J. Phys. Chem. 1981, 85,1126-1132
1126
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2l E 54.
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mole fractions of TFE, this positive deviation begins to diminish, indicating that the system is consistent with 1). Raoult’s law in the limit (as XTFE The slight negative deviation found in the 2-butanolTFE liquid-vapor plot may be attributed to relatively strong hydrogen bonding in the liquid phase. The negative deviation of the liquid curve in the case of ethanol-TFE is again indicative of complexation. In this case, an azeotrope is observed at P = 48.10 torr, RWE= 0.41. As mentioned before, methanol-TFE displays a maximum azeotrope (negative deviation from Raoult’s law). The presence of or lack of an azeotrope for these three systems displaying negative deviation from Raoult’s law is consistent with Bancroft’s ruleen The ratio of the vapor pressures of TFE and ethanol is very close to one, and so a slight amount of nonideal behavior attributable to the formation of hydrogen bonds will cause an azeotrope to form.
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/
,
1.0
OF TFE Flgure 7. Dependence of pressure on composition for the binary system 2,2,2-trlfluoroethanol-2-butanol at 25 ‘C: (+) experimental data for five separate runs: (A) least-squares points (Pvs. &&; lines are IlquMus (Pvs. fim)and vapor (Pvs.Fm) inferred from analysls of data from all runs. MOLE F R A C T I O N
The Hansen-Miller parameters are given in Table 11. From the least-squares program, calculated liquid and vapor mole fractions are found for each pressure and appear in Figures 5-7 as the liquid-vapor curves for waterTFE, ethanol-TFE, and 2-butanol-TFE, respectively. A pronounced positive deviation from Raoult’s law is observed in the water-TFE liquid-vapor curve. At higher
Acknowledgment. We thank William Hartsell, Dale Christian, and Mohit Nanda for collecting the data and Kevin Stellner and Richard Shahan for preparing the graphs for publication. This research has been supported by the National Science Foundation through Grant No. CHE77-03668. (22) M. B. King, “Phase Equilibrium in Mixtures”, 1st ed., Pergamon Press, Elmsford, NY, 1969, pp 373-5.
Oxidation of Sulfur Dioxide by Methylperoxy Radicals Charles S. Kan,t Jack 0. Calvert,’t and John H. Shaw’ Department of Chemistry and Department of Physics, The Ohio State UniversW, Columbus, Ohio 43210 (Received: Ju& 3, 1980)
This study was made to resolve the apparent discrepancy between the finite rate constants observed for the CH302reaction with SO2in high-intensity flash photolysis and the near zero values observed recently by us and others for this reaction in NO-free, CH302-SO2 experiments at low intensity. In kinetic flash experiments we have found the apparent second-order rate constant for CH302S02reaction to be dependent on flash intensity or CH302radical concentration. Low-intensity photolysis of dilute azomethane-SO2 mixtures in air were made by employing FT IRS for product identification and rate measurements. Very little reaction between SO2and CH302was seen for these conditions. However, in similar experimentswith added NO, a new metastable product (conceivably CH302S020N02) of 22-min lifetime formed in the system. Alternative explanations for these seemingly contradictory findings were tested experimentally. It was concluded that the possible complicating reactions of CH3,CH30, H02,or HO radicals with SO2 were unimportant for the conditions employed in our flash experiments. All of the results can be rationalized well in terms of the reversible addition of CH302to SO2: CH302 + SO2 + CH302S02(lb,9); CH302S02+ O2 + CH302S0202(10,ll). The high-intensity flash cm3 molecule-l s-’; cm3 molecule-l s-l and kl&1&12/k9k11 = 4 X data suggest k l b N (1.4 f 0.2) X CH302S0202+ CH302 CH302S020+ CH30 + O2(12). In the systems with NO added, reaction 5 may occur to entrap the CH302S0202 radical: CH302S0202 + NO CH302S020+ NO2 (5). In terms of the mechanism favored here the net oxidation of SO2 which occurs in a system following reaction l b depends on the concentration of CH302and NO present in the system and the extent to which reactions of the CH302S02and CH,02S0202 species with CH3O2 and NO can compete with the decomposition reactions 9 and 11 of these species.
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Introduction Recently Kan et al.,1 Sanheuza et a1.,2 and Simonaitis and Heicklen3 have reported studies of the CH302-S02 reaction. Measurements in the first two studies were made by flash photolysis of azomethane-O2-SO2 mixtures by following the CH302radical through its ultraviolet ab‘Department of Chemistry. *Department of Physics.
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sorption band (290-200 nm). The products of the reaction were not determined, but it was speculated that in the Primary elementary step CH302either transferred an 0 atom to soz in reaction l a or added to SOz in reaction lb. (1) C. S. Kan, R. D. McQuigg, M. R. Whitbeck, and J. G. Calvert, Znt. J. Chem. Kinet., 11, 921 (1979). (2) E. Sanhueza, R. Simonaitis, and J. Heicklen, Znt. J. Chern.Kinet., 11, 907 (1979). (3) R. Simonaitis and J. Heicklen, Chem. Phys. Lett., 65,361 (1979).
0022-3654/81/2085-1126$01.25/0
0 1981 American Chemical Society
SOz Oxidation by CH302
+
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The Journal of Physical Chemistry, Vol. 85, No. 9, 198 1
CH30
+ SO3
(la) CH30zS02 (1b) For the flash conditions employed in the previous studies the dominant loss of CH3O2 occurred in the self-reactions 2. In a series of experiments with increasing SO2 con2CH302 2CH30 + Oz (24 CH20 CH3OH 02 (2b) CH3OOCH3 0 2 (2~) centration the observed increase in rate of decay of CH302, presumably from reactions l a and/or lb, was used by Kan et al.' to derive Itl, + k l b = (1.1f 0.2) x W4cm3 molecule-' s-'. In similar experiments Sanheuza et a1.2found k1, + k l b = (8.2 f 0.5) X (23 "C, 1 atm pressure). Simonaitis and Heicklen3 determined the competitive rates of CH302reactions with NO and SOz using steadystate photolysis of a~omethane-SO~-N0-0~ mixtures. The addition of SO2 to the azomethane-N0-OZ mixture increased the quantum yield of NO oxidation, presumably as a result of the additional reactions involving the CH302 interaction with SO2: CH3Oz + NO CH30 + NO2 (3) CH3O2 + SO2 CH302S02 (1b) (4) CH302S02 + 0 2 CH30zS0202 CH302S0202 + NO CH302S020 + NO2 (5) CH3O2SO20 CH3O2 + SO3 (6) From an analysis of the rate data they derived the estimate klb/k3 = (2.5 f 0.5) X The current most direct estimates give k3 = (3.0 f 0.2) X 6.5 X 10-12,5(8.0 f 2.0) X 10-12,6and (7.1 f 1.4) X cm3 molecule-l s-l,' with an average value of k3 = (6.2 f 3.1) X cm3 molecule-' s-'. This coupled with Simonaitis and Heicklen's rate constant ratio give klb = (1.6 f 0.8) x cm3 molecule-' s-', in reasonable accord with the more direct measurements. In recent subsequent experiments in our laboratory attempts were made to monitor the CH3O2-SOZreaction in steady-state photolyses of azomethane-Oz-SOz mixtures using Fourier transform infrared spectroscopy to follow reactants and products. Contrary to our expectations based upon the recent measurements of k1 from which we anticipated that over 50% of the CH30zradicals formed would react with SO2, we found very little reaction; the cm3 molecule-' 8,two results suggested kl I1 X orders of magnitude lower than the previous estimates. We have learned subsequently of other low-intensity studies of Sander and Watsons and Heicklen and co-workersgin which similar results suggest k l I5 X cm3molecule-' s-l. The present study was made to rationalize these apparently conflicting results and to establish the true nature of the CH302-S02reaction.
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Experimental Section The Fourier transform infrared spectroscopic system and the associated large photolysis cell employed in the steady-state studies reported here have been described in (4) H. Adachi and N. Basco, Chem. Phys. Lett., 63,490 (1979). (5)R. A. Cox and G. S. Tyndall, Chem. Phys. Lett., 65,357 (1979). (6)I. C. Plumb, K. R. Ryan, J. R. Steven, and M. F. R. Mulcahy, Chem. Phys. Lett., 63,255 (1979). (7)S.P. Sander and R. T. Watson, J. Phys. Chem., 84, 1664 (1980); we are grateful to the authors for a preprint of this work before publication. (8)S.P. Sander and R. T. Watson, Chem. Phys. Lett., 77,473(1981); we are grateful to the authors for a preprint of this paper. (9)J. Heicklen, personal communication.
I rime,mwo
Figure 1. Plot of the reciprocal of the absorbance of the CH302radical (265 nm) vs. time for the flash photolyses of azomethane (21.24 torr), 0,(678.2 torr), triangles; azomethane (21.24 torr), SO2 (0.21 torr), O2 (50.31 torr), N, (28.26 torr) circles.
detail previously.l0J1 A 170-m pathlength was employed with 1-cm-' resolution. The flash photolysis studies were carried out as described by Kan et a1.l The azomethane reactant was prepared by the method of Renaud and Leitch12and purified by repeated vacuum fractionations. Methyl nitrite was prepared and purified by the method of Hanst and Ca1~ert.l~
Results and Discussion Based upon the recent estimates of the rate constants for CH3O2 reacting with S02,1-3we carried out a series of studies to determine the nature of the products of this reaction. Using the Fourier transform infrared spectrometer and large cell system we photolyzed for 19.25 min a mixture of 30 ppm of azomethane with 100 ppm of SOz, 100 torr of Oz,and 600 torr of N2 From the measured rate constants we expected that about 50% of the CH302 radicals formed would react with SO2 for these conditions. However, the nature and amounts of the products observed, CHzO, CH30H, CH3OZH,HCO2H, and CO, were nearly the same as those seen in similar photolyses in the absence of SOz. Less than 2% of the SO2 was removed in the experiment. Thus reaction 1did not appear to occur significantly in these steady-state experiments although rate constant measurements from flash photolyses would predict otherwise. CH302 + SO2 CH30 + SO3 (la)
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-+
CH302S02
Ob)
In view of these unexpected findings we repeated the flash photolysis experiments of Kan et al.' and found results similar to those reported by them. Note in Figure 1 that the presence of only 0.21 torr of SO2 in a flashed mixture with azomethane (21.2 torr) and O2 (50.3 torr) did perturb the kinetics of the CH302.decay. It causes a change in the initial slope of the reciprocal of the CH302 absorbance (265 nm) vs. time plot. If the disappearance of CH3O2 is described by relation 7, and these kinetic data -d[CH,O,]/dt
= 2kz[CH30Zl2+ k,[CH302][S02]
(7)
(10)W. M. Uselman, S. Z. Levine, W. H. Chan, J. G. Calvert, and J. H. Shaw in "Nitrogenous Air Pollutanits", D. Grosjean, Ed., Ann Arbor Science Publication, Ann Arbor, 1979,p 17. (11)F. Su, J. G. Calvert, C. R. Lindley, W. M. Uselman, and J. H. Shaw, J. Phys. Chem., 83,912 (1979). (12)R. Renaud and L. C. Leitch, C:an.J. Chem., 32, 545 (1954). (13) P. L. Hanst and J. G. Calvert, J. Phys. Chem., 63,2071 (1959).
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Figure 2. Plot of kl[S02] vs. azomethane pressure; data were derived from [CH302] decay data measured In the flash photolysis of azomethane mixtures; reactant pressures, 02,50 torr; SO2, 0.21 torr; N,, added to make total pressure 100 torr; the two curves show the theoretical variation of the parameters expected by computer simulation assuming klbk ,k1 l k s k l l values of (A) 7 X lo4' and (B) 2 X 10-47 cm3 molecule-) s-!
are analyzed as before,, the kl[S02]term can be calculated. The removal of CH302from the reaction 8 is not important products (8) CH3O2 + CH3N=NCH3 for the time scale employed here. In a series of experiments the [SO,] was held constant and the initial azomethane pressure was varied to alter the CH302radical concentration. For these conditions the yield of CH3O2 radicals was proportional to the azomethane pressure, and only about 0.29% of the azomethane was decomposed per flash. The values of the k1[SO2] term in relation 7 were determined. These are plotted vs. the initial azomethane pressure in Figure 2. Although there is considerable scatter in the data, a definite dependence of kl[S02] on PM~ is seen ~ Nat~low azomethane pressures. It appears to decrease at low PMezNz and reach a limiting value at high PMe2y2values. Obviously, the simple kinetics expected in relation 7 is not followed. Hence the simple mechanism accepted in the previous work cannot be complete. There are several possible explanations for the results observed here and the apparent disagreement between experiments designed to measure k,. (1)The effect of SO, observed in Figure 1 which has been interpreted as an indication of a reaction between CH3O2 and SOz may be an artifact resulting from the reaction of the CH3 radical with SO2 followed by CH302 removal by CH3S02 or CH3S0202radicals peculiar to the SO2-containingsystem. (2) The CH30 radical product of reaction 2a may react with SO2,and the initial product CH30SOzor CH30S0202 may react to remove additional CH3O2 radicals. (3) A possible third explanation is that decomposition of the CH302S02initial product of reaction l b (in reaction 9) may dominate in experiments at low radical concentrations; capture of this radical or a subsequently formed CH3O2SOzOzradical by reaction with another CH302radical may be favored in experiments at high methylperoxy radical concentrations: CH3O2 + SO2 CH302S02 Ob) CH302S02 CH302+ SO2 (9) CH302S02 + 0 2 ;=+ CH302S0202 (10,ll) CH3O2SO2O2+ CH30z CH3OZSO20+ CH30 0, (12) Experiments were designed to test each of these alternatives.
I
C
Figure 3. Infrared spectra from the photolysis of CH30N0(10.1 ppm), O2(50 pprn), in N2 (700 torr); no SO2 added; A, initial mixture: B, after 19.3-min photolysis; C, standard spectrum of CH30N02; note thls compound is not formed in thls S02-free system.
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Figure 4. Infrared spectra from the photolysis of CH30N0 (10.2 ppm), SO2 (49.5 pprn), O2 (50 ppm), N2 (700 torr): A, initial mixture; B, spectrum foilowlng 11.3-mn photolysis: note the presence of CHBON02 among the products In thls case.
The Possible CH30-SO2 Reaction as a Complication in the CH302-SO2Systems. In an FT IR spectroscopic experiment we photolyzed for 19.3 min CH30N0 (10.1 ppm) in small amounts of O2 (50 ppm) and excess N2 (700 torr). The products observed (Figure 3), NO (3.0 ppm), CH20 (1.8 ppm), CH30H (1.3 ppm), and N20 (concentration not measured), are expected in terms of the mechanism 13-17. CH30N0 + hv (410 > X > 290 nm) CH30 NO (13) 2CH30 CH30H CH20 (14a) CH302CH3 (14b) CH30 + Oz CHzO H 0 2 (15) CH30 + NO CH30N0 (16a) HNO CH2O (16b) 2HNO N2O HzO (17) A similar experiment was performed in which SO2 was present in the initial mixture: CH30N0 (10.2 ppm), SOz (49.5 ppm), 0,(50 ppm), Nz (700 torr). Photolysis for 11.3 min gave the spectra of products shown in Figure 4: CH30N0 (reacted), 3.3 ppm; SO2 (reacted), 2.5 ppm; NO, 2.8 ppm; CH20,0.96 ppm; CH30H, N20, and CH30N02 (amounts not measured). Here there is striking evidence
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SO2 Oxidation by CH302
The Journal of Physical Chemistry, Vol. 85, No. 9, 1981 1129
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Flgure 5. Infrared spectra from the photolysis of azomethane (30.1 pprn), SO2 (10.1 pprn), O2 (50 pprn), N2 (700 torr): A, Initial mixture: B, after 19.3-min photolysis.
that SO2 does react with CH30 to form products. Presumably the production of the new product CH30N02in the SO2-containingsystem arose from the subsequent reactions of the CH30S02radical: CH30 + SO2-,CH30S02 (18) CH3OSO2 + 0 2 CH30S0202 (19)
+ NO CH30 + NOz
CH30S0202
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-.+
CH30S020
+ NO2
CH30N02
(20) (21)
From the SO2reacted and the other products formed here we have estimated k18 N 5.5 X cm3molecule-l s-l. If we couple this estimate with that for k15 N 6.1 X cm3 molecule-I s-I,l4 then we would anticipate that CH30 radicals formed in reaction 2a in our OZ-richflash photolysis experiments would be removed effectively in reaction 15. Thus the CH30 radical reaction with SO2 cannot be CH30 + O2 H 0 2 + CH20 (15)
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the source of the reaction attributed previously to the CH302-S02reaction 1. The Possible CH3-SO2 Reaction as a Complication in the CH302-SO2 System. A further test of alternative mechanisms was made by photolyzing azomethane (30.1 ppm), SO2 (10.1 ppm), in low O2 (50 ppm), and N2 (700 torr) mixtures. The spectra of the products observed after 19.3-min photolysis are shown in Figure 5. In addition to the expected absorption from the products of the CH3, CH3O2, and CH30 reactions (CH30H, CH20, CH302H, HC02H, and CO), additional unidentified absorptions appeared at 835,930, and 1220 cm-'. After subtraction of the absorption due to CH20, CH30H, CH302H, and HC02H, the residual spectrum showed unidentified components with somewhat different lifetimes (in the range 5-10 min) so that a resolution into two spectral components could be made; see Figure 6. The nature of the spectra of component B and the visible light scatter which could be seen in similar small cell experiments suggest that this component may be in aerosol form. At low O2 concentration the CH3radical reaction with SO2can compete successfully with the CH3 radical reaction with 02: CH3 + SO2 (+M) CH3S02 (+M) (22) +
CH3 + O2(+M) -.+ CH302(+M) (23) The new products seen are thought to arise following reaction 22. The previous experiments with azomethane photolysis in SO2 mixtures at high [02]showed no such
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Figure 6. Residual infrared absorption due to two Unidentified components of the reaction of CH, radicals with SO2.
TABLE I: Products Concentrations (ppm) from the Photolysis of Azomethane in Mixtures with SO," CH,CH,CH,O OH 0,H CO HC0,H -SO, 100.1 2.68 2.04 2.74 0.77 0.30 8.6 29.98 3.50 1.67 1.13 2.61 0.24 3.7 10.07 4.43 3.13 1.67 1.01 0.20 2.3 5.08 4.43 3.67 1.22 1.66 0.15 1.2 " Initial concentrations (ppm): azomethane, 30; 0,, about 50;N,, 700 torr; photolysis time, about 19 min; photolysis was carried out in the FT IRS, long path photochemical reactor. SO, (initial)
products and rule out the CH302 radical as a source of these. The experiments with CH30N0 at low [O,](described in the previous section) also do not give the residual unidentified products seen here; obviously the CH30-S02 reactions are not involved. We may test the hypothesis of CH3 radical involvement through a series of kinetic studies using azomethane photolysis (30 ppm) in mixtures with varied SO2 (5-100 ppm), constant low O2 (50 ppm), and high Nz (700 torr). These results are summarized in Table I. We have estimated an upper limit to kz2from the function 24 and existing kzs estimates. For these runs -RsoJ(RcH~o+ RCH,OH + R C H ~ O+~ H R H C O+ ~H Rco) k22[S021 /k23[021 (24) at constant [02] a plot of the function 24 vs. [SO,] gives a linear relation within the experimental error as is anticipated from relation 24; see Figure 7. Taking k23 = 2.2 X 10-12cm3molecule-l s-I,l5these data give k22I9 X cm3molecule-l s-l. This represents an upper limit in that some SO2would react in 18 with CH30 radicals as well for these conditions. The estimate of kz2 derived here is in qualitative agreement with that derived by direct CH radical observations by James et a1.,16 3 X cmP molecule-' s-l. Even if we employ our upper limit estimate of k22 we calculate that for the conditions of our flash photolysis experiments (50 torr of 0,) less than 0.2% of the CH3 radicals would react with SOz. It can be shown by computer simulation that this small extent of reaction cannot explain the magnitude of the perturbation seen in the CH302 decay in our flash photolysis experiments. The Reversible Addition of CH302to SO2 as a Possible Complication in the CH302-SO2 System. The third (15) C. J. Hochanadel, J. A. Ghormley, J. W. Boyle, and P. J. Ogren,
(14) C. S.Kan,J. G. Calvert, and J. H. Shaw, J.Phys. Chem., 84,3411 (1980).
J. Phys. Chem., 81, 3 (1977).
(16) F. C. James, J. A. Kerr, and J. P. Simons, J.Chem. SOC.Faraday Trans. I, 69, 2124 (1973).
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The Journal of Physical Chemistfy, Vol. 85, No. 9, 1981
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Figure 7. Plot of function 24 of the text vs. Psop; the slope in theory provides an estimate of k 2 2 / k 2 3 [ 0 2 ] .
possible alternative, reversible addition of CH3O2 to SO2 (reactions l b and 9), should be considered in explanation CH3O2 + SO2 CH302S02 (1b) CH302S02 CH3O2 + SO2 (9) of the anomalous behavior of this system. One anticipates that the CH3OZSO2radical formed in reaction l b would be equilibrated rapidly with 02: CH302S02 + 0 2 + CH302S0202 (10,ll) The peroxy radical product of reaction 10 may be involved in reactions with CH3O2 such as reaction 12 which prevent CH,O2SO2O2+ CH3O2 CH302S020+ CH30 + O2 (12) the re-formation of CH3O2 and SO2 and lead to net SO2 oxidation. Other radicals such as CH30 and H02may also react to entrap the CH302S0202radical, but we may neglect these for our present conditions. The flash photolysis data for kl[S02]vs. PMeaNz shown in Figure 2 can be rationalized reasonably well in terms of this mechanism choice. In our attempt to computer simulate these results reactions l b and 9-12 have been used together with the complete reaction mechanism for azomethane photooxidation given previously in Table I1 of ref 14 and the measured flash intensity vs. time profile. The critical factor in the fit of the data was the magnitude of the rate function kl&l&E/k&ll. We have taken two choices of this parameter, 7 X and 2 X cm3 molecule-' s-' in the calculation of curves A and B, respectively, given in Figure 2. The theoretical curves show the general trend observed experimentally. In terms of this mechanism reaction l b becomes rate limiting at high azomethane pressures (high [CH302]). From this limit we estimate klb cm3 molecule-l s-', This is in reaN (1.4 f 0.2) X sonable accord with previous high-intensity flash experiments of Kan et al.' and Sanheuza et aL3 In the FT IR spectroscopy experiments carried out with steady illumination and hence much lower radical concentrations than in the flash system, our present mechanism and the flash photolysis data predict that the net reaction of the CH302radical with SO2will be unimportant; for these conditions decomposition of the CH3O2SOZ and CH302S0202species should be favored over capture by CH302radicals in reaction 12. Sander and Watson have recently carried out flash photolysis studies of CI2-CH4-SO2-O2 mixtures in experiments at flash intensities which were about one onethousandth of those used by Kan et a1.l and Sanheusa.2 Thus in terms of the mechanism suggested here one ex-
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pects less net CH302-S02 reaction for the Sander and Watson experiments. These investigators report little or no reaction and suggest kl I 5 X cm3 molecule-' s-'. Unfortunately, the results are not quantitatively comparable to our own since the radical concentration is very much lower in their study, and the wavelength they employed to follow the CH302 was 242.5 nm while 265 nm was used in this study. Since both CH3O2 and CH302S0202 have similar chromophores, both species may contribute to the observed absorption to an undetermined extent. Note if CH302S0202is formed in our flash experiments, its absorption cross section at 265 nm is not the same as that for CH3O2 and/or the rate constant for CH302S0202 radical reaction with CH3O2 is not the same as that for CH3O2 with CH3O2; we would have seen no alteration in the CH3O2 kinetics on SO2 addition if this were not the case. We have arbitrarily assumed that only CH3O2 absorbs significantly a t 265 nm in our treatment of the data given here. If CH302S0202absorbs significantly at 265 nm as well, then our estimates of k1 represent lower limits to the true constant. It is possible to fit the Sander and Watson reciprocal absorbance vs. time data (Figure 2 of their stud?) using the mechanism outlined here and the same rate constants we employed to fit our data provided that CH302S0202 does absorb with c242.5 = 1450 f 50, 9.65 = 230 i 115 L mol-' cm-' (base e) and taking klo[02]/kll = 1.3 X 10' and k12 = 3.3 X cm3 molecule-' s-'. If the mechanism outlined here is correct in involving the reversible addition of CH3O2 to SO2,then in principle the CH302-S02data obtained in our runs at high flash intensity and long wavelength analytical beam for CH302should provide the best conditions to observe the CH3O2-SOZreaction. Note that although the Simonaitis and Heicklen study of the competitive rates of the CH302reaction with NO and SOz was made at low intensities and low radical concentrations, a value of klb near equal to that found in our flash experiments was derived, k l b N (1.6 f 0.8) X cm3 molecule-' s-'. In this system the NO can act to remove peroxy radicals in reaction 5 before significant decay to re-form SO2may occur. It is possible to rationalize these results by using a reasonable set of rate constants which include those derived previously. Thus taking k l o / k l l = 1.7 X cm3molecule-', k4 < 2.4 X lo's-', and k5 = 6.2 X cm3 molecule-' s-' then a t least 90% of the CH302S02formed under the experimental conditions employed by Simonaitis and Heicklen should proceed to oxidize NO and result in fixation of the higher oxidation state of sulfur. Further FT IRS experiments were carried out in this work to test the present interpretation of the CH3O2S02-N0, system. Direct evidence was obtained for the CH302reaction with SOz in photolyzed mixtures of azomethane (90.1 ppm), CH30N0 (9.98 ppm), SO2(48.7 ppm), and O2 (2000 ppm). After 4.35 min of irradiation in the large chamber the major products formed were CH20, CH30H, CH30N0, CH30N02, and CH302N02. The photolysis of azomethane formed CH3 radicals and ultimately CH302radicals for these conditions, while CH30N0 photolysis acted as an internal source of small amounts of NO: (CH3)2N2+ hv 2CH3 + N2 (25) CH302 (+M) (23) CH3 + O2 (+MI CH30N0 hv CH30 + NO (13) The O2 pressure was kept low to prevent significant generation of H 0 2 radicals from CH30 yet high enough to entrap completely the CH3 radicals formed. The me-
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The Journal of Physlcal Chemlstry, Vol. 85, No. 9, 1981 1131
SOz Oxidation by CH3Oz
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Flgure 8. Infrared spectra from the photoiysls of azomethane (90.1 ppm), CH,ONO (9.98 ppm), SOz (48.7 pprn), and O2(2000 ppm): A, initial mixture; B, after 4.35-mln photolysis, C, resklual spectrum after reactant and known products were subtracted; D, standard spectrum Of CH302N02.
thylperoxy radicals oxidized NO to NO2 and CH30N02 and CH302N02resulted from NO2interactions with CH30 and CH302radicals: CH302+ NO CH30 + NO2 (3) CH30 + NO2 CH30N02 (21) CH302 + NOz + CH302N02 (26) However, in addition to these products SO2is removed at a significant rate, and a new metastable compound X formed; see Figure 8. This compound has characteristic absorption bands at 730,785,1013,1217,1303, and 1767 cm-l with a lifetime of 22 min. The same metastable species is formed, although less efficiently, in photolyzed azomethane-S02-N0,-02 mixtures. The inefficiency in the latter experiment results from the removal of a large fraction of the CH302radical through the reaction 3 which occurs at the high levels of NO used here. In the NO,-free system with CH30N0 present, its photolysis produces a low steady level of NO which cannot compete well with SO2 for CH3O2 radicals but which can react well with CH302S0202 radicals formed. This metastable compound X is not formed in similar experiments using azomethane-S02-02 mixtures and no SO2 is removed in this case. It appears that CH302,NO,, and SO2 are all necessary ingredients to form the compound X. The characteristic absorption spectrum of this compound can be compared with those of some compounds of related structure in Figure 9. These data suggest that X may be CH302S020N02(or possibly CH302S0202N02)which could arise in the following sequence: CH3O2 + SO2 + CH302S02 (lb,9) CH302S02 + 02 + CH302S0202 (10,ll) CH302S0202 + NO CH302S020 NO2 (5) CH30zS020 + NO2 CH302S020N02 (27) CH302S0202 + NO2 + CH302S0202N02 (28)
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Flgure 9. Comparison of infrared spectra of several structurally related compounds and the metastable product X of the CH30z-SOz-N0, system.
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Flgure 10. Infrared spectra from the photolysis of Ciz (4.93 ppm), CH30H (9.68 pprn), SOz (48.9 ppm), and NOz (2.8 ppm): A, initial mixture; B, spectrum following 3.42-min photolysis; C, residual spectrum after the reactants and known products were subtracted (compound Y of the text): D, spectrum of product X obtained in the CH,Oz-SOz-NO, experiments.
The alternative origin of compound X from CH30 radical attack on SO2 can be ruled out in view of the results of additional experiments which we have carried out. In one experiment CH30 radicals were generated from the photolysis of a mixture of C12 (4.93 ppm), CH30H (9.68 ppm), SO2 (48.9 ppm), and NO2 (2.8 ppm) for 3.42 min; see Figure 10. The products formed were HCl, CH20, HON02, CO, CH30N02,CH,ONO, NOC1, HC02H, and
J. Phys. Chem. 1981, 85, 1132-1137
1132
a new metastable compound Y which absorbs at 787,854, 1023, 1217, and 1435 cm-l; this compound has a much shorter lifetime (3.7 f 0.4 min) than that of X. This same metastable compound is formed also in photochemical experiments in which CH30N0 photolysis is used as the source of CH30: mixtures of CH30NO-S02-low O2 or CH30NO-S0z-N02-low 02.However it is not formed in the photolysis of mixtures of C12-CH30H-S02 or ClzCH30H-N02 (low Oz in each case). It absorption spectrum is similar to but not the same as that of dimethyl sulfate which absorbs at 760,820,1010,1210, and 1420 cm-'; see Figure 10. In the case of compound Y there is no band characteristic of the 0-NO2 group which is seen in the CH302-S02-N0, system product X. A reasonable postulate for the structure of Y is CH30S020H;this could be formed in the following sequence for the CH30NO-S02 mixture photolysis: CH30N0 + hv CH30 + NO (13) (18) CH30 + SO2 CHBOS02 CH3OS02 + 0 2 ---* CH30S0202 (19) CH3OSOzOz + NO CH3OS020 + NO2 (20) CH30S020+ CH30 CH30S020H+ CHzO (29) In any case both the spectrum and the lifetime of the product of the CH30 radical attack on SOz are very different from those of the metastable species X of the CH3O2-SOZ-NO, system, so the participation of CH30 in the formation of X can be ruled out. The origin of the metastable compound X from H 0 2or HO attack on SO2 can be excluded from the results of other
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experiments. The photolysis of dilute ClZ-H2-SO2-NO, mixtures in O2 do not give the metastable compounds X or Y, but they lead to a product identified as H2S04aer0~01.'~ A further study of the chemistry of the metastable compound X is underway, and these results will be published in a subsequent report. However, we may conclude from the present evidence that CH3O2 does react with SOz. SO3 and its ultimate product, H2S04,were not identified as primary products of this interaction, so we conclude that the only reactive channel for this reaction is reaction lb. The initial adduct may decompose in the absence of other suitable reactants such as CH3O2, NO, or conceivably other radical transients present in the given mixture. Thus the net oxidation of SO2 which occurs from CH3O2 attack depends upon the concentration of the peroxy radicals, NO, and possibly other species in the system. Further studies are now underway in our laboratory to evaluate the influence of atmospheric levels of NO on the rate of SO2 oxidation by CH3O2 radicals. Acknowledgment. This work was supported by a research grant from the Environmental Protection Agency (R-806479-02-0). We are grateful to Dr. Fu Su for his help in part of this study, Drs. S. P. Sander and R. T. Watson for a preprint of their recent work before publication, and these workers and J. Heicklen for their helpful discussions of this work with the authors. (17)F. Su, J. G. Calvert, and J. H. Shaw, J. Phys. Chern., 84, 239 (1980).
Low Resolution Microwave Spectroscopy. 12. Conformations and Approximate Barriers to Internal Rotation in Ethyl Thioesters' Nancy S. True,+ Clarence J. Silvla, and Robert K. Bohn" Department of Chemistry and Institute of Materials Science, University of Connecticut, Sforrs, Connecticut 06268 (Received: September 22, 1980)
S-Ethyl thiochloroformate,thiocyanoformate,thiotrifluoroacetate,and thiofluoroformateproduce low-resolution microwave band spectra which have been assigned to stable conformationalisomers and to torsionally excited species. Each compound has a stable conformation with a syn configuration of the O=CSC fragment and a gauche configuration of the CSCC fragment. Only the fluoro and chloro compounds each have a stable syn(O=CSC)-anti(CSCC) conformation. The fluoro-,chloro-, and trifluoromethyl esters also display broad bands of less stable species. These bands are composed of unresolved lines of species excited above a low internal barrier (-1 kcal/mol) about the S-C (ethyl) bond. The relative intensities of the band series and their temperature dependence reveal the relative energies of the species and infer approximate values for the internal rotation barriers. Introduction Despite the biological importance of the thioester linkage, conformational preferences of thioesters have not been extensively investigated. A recent microwave study of S-methyl thioformate2 demonstrates that this molecule exists in a syn (?(O=CSC) = 0') planar conformation analogous to that previously observed for methyl formate? The study suggests that the barrier hindering internal +Department of Chemistry, University of California, Davis, CA 95616. 0022-3654/81/2085-1132$01.25/0
rotation of the methyl group in S-methyl thioformate is different from the corresponding value for methyl formate (1190 f 40 cal/m01)~since no internal rotation doublets were observed in the microwave spectrum of the ground vibrational state. The authors concluded that the methyl barrier in the sulfur compound is greater than 2 kcal/mol. (1)Paper 11 on this series: L. P. Thomas, N. S.True, and R. K. Bohn, J. Phys. Chern., 82, 480 (1978). (2)G. I. L.Jones, D. G. Lister, N. L. Owen, M. C. L. Gerry, and P. Palmieri, J. Mol. Spectrosc., 60, 348 (1976). (3) R. F. Curl, Jr., J. Chern. Phys., 30, 1529 (1959).
0 1981 American Chemical Society
