J. Phys. Chem. 1987,91, 5743-5749 Because Laufer and Bass appeared to measure the rate at only one acetylene pressure (75 mTorr), a reaction scheme involving a C4H3intermediate seems also to be possible since the order of the reaction producing C4H2 with respect to C2H2was not determined. Likewise, in the case of CzH 02,Laufer and Lechleider measured the rate of C O formation for only one O2 pressure so that they did not measure the order of the reaction with respect to 02.In this case a more general multistep reaction such as
+
+ O2 -% H C 2 0 + 0 HC20 + O222CO + OH C2H
is not ruled out by O2 reaction order. For any two-step mechanism the initial slope of the product C.,H2 rise will always be zero, but the time domain of the resulting kink in the concentration versus time curve decreases rapidly as kC[C2H2]/ k Dincreases. Because of interference caused by the photoflash, Laufer and Bass could not make observations during the first 10 ps. As long as the creation rate of the isomer, kc[C2H2],is much greater than its destruction rate, kD,the existence of the two-step mechanism will not be revealed by following only the product C4H2 unless the product is followed at very short
5143
reaction times. Test calculations of the size of this initial effect using our k values for kc and Laufer et al.3 k values for kD indicate that such an initial kink would be undetected by Laufer et al. for both C2H + C2H2and C2H 02.(The time analysis for CzH O2done by Laufer and Lechleider6used a much smaller value of kc than ours and is not relevant here.) We suggest that the mechanism for both these reactions may be more complex than a single step. Unfortunately, we do not have the sensitivity to follow the time evolution of precursor or product concentrations with the current apparatus. The ethynyl radical absorptions, which are easily seen, are especially intense since they invo_lve-vibronictransitions which borrow oscillator strength from the A-X transition. In the future, we plan to incorporate a multipass cell into the present apparatus which should enable us to observe CF3CzH, C2H2,and C4Hz. Also, we hope to monitor H C O and C O by using a diode laser. The results of such investigations should enable some of the questions raised by the present study to be resolved.
+
+
Acknowledgment. This work was supported by the Department of Energy under Grant DE-FG05-85ER 13439 and the Robert A. Welch Foundation under Grant C-07 1. Registry NO.CF,C2H, 661-54-1; CIH, 2122-48-7; H2, 1333-74-0; 02, 7782-44-7; NO, 10102-43-9; HCCH, 74-86-2.
Temperature Dependence of the Rate Constant for the Reaction HS
+ NOP
Niann S. Wang, Edward R. Lovejoy, and Carleton J. Howard* NOAA Aeronomy Laboratory, R/E/AL-2, Boulder, Colorado 80303, and the Department of Chemistry and Biochemistry and CIRES, University of Colorado, Boulder, Colorado 80309 (Received: February 9, 1987; In Final Form: May 19, 1987)
-
-
Two reactions of atmospheric importance involving the HS radical have been studied by using a discharge flow laser magnetic resonance technique: (1) HS + NO2 products; and (2) HS + O2 products. The rate constant for reaction 1 was measured at low pressure (- 1 Torr) between 221 and 415 K and a negative temperature dependence was observed: k l = (2.9 f 0.5) X lo-" exp((240 f 5O)/T) cm3 molecule-' s-'. The room temperature value for kl from this study is higher than three previous values. Secondary chemistry associated with high H2S concentrations in the earlier studies is believed to contribute to the discrepancies. DS was used to investigate the isotope effect for reaction 1 and no evidence has been observed for a primary.isotope effect, k(DS NO2) = (7.3 f 1.1) X lo-'' cm3 molecule-' s-', at 299 K. No reaction was observed with O2 and an upper limit of 1.5 X cm3 molecule-' s-I was assigned to reaction 2.
+
Introduction The increasing interest in acid precipitation problems in recent years has led to greater attention being given to the atmospheric reactions of sulfur compounds. It is becoming clear the the oxidation of reduced sulfur compounds such as H2S, OCS, CS2, and CH3SCH3contributes significantly to the global sulfate and sulfuric acid production. For HzSthe oxidation process is believed to be initiated by the well-characterized reaction with an OH radical which produces HS as a It is necessary to understand the subsequent steps in order to assess the relationship between the sources of reduced sulfur compounds and the sulfate deposition sites. Therefore, it is of interest to study HS kinetics in order to evaluate its role and importance in acid precipitation chemistry. In the present study we focus on two reactions of the HS radical: HS + NOz products (1) HS
+ 0,
--
products
(2)
agreement on the rate constant. Black3 measured kl = (3.5 f 0.4) X lo-" cm3 molecule-' s-' with a flash photolysis laser-induced fluorescence (LIF) method. A discharge flow LIF study by Friedl et aL4 gave kl = (3.0 f 0.8) X lo-'* cm3 molecule-I s-'. Bulatov et a1.5 reported a somewhat lower value of k , = (2.4 f 0.2) X lo-" cm3 molecule-' s-I at 295 K using a less direct method of monitoring the product concentration, [HSO], in a flash photolysis intracavity laser absorption experiment. No reaction between HS and O2 has been observed in three previous studies. The work by Tiee et ale6and Black3 with flash photolysis L I F experiments assigned upper limits for k2 (in cm3 and 4 X respectively. The molecule-' s-') of 3.2 X discharge flow study of reaction 2 by Friedl et aL4 gave k2 5 1 x io-'' cm3 molecule-' s-I. (1) Sze, N. D.; KO, M. K. W. Almos. Environ. 1980, 14, 1223. (2) Leu, M. T.; Smith, R. J. J . Phys. Chem. 1982, 86, 73. (3) Black, G. J . Chem. Phys. 1984,80, 1103. (4) Friedl, R. R.; Brune, W. H.; Anderson, J. G. J. Phys. Chem. 1985,89,
Two previous studies of reaction 1 at 298 K are in very good
5505. ( 5 ) Bulatov, V. P.; Kozliner, M. Z.;Sarkisov, 0. M. Khim. Fir. 1984, 3,
'Author to whom correspondence should be addressed at NOAA R/E/ AL-2, 325 Broadway, Boulder, CO 80303.
1300. (6) Tiee, J. J.; Wampler, F. B.; Oldenborg, R. C.; Rice, W. W. Chem. Phys. Lett. 1981, 82, 80.
0022-3654/87/2091-5743$01.50/0
0 1987 American Chemical Society
5744
The Journal of Physical Chemistry, Vol. 91, No. 22, 1987' M ,.C IROWAVE DISCHARGE -REACTANT INLET CARRIER GASlREACTANT
TEMPERATURE REGULATED JACKET
MOVEABLE SOURCE REACTION REGION
FLUSH GAS\
JlkvV&'
Wang et al. inhibit back diffusion into the discharge. The pressure in the source reactor (- 2 Torr) is not measured directly but is calculated by using the Poiseuille equation. The main reactor utilizes a moveable inlet (9 mm 0.d.) to vary the reaction time. This inlet has a double-wall design so that source reactions can be carried out in the tip. The inside tube transports discharge products to the source reaction region (8 cm X 6 mm i.d.) at the end of the inlet. Other source reactants are admitted through the annulus between the inner and outer tubes. The reaction mixture from the flow tube passes into the detection zone of an optically pumped LMR spectrometer. The detection zone is defied by the intersection of a far-infrared laser, an adjustable magnetic field, and the gas stream from the flow tube. Paramagnetic radicals are detected by Zeeman tuning a rotational transition into resonance with the fixed frequency radiation of the far-infrared laser. The magnetic field is modulated and the absorption is monitored by using phase-sensitive detection. It has been demonstrated that the resultant absorption signal is proportional to the radical concentration.' The LMR spectroscopy of the HS molecule has been studied by Davies et ale9 Several components of the J = 3/2 5/2 transition in H3,S (X2113/2)were obtained by the 216-pm CH30D laser at magnetic field strengths between 5.8 and 10.5 kG. The M J = -1/2 +1/2 transition at a magnetic field strength of 10.455 kG was used to measure [HS] in our study. The HSO radicals were observed by the same laser line at a magnetic field strength of about 200 G with a detection limit of about 5 X lo9 molecule cm-3 as described in the following paper.I0 Four HS radical sources employing the atomic species F, H, and C1 were evaluated and two of them were used in our kinetic measurements. In the first one, F atoms were used in a very fast reaction
-
mN i
-\
L FAR INFRARED
PUMP
Figure 1. Schematic of discharge flow apparatus. MICROWAVE DISCHARGE
MOVEABLE TEFLON INLEl
15 cm-
n \'\ TEFLON INSERT
-
F
\ 'HALo~~~BoN#
0 RING SEAL
71
Figure 2. Schematic of the fixed radical source reactor.
The laser magnetic resonance spectrometry (LMR) used in the present study offers sensitive detection of the HS radical. In addition, the detection of other radicals, such as HSO, DS, OH, and HOz, by the LMR technique provides the ability to identify reaction products and study the reaction mechanism. In this paper, which is the first of a series on HzS oxidation, we report measurements of the rate coefficient and mechanism for reaction l. An upper limit for kz at room temperature is presented. In addition, the atmospheric relevance of reactions 1 and 2 are discussed.
Experimental Section The discharge flow laser magnetic resonance (LMR) system used in these studies has been described in detail.',* Therefore, only a brief discussion of the apparatus is presented here. The discharge flow portion of the apparatus is shown in Figure 1. The flow tube (115 cm long X 2.5 cm i.d.) is lined with a Teflon sleeve (2.2 cm i.d.) which reduces radical loss at the walls. The sleeve extends from the detection zone to above the carrier gas inlets. A radical source inlet, located between the main carrier gas inlet and the beginning of the reaction zone, is equipped with an O-ring sealed glass joint to allow the use of various fixed radical source reactors. The HS source reactor, as shown in Figure 2, uses a '/&-o.d. moveable Teflon inlet to adjust the reaction time within the source. The source reaction region (10 cm X 1 cm i d . ) is coated with halocarbon wax to reduce radical loss in the walls. The source reactor also incorporates two microwave discharge tubes for atom production. One tube is used exclusively for fluorine atom production, because fluorine etches the glass in the discharge region making it very lossy for other atoms, and the other is used for all other atoms. These tubes each contain a constriction (1 5 mm X 2 mm i d . ) between the discharge region and the reactor to
k3 = 1.5
+ H2S
-
HS
+ HF
(3)
lo-'' cm3 molecule-' s-',ll to generate HS. This reaction was used with both source reactors and at every temperature of the study. F atoms were produced by discharging a 0.2% CF, in He mixture and reacted with H2S in the source reaction zone. The H2S concentration was (0.3-1.0) X 1014 molecule cm-3 in the radical sources with a flow velocity of between 700 and 1000 cm s-I. The second method used to generate HS radicals was the reaction of H atoms, which were produced by discharging a 1% Hz in He mixture, with ethylene sulfide, C,H,S, X
H t
H2C-CH2
\/
---
HS t
C2H4
(4)
S
k4 = 1.2 X lo-', cm3 molecule-' s-I.l2 This source reaction was used only in the fixed sidearm reactor for kinetic studies at T 1 295 K. The concentration of ethylene sulfide in the source reactor was about (3-6) X lOI4 molecule ~ m - ~ . The reactant concentrations in the source reactors were high and the flow velocity was slow so that the reactions went to completion, >99.4%. The source gases were diluted by a factor of 10 or more upon entering the main flow tube. This minimizes interference from secondary reactions such as HS + HS, HS + CF,, O H H2S, etc., and from species generated by these secondary reactions in the source reactors. Two other H S source reactions were tested but not used in our kinetic measurements. One employed the reaction H + H,S HS + H, (5)
+
-
k5 = 7
X
cm3 molecule-'
s-j,l3 which
is much smaller than
(7) Howard, C. J.; Evenson, K. M. J . Chem. Phys. 1974, 61, 1943. (8) Stimpfle, R. M.; Perry, R.A,; Howard, C. J. J. Chem. Phys. 1979, 7 1 , 5 183. (9) Davies, P. B.; Handy, B. J.; Murray Lloyd, E. K.; Russell, D. K. Mol. Phys. 1978, 36, 1005. (IO) Lovejoy, E. R.; Wang, N. S.; Howard, C. J., the following paper in this issue. (11) Agrawalla, 0. S.; Setser, D. W. J . Phys. Chem. 1986, 90, 2450. (12) Lee, J. H.; Stief, L. J. J . Chem. Phys. 1977, 67, 1705.
Rate Constant for the Reaction HS
+ NO2
The Journal of Physical Chemistry, Vol. 91, No. 22, 1987 5745
i
2.0$\
5
W 3
1.5 -
W -
p-
1.0 -
e-
F
0.5 -
0' 0 1
2
3
4
5
k3. In order to titrate all the H atoms, a much higher [H2S]must be used than was required to titrate the F atoms. High [H2S] in the flow tube can cause complications from secondary chemistry as discussed later. Although the reaction of H atoms with ethylene sulfide is also relatively slow for a source reaction, the presence of a high concentration of ethylene sulfide in the flow tube did not cause problems from secondary chemistry. In fact this reagent was found to be an effective scavenger of OH radicals and did not give rise to the regeneration of HS as discussed later. The reaction of C1 atoms with HIS was also tested as a possible HS source:
HS
+ HCl
(6)
Although reaction 6 is fast enough to be a good source reaction, k6 = 6.3 X lo-" cm3 molecule-' the undissociated c12 molecules from the source discharge propagated a chain reaction15 C12 + HS
+
HSCl + C1
,
,
15
20
25
30
35
HS Signal (arbitrary units)
Figure 3. [HS] calibration plot at 295 K and 1 Torr. HS is generated by reacting HIS with excess F atoms. HS signal is measured as the peak-to-peak height of the LMR signal.
-
10
6
[H2S](10" molecule ~ m - ~ )
C1+ H2S
,
5
(7)
followed by reaction 6 which complicated the source chemistry. The moveable radical source reactor was used in this study to solve two problems at low temperatures. At temperatures below room temperature, the flow tube surfaces became increasingly lossy to HS radicals, k, 1 20 s-l. When the fixed radical source reactor was used a t these low temperatures, most of the HS radicals were lost on the moveable inlet and flow tube walls, making kinetic measurements difficult. An adequate supply of HS radicals was made by using the radical source reactor on the moveable inlet. This reduced the HS loss by decreasing the amount of exposed flow tube surface area. Also in this configuration, the radicals were not exposed to the outer surface of the moveable inlet. Another problem associated with the low-temperature measurements was the dimerization of NO2. A calculation using thermochemical datal6 showed that at 200 K over 40% of the NO2could be dimerized when it was diluted in 1 STP cm3 s-I H e (STP = 273 K, 1 atm) and added to the flow tube through the moveable inlet a t a total pressure of 1 Torr. On the other hand if the NOz were added to the system with the carrier gas (He) and diluted before it reached the cold reaction region, the ratio of [N204]to [NO2] could be reduced to about 8% at 200 K and less than 1% at 221 K. (13) Kurylo, M. J.; Peterson, N . C.; Braun, W. J . Chem. Phys. 1971, 54, 943. (14) Nava, D. F.; Brobst, W. D.; Stief, L. J. J. Phys. Chem. 1985, 89, 4703. (15) Nesbitt, D. M.; Leone, S. R. J. Chem. Phys. 1980, 72, 1722. (16) Stull, D. R., Prophet, H., Eds. JANAF Thermochemical Tables; National Bureau of Standards: Washington, DC, compiled and calculated by the Dow Chemical Co., Midland, MI, including revised tables issued through 1979.
Figure 4. Calibration of [HS] using OH + H2S. The 20 data points are from four [H2S] at five different reaction times.
An accurate measurement of [HS] was not required for the kinetic measurements, but it was desired to establish pseudofirst-order kinetics conditions and to evaluate the product channels and secondary chemistry. The L M R HS signal was calibrated by two methods. First, a small measured [H2S] was added to the flow tube through the moveable inlet with its tip a few centimeters above the detection region. The HIS was titrated by an excess of F atoms which were generated in the side arm by discharging a CF4/He mixture. It was assumed all of the H2S reacted with F atoms to give HS molecules. A plot of HS signal vs. [H2S] is shown in Figure 3. Very good linearity was observed in the [H2S] range from 3 X loio to 5.5 X 10" molecule ~ m - ~The . second calibration method used the reaction OH
+ H2S
-
HS
+ H20
(8)
k8 = 4.7 X cm3 molecule-' sW1.l7 Here an excess of H2S, [H2S] = (0.4-2.2) X 1013molecule ~ m - was ~ , added through the moveable inlet and reacted with a small known concentration of ~ . O H was produced OH, [OH], = 2.1 X 10" molecule ~ m - The by reacting F atoms with an excess of H 2 0 molecules, [ H 2 0 ] = 3 X 1013molecule ~ m - ~[H2S] . and reaction time were varied such that different fractions of O H reacted with H2S. The concentration of the unreacted O H was measured along with the signal due to HS. It was assumed that each O H reacted gave one H S radical. Figure 4 shows a plot of the concentrations of the unreacted OH vs HS signal. A 163-pm C H 3 0 H laser line was used to detect OH radicals at about 3.7 kG magnetic field and gave a 6 X lo7 molecule cm-3 detection limit. The [OH] was calibrated by the titration reaction
H
+ NO2
+
OH
+ NO
(9)
k9 = 1.3 X lo-', cm3 molecule-' s-l,I8 with measured [NO,] in excess H. Reaction 8 was the preferred HS calibration method because the chemistry in this system was simple with no significant secondary reactions to remove HS. The detection limit for HS was found to be about 1 X lo9 molecule cm-3 (S/N 2: 1 at 0.4 s time constant) from this calibration method. The F HS reaction was thought to interfere in reaction 3 which gave a detection limit which was about 40% higher. There are several factors that determine the detection limit for a molecule in our LMR apparatus. These include the transition moment of the absorbing species, the population of the absorbing level, the stability of the laser oscillator, and the line shape of the
+
(17) DeMore, W. B.; Margitan, J. J.; Molina, M. J.; Watson, R. T.; Golden, D. M.; Hampson, R. F.; Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R. NASA Panel for Data Evaluation; "Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling"; JPL Publication 85-37; Jet Propulsion Laboratory: Pasadena, CA, 1985. (18) Michael, J. V.; Nava, D. F.; Payne, W. A,; Lee, J. H.; Stief, L. J . J. Phys. Chem. 1979, 83, 2818 and references cited therein.
5146
Wang et al.
The Journal of Physical Chemistry, Vol. 91, No. 22, 1987
TABLE I: Summary of HS + NO2 Reaction Data HS source reaction' and reactorb
no. of T. K
exDts
22 1 262 295 347 383 415
11 9 36 30
1, 1, 1, 1, 1, 1,
15 20
+
4 4 2, 2, 2, 2,
3, 4 3 3 3
u range,
[No:; range,
cm s-'
10 cm-'
k: lo-" cm3 molecule-I s-I
2450 2990 2750-3800 1930-4070 2040-4600 2200-4690
0.42-4.08 0.42-5.05 0.48-7.64 0.56-6.34 1.02-8.59 0.68-8.59
8.06 f 1.21 7.01 i 1.05 6.69 f 1.00 5.66 i 0.85 5.24 f 0.73 4.87 f 0.73
+
'HS source reaction: 1 = F H,S; 2 = H C2H4S. b H S source reactor: 3 = fixed side arm reactor: 4 = movable reactor. 'Error limits represent the estimated accuracy, f 1 5 % , at the 95% confidence level.
observed transition. The absorption intensity for a transition from Mj!t to MJt is given byI9
Iabs a P ( v ) M ~ v ~ N (/ M NJ~~)
A
(10) B
where p ( v ) is the radiation intensity a t frequency v, p is the transition moment, and N ( M l t ) / N is the fraction of the molecules in the MJt,level. For the laser oscillators employed, X(OH) = 163 pm, 'i = 61.4 cm-I, h(HS) = 206 pm, 'i = 48.5 cm-I. Since the L M R experiment uses a bolometer (energy) detector, the L M R signal incorporates an additional frequency factor, Le., L M R signal = L M R S
0:
The absolute absorption intensity for intracavity absorption also depends on other factors including optical properties of the laser which are not known, so L M R S cannot be predicted accurately. However the relative absorption intensities of different species can be estimated. This estimate is most reliable if a single oscillator (laser frequency) is employed since many factors will cancel when the ratio is calculated. Because HS and O H are detected at different frequencies, we can only crudely estimate the detection limit as follows: The observed transitions are pure rotational J ' = 3/2 in O H and XZn3z, J" transitions XzIIl,2,J"= 1/2 = 3/2 J' = 5/2 in HS. Therefore p is the permanent dipole moment: p ( 0 H ) = 1.98 D and p(HS) = 0.76 DsZo The population factors N ( M J t t ) / Nare 0.035 for OH and 0.039 for HS. Thus we obtain LMRS(OH)/LMRS(HS) = 14.2, which is comparable to the experimental result, 17. The two factors which we have not included in this analysis, the noise level or stability of the laser oscillators and the tuning rate or line width of the observed transitions, can easily account for the small discrepancy between the observed and predicted detection limits. The He (>99.9996%analyzed) was passed through a molecular sieve filled trap cooled with liquid N,. CF4 (>99.9%), H2S (>99.999%), and H2 (>99.999%) were diluted in H e and used without purification. Ethylene sulfide (Aldrich, 99%) was degassed a t -77 OC. NOz was synthesized by reacting purified N O with an excess of O2 (>99.97%) at a pressure of about 900 Torr, was distilled under vacuum, and was metered from an ice-cooled trap. The gas flow meters were calibrated with a wet test meter or by the rate of pressure change in a calibrated volume. The pressure transducer was calibrated with a precision water micromanometer.
-
C
p(v)p2v3N(MJt.)/N (11)
-
Results A . The Rate Constantfor HS NO,. The pseudo-first-order conditions, [NO,] = (7.5-75) [HS],, were maintained during all the kinetic measurements with [HSIoI 1.5 X 10" molecule The rate equation for reaction 1 is expressed as [HSI) -V = k l [ N 0 2 ] = k' dz where u is the average flow velocity in the flow tube and z is the reaction distance measured from the tip of the moveable inlet to the detection zone. The flow velocity was between 1900 and 4700 cm s-l and z was varied by about 30 cm which corresponded to
+
(19) Hollas, J. M. Hiah Resolution Spectroscopy; .. Butterworths: London, 1982; pp 98-99. (20) Huber, K. P.;Herzberg, G. Consrants of Diatomic Molecules; Van Nostrand Reinhold: New York, 1979.
I
I
25
5
1
I
7 5 1(10-%]
10
125
Figure 5. Semilog plot of [HS] vs. reaction time. T = 295 K, P = 1.04 Torr, u = 3600 cm s-I. (A) blank, [NO,] = 0; (B) [NO,] = 0.75 X 10l2; (C) 2.78 X 10l2;(D) 5.44 X IOt2 molecule ~ m - ~ .
maximum reaction times of about 16 and 6 ms. Figure 5 shows a set of typical HS decay plots. The blank measurements, with [NO,] = 0, gave the loss of HS radicals on the inlet wall when the fixed radical source was used. The loss of radicals on the flow tube wall, k,, was measured by using the moveable radical source reactor. The HS radical loss was a first-order process with k , 5 15 s-l on the halocarbon wax coated inlet and k , I 20 s-I on the Teflon sleeve of flow tube at T 1 295 K. k , = 20 s-' corfor HS on Teflon. responds to a removal efficiency of y = 1 X At temperatures below room temperature, the H S radical loss on the surfaces increased significantly. The k, value for the Teflon flow tube was 1 3 0 s-I at 262 K and 165 s-l at 221 K. When the moveable radical source was used, the measured k, was subtracted from the HS decay rate constants obtained in the presence of NO2:
k' = kfdccay)- k, The k' was also corrected for axial diffusion
kIcar= k'(1
+ D k l / u z ) = kl[NOz]
where D is the diffusion coefficient in cm2s-'. The HS diffusion coefficient in He was estimated at 500( T/295)'.'P1 Torr cmz s-1.21 The largest correction was 6% a t 415 K and u = 2200 cm s-I. At 295 K two HS source reactions, F H2Sand H + CzH,S, and both the fixed and moveable radical source reactors were used. A halocarbon wax coating was also used on the flow tube interior for some measurements at this temperature. A k' vs. [NO,] plot for the 295 K data is shown in Figure 6 and gives a second-order rate constant k , = (6.69 A 0.26) X lo-" cm3 molecule-' s-] as the slope of the line. The uncertainty represents two standard deviations in the linear least-squares fit. The reaction rate constant
+
(21) Diffusion coefficient was estimated by using data from: Marrero, T. R.; Mason, E. A. J. Phys. Chem. Ref Data 1972, I , 3 .
Rate Constant for the Reaction HS
+ NOz
The Journal of Physical Chemistry, Vol. 91, No. 22, 1987 5147
5001
T(K)
500
333
r
250
200
I
400
$ . O O 200 I
100
1 I /
0
1
2
3
4
i
I
l
l
5
6
7
2.0
3.0
5.0
4.0
1000/T 6
Figure 8. Arrhenius plot for reaction 1.
[NO21 (10" molecule cmW3)
Figure 6. Plot of k' vs. (NO,] at 295 K with different source reactions, source reactors, and flow tube coatings: (0)H + C2H4S,fixed reactor, halocarbon wax wall; (0)H + C2H4S,fixed reactor, Teflon wall; (A) F + H2S, fixed reactor, Teflon wall; ( 0 ) F + H2S, moveable reactor, Teflon wall. 500
400
300
- I
I
I
-YLo
200
I 40
I 50
I 60
I 70
I 80
I 90
t ( 10-3s)
100
Figure 9. HS decay plots. Blank run in 6 Torr of N2 (0),k' = 14 s-l, and 0, (a), kK 5 17 s-'. T = 295 K, P = 6.4 Torr, [HS], = 6 X 1OI0 molecule cm-'. The line through the 0, data shows the slope that corresponds to the upper limit k2 5 1.5 X 1O-I' cm3 molecule-I s-l.
a [NO,] (10"
molecule
Figure 7. Plot of k' vs. [NO2] at T = 221 K (0)and 415 K (0). kl was measured at about 1 Torr pressure and six different temperatures between 221 and 415 K. At 415,383, and 347 K the HS was produced in the fixed reactor with both source reactions. At the two lowest temperatures, 221 and 262 K, only the moveable F H2S source was used. Figure 7 shows the k' vs. [NO,] plots for 221 and 415 K data. The experimental conditions and the k , measurements are summarized in Table I. The estimated accuracy of the rate constant measurements is derived from error estimates for the gas flow rate (f3%), temperature (fl%), pressure (fl%), NO, concentration (i4%), flow tube radius (fl%), and the slope of the decay plot (f4%). That gives a total error of about f9% at the 95% confidence level. This can be compared to the uncertainties in the slope of the k1 vs. [NO,] plots, which are between 2.5 and 6%. Combining the estimated error with a factor for possible systematic errors yields a value of about k15% at the 95% confidence level for the overall uncertainty in the value of k l at each temperature. The Arrhenius plot of our data for reaction 1 is shown in Figure 8. A least-squares fit gives k , = (2.85 f 0.35) X lo-'' exp((240 f 39)/T) cm3 molecule-' s-I, where the error limits are 2 times the standard deviations from the fit. B. DS + NO2 Reaction. The rate constant for reaction 15 was measured at room temperature to investigate the possibility of an isotope effect in reaction 1 DS + NO, products (15) The DS radical was produced in the side-arm reactor by reacting D atoms with C2H4S. It was detected by the CH30D laser at 216 g m and magnetic field strength of 7.7 kG9 with a detection . measurements limit of about 1.5 X IO9 molecule ~ m - ~Eleven were made at 1 Torr and 299 K and [NO,] ranged from 0.3 X
+
-
TABLE 11: Comparison of the HS
Measurements T, K 295
298 298 293
P,Torr 1.0-1.1 28.5-300 2-8 100
method" DF-LMR LP-LIF DF-LIF FP-IA
+ NO2 Reaction Rate Constant k , . lo-" cm3 molecule-' s-l 6.7 f 1.0 3.5 f 0.4 3.0 f 0.8 2.4 f 0.2
ref this workb
Black' Fried1 et aL4 Bulatov et aLs
" DF = discharge flow; LMR = laser magnetic resonance; LP = laser photolysis; LIF = laser-induced fluorescence; FP = flash photolysis; IA = intracavity absorption. * & ( T )= (2.9 f 0.5) X lo-" exp[(240 f 50)/7-l.
lo', to 3.5 X 10l2molecule ~ m - A ~ .rate constant of (7.3 f 1.1) cm3 molecule-' s-l for reaction 15 was determined from the k' vs. [NO,] plot. C. HS 4 Reaction. The HS was generated in the moveable inlet reactor via reaction 3. The H,S concentration was kept low ~ avoid , HS in the flow tube, [H$] I2 X lo1, molecule ~ m - to regeneration by reaction 8 in case OH was a product of the reaction of HS with 0,. Blank runs were carried out by replacing the O2 with H e or N,. We estimated a limit of 3 s-l for the smallest first-order HS decay measurable in these experiments. No decay of HS was observed at up to [O,] = 1.94 X lo1' molecule cm-3 as shown in Figure 9. That gave an upper limit of 1.5 X 1O-I' cm3 molecule-' s-' for reaction 2. X lo-"
+
Discussion The various measurements of k , from the present and previous work are summarized in Table 11. The temperature dependence of k l has not been reported before. There are large discrepancies between our room temperature value and those from other laboratories. Two similarities are apparent in the three previous
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studies: (1) they all reported lower kl values and (2) a high [H2S] was used to generate the HS radical. We propose that HS regeneration associated with the high [H,S] can account for the lower kl values. Black3 and Bulatov et aL5 photodissociated H2S to generate HS: H2S + hv --* HS + H (16) '
Equal concentrations of H atoms and HS radicals are produced by this process. In the presence of NO,, the H atoms also generate HS radicals:
Wang et al. TABLE 111: Summary of Measurements of the HS + O2 Reaction Rate Constant at Room Temperature k2,cm3 molecule-' SKI [O,], Torr T, K ref
