much of a contributi~n.~~~ In a paper describing the kinetics of reaction 1, we reported a few experiments pertaining to the me~hanism.~ Oxygen atoms were allowed to react with a large excess of H2S, and the yields of HzO and H2 were determined. If these products arise from reactions l a and 1b followed by OH + H2S -,H 2 0 SH and H H2S-,H2 SH, then the yields of HzO and H2 per oxygen atom reacted would give the relative importance of paths l a and lb. The results indicated kla/kl = 0.52 and klb/kl I0.11. However, because accurate quantitative analysis of small amounts of water is difficult, systematic errors could potentially be present. Furthermore, in the interpretation of the data,
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N.R.C.C. No. 20208. 0022-3654/8212086-2605$01.25/0
it was assumed that no alternative path exists for the OH H2S reaction. It is conceivable that an addition path could occur at least to some extent, especially if the analogous oxygen atom reaction were to proceed by addition. Because of these arguments, another method was used to determine kla/kl in the present work. Carbon monoxide was added to the reaction mixture in order to trap the OH radicals, formed in reaction la, by the reaction OH CO C02 H. This method has the advantage that C02is more easily recovered and analyzed than HzO.
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Experimental Section Oxygen atoms were generated by mercuryphotosensitized decomposition of N20 at 253.7 nm with a low-pressure mercury lamp and a Corning 9-54 filter to remove the 184.9-nm radiation. The reactants N20, H2S, and CO were continuously circulated through a cylindrical quartz cell (50 mm 0.d. X 100 mm) with Suprasil windows. The circulating pump was made of Pyrex and Teflon and contained a drop of mercury in the connecting tubing to maintain a constant concentration of mercury vapor. The volume of the reaction cell and circulating pump was 268 cm3. Because direct analysis of small amounts of Nz in a large excess of CO was not possible, the noncondensible (at 77 K)reactant, CO, and products, N2 and H2,were removed from the reaction cell with a Toepler pump and passed through a column of CuO at 513 K to oxidize the CO to C02. In the process, H2 was oxidized to H20. After removal of the C02 and H20 with a trap at 77 K, the N2and (1)J. H.Lee,R. B. Tmmons, and L. J. Stief, J. Chem. Phys., 64,300 (1976). (2) I. R.Slagle, R. E. Graham,and D. Gutman, Int. J. Chem. Kinet., 8, 451 (1976). (3)R.J. Cvetanovic, D. L. Singleton, and R. S. Irwin, J. Am. Chem. Soc., 103, 3530 (1981). (4) D. L. Singleton, R. S. Irwin, W. S. Nip, and R. J . Cvetanovic, J. Phys. Chem., 83, 2195 (1979).
0 1982 American Chemical Society
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any residual CO were analyzed by gas chromatography on a molecular sieve column. The material in the reaction cell condensible at 77 K (N20,H2S, C02, H20) was analyzed by gas chromatography, and C 0 2yields were measured quantitatively by using propane as an internal standard. The C 0 2 yields based on the very large N 2 0 peak as an internal standard were within 10% of those based on propane. The COz was completely resolved and was eluted just prior to N 2 0 on a 0.25 in. 0.d. X 20 f t stainless steel column packed with Porapak Q. The gas chromatographic results of three or four aliquots were averaged for each experiment. The reactants NzO (Matheson, 99.99%) and H2S (Matheson, 99.5%) were repeatedly degassed by freezepumpthaw cycles and distilled from trap to trap. Before each experiment, the reactant CO (Matheson, 99.99%) was passed through Oxisorb (which, according to the manufacturer, removes O2and other impurities) and three traps at 77 K. In about 10 experiments, the material in the three traps was collected, combined, and later analyzed by gas chromatography-mass spectrometry. Only HzO and COP were detected, and there was no evidence of iron carbonyl. There was no detectable C 0 2 in the N20, HzS, or CO, and upper limits of C 0 2 in these three reactants were 0.0014, 0.02, and 0.005%, respectively. Although there was not any N2 detectable in the N20 or H2S, a small but variable amount of N2was present in the CO which may have arisen to some extent by displacement of N2 by CO on the Oxisorb.
Results Several experiments were done to establish the amount of Nz and COz formed with mixtures of NzO,CO, H2S, and Hg without photolysis (dark runs). Most if not all of the Nz found in the dark runs was an impurity in the CO, as discussed above, but the C 0 2 detected was a result of a dark reaction. This has been reported p r e v i ~ u s l yfor ~>~ mixtures of N20, CO, and Hg. Although there appeared to be a slight trend in C 0 2 and N2yields with the amount of H2S present in the dark runs, the trend could not be reproduced with a new cylinder of CO after the original cylinder was depleted. For four dark runs at 741 torr total pressure, 5.5 < PHzs< 21 torr, 100 < Pco 200 torr, the average yields and their standard deviations in micromoles were as follows: N2,0.438 f 0.095; COz, 0.224 f 0.049. For one dark run at 370 torr, PH = 5 torr, Pco = 49 torr, and with a new cylinder of CO, t f e yields were Nz,0.147 pmol; COz, 0.149 pmol. The results of experiments with photolysis are given in Table I. The yields of N2 and C 0 2 have been corrected for the yields in the dark runs. The corrected yield of N2 gives the number of oxygen atoms formed, according to the following two reactions:' Hg6('So) + hu (253.7 nm) Hg6(3P,)
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Hg6(3P1)+ N 2 0 Hg6('So) Nz + O(3P) The yield of C02, when expressed relative to that of Nz, gives the yield of C 0 2 per oxygen atom. The ratio of the yields C 0 2 / N 2 appears to be independent of irradiation time and of pressure of H2S within the experimental uncertainty, as seen in Figures 1 and 2. The second data point in Table I was not included in Figures 1 and 2 because the ratio C02/N2 appears to be too large, as is evident in Figure 3. Because a replicate experiment, the third (5) R. Simonaitis, J. Heicklen, M. M. Maguire, and R. A. Bernheim, Chem., 75,3205 (1971). (6)R. Simonaitis and J. Heicklen, J . Chem. Phvs., 56. 2004 (1972). (7)R. J. Cvetanovic, h o g . React. Kinet., 2, 39 (1964).
J.Phys.
Singleton et al.
The Journal of Physical Chemistry, Vol. 86, No. 14, 1982
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t (min)
Figure 1. Yield of CO, per oxygen atom, C02/N2, as a function of irradiation time. The error bars are 2a. The llnes are calculated yields based on eq I, assuming k lalk = 0.52. For the solid line, the recommended value (ref 8) of k , Is used, and for the dashed lines, one-half and twlce the recommended value are used: P H,S = 11 torr: Pco = 100 torr; PNzO= 630 torr. 0.4
(u
o.l 0
t
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IO
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P,2,(TORR)
Fbgwe 2. Yield of CO,per oxygen atom, C02/N,, as a function of H,S partlal pressure. The error bars are 2a. The lines are calculated as in Figure 1. [H,S]I[CO] = 0.11; P,ou, = 741 torr; t = 10 min. IO
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