J . Phys. Chem. 1986,90, 5393-5396 the relative separation and orientation of the two CH3 groups together determine the interfragment atomic separations and, hence, V, in eq 111.4. In addition, the CH3structure determines the three principal moments of inertia IA(R),ZB(R),and Zc(R), used to assign a (rigid-rotor) rotational energy to each fragment. In a theoretical study of the potential energy for colliding methyl radicals, Yamabe, Minato, Fujimoto, and Fukui9 found that the geometrical deformation of the radicals (planar pyramidal) facilitates the recombination by bringing about a decrease in the exchange repulsion energy. The parameters appearing in eq B.l and B.4 as well as other relevant parameters are listed in Table XI. In Figure I b the various bond angles and bond distances that determine V, are depicted.
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Appendix C. Orientational Analysis of V , A particular relative separation and orientation of the methyl radicals is achieved from a standard configuration as follows: Consider the two radicals to be. superimposed with centers of mass at the origin of the ( x , y , z ) coordinate system used in section I1 with the molecular symmetry axes aligned along the z axis. Radical 1 is then translated along the positive z axis by a distance R . The final orientation of radical i ( i = 1, 2) is specified by the Euler angles (y,, 6i,x,)which directly connect the ( x , y , z ) system to the double-primed system (x,", y,", q")in which the molecular symmetry axis is aligned along the z," axis. x, is the rotation in
5393
the xy plane about the z axis, 6, is the angle between the z and z? axes, and y i is the rotation in the x?yT plane about the zp axis (0 Ixi, yi I2a, 0 I 6{ Ia). Since both x1and xz specify rotations in the plane and since only different values of x1- x2 can yield different relative orientations of the radicals, we simply set xl = 0 and retain x2. This elimination rotations of the entire system about the z axis, thereby reducing the dimensionality of the angular space from six to five. The volume of the five-dimensional angular space can be reduced by taking account of symmetry. Since each radical, as modeled, has C,, symmetry for all R values, y, can be confined to the range 0 I6i I2 ~ / 3( i = 1, 2). Furthermore, since the radicals are identical, x2 can be restricted to the range 0 Ix2 Ia;values of x2 between A and 27 yield configurations different only in the exchange of the two fragments. The range of the angles 6, and b2 is unaffected by symmetry considerations in this case. The values of these two angles provide a convenient and simplified description of the gross relative orientation of the two radicals: the "back-to-back" orientation with 6, < x/2 and ti2 > ~ / is2 denoted configuration A, the "back-to-face'' orientation with 6' > a/2 and 62 > ~ / or2 6, < a/2 and 62 < 712 is denoted configuration B, and the "face-to-face" orientation with 6, > ~ / and 2 62 < rJ2is denoted configuration C. A grid with As, = Ati2 = Ayl = Ay2 = ~ / 1 and 8 Ax2 = a/9 proved sufficiently fine for our examination of Vt. Registry No. Methyl radical, 2229-07-4.
Absolute Rate Constants for the Gas-Phase Reactions of the NO3 Radical with CH3SH, CH3SCH3,CH3SSCH3,H,S, SO,, and CH30CH3over the Temperature Range 280-350 K Timothy J. Wallington? Roger Atkinson,* Arthur M. Winer, and James N. Pitts, Jr. Statewide Air Pollution Research Center, University of California, Riverside, California 92521 (Received: March 26, 1986)
Using a flash photolysis technique, absolute rate constants have been determined for the gas-phase reactions of the NO3 radical with CH3SH, CH3SCH3,CH3SSCH3,H2S, SO2,and CH30CH3. For CH,SH, CH3SCH3,and CH3SSCH3,rate constants were obtained over the temperature range 280-350 K,and the measured limiting high-pressure rate constants are cm3 molecule-' s-'; CH3SSCH3,(1.9 given by the following Arrhenius expressions: CH3SCH3,(4.7T2;) X 10-13e('70*'30)/T & 0.3) X 10-13e(290*So)/T~m3 molecule-' s-l; and CH3SH, (l.O$$) X 10-13e(6w*4"'')/T cm3 molecule-' s-'. For the reactions of the NO3 radical with SOz, CH30CH3,and H2S, upper limits to the rate constants of 4 X 3X and 3 X cm3molecule-' s-l, respectively,were determined at 298 & 2 K. These data are discussed with respect to the previous literature data, the mechanisms of the reactions of NO3 radicals with sulfur-containing organic species, and the atmospheric chemistry of these species.
Prompted by evidence of acidic deposition in the troposphere and the formation of sulfate aerosols in the stratosphere,'J there has been a mounting interest concerning the atmospheric chemistry of sulfur compounds during the past decade. Natural sulfur emissions are mainly comprised of the reduced sulfur compounds hydrogen sulfide (H,S), carbonyl sulfide (COS), carbonyl disulfide (CS2), methanethiol (CH3SH),dimethyl sulfide (CH3SCH3),and dimethyl disulfide (CH3SSCH3).>' To understand the role played by these sulfur compounds in the chemistry of the atmosphere, it is necessary to quantify their sources and sinks. Until recently it was believed that reaction with OH and, to a limited extent, with O('P) atoms" were the important atmospheric chemical loss processes for these reduced sulfur compounds. However, recent measurements of the rate constants
for the reactions of the NO3radical with CH3SCH3,12-14 CH3SH,15 C2H@-L'5 and CH3SSCH3,'' together with observations of the
*Author to whom correspondence should be addressed. +Present address: Center for Chemical Physics, National Bureau of Standards, Gaithersburg, MD 20899.
I 65 .,
~
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(1) Wayne, R. P. Chemistry of Atmospheres; Oxford University: London, 1985. (2) Finlayson-Pitts, B. J.; Pitts, J. N., Jr. Atmospheric Chemistry: Fundamentals and Experimental Techniques; Wiley: New York, 1986. (3) Graedel, T. E. Geophys. Res. Lett. 1979, 6, 329. (4) Nguyen, B. C.; Gaudry, A.; Bonsang, B.; Lambert, G. Nature (London) 1978, 275, 637. (5) Cullis, C. F.; Hirschler, M. M. Atmos. Environ. 1980, 14, 1263. (6) Adams, D. F.; Farwell, S. 0.;Robinson, E.; Pack, M. R.; Bamesberger, W. L. Environ. Sci. Technol. 1981, 15, 1493. (7) Sze, N. D.; KO, M. K.W. Atmos. Enuiron. 1980, 14, 1223. (8) Graedel, T. E. Rev. Geophys. Space Phys. 1977, 15, 421. (9) Wine, P. H.; Kreutter, N. M.; Gump, C. A,; Ravishankara, A. R. J . Phys. Chem. 1981,85, 2660. (10) Jones, B. M. R.; Cox, R. A.; Penkett, S. A. J . Atmos. Chem. 1983, (1 1) Nip,
W. S.; Singleton, D. L.; Cvetanovic, R.J. J . Am. Chem. SOC.
1981, 103, 3526.
0022-3654/86/2090-5393.$01 SO10 0 1986 American Chemical Society
5394 The Journal of Physical Chemistry, Vol. 90, No. 21, 1986
NO, radical at mixing ratios of up to -430 parts per trillion (ppt) in polluted atmospheres,I6 show that reaction with NO, radicals can be an important tropospheric loss process for these naturally emitted reduced sulfur compound^.^^ With the exception of the recent absolute rate studies by Burrows et aI.l3 and Wallington et al.14 of the kinetics of reaction 1, the remaining kinetic data available for these NO, radical NO,
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+ CH3SCH3
products
(1)
Experimental Section The flash photolysis-long pathlength optical absorption apparatus and experimental techniques used have been described in detail previo~sly,'~ and hence only the relevant details are given here. The reaction vessel consisted of a 1 m long, 51 mm i.d. quartz tube surrounded by two annular jackets. Distilled water was circulated through the innermost of these to control the temperature of the reaction vessel over the range 280-350 K to within f0.5 K. The outer annular jacket, fitted with two tungsten electrodes, was filled with 10-30 Torr of argon or xenon and served as the flash lamp. This flash lamp was typically operated at 100-500 J/flash at a repetition rate of 1 flash every approximately heating of the reactant gas from the 6 s. As shown previo~sly,'~ flash was negligible (
