1780
-
+
J . Phys. Chem. 1992, 96, 1780-1785
+
exothermic reaction IO H 2 C 0 HOI HCO (AH= -27.5 kcal mol-') is another possible source of HOI, the importance of which is dependent on its rate constant which is presently unknown. a complement of labratory and modeling studies, field observations of HOI are also required in order to assess the role of this species in the photochemistry of the atmospheric marine
boundary layer.
Acknowledgment. We thank the Commission of the European C0"UnitieS for Support in the fmme Of their fourth Environmental Programme. Registry No. IO, 14696-98-1; H 0 2 , 3170-83-0; NO2, 10102-44-0.
High-pressure Discharge Flow Kinetics Study of OH -I- CH,SCH,, CH,SSCH, Products from 297 to 368 K
-
J. P. D. Abbatt,*gt F. F. Fenter,t and J. G. Anderson Department of Chemistry and Department of Earth and Planetary Sciences, Haruard University, Cambridge, Massachusetts 02138 (Received: August 22, 1991)
Due to a host of experimental and chemical complications, there has been extremely high variability in the literature results for the kinetics of the H atom abstraction reaction between OH and CH3SCH3(DMS). In particular, as a result of a wall reaction between OH and DMS, this reaction stands as the prime example of one which severely restricts the capability of the low-pressure discharge flow technique to measure gas-phase, homogeneous rate constants. In this study, the "wall-less" high-pressure discharge flow technique is used to measure the kinetics for this reaction, and for the related reaction between OH and CH3SSCH3(DMDS), over the temperature range from 297 to 368 K. To the 95% confidence level, the results (4.98 f 0.46) X at 297 K, and for OH/CH&SCH3, are as follows: for OH/CH3SCH3,(1.35 0.62) X 10-"d-28S*135)~Tand (6.2 f 4.9) X 10-11e(410*2'0)/T and (2.39 f 0.30) X at 297 K, where the units are cm3 molecule-' s-l. The results for the OH/DMS reaction are similar to those of specific studies performed with the complementary flash photolysis technique. The agreement between two dissimilar approaches increases our confidence in the literature database for this reaction. The mechanisms of these reactions are discussed in terms of orbital interactions.
*
Introduction Dimethyl sulfide (DMS) is the primary natural source of sulfur to the atmosphere.' As a result, the gas-phase reaction between hydroxyl radicals (OH) and dimethyl sulfide (DMS), which is the main loss mechanism for DMS in the atmosphere, has been the subject of no fewer than 13 studies in the past decade or so. The reason for the profusion of recent work is that a number of complications plague most experimental kinetics approaches to this reaction. In this paper we report kinetics results for the OH/DMS reaction, measured by the high-pressure discharge flow technique which is particularly well suited to deal with the complications of sulfur kinetics. We believe this study removes the errors which have arisen in previous flow tube investigations and in a number of other approaches as well. The reaction is thought to proceed via both H atom abstraction and CH3S(OH)CH3adduct formation? OH
+ CH3SCH3
+
CH$(OH)CH3
(1b)
Close to room temperature, the adduct is thermodynamically mstable with respect to dissociation back to reactants. Consequently, a kinetics experiment performed on a time scale which is long relative to the lifetime of the adduct, and where the concentration of OH is monitored, is sensitive to channel l a alone. For a simple H atom abstraction it comes as a surprise that the reported rate constants for reaction l a consistently show wide imprecision with respect to each other. Although there is now a consensus on the possible errors inherent in some of the preliminary kinetics studies which measured large room-temperature rate constants ((8 - 10) X 10-l2cm3 molecule-I s-l), within the more recent results there still remains scatter in the rate constant To whom correspondence should be addressed. Present address: Department of Earth, Atmospheric and Planetary Sciences, Bldg. 54-1312, MIT, Cambridge, MA 02139. *Present address: Laboratoire de Photophysique et Photochemie Moleculaire, Universite de Bordeaux 1, 33405 Talence, France.
((3.2-5.5) X cm3 molecule-' s-I) and in the magnitude and sign of the Arrhenius activation energy. The dominant experimental complications which have been identified fall into four groups. First, in all low-pressure flow tube studies, there has been evidence of heterogeneous wall effects. MacLeod et al.3 observed a large intercept in their pseudofirst-order rate constant versus DMS concentration plots which is indicative of a wall reaction of OH with adsorbed DMS, comparable in rate to the homogeneous reaction in the gas phase. In our lab, unpublished low-pressure discharge flow studies of this reaction exhibited extremely variable behavior where the first-order loss of OH was not linear with the concentration of DMS. The severity of the wall effects changed with different wall coatings-phosphoric acid being the worst and halocarbon wax the best. Similar effects have been observed by Hsu et ala4 Although, it is strictly possible to measure gas-phase rate constants if the heterogeneous first-order OH loss rate constant is independent of the gas-phase DMS concentration, one can have little confidence in these measurements. The second complications arises from the likely presence in commercial DMS samples of reactive sulfur impurities, CH3SH and CH3SSCH3.Because the respective rate constants for reaction with OH for these compounds are 7 and 50 times larger than that for DMS, the purity of the DMS sample must be high. A third complication arises in relative rate studies with the presence of NO within the reaction chamber. Product chemistry which involves NO is thought to enhance the overall loss rate of DMS, possibly via reaction with the CH3Sradical. Finally, for studies performed in air there is evidence that the rate constant for OH loss is enhanced by reaction of the CH3S(OH)CH3 adduct with 02.
(1) Finlayson-Pitts, B. J.; Pitts, Jr., J. N. Atmospheric Chemistry; Wiley and Sons: New York, 1986. (2) Atkinson, R.J . Phys. Chem. Ref.Data 1981, Monograph 1. (3) MacLeod, H.;Poulet, G.; LeBras, G. J . Chim. Phys. 1983, 80, 287. (4) Hsu,Y.-C.; Chen, D.-S.; Lee., Y.-P. Inf. J. Chem. Kinet. 1987, 19,
1073.
0022-3654/92/2096- 1780%03.00/0 0 1992 American Chemical Society
OH
+ CH3SCH3, CH3SSCH3
-
The Journal of Physical Chemistry, Vol. 96, No. 4, 1992 1781
Products
N Figure 1. Schematic diagram of the high-pressure flow system (not to scale): A, mass flow controllers (50 and 200 SLM); B, settling chamber; C, flow straighteners; D, nozzle; E, flow preparation region; F, radical injectors; G, copper vapor laser; H, dye laser; I, pitot static probe; J, laser-induced fluorescence axes; K, resonance fluorescence axis; L, differential pressure transducer; M, UV lamp; N, vacuum spectrometer; 0, ultraviolet absorption system; P, flat mirror; Q,gate valves; R, bypass; S, Roots blower (9200 LM); T, roughing pump (500 LM).
In this paper, we report kinetics data for reaction l a over the temperature range 297-368 K. The measurements were made in a "wall-less" high-pressure discharge flow system which is not susceptible to heterogeneous effects,s and careful attention was given to other possible systematic errors which could affect the accuracy of the results. The particular aim of this study is to eliminate the heterogeneous effects which have arisen in previous flow tube studies and to compare the results to those from flash photolysis, the other commonly used direct kinetics approach. Agreement between these two complementary methods will sharply reduce the uncertainty in the kinetic parameters for this reaction. The O H kinetics with dimethyl disulfide (DMDS) have also been measured as part of this study:
OH
-
+ CH3SSCH3
products
(2)
Although DMDS has been identified in tropospheric air, it is thought to be a much smaller source of sulfur to the atmosphere than DMS.'
Experimental Section The high-pressure discharge flow experimental approach has been recently developed to alleviate the constraints historically imposed onto the flow tube kinetic technique by the plug-flow approximation. A detailed description of the apparatus and approach is given elsewhere,s but a general summary is given below. The plug-flow approximation assumes that rapid molecular diffusion flattens molecular concentration gradients radially across the flow tube. As a result, low pressures, small flow tube diameters, and small rate constants for radical wall loss are a necessity. At high pressure and/or when there is a large sink for radicals at the wall (often due to heterogeneous reactions), the plug-flow approximation breaks down and the flow tube technique is rendered useless. In the high-pressure discharge flow technique, the capability of measuring axial/radial radical concentration gradients within the reaction zone allows the continuity equation to be solved numerically at any point in the flow. In this manner, flat concentration gradients are not required to extract a rate constant since the solution of the continuity equation decouples chemical loss from mixing/diffusion processes in the flow. In the limit of radially symmetric radical concentration profiles and in fully developed laminar or turbulent flow, the apparent axial loss of radicals from the central radial position, which arises from mixing of the radicals in the direction of the wall, remains constant as the concentration of the excess reagent varies. As a result, a plot of the axial decay rates of the radical concentration versus the excess reagent concentration has a slope which is the bimolecular rate constant and an intercept which reflects mixing of radicals from the center of the flow toward the wall. ( 5 ) Abbatt, J. P. D.; Demerjian, K. L.; Anderson, J. G. J. Phys. Chem. 1990, 94, 4566.
Because of the large flow tube diameter and the relative time scales for transport and mixing/diffusion which exist at high pressure (>lo Torr of NJ, radicals injected into the center of the flow do not encounter the walls within the reaction zone. As a result, heterogeneous effects are negligible in a reactor of this type. 1. Row System. The 12.4-cm4.d.flow system is a closed-loop design (see Figure 1). After leaving a Roots blower, the bulk flow of Nz enters a flow preparation region which is sufficiently long that fully developed laminar and turbulent flow (i.e., with a steady radial velocity profile) is present in the reaction zone. The velocity is measured with a pitot static probe. After the reaction zone, but prior to the Roots blower, the bulk flow passes through an absorption cell along the return path. On each cycle, approximately 5% of the flow is removed from the system with a roughing pump and replenished with flow controllers. In this way, a steady-state pressure is achieved in the system. For Arrhenius studies, the temperature of the gas is raised by heating 5.5 m of the flow preparation region immediately upstream of the reaction zone (see ref 6 for more details). 2. Radical Source and Detection. OH radicals are added to the bulk flow via injection from four radially oriented needle injectors located just prior to the reaction zone (see Figure 2). In a small flow of He (less than 0.2%of the bulk flow of N2), H atoms are generated from H2 in a microwave-induced plasma. The H atoms are then added to the four injectors, together with NOz, allowing OH to be generated by the fast reaction H NO2 OH + NO. The OH is injected into the center of the flow tube in a radially symmetric manner. Over 95%of the OH is within the central 5-cm-diameter core of the flow tube. At five downstream axes the concentration of OH is detected by laser-induced fluorescence using the A28-X211 (1,O) transition. The exciting radiation at 282 nm is generated from a copper vapor laser pumping an etalon-narrowed dye laser with frequency doubling into the ultraviolet region. The laser beam is passed sequentially from one detection axis to another using high-quality mirrors and windows (seeFigure 2). The OH fluorescence at 309 nm is detected in the center of the flow tube using an electronically gated and optically filtered bialkali photomultiplier tube at each axis. The high radial resolution of the detection system (-90% of the fluorescence originates from a volume element 1 cm long) and the large diameter of the flow tube (12.4 cm)ensure that only OH in the center of the flow tube is being detected. OH radical detection limit is on the order of 1 X lo7 molecules cm-3 at 10 Torr of N2. For the majority of experiments, the maximum concentrations of OH in the center of the tube were
