1134
J. Phys. Chem. 1988, 92, 1134-1 138
Reactions of i-C3H7Swith 0,, NO2, and NO at 296 K Graham Black,* Leonard E. Jusinski, Chemical Physics Laboratory, S R I International. Menlo Park, California 94025
and Roger Patrick LSI Logic, Santa Clara, California 95050 (Received: March 20, 1987: In Final Form: July 28, 1987)
The laser-induced fluorescence technique has been used to study the reactions of the i-C3H7Sradical with 02,NO2, and NO at 296 K. The i-C3H7Sradicals were made by photodissociation of i-C3H7SHat 248 nm. The reactions with O2 and NO2 have rate coefficients of (1.1 f 0.1) X and (5.9 & 0.6) X lo-" cm3 molecule-l s-l, respectively. The reaction of i-C3H7Swith NO involves a third body and has been shown to be in the transition region between the low- and high-pressure limits (although much closer to the latter). With the use of an expression developed by Troe to fit the results, values of k , = (4.0 & 0.4) X lo-" cm3 molecule-I s-I and ko = (1.7 2:)X cm6 molecule-2 S-I have been obtained.
Introduction Alkylthio radicals (RS) are important intermediates in the photolysis and mercury-photosensitized decompositions of reduced sulfur compounds (RSH, RSR', and RSSR'). They are also important intermediates in the reactions of these compounds with O H radicals in the atmosphere. Although these compounds are minor constituents, they may play a part in the atmospheric sulfur cycle and contribute to the acid precipitation problem. It is, therefore, important to understand the atmospheric chemistry of these radicals. Until recently, the only reported absolute rate coefficient measurements involved the HS'-6 and CH3S7.*radicals. Very recently rate coefficients for the reactions of C2HSSwith 02,NO2, and N O at 296 K were r e p ~ r t e d . ~This paper reports the first direct measurements of the rate coefficients for the reactions of i-C3H7S with 02,NO2, and NO at 296 K. Experimental Section The apparatus has been described previously.2 Briefly, an excimer laser was used to generate i-C3H7S radicals by the photolysis of i-C3H7SH (isopropyl mercaptan, IPM) at 248 nm. The Quanta-Ray Nd:YAG dye laser, using either doubled Exciton LDS 821 or Exciton Stilbene 420, provided 2-10 mJ of light! i the region 395-435 nm for exciting fluorescence on the A-X transition of i-C3H7S.Io Excitation was carried out at one of several wavelengths, mainly 405.3, 411.3, or 417.3 nm (tentative1yl0 assigned as transitions from the ground state to various levels of the C-S stretching mode in the excited state). The dye laser beam was propagated through the cell in a direction opposed to that of the excimer beam. The lasers were operated at 10 H z and a variable delay could be introduced between them to follow the decay of the i-C3H7S radical. Experiments were performed under pseudo-first-order conditions. In the absence of any reactant, the i-C3H7Sradicals decayed by self-reaction (combination or disproportionation) and diffusion. When reactants were present in sufficient concentration, the decay (1) Tiee, J. J.; Wampler, F. B.; Oldenborg, R. C.; Rice, W. W. Chem. Phys. Lett. 1981, 82, 80. (2) Black, G. J. Chem. Phys. 1984,80, 1103. (3) Black, G.; Patrick, R.; Jusinski, L. E.; Slanger, T. G. J. Chem. Phys.
1984,80, 4065.
(4) Friedl, R. R.; Brune, W. H.; Anderson, J. G. J. Phys. Chem. 1985,89, 5505. (5) Stachnik, R. A.; Molina, M. J. J. Phys. Chem. 1987, 91, 4603. (6) Wang, N. S.; Lovejoy, E. R.; Howard, C. J. J . Phys. Chem. 1987, 91, 5743. (7) Balla, R. J.; Nelson, H. H.; McDonald, J. R. Chem. Phys. 1986, 109, 101.
( 8 ) Black, G.; Jusinski, L. E. J . Chem. Soc., Faraday Trans. 2 1986, 82, 2143. (9) Black, G.; Jusinski, L. E.; Patrick, R., submitted for publication in J . Chem. Phys. (IO) Black, G.; Jusinski, L. E., submitted for publication in Chem. Phys. Lerr.
of the i-C3H7S radicals could be fitted by a single exponential over at least 3 lifetimes. The detection system consisted of a Heath EU-700, 0.35-m monochromator equipped with an RCA C3 1034A photomultiplier. For most of the experiments, the monochromator was operated in zero order and filters (Wratten no. 3 and a short-pass filter) were used to block laser-scattered light and transmit fluorescence in the 440-520 nm range. For measurements with the lowest concentrations of IPM, the monochromator was removed, and the photomultiplier and filters were used directly. The photomultiplier output was fed via a fast 100 MHz, gain 100 amplifier (Pacific Photometrics Model 2A50) to a boxcar averager and then to a chart recorder. The IPM (Aldrich, 98%) was degassed by several freezepump-thaw cycles and then used alone or as mixtures in argon. Most of the experiments used IPM (vapor pressure = 7.4 X 10I8 molecule cm-3 at 296 K) as a 1:25 mixture in argon. The other gases used were supplied by Matheson Gas Products with purity 99.6% minimum and were used without further purification. The NO2 was a 1% mixture in helium. The N O was used either pure (CP Grade; 99% minimum) or as 5 or 10% mixtures in argon. The gases passed through flowmeters and were mixed prior to entering the photolysis cell. The cell was equipped with MKS pressure gauges and was evacuated with a small rotary pump, which gave =2 s for the residence time of the gas in the cell.
Results and Discussion In earlier work" on the 253.7-nm photolysis of dilute solutions of aliphatic thiols in 3-methylpentane glasses at 77 K it has been shown that the major products formed initially are thiyl radicals and hydrogen atoms. Assuming this holds at 248 nm in the gas phase then i-C3H7SH
+ hu (248 nm)
-
i-C3H7S
+H
(1)
is a convenient source of i-C3H,S radicals for this study. The H atoms produced in reaction 1 can give rise to secondary production of i-C,H7S radicals by the reaction H
-
+ i-C3H7SH
i-C,H7S
+ H2
AH = -12 kcal/mol (2)
There does not appear to be a determination of the absolute rate coefficient for reaction 2. It seems likely to be similar to the value12 of 2.0 X cm3 molecule-' s-' at 296 K for the reaction
H
+ CH3SH
-
CH3S
+ H2
(3)
since D(CH3S-H) and D(i-C3H7S-H) are approximately equal.I3 ( 1 1 ) Elliot, A. J.; Adam, F. C. Can. J . Chem. 1974, 5 2 , 102. (12) Wine, P. H.; Nicovich, J. M.; Hynes, A. J.; Wells, J. R. J. Phys.
Chem. 1986, 90, 4033. (13) Benson, S . W. Chem. Reu. 1978, 78, 23.
0022-365418812092-1134$01.SO10 0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 5, 1988 1135
Reactions of j-C3H7S with, 02,NO2, and NO
'"
+
+ 0, + M
H
I
ecule cm.3
2
0
6
4
E
[02](10'8 molecute cm-3)
Figure 1. Decay rate of the i-C3H7Sradical vs O2 addition. IPM = (0.6-1.6) X l O I 4 molecule cm-3 in various argon concentrations undergoing photodissociation at 248 nm.
In the absence of added reactants, a long-lived component to the decay of the i-C3H7S radicals in IPM was observed and might originate from reaction 2. In the subsequent studies the IPM was varied by up to a factor of 15 in concentration (6 X 1013-9 X IOl4 molecule ~ m - to ~ be ) sure that the results obtained did not change and hence where unaffected by the secondary chemistry that can occur in these systems. i-C3H7S+ 02.A slow reaction of i-C3H7Swith 0, could be measured and the results are shown in Figure 1 . The slope of cm3 the line shown gives a rate coefficient of ( 1 . 1 f 0.1) X molecule-' s-' for the reaction of i-C3H7Sradicals with 0,. The results obtained were independent of the pressure of argon buffer gas over the range shown. It may be that if three-body processes are involved that they are close to the high-pressure limit and hence not affected by the argon additions in this experiment. Previous CH3S,7*8and C2H5S9has only established upper work on HS,',2*4,5 limits for the rate coefficients of the reactions with 0,. Energetically allowed reaction pathways for i-C3H7S 0, are as follows:
i-C3H7S+ 0,
-
-
+
(CH3),CS
+ HO,
(4)
i-C3H7 + SO,
-
AH = -14 kcal/mol
AH = -74 kcal/mol
(5)
M
i-C3H7SOZ
AH = -96 kcal/mol
(6)
where the exothermicities are calculated from AHfo(i-C3H7S)= 21 kcal/mol (ref 13), AHf0[(CH3),CS] = 2 kcal/mol (estimate), AHfO(H0,) = 5 kcal/mol (ref 14), AHfo(i-C3H7)= 18.2 kcal/mol (ref 15), AHfo(S02)= -70.9 kcal/mol (ref 14), and AHf"(& C3H7SO2)= -75 kcal/mol (ref 16). Further work, including a determination of the products in the photooxidation of IPM under a variety of conditions would be required to determine the contributions of these different pathways. In previous workI7 on CH3S + O,, evidence has been presented to support
CH3S
+
the main reaction for both CH3S O2and CzHSS 0,. Clearly much further work will be required before our understanding of the chemistry in these systems is complete. Such studies are important since this study shows that, at least for i-C3H7Sradicals, the reaction with 0, is the dominant atmospheric removal process. In this system, the H atoms generated by the photodissociation of IPM will be rapidly converted to HO, by the reaction
I
+ O2
M
CH3S02
A H = -89 kcal/mol
(7)
(the analogue of reaction 6) as the important reaction in air. Earlier workisJ9 concluded that the analogue of reaction 5 was (14) Stull, D. R.; Prophet, H. "JANAF ThermochemicalTables," 2nd ed.; Natl. Stand. Ref. Data Ser. (U.S., Natl. Bur. Stand.) 1971, 37. (15) McMillen, D. F.; Golden, D. M. Annu. Rev. Phys. Chem. 1982,33, 493. (16) Mackle, H. Tetrahedron 1963, 19, 1159. (17) Hatakeyama, S.; Akimoto, H. J . Phys. Chem. 1983, 87, 2387. (18) Cullis, C. F.; Roselaar, L. C . Trans. Faraday SOC.1959, 55, 272. (19) Kirchner, K.; Vetterman, R.; Indruch, H. Ber. Bunsen-Ges. Phys. Chem. 1978, 82, 1223.
-+
HO,
+M
(8)
which has a rate coefficientZoof 5.5 X cm6 molecules-2 s-l at 296 K for M = 02.Hence HO, will be formed on a faster time scale than that of the i-C3H7Sradical decay (which itself may produce H 0 2 by reaction 4). The role of HO, must therefore be considered in this system. Because most of our experiments were carried out with initial radical and atom densities ~m-~, removal of i-C3H7Sradicals by HO, cannot compete with the observed fast (104-105s-I) removal by 0,. There is, however, the possibility of i-C3H7Sradical regeneration by
-
HOz + i-C3H7SH
H,O2 + i-C3H7S
AH = 2 kcal/mol (9)
This reaction will be considerably slower than the corresponding H-abstraction reaction with O H OH
-
+ i-C3H7SH
H20
+ i-C3H7S
AH = -27 kcal/mol (10)
for which the rate coefficient2' is 4.1 X lo-'' cm3 molecule-' s-I. Hence we would estimate an upper limit for the rate coefficient of reaction 9 of 2 X lo-', cm3 molecule-' s-'-simply based on adding an additional 2 kcal/mol, the endothermicity of reaction 9, to the barrier height. Therefore, it does not appear that reaction 9 can regenerate i-C3H7Sradicals on the time scale of the measured decays (10-60 ps), particularly for the measurements made with [IPM] = 6 X loi3 molecule ~ m - ~ , There are other reactions that could lead to i-C3H7Sregeneration in this system. A scheme similar to that proposed by Balla and Heicklen2, in their study of CH3SSCH3photodissociation in the presence of 0, is as follows:
+ -
i-C3H7S + Oz + M i-C3H7S02
+0 2
-
i-C3H7S04
M
(CH3)zC0
i-C3H7S02
(11)
i-CjH7S04
(12)
+ SOz + OH
(13)
followed by reaction 10. At the lowest Oz additions, there is the possibility that this scheme could slightly affect the measured radical decay rates even for measurements with [IPM] = 6 X loi3 molecule ~ m - ~In. Figure 1, it can be seen that the points at low O2concentrations lie on the same line as those measured at the highest O2concentrations, and the measurements at the highest 0, concentration were unaffected by increasing the IPM con). centration (from 0.6 X lOI4 to 1.6 X loi4molecule ~ m - ~ Hence the measurements are not sensitive to regeneration of i-C3H7S by this sequence of reactions. This may be due to the finite time for OH generation in the reaction sequence 11-13, which will further reduce the effect on the i-C3H7Sradical decay rates. i-C3H7S+ NO,. Initial measurements indicated that NO, fluorescence, excited by the dye laser, was long-lived (at low buffer-gas press_ures)and, because it occurs in the same region as the i-C3H7S(A)fluorescence, interfered with the measurements of the i-C3H7Sradical decay. This problem was solved by working at a high buffer-gas pressure (Ar 1.3 X lOI9 molecule ~ m - ~ ) , which greatly shortened the NOz fluorescence lifetime to =2 ns (the quenching rate coefficient measured for argon wasj.4 X IO-" cm3 molecule-' 8)while only shortening the i-C3H7S(A) emission lifetime to =40 ns (ref IO). While removing the interference from NO, fluorescence, this solution did prevent measurements of the
-
(20) Wong, W.; Davis, D. D. Inr. J . Chem. Kinet. 1974, 6, 401. (21) Wine, P. H.; Thompson, R. J.; Semmes, D. H. Int. J . Chem. Kinet. 1984, 16, 1623.
(22) Balla, R. J.; Heicklen, J. J . Photochem. 1985, 29, 297.
1136 The Journal of Physical Chemistry, Vol. 92, No. 5, 1988
Black et al.
TABLE I: Vibrational Frequencies and Moments of Inertia for i-C,H,SNO
mode
frequency, cm-'
CH3 asym stretch C H stretch CH, sym stretch N O stretch CH, asym deformation CH, sym deformation C H deformation C C stretch C H I rock
2981, 2971, 2964, 2939 2925 2877, 2864 1534 1476, 1468, 1456 (2) 1388, 1372 1314, 1250 1162, 893 1125, 1089, 956, 929 655 622 396 333, 301 239, 319 250 250 70 65
NS stretch C S stretch CCC deformation CCS deformation C H I torsion S N O bend CSN bend
i
,y , 2
0
I
I
4
6
[NOp]
I 8
I
NO torsion SNO torsion
J
1 0 1 2
molecule cm 3,
Figure 2. Decay rate of the i-C3H7Sradical vs NO2 addition. IPM = (0.6-9) X loi4molecule cm-3 in Ar z 1.3 X lOI9 molecule undergoing photodissociation at 248 nm. rate coefficient as a function of buffer-gas pressure. The results are shown in Figure 2, and the slope of the line gives (5.9 f 0.6) X lo-" cm3 molecule-' s-l for the removal of i-C3H,S by NO,. This is to be compared with similarly large values of 1.09 X and 9.2 X lo-'' cm3 molecule-I s-' for CH3S7and CzH5S,9respectively. Because of the large rate coefficient observed, and because the extent of NO, photodissociation at 248 nm is
