J. Phys. Chem. 1987, 91, 5749-5755 111. Even with the lowest upper limit for kl, 1 X cm3 molecule-' s-I, reaction 2 is stili potentially the most important HS oxidation process in the atmosphere. Further study on this reaction is needed to determine its atmospheric importance. The only published rate coefficient for the reaction between HS and O3was given by Friedl et ale4who reported k20 = (3.2 f 1.0) X lo-', cm3 molecule-' s-' at 298 K.
HS + 03
+
HSO
+ 03
5749
to determine the details of this scheme.
Note Added in Proof. A new study (Schonle, G.; Rahman, M. M.; Schindler, R. N. Ber. Bunsen-Ges. Phys. Chem. 1987,91, 6 6 ) of the HS + NO2 reaction was published recently. A flow tube reactor with mass spectrometric detection was employed to cm3 molecule-' s-' at 298 K. determine k, = (1.2 f 0.2) X HSO was sueeested to be the oroduct of the reaction. 1 1
(20)
This indicates that HS reacts with NO, about 20 times faster than with 03.They also observed HS regeneration from the reaction of O3with HSO radicals. Thus in the troposphere NO2 may compete with O3in polluted air to oxidize HS. Further studies of reactions 2 and 19 and the fate of the HSO radical are necessary
Acknowledgment. This work was supported by NOAA as part of the National Acid Precipitation Assessment Program. -We thank our colleague Dr. A. R. Ravishankara for helpful comments on this paper. Registry No. HS, 13940-21-1; NO2, 10102-44-0; 02,7782-44-7; deuterium, 7782-39-0.
Kinetic Studles of the Reactions of HSO with NO2, NO, and O2 Edward R. Lovejoy, Niann S. Wang, 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)
A far-infrared rotational transition in HSO has been observed by using an optically pumped laser magnetic resonance (LMR) spectrometer. LMR detection of HSO was utilized to study the reactions of HSO with NO,, NO, and 0,. Rate coefficients were measured under pseudo-first-order conditions in a discharge flow system coupled to the spectrometer. The results are k(HS0 NO,) = (9.6 f 2.4) X lo-'* cm3 molecule-' s-,; k(HS0 NO) I1 X lo-'' cm3 molecule-' 8;and k(HS0 0,) I 2 X lo-'' cm3 molecule-' s-'. All measurements were made at low pressures (1-5 Torr) and room temperature (296 f 3 K). Experiments were also conducted to determine the products of the HSO + NO2 reaction. The results show that a species is produced which reacts with O2to form H 0 2 with a rate coefficient of (3 2 ) X cm3 molecule-' s-' which was determined from the rate of HOz formation. We-propose that the unidentified intermediate is HSO,.
+
+
+
*
Introduction The oxidation of reduced sulfur species, namely H2S, CS,, and COS, in the atmosphere is believed to be initiated by reactions with the hydroxyl radical.'-3 Leu and Smith have studied two of these species, O H + H2S4and OH + OCS5 In both cases HS was detected as a primary product. The products of the reaction of hydroxyl with CS, have not been reported, but HS production is exothermic. The fate of the HS radical is less clear. Reactions 1-3 are possible loss channels for HS in the atmosphere. HS + 0 2 + OH SO (1)
+
HS+03+HSO+02 (2) HS + NO2 HSO N O (3) Several workers- report that reaction 1 is slow and have assigned upper limits for the rate coefficient of (1-4) X cm3molecule-' s-I. The rate coefficient for reaction 2 was recently measured by Friedl et aL6 and they obtained k2 = 3.2 X lo-', cm3 molecule-' SI. There have been a number of investigation^^-^ of reaction 3 which have yielded rate coefficients ranging from 2 X lo-'' to 7 X lo-" cm3 molecule-' s-'. Assuming an ambient ozone concentration of lo1,molecules cm-3 at 1 atm, the loss of HS via (1) cm3 molecule-' s-I. N O 2 conwill dominate if kl 2 1 X centrations are significantly lower than O3 in clean air but can reach comparable levels in polluted air ( ~ 4 ppbv).'O 0 This suggests that a significant fraction (possibly >0.10) of the HS reacts with ozone and nitrogen dioxide. Recent studies have shown that HSO is a product of reactions 2"J2 and 3.4,59'3 Other known HSO production schemes include the reaction +
+
0 + RSH
-.
HSO
+R
(4)
where R = H, CH3,and C2H5. The HSO production has been and LIFI5 verified in these systems by using mass ~pectrometric'~ detection of reaction products. There have been several spectroscopic studies of the HSO radical following the assignment of its chemiluminescence spectrum produced in reaction 2.12 High-resolution laser-induced fluorescence (LIF) studies of the (003)-(000) band of the A-X transition for HS0I6 and DSO17 have yielded the structures in (1) Gradel, T. E. Reu. Geophys. Spoce Phys. 1970, 15, 421. (2) McElroy, M. B.; Wofsy, S. C.; Sze, N . D. Atmos. Enuiron. 1980, 14, 159. (3) Sze, N . D.; KO, M. K. W. Atmos. Enuiron. 1980, 14, 1223. (4) Leu, M. T.; Smith, R. H. J . Phys. Chem. 1982, 86, 73. ( 5 ) Leu, M. T.; Smith, R. H. J . Phys. Chem. 1981,85, 2570. (6) Friedl, R. R.; Brune, W. H.; Anderson, J. G. J . Phys. Chem. 1985,89, 5505. (7) Black, G. J . Chem. Phys. 1984, 80, 1103. (8) Wang, N. S.; Lovejoy, E. R.; Howard, C. J., preceding paper in this issue. (9) Bulatov, V. P.; Kozliner, M. 2.; Sarkisov, 0. M. Khim. Fiz. 1984, 3, 1300. (10) Logan, J. A. J . Geophys. Res. 1983, 88, 10785. (1 1) Kendall, D. J. W.; O'Brien, J. J. A,; Sloan, J. J.; MacDonald, R. G. Chem. Phys. Lett. 1984, 110, 183. (12) Schurath, U.; Weber, M.; Becker, K. H. J . Chem. Phys. 1977, 67, 110. (13) Bulatov, V. P.; Kozliner, M. 2.; Sarkisov, 0. M. Khim. Fiz. 1984,3, 988.
(14) Slagle, I. R.; Graham, R. G.; Gutman, D. Int. J . Chem. Kinet. 1976, 8. 451.
*Author to whom correspondence should be addressed at NOAA, R/E/ AL-2, 325 Broadway, Boulder, CO 80303.
0022-3654/87/2091-5749$01.50/0
(15) Kawasaki, M.; Kasatani, K.; Tanahashi, S.; Sato, H.; Fujimura, Y. J . Chem. Phys. 1983, 78, 7146.
0 1987 American Chemical Society
5750 The Journal of Physical Chemistry, Vol. 91, No. 22, 1987
both states. Endo et a1.I8 have recorded a number of microwave transitions and determined accurate molecular parameters as well as the magnetic hyperfine interaction between the hydrogen nucleus and the unpaired electron. The electric dipole moments in both the ground and first excited electronic states were measured by Webster et al.I9 in an LIF Stark experiment. Sears and McKellar20 observed vibration-rotation transitions of HSO in the u3 fundamental band using an infrared laser magnetic resonance experiment. Other workers report the fluorescent lifetimes of single vibrational levels in the A state2] and the dynamics and energy partitioning of HSO* produced in reaction 2." The thermochemistry of HSO is uncertain. Present estimates are based on ab initio calculations and on observations of the radical as a reaction product. Schurath et al.I2 observed HSO chemiluminescence from reaction 2 corresponding to an excess HSO energy of about 55 kcal mol-' and concluded AH?298(HSO) 5 14.9 kcal mol-'. Slagle et aLz2suggested that if oxygen atoms add to H2Sto form HSO, as in reaction 4, this would reduce the upper limit for AHfDzss(HSO)to 2.7 kcal mol-'. Kakimoto et a1.I6 more recently observed L I F signals from 0 H2S which matched the HSO chemiluminescence transitions assigned by Schurath et a1.I2 However, they were careful to point out that their results do not establish HSO as a primary product from 0 H2S. Davidson et al.23have subsequently shown that HSO is indeed a primary product from the reaction 0 H2Sin a crossed molecular beam mass spectrometer experiment. They estimate = -1.4 f 1.9 kcal mol-'. Ab initio calculations AHfD298(HSO) by Luke and M ~ L e a agree n ~ ~ with this estimate and they report = -0.4 f 3.0 kcal mol-]. Based on these data we AHfD298(HSO) adopt the value AHfD(HSO)= -1 kcal mol-' for all subsequent discussions of HSO thermochemistry. The chemistry of HSO is not well established, although a few kinetic studies for the radical have been reported. Fried1 et aL6 indirectly derived a rate constant for the reaction with ozone based on the observation of HS regeneration in the HS O3 system. They argued that the reaction proceeds as HSO O3 HS + 202, and obtained k = (1.0 f 0.4) X cm3 molecule-] s-'. Bulatov et al. have measured the rate constants for HSO + NO2, k = 4.1 X cm3 molecule-' s - ' , ~ and HSO NO, k = 2.6 X cm3 molecule-' s - ' , ~by ~ observing the HSO decay using intracavity laser absorption. In their experiments the HSO radical was generated in a reaction sequence initiated by photolyzing H2S in the presence of NO2. The present study is part of a systematic examination of the mechanism for the atmospheric oxidation of reduced sulfur compounds, specifically HIS. A kinetic study of HS, which is the first radical produced in the H2S oxidation sequence, is presented in the preceding paper.8 In that study the rate coefficient for HS NO2was measured and the HSO production was verified. The HS + NO2 rate coefficient is significantly different than a number of previously reported values. Secondary chemistry was suggested as the cause for the discrepancies. The present study investigates the chemistry of HSO which is also important in the H2Soxidation sequence. H S O is detected with far-infrared laser magnetic resonance (LMR) spectroscopy. A direct kinetic measurement and a product analysis for the reaction HSO NO2 are performed. Upper limits for the rate coefficients of the HSO O2 and H S O NO reactions are also determined.
+
+
+
+ +
-
+
+
+
+
+
(16) Kakimoto, M.; Saito, S.; Hirota, E. J . Mol. Spectrosc. 1980, 80, 334. (17) Ohashi, N.; Kakimoto, M.; Saito, S.; Hirota, E. J . Mol. Spectrosc. 1980, 84, 204. (18) Endo, Y . ;Saito, S.; Hirota, E. J . Chem. Phys. 1981, 75, 4379. (19) Webster, C. R.; Brucat, P. J.; Zare, R. N. J . Mol. Spectrosc. 1982, 92, 184. (20) Sears, T. J.; McKellar, A. R. Mol. Phys. 1983, 49, 25. (21) Kawasaki, M.; Kasatani, K.; Sato, H. Chem. Phys. Lett. 1980, 75, 128. (22) Slagle, I. R.; Baiocchi, F.; Gutman, D. J. Phys. Chem. 1978, 82, 1333. (23) Davidson, F. E.; Clemo, A. R.; Duncan, G. L.; Browett, R. J.; Hobson, J. H.; Grice, R. Mol. Phys. 1982, 46, 33. (24) Luke, B. T.; McLean, A. D. J . Phys. Chem. 1985, 89, 4592. (25) Bulatov, V. P.; Kozliner, M. Z.; Sarkisov, 0.M. Khim. Fiz. 1985,4, 1353.
Lovejoy et al. TABLE I: Radical Detection Information
radical HS HSO
OH HOz
far-IR laser gas/wavelength, Nm CH,OD/216 CH,OD/216 CH,OH/163 CH,OH/163
magnetic field strength,
kG
re1 polarizna
10.4 0.2 3.1 2.2
U
detection limit,* molecule cm-3 1x 6X 6 X 4x
U U U
109 lo9 lo7 108
'Polarization corresponds to the orientation of the electric vector of the laser radiation relative to the magnetic field: u = perpendicular. Detection limit refers to the approximate radical concentration at S/N = 1 with a 0.4-s time constant.
Experimental Section The laser magnetic resonance (LMR) discharge flow system used in these studies is described in the preceding paper8 and will not be discussed here. The L M R spectrometer is capable of detecting a number of radicals important in this study. Table I lists the radicals and the pertinent LMR detection information. Two HSO source reactions are used in these studies. The first cm3 molecule-' s - ' ) , ~has reaction, 0 + CH3SH ( k = 2 X been investigated by Slagle et al.I4 in a crossed jet reactor coupled to a photoionization mass spectrometer. They identified three product channels. 0 + CH3SH H S O CH, (5a)
----*
+
CH3SO
+H
CH3SOH
(5b) (5c)
This identification is based on the observation of ion peaks assigned to CH3, C H 3 S 0 , and CH3SOH. HSO was not observed. Branching ratios were not determined, although the authors predicted that decomposition of the hot CH3SOH adduct via channel Sa predominates. HSO was positively identified as a product of reaction 5 in a number of spectroscopic s t ~ d i e s . ' ~ - ~ ~ In our studies oxygen atoms are produced by using the reaction N NO N2 0 ( k = 3.4 X lo-'' cm3 molecule-' s - I ) . ~ ~ Slightly larger [HSO] are obtained with this oxygen atom source as compared to a discharge in 02.Nitrogen atoms are formed by discharging a mixture of 1% N2 and He. A 3% N O in He mixture is added slightly downstream from the discharge so that 299% of the nitrogen atoms are consumed in the first 2 cm of the source reaction zone ([NO] i= 7 X 10I3 molecule ~ m - ~ ) . Methyl mercaptan is injected further downstream through the moveable Teflon inlet ([CH3SH] i= 5 X IOl4 molecule cm-3 in the source). Small He flows, totalling less than 3 STP cm3 s-I (STP = 273 K and 1 atm), are added to each stream resulting in typical residence times of 10-20 ms in the source reactor. The source reaction mixture is diluted by an additional 5-10 times upon entering the flow tube. Under these conditions, the HSO signal is insensitive to larger N O concentrations, although increasing nitrogen atom concentrations in excess NO sharply decreases the HSO signal. This suggests that an important loss for HSO in the source may be reaction with oxygen atoms. Typically, the maximum [HSO] obtainable with any source was about 5 X 10" molecule cm-3 in the flow tube which may be the limit due
+
-
+
t o a rapid self-reaction for HSO.
+
The second source reaction utilized in these studies is HS NO2 ( k = 6.7 X IO-'' cm3 molecule-' Leu and Smith have observed HSO using mass spectrometric detection in two systems containing HS and NOz. These systems, OH H2S4and O H + COS5,each contained NO2 from the OH source and HS as a reaction product. In both studies the HSO appearance was characteristic of a primary product from HS NO2. Bulatov et ale9have detected H S O using intracavity laser absorption following photolysis of H2S in the presence of NOz.
+
+
(26) 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. "Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling"; Jet Propulsion Laboratory, 1985; Evaluation No. 7 .
Reactions of HSO with NO2, NO, and
The Journal of Physical Chemistry, Vol. 91, No. 22, 1987 5751
0 2
+
In our studies, both F H2S ( k = 1.5 X cm3 molecule-' and H C2H4S,ethylene sulfide ( k = 1.2 X cm3 ~ used as HS sources. Fluorine atoms are molecule-' s - ' ) ~ are produced in a microwave discharge of a 0.2% CF4 in H e mixture and hydrogen atoms are generated by discharging a 1% H2in H e mixture. The C2H4Sflows directly from a reservoir containing the liquid and HIS is used as a 3% in He mixture. Typically, the hydrogen sulfide and ethylene sulfide concentrations are about 5 X 1013and 1 X 1014 molecule ~ m - respectively, ~, in the source reactor. For both sources the HS production reaction is 299% complete prior to the addition of a 1% NO2 in He mixture through the Teflon inlet. Optimization of this source requires adding in the source) to enough N 0 2 ( [ N 0 2 ]i= 1 X 1013molecule minimize the unreacted HS without consuming excessive amounts of the HSO produced. The source conditions are adjusted while monitoring both the H S and H S O species. The H S O loss due to reaction with NOz from the source does not interfere with the kinetic measurements because it is a constant first-order loss which is independent of the reaction distance. This loss is similar to a wall loss and is typically 120 s-l. Preliminary HSO + NO2 kinetic studies using the F/H2S/N02 source indicated significant HSO regeneration at high [NO,]. A significant production of hydroxyl radicals was also observed. This suggests the following H S O regeneration scheme: O H + H2S HS + H2O (6)
+
s-I)~'
HS
+ NO2
-
+
HSO
+ NO
(3)
It is not known what process leads to the O H production, but possible OH generation schemes are discussed later. To remove the OH, C2F3C1is added to the carrier gas ([C2F3Cl]= 2 X 1015 molecule ~ m - ~ The ) . reaction OH C2F3C1is reasonably fast ( k = 6 X lo-'* cm3 molecule-l s-I) and has been proven as an effective OH scavenger in other experiment^.^^ The reaction of HSO with the scavenger is negligible, k 1 2 X an3molecule-' s-', as measured in this work. An added scavenger is not required for the H / C 2 H 4 S / N 0 2source since the reaction O H + C2H4S is fast ( k i= 6 X lo-" cm3 molecule-' s - ' ) ~ and does not lead to HS regeneration. For the kinetic study of HSO NO2 all three HSO production methods are utilized. In each case the HSO is made in the side-arm reactor and the NO2reactant is admitted with a small H e flow (=0.5 STP cm3 s-I) through the moveable inlet. In the HSO O2 and the HSO N O studies only one source, 0 CH,SH, is used. The source reactions are carried out in the tip of the moveable reactor inlet. Methyl mercaptan and NO (3% in He) are mixed and flow through the outside passage of the inlet while nitrogen atoms are passed down the center tube. H S O decays are measured by substituting reactant, N O or 02,for N 2 as a carrier gas. The total flow rate ie held constant so that other HSO loss processes, such as first-order wall loss, are unchanged. Thus changes in the H S O decay rate are due to reactions with NO or 0,. The moveable radical source is utilized so that unusually large reactant flow rates, up to 90% of the total flow, can be used without disturbing the flow characteristics in the reaction zone of the flow tube. Relative calibrations for the radical species listed in Table I are performed as follows. The OH response relative to HS is estimated by adding H2S to OH and monitoring the HS appearance and the OH disappearance. The OH is made in the fixed radical source by F H 2 0 ( k = 1.1 X lo-" cm3 molecule-' s-')*~ and H2S is admitted througb the moveable inlet. Assuming quantitative conversion of OH to HS, the response ratio for 0H:HS is about 17:l. In other words, the OH signal is about 17 times stronger than that of HS for an equal radical concentration. Sample data and details of the calibration procedure are presented in the previous paper.* A relative calibration for HO, vs. O H is made using the reaction H 0 2 + NO O H + NO, ( k
+
+
+
+
+
+
-
(27) Agrawalla, B. S.; Setser, D. W. J . Phys. Chem. 1986,90,2450. (28) Lee, J. H.; Stief, L. J.; Timmons, R. B. J . Chem. Phys. 1977,67, 1705. (29) Zahniser, M. S.; Howard, C. J. J . Chem. Phys. 1980, 73, 1620.
= 8.3 X cm3 molecule-' s-').~~ Nitric oxide is added through the moveable inlet and H 0 2 is made in the fixed radical source by C1+ C H 3 0 H C H 2 0 H HC1 and C H 2 0 H O2 HO, CHzO. The HO, and O H signal changes are monitored upon addition of increasing amounts of NO. This method yields a response ratio for OH:H02 of about 7:l. Reaction 2 is used to estimate the HSO signal strength relative to HS. This calibration is complicated by the fast reaction of HSO with NO,. To suppress the secondary chemistry, the NO2 is added at a short reaction distance ( t i= 6 X s) and at low concentrations ([NO,] i 3 X 10l2molecule ~ m - ~ A ) , plot of the ratio of the changes of the HSO and HS signals extrapolated to [NO,] = 0 yields a response ratio of about 6:l for HS:HSO. Each of these calibrations is based on the assumption of quantitative conversion of the radical species by the reactions used. Howard30 has shown that the O H yield from HOZ NO is 299% which justifies the use of this reaction for an OH:H02 calibration. The HS yield from O H HzS has not been measured but it is unlikely that any other major product channels exist. The accuracy of the HSO calibration is questionable because of the uncertainty in the yield of HSO from HS + NO2. However, if other product channels do exist then the measured HSO response represents a lower limit and HS:HSO 16:l. The helium (299.999%) is passed through a liquid nitrogen cooled trap containing molecular sieves. The nitrogen (299.999%), oxygen (299.97%), C2F3C1(299.0%), and CH3SH (299.5%) are used without purification. The NO/He mixture is prepared from N O (299.99%) which is purified by passing it through a trap filled with silica gel cooled in a dry ice and ethanol bath. The NO2 is prepared by reacting purified N O with excess 0, at about 900 Torr and purified by trap to trap distillation in excess 0,. The hydrogen (299.999%), CF4 (299.9%), and nitrogen are used without purification to make mixtures in helium. All gas flows except that of NO2are measured by using linear mass flow meters. These meters are frequently calibrated with a wet test meter or by the pressure rate of change in a calibrated volume. The NO2 flow rate is determined by the pressure rate of change method. A correction is made to account for the dimerization equilibrium and is typically less than 2% of the calculated flow. The flow tube pressure is measured with a capacitance manometer through a port at the center of the reaction zone. This pressure meter is calibrated with a precision water micromanometer. The precision of the factors contributing to the rate coefficient are approximated as follows: flow f3%, temperature fl%, pressure f 1%, flow tube radius f 1%, NO2flow f4%, and decay plot slope 45%. These values yield a precision in the second-order rate constant of about 10%. Other contributions, including estimated systematic errors, add 10-15% to the ~ n c e r t a i n t y , re~' sulting in a total uncertainty in these measurements of about 25% at the 95% confidence level.
-
+
+
+
-
+
+
Results HSO Detection. Rotational energies of the HSO radical were calculated by Sears32using the molecular parameters determined by Endo et al.'* in microwave experiments. A thorough search for HSO LMR absorptions was conducted using 12 far-infrared laser wavelengths between 190 and 930 fim. The difference between the laser frequency and the predicted rotational transition was typically less than 2%. The magnetic field was scanned from 0 to 15 kG for each wavelength and only one reasonably strong absorption was found. This absorption spectrum is shown in Figure 1 and has been tentatively assigned to a b-type transition by Sears.32 The strong feature at low field is probably an unresolved cluster of transitions. Without a complete set of absorptions, a definitive spectroscopic identification is impossible. Chemical tests were performed to reinforce Sears' assignment. Four known (30) Howard, C. J. J . Chem. Phys. 1979,71,2352. (31) Howard, C. J. J. Phys. Chem. 1979,83,3. (32) Sears, T. J., Brookhaven National Laboratory, personal communication, 1985.
5752 The Journal of Physical Chemistry, Vol. 91, No. 22, 1987
I
0
i
500
10
1000
Figure 1. HSO LMR spectrum. CH30D 216 pm laser polarized perpendicular ( a ) to the magnetic field. [HSO] = 2 X 10" molecule ~ m - ~ .
+
I
I
I
I
15
20
25
30
Reaction Time (10-3s) Figure 2. HSO NO, decay plots. HSO source, O/CH,SH; T = 295 K, P = 1 Torr, u = 1760 cm d. [NO,] ( l o i 2 molecule cm-3) = 0 (0); 2.4 (W); 4.9 (A); 8.9 ( 0 ) .
+
MAGNETIC FIELD (GAUSS)
+
Lovejoy et al.
+
+
sources of HSO (0 CH3SH, HS NO2, HS 03,and 0 H2S) all produced the spectrum shown in Figure 1. Further evidence was obtained by labeling the radical with lSOand deuterium. HSI80was made by reacting either CH3SHor HIS with the products from an discharge. The deuteriated species was generated by replacing H2Swith D2S in the F/H2S/N02 source. For both species, HS180 and DSO, the absorption assigned to HSO was not evident, indicating that the absorbing species contained 0 and H. OH and HO, were made independently and neither species exhibited absorptions which coincided with the assigned HSO absorption. These chemical tests are unable to distinguish between HSO and its isomer, SOH. The SOH radical is predicted to be more table,^^,^^ although no spectroscopic observation of this radical has been reported. The spectrum in Figure 1 occurs a t low magnetic field strength (=200 G) which indicates that there is a near-coincidence between the laser and the rotational transition frequencies. The HSO rotational energies calculated by Sears are based on microwave data and should be accurate. Therefore, based on the combined chemical and spectroscopic evidence, the spectrum shown in Figure 1 has been confidently assigned to the HSO radical. Kinetic Results. The kinetic measurements were made under pseudo-first-order conditions, with the concentration of the reactant, R = NO2, NO, or 02,much greater than the H S O radical concentration. Under these conditions with a fixed radical source the usual analysis applies
g
100
Y
0
I
:4
Yo"
,
5
0
.
,
15
10
I
20
[NOz] (10'' molecule cm-3)
+
Figure 3. k' vs. [NO,] for HSO NOz. T = 295 K, P = 1 Torr, slope = 9.6 X cm3 molecule-' s-I. HSO source: (0) O/CH3SH; (A)
H/C,H,S/NO2;
(0) F/H2S/N02/C2F3CI.
that either HSO does not react appreciably with O2or HSO reacts and is rapidly regenerated via secondary chemistry. Possible reaction channels include HSO 0 2 HO2 SO (7a)
+
-
+
+ O H + SO2
(7b)
M
where k' is the first-order rate constant and k is the second-order rate constant. The reaction time t is the reaction distance divided by the average flow tube velocity, and [HSO] is proportional to the LMR signal. Therefore, a plot of In (HSO signal) vs. reaction time yields a straight line with slope of -kl. The second-order rate constant is the slope when k' is plotted vs. [ R ] . A total of 37 individual k' measurements for HSO NOz were made at room temperature and 1 Torr pressure in He. Each k' was corrected for the loss of radicals on the moveable inlet (55 s-I) and for axial diffusion (52%). A set of typical decay plots is presented in Figure 2. The first-order rate coefficients measured with three different HSO sources are plotted vs. [NO,] in Figure 3. A linear least-squares fit to k' vs. [NO,] yields k = 9.6 X cm3 molecule-' s-' with one standard deviation of 0.4 X The intercept, 2.3 f a = 3.3 s-l, is not statistically significant. The HSO 0,reaction was studied by using a moveable source of HSO (0+ CH3SH) and 0, as the carrier gas. No loss of HSO was observed in up to 1 X lOI7 molecule cm-3 of 02.This implies
+
+
-adduct (7c) Neither H 0 2 nor OH were observed in this system, but some O H would be consumed by reaction with CH3SH from the source ( k = 3.4 X lo-'' cm3 molecule-' s-').~~ The most facile HSO regeneration scheme requires 0 atom production followed by 0 + CH3SH. However, it is difficult to conceive of a scheme to regenerate 0 atoms in this system. Therefore, we report k7 5 2X cm3 molecule-' s-l at 4.5 Torr and 296 K. The reaction H S O NO was examined by using the O/ CH,SH moveable source of HSO with NO added to the He carrier gas flow. A small loss of HSO in excess NO ([NO] = 2 X 10l6 molecule ~ m - was ~ ) observed at 1 Torr total pressure and room temperature. Possible reaction channels include HSO + N O H N O SO @a)
+
-+
+
M
-adduct (8b) Channel 8a is slightly endothermic ( A H , i= 5 kcal mol-'). A pressure dependence for k8,which would be evidence for channel 8b, was not analyzed due to the difficulty in obtaining an HSO signal at higher pressure in this system. We conclude that the
(33) Sannigrahi, A. B.; Peyerimhoff, S.D.; Buenker, R. J. Chem. Phys. 1977, 20, 381.
(34) Atkinson, R.; Perry, R. A,; Pitts, J. N. J . Chem. Phys. 1977, 66, 1578.
Reactions of HSO with NO2, NO, and 0, 1.0
,
I
I
I
The Journal of Physical Chemistry, Vol. 91, No. 22, 1987 5753
1
I
* H
2
0 =
a a
L
1 I tp
5a
II-
.8
I I
2
2
w
0
w
0
z
0
0
z
.4
I
ll
0
0 4
a 0 0 a
0n a
K 0
2
8
4
8
10
-
[NOz] ( 1 0 1 2 molecule ~ m - ~ ) Figure 4. Radical profiles for HS + NO2 + O2 products. T = 298 K, P = 1 Torr, t = 0.023 s, [O,] = 3.4 X 1015molecule HS source, H/C2H4S; [ C , H S ] = 1 X IOl3 molecule cm-3, [HSIo= 1 X 10" molecule ~ m - ~ (0) . HS, (A)HSO, (0)H02. Lines are hand drawn through the data.
reaction is too slow to measure using our apparatus and we report cm3 molecule-' s-l at 297 K. k8 I1 X H S O NO2 Product Analysis. Possible reaction pathways for HSO NO2 include
+
+
-+ +
HSO
-
NO, H Soz+ N O
+ NO H O N O + SO HSO,
M
AH298
AH298
= -4 kcal mol-'
(9a)
= -38 kcal mol-'
(9b)
AHzg8= -23 kcal mol-'
(94
The reaction 9b enthalpy is calculated by using AHf,298(HS02) = -53 kcal mol-', which is derived from an ab initio calculation of the H-S02 bond strength.35 Adduct formation can probably be ruled out as a major channel because of the rapid rate measured for reaction 9. To test for H atom production by channel 9a, ~ ,3 Torr total sufficient 0, (IO,] = 6 X 10l6 molecule ~ m - at pressure) was added to make H 0 2 sH02 ( k , = 5.5 X cm6 molecule-2 s-' a t 300 competitive with H NOz OH N O ( k = 1.4 X cm3 molecule-' s - ' ) . ~ ~ A significant production of H02 was observed. At lower O2 concentrations ([O,] = 3 X lOI5 molecule ~ m - P~ =, 1 Torr), where H O2was negligible compared to H + NO2, H 0 2 production was still significant. Figure 4 shows the HS, HSO, and HOz radical profiles as a function of [NO,] for fixed [O,] and reaction time. Similar radical profiles were generated with both the H/C2H4S and the F/H2S sources of HS. For these experiments HS was made in the fKed radical source and NO, was added through the moveable inlet fixed at a specific reaction distance. The 02 was added upstream of the fmed radical source. The data presented in Figure 4 indicate that the HSO + NO, reaction produces an undetected species which reacts with O2to form HO,. The HOz production occurs at O2concentrations (IO,] < 1 X 1OI6 molecule ~ m - P~ , = 1 Torr) for which H 0, is negligible. This may be evidence for reaction 9b followed by
-
+
+
+
+
HS02
+0 2
--*
HOZ
+ SO2
(10)
Reaction 10 is exothermic by about 15 kcal mol-'. Insignificant quantities of OH ([OH] < 0.01 [HS],) were observed in this system, but they do not reflect the actual OH production because of OH loss by reaction with C2H4S( k = 6 X lo-'' cm3 molecule-' (35) Boyd, R.J.; Gupta, A.; Langler, R. F.; Lownie, S. P.; Pincock, J. A. Can.J . Chem. 1980, 58, 331. (36) Michael, J. V.;Nava, D.J.; Payne, W. A,; Lee, J . H.; Stief, L. J. J . Phys. Chem. 1979,83, 2818.
2
0
6
4
[NO,]
8
I O
(10'2 molecule cm-3)
Figure 5. OH production in the HS + NO, system. T = 298 K, P = 1 Torr, t = 0.028 s; HS source, F + H,S; [HZS] = 2 X 10" molecule ~ m - [HS], ~ , = 3 X loLomolecule cmF3. (0)HS, (0)OH, and (0) HOz with [O,] = 3.1 X l0l5molecule ~ m - (e) ~ ; OH with [02]= 0. Lines
are hand drawn through the data. s-I).~ To investigate the OH production in the H S / N 0 2 system, the F/H2S source of HS was used. Figure 5 shows the O H production using a low HIS concentration ([H2S] = 2 X 10l2 molecule cm-3 in the flow tube) in the HS/N02 system. At lower ~ ) OH was concentrations ( [H2S] = 5 X 10" molecule ~ m - more observed which was characterized by a rapid rise at low NO, This OH was concentrations ([NO,] 5 2 X lo', molecule due to H atom production in the source, possibly by
(9c)
adduct
+
U
F + HS -HF
(1 1)
S + HS
(12)
---L.
+S H + Sz
The H from the source was rapidly converted to OH in the flow tube by reaction with NO,. To confirm the source H atom = 5 X 10l6 molecule cmb3,[NO,] production, 0, was added ([O,] = 0) to convert H to H02. The resultant HOz levels were consistent with the amount of OH formed at low [NO,] without 0, present. The source contribution to the OH profile in Figure 5 was minimized by operating a t low microwave discharge power ( I 1 0 W) and at moderate H2S concentrations. HO, appeared = 3 X lOI5 molecule and the OH decreased upon addition of [02] cmF3to the system. These profiles, as a function of [NO,], are also shown in Figure 5. The following reaction scheme is consistent with our observations. HSO NO2 H SO, NO A H 2 9 8 = -4 kcal mol-' (sa)
-+ + +
HSO,
HSO2
+ NO,
H + NO2
-
+
+ NO
-
OH
HS02 + O2 H
+ O2 + M
-+
-
OH
AH298
+ SO2 + NO
(9b)
= 5 kcal mol-I (13) AHzg8= -29 kcal mol-' (14)
+ NO H 0 2 + SO,
H02
= -38 kcal mol-'
+M
AH298
AHz9*= -15 kcal mol-I (10) AH298 = -49 kea1 mol-' (1 5)
Based on the available thermochemical data O H production in reaction 13 is endothermic. However, the heat of formation of HSO, is not well established and reaction 13 cannot be ruled out. Reactions 9a and 14 are included as another possible source of the observed OH. Quantitative evaluation of the HO, and O H yields in the H S / N 0 , / 0 2 system is difficult. W e observe that about 50% of the initial H S is converted to HOz. The H 0 2 calibration is based on only the HS/OH and OH/H02relative calibrations which are accurate to =t15%. At high [NO,] we can only account for about 70% of the initial HS in the form of HSO and HOz. This is
5754 The Journal of Physical Chemistry, Vol. 91, No. 22, 1987
Lovejoy et al. TABLE II: Summary of Data for the HS02 + O2 Reaction Rate Coefficient Measurement by Nonlinear Least-Squares Fits to the H 0 2 Appearance Data HS source
kwHs~,,"s-'kWH~,,'s-' 40 0 40 0 40 0 40 0 40 0 40 0
F + H2S F + H2S F + HIS F + H2S H H
+ C2H4S + C2H4S
s 0.040 0.067 0.013 0.025 0.038 0.020 ',f
k,,,,b lo-') cm3 molecule-' s-' 2.2 2.3 3.7 3.2 3.0 3.5
(3 f 2)'
Fixed parameter. bVariable parameter. Recommended value. 10 0
0
[O,] ( 1 0 ' 4 molecule ~ m - ~ )
Figure 6. H 0 2 production in the HS/N02/02 system. T = 298 K, P = 1 Torr. Fixed parameters: kwHSo2 = 40 s-l, kWHo2 = 0 s-l, f = 0.038 s. Fit results: [HS0210= 13.8 arbitrary units, klo = 3.0 X lo-" cm3 molecule-I s-'. Curve represents calculated fit and (0)experimental data.
probably due to heterogeneous loss of the radicals and to conversion into nondetectable species. The HSO calibration may also contribute to this discrepancy. We have not corrected the data for any secondary loss processes. Therefore, the observed HO, production can probably be interpreted as a lower limit for the actual yield. The OH production is more difficult to quantify. Modified HS source conditions are required to observe the OH production because the HS precursors (H2S and C,H4S) react rapidly with OH. Using low [H2S] we observe that about 15% of the initial HS is converted to OH and only 30% is converted to HO, at high [NO,]. The reduced H0, production may indicate an enhancement of radical loss processes associated with the modified source conditions. In view of the possible complications in the OH production experiments we report that H and OH are minor products (150%) and H 0 2is the major product (250%). The HO, appearance in the HS/NOJ02 system was analyzed as a function of [O,] to estimate a rate constant for reaction 10. With the oxygen reactant in excess, the concentration of H0, is described by
(16) where kwx is the first-order loss of species X in the absence of added reactant and t is the reaction time. To determine klo, H0, was monitored vs. oxygen concentration at a series of fxed reaction times. HO, production via reaction 15 is negligible under the data were fitted conditions of these experiments. The H0, vs. [O,] to eq 16 using a nonlinear least-squares routine. Representative H0, appearance data and the corresponding fit are shown in Figure 6. For these experiments the HS0, was made in the fixed radical source by reacting HS with enough NO2 to remove all the HS and HSO. The 0, was added through the moveable inlet set at a fixed reaction distance. Under these conditions the HO, production exhibited a maximum vs. [NO,], suggesting a possible reaction between HSO, and NO2 such as reaction 13. k , for HSOz was measured by making HS0, in the moveable inlet and flooding the LMR detection zone with oxygen added through the flush gas ports. The resultant H 0 2 signal was used as a measure of [HSO,]. The HSO, wall loss, determined by monitoring the HOz signal as a function of inlet position, was in the range from 20 to 60 s-I. This measured loss includes contributions due to reaction with source species such as NO2 as well as wall loss. kwHSO2 was set to 40 s-' for all the fits. Variation in the range from 20 to 60 s-l resulted in less than a 20% change from the k , , determined with kwHS02= 40 s-l. The k, for H 0 2 was set equal to zero for all the fits based on observed negligible loss (kwH02 5 2 s-'). The initial HSO, concentration [HSO,],, was a variable parameter for all the fits. Two different HS sources, H/C2H4S
and F/H2S, were used to test for source interference and no significant deviation between fit results was observed. The results for six data sets are presented in Table 11. Analysis of these data yields k l o = (3 f 2) X cm3 molecule-' s-l. The assigned uncertainty is significantly larger than the standard deviation of the mean which is about 10%of the mean. The large error limits reflect the difficulty in measuring the rate coefficient of a reaction involved in a complex reaction sequence by monitoring the appearance of a product.
Discussion Two of the three reactants examined in this study were the subjects of previous studies. Bulatov et al. have reported rate coefficient measurements for HSO + NO? and HSO + NO.25 In both experiments they photolyzed H2Sin the presence of NO, to produce HSO radicals which were detected by using intracavity laser absorption. For the HSO + NO, measurement they used [H2S] from 0.08 to 0.8 Torr and varied [NO,] from 0.01 to 0.08 Torr at 100 Torr total pressure. They obtained k9 = 4 X lo-', cm3 molecule-' s-l which is about one-half of our value. As described earlier, in the HS/H2S/NO2 system we observed appreciable HSO regeneration which we attributed to OH production via secondary chemistry. At the [H,S] levels used by Bulatov et al., this interference is significant and may explain the discrepancy between our results. For H S O + NO, Bulatov et al. cm3 molecule-' s-')~~ report a rate coefficient (k8 = 2.6 X which is approximately 25 times faster than our upper limit measured at 1 Torr. They did not detect any pressure dependence between 10 and 100 Torr in Ar. A pressure dependence below 10 Torr for reaction 8 cannot explain a 25-fold increase in the second-order rate coefficient between 1 and 10 Torr and seems unlikely if the reaction is pressure independent between 10 and 100 Torr. Interference from secondary chemistry is a more likely explanation of the discrepancy. Our HSO NO, product studies indicate that a species is produced which reacts with 0, to yield H0,. We propose that HSO, is the product based on the available thermochemical data and the lack of other possible products which could exhibit a similar reactivity toward 0,. This assignment is speculative and further work is required. For example, the observation of the SO, product in reaction 10 would be important added evidence. The major loss channels for HSO in the atmosphere are probably by reaction with 02,NO,, or 03.Our upper limit for HSO 0, does not eliminate this channel as the most important loss process. However, our measurements do establish NO, as an important reactant relative to 03.Based on the Fried1 et aL6 data for HSO + 03,our measurements show that the NO2 reaction is about 100 times faster than the O3 reaction. In the troposphere the ratio N 0 2 / 0 3 varies from 50.01 in clean areas up to unity in polluted areas.', Consequently, the NO2 channel will dominate in polluted areas while the O3 reaction is more important in clean areas. Our product analysis shows that the oxidation of H2Sto SO, in the atmosphere appears to conserve odd hydrogen radical species when NO2 is the main oxidant. The initial step, reaction with OH, is well e~tablished,'-~ and evidence for HOz production in the oxidation process is presented here. The OH detected in these experiments is probably produced via reactions 13 and 14 which
+
+
J. Phys. Chem. 1987, 91, 5755-5760 TABLE 111: Sulfur Radical Reaction Rate Coefficients
reaction
---+ - + + -
S + NO2 SO + NO HS + NO2 HSO + NO CH3S + NO2 products SO + NO2 SO2 + NO HSO + NO2 products S + O3 SO + O2 HS + 0 3 HSO + 0 2 +
so
HSO
0 3
O3
so2
0 2
products
rate coeff (298 K), cm3 molecule-' s-I 6.0 X lo-" (ref 26) 6.7 X lo-" (ref 8) 8.0 x lo-" (ref 37) 1.4 X lo-" (ref 26) 9.6 X (this work) 1.2 X lo-" (ref 26) 3.2 X (ref 6) (ref 26) 9.0 X 1.0 X lo-" (ref 6)
will not be important in the atmosphere because of the rapid competing reactions involving 02.These oxygen reactions, (10) and (15), both produce H02. The role of ozone in the sequence requires further study. The effect of the reaction of ozone with HS is similar to that of NOz since it also produces an H S O product. An interesting ozone influence may occur in the reaction with HSO. There are a number of possible reaction pathways and no definitive product analysis has been performed. HSO + O3 AH298 = -22 kcal mol-' OH SO O2 (17a)
--
+ + HS + 2 0 2 AHzss = 1 kcal mol-' A H 2 9 8 = -52 kcal mol-' H + O2+ SO2
+ O2 H 0 2 + SOz HS02
(17b) (17c)
AH298
= -86 kcal mol-'
(1 7 4
AH298
= -101 kcal mol-'
(17e)
Reactions 17b and 2 represent a catalytic cycle for O3destruction as discussed by Fried1 et aL6 This cycle would be interrupted by
5755
other loss channels for HSO and HS. HSO removal by reaction with NOz is thus the only established path for HSO oxidation. Prior to the availability of kinetic data for HS, its reactivity was often predicted to be similar to OH. However, recent results indicate that this is not a good analogy and other comparisons, particularly with Br,6 have been discussed. With regard to trends in reactivity, it is interesting to note the similarities among the homologues of the two sets of sulfur radicals: S/HS/CH3S and SO/HSO. The rate coefficients for the reactions of these species with NOz and O3are presented in Table 111. Each set of reactions appears to proceed by a similar mechanism and the homologues exhibit similar reactivities at room temperature. It is interesting to note that the mechanism of 0 atom transfer to the sulfur is consistent with HS02 production via reaction 9. When there is not an analogous exothermic channel available, the homologue reactivities vary. For example, the reaction
s+0 2
- so +
0
(18)
is reasonably fast (k18= 2.3 X cm3 molecule-l s-')~~ but the similar reactions of HS and CH3S are endothermic and are not cm3 molecule-' s - ' ) , ~ , ~ ' observed ( k C 2 X
Acknowledgment. This work was supported by NOAA as part of the National Acid Precipitation Assessment Program. We are grateful to Dr. T. J. Sears for assistance with the LMR spectroscopy of HSO and to Dr. A. R. Ravishankara for his useful comments on the manuscript. Registry No. HSO, 62470-71-7; NO2, 10102-44-0;NO, 10102-43-9; 0 2 , 7782-44-7. (37) Balla, R. J.; Nelson, H. H.; McDonald, J. R. Chem. Phys. 1986, 109,
101.
Effects of Pressure on the Sucrose Inversion over an Immobilized Invertase Catalyst Masanori Sato, Sentaro Ozawa, and Yoshisada Ogino* Department of Chemical Engineering, Faculty of Engineering, Tohoku University, Aramaki-Aoba, Sendai 980, Japan (Received: February 9, 1987; In Final Form: May 19, 1987)
Kinetics of the sucrose inversion over an invertase catalyst immobilized by porous glass particles has been studied under high pressures up to 127 MPa and at a temperature of 30 f 0.1 OC. The rate equation derived by using the Michaelis-Menten type kinetic model well represents reaction rates observed for E + S + ES EF + G E + F and E + G + EG, where E, S, ES, F, G, EF, and EG denote enzyme, substrate, Michaelis complex, fructose, glucose, enzyme-fructose complex, and enzyme-glucose (inhibitor) complex, respectively. Utilizing the rate equation, one can evaluate kinetic parameters such as the maximum reaction rate V-, the Michae1i:constant K,, and the inhibitor constant Ki as a function of pressure. The activation volume (AVJ), the volume change (AV,) for the dissociation of the Michaelis complex, and the volume change (AVJ for the dissociation of the inhibitor complex have been determined to be -29 f 3 mL/mol, +20 f 3 mL/mol, and +1 f 0.1 mL/mol, respectively, at 0.1 MPa. A strong polarity induced by the enzyme upon the transition state together with an incorporation of one water molecule into the transition state would account for the fairly large negative activation volume.
-
Introduction Little information about the effect of pressure upon the beterogenmus a&lysis in solution is available in the literature, though numerous papers reporting the pressure effects upon chemical reactions in solutions have hitherto been published.'+ Considering (1) Hamann, S. D. Physico-Chemical Effects ofpressure; Butterworths: London, 1951. ( 2 ) Weale, K. E. Chemical Reactions at High Pressures; E.& F.N. SPN: London, 1967. (3) Kelm, H., Ed. High Pressure Chemistry; D. Reidel: London, 1978.
0022-3654/87/2091-5755$01.50/0
-
this situation, the present authors have attempted to make a series of high-pressure studies on solid-liquid interfacial phenomena, andtar€' ofthe results have already been The Present (4) Isaacs, N. S . Liquid Phase High Pressure Chemistry; Wiley: New York, 1981. ( 5 ) Noguchi, H.; Uchiyama, G.; Ozawa, S . ; Ogino, Y. Nippon Kagaku Kaishi 1980, 1195. ( 6 ) Ozawa, S.; Gotoh, M.; Kimura, K.; Ogino, Y. J. Chem. SOC.,Faraday Trans. 1 1984, 80, 1049. (7) Ozawa, S.; Kawahara, K.; Yamabe, M.; Unno, H.; Ogino, Y. J . Chem. SOC.,Faraday Trans. I 1984, 80, 1059.
0 1987 American Chemical Society
