J . Phys. Chem. 1991, 95, 1232-1 240
1232
of isopropyl alcohol, equilibrium constants of 0.038 f 0.005 mol dm-3 and 0. I8 f 0.02 mol dm-' were obtained in acetonitrile and benzene solvents, respectively. These results arc in line with the association constant reported' for the Me2CO-MeOH system in acetonitrile ( K = 0.08 mol d m 3 ) and with that estimated'* for the Ph2CO-EtOH system in hexane ( K = 0.4 mol d ~ n - ~ ) . The infrared results show that the benzophenone-isopropyl alcohol complex is about 4.5 times more stable in benzene than in acetonitrile. I f roughly the same is true for the stability of the benzophenone-dimethylketyl radical complexes, then the agreement between the ratio of the K, values in the two solvents and the ratio of the appropriate rate coefficients (Le., k5(in benzene)/k5(in acetonitrile) = 4.4 f 1 . I ) may be considered as support for the mechanism presented in Scheme 11.
bl c
a
. i
P
b
8
Appendix 1700
1650
1600 1700
1650
1600
w n e ~ l b ricm-1 r Figure 6. I R spectra of solutions of 0.05 mol dm-' benzophenone and
isopropyl alcohol: (a) iicctonitrilc solvcnt, [Me,CHOH] = 0 ( I ) , 2.62 (2). 4.58 (3). 6.55 (4) mol d ~ n - (b) ~ : benzene solvent, [Me2CHOH]= 0 ( I ) %2.62 (2). 5.24 (3) mol dm-'. the C=O stretch vibration of benzophenone, while the new band which appears at 1653 cm-' in the presence of isopropyl alcohol and increases with increasing alcohol concentration may be assigned to thc hydrogcn-bonding complex. From the decrease of the 1662-cm-' absorption of the nonbonded molecule on addition
Reevaluation of the Arrhenius Parameters A5 and E5from the Results of Naguib et al.3 (i) E4;,- E6/2 - E5 = -1.8 kcal mol-' (a result of Naguibet aL3). (ii) Eh = 3.13 kcal mol-' (a literature value4 which is in agreement with unpublished results obtained in the Ph2CO-Ph2CHOH system.) (iii) E4a= Eh (a reasonable assumption for two combination reactions. (iv) E5 = 3.35 kcal mol-' (derived from the data given above). (v) k5 = 3 X IO5 dm3 mol-' s-' (obtained from the reevaluation of the results of Naguib et aL3, see text). (vi) A5 = 9 X IO7 dm3 mol-' SKI (derived with k 5 and E5 given above). (12) Brealey, G. J.; Kasha, M. J . Am. Chem. Soc. 1955, 77,4462-4468.
Kinetics and Mechanism of the Reaction of Hydroxyl Radicals with Acetonitrile under Atmospheric Conditions A. J. Hynes* and P. H. Wine Physical Sciences Laboratory, Georgia Tech Research Institute, Georgia Institute of Technology, Atlanta, Georgia 30332 (Receiord: March 16, 1990; In Final Form: August 21, 1990)
The pulsed laser photolysis-pulsed laser induced fluorescence technique has been employed to determine absolute rate coefficients for the reaction OH + CH3CN ( I ) and its isotopic variants, OH + CD3CN (2). OD + CH3CN (3). and OD + CD3CN (4). Rcactions 1 and 2 were studied as a function of prcssurc and tcmpcraturc in N,, N 2 / 0 2 , and He buffer gases. In the absence of O2 all four reactions displayed well-behavcd kinetics w i t h cxponcntial OH decays and pscudo-first-order rate constants which were proportional to substrate concentration. Data obtained in N, over the range 50-700 Torr at 298 K are consistent with k , showing a small pressure dependcncc. The hrrhcnius cxprcssion oblaincd by averaging data at all pressures is k , ( T ) =) :( I: .! I X exp[(-l I30 h 90)/7'l cm3 molecule-' s-I. The kinetics of reaction 2 are found to be to (2.16 f 0.1 I ) X cm3 molecule-' s-I over prcssurc dcpcndcnt with k 2 (298 K ) increasing from ( I .21 f 0.1 2) X the prcssurc rangc 50-700 Torr of N 2 at 298 K. Data a t prcssurcs >600 Torr givc k 2 ( T ) = (9.4!i,:4) X IO-'' cxp[(-l 180 f 2 5 0 ) / q cm3 molcculc-' s-l. The rates of reactions 3 and 4 are found to be independent of pressure over the range 50-700 Torr of N 2 with 298 K rate coefficients given by k 3 = (3.18 f 0.40) X mi3 molecule-' s-l and k4 = (2.25 f 0.28) X cm3 molcculc-' s-I. I n the presence of O2each reaction shows complex (non-pseudo-first-order) kinetic behavior and/or an apparcnt dccrcasc in the observcd rate constant with incrcasing [ 0 2 ] ,indicating thc prcscncc of significant OH or OD regeneration. Observation of regeneration of OH in (2) and OD in (3) is indicative of a reaction channel which proceeds via addition followed by reaction of the adduct, or one of its decomposition products, with 0,. The observed OH and OD decay profiles havc been modeled by using a simplc mechanistic scheme to cxtract kinetic information about the adduct reactions with O2 and branching ratios for OH regeneration, A plausible mechanism for OH regeneration in (2) involves OH addition to thc nitrogen atom followed by 0, addition to the cyano carbon atom, isomcrixation. and decomposition to D2C0 + DOCN + OH. Our rcsults suggest that the OH + CH3CN reaction occurs via a complcx mechanism involving both biniolecular and tcrmolecular pathways, analogous to the mechanisms Tor thc important atmospheric reactions of OH with CO and HN03.
Introduction
I t is now generally accepted that acetonitrile (CH3CN) is present at ppt levels in the stratosphere.' Attempts to understand the role of acetonitrile in stratospheric positive ion chemistry2 and its contribution to the stratospheric NO, budget3 require a detailed *Author to whom correspondence should be
addressed.
0022-3654191/2095-1232$02.50/0
understanding of its atmospheric sources, emission rates, and oxidation mechanism. Acetonitrile was first proposed as a com( I ) Schlager, H.; Arnold, F. Planer. Space Sci. 1986, 34, 245. (2) Arijs, E.; Ncvcjans, D.; Ingcls, Inr. J . Mass Specrrom. Ion Processes 1987, 81, 15. ( 3 ) *Almospheric Ozone 1985." WMO-Report No. 16, Vol. I, World Mctcorological Organization, Gcncva, 1985.
0 I99 I American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 1233
Reaction of Hydroxyl Radicals with Acetonitrile ponent of stratospheric positive cluster ions by Arnold et al.? who used a rocket-borne ion mass spectrometer to identify cluster ions of the type tl+X,(H?O),,.where X = mass 41, I = I , 2, and n = 0, I . 2, 3. Thcsc ions have been termed non-proton hydrates ( N P H ) s to distinguish them from proton hydrates (PH) of the form H + ( H 2 0 ) , which had been predicted to be present in the stratosphere6 and wcrc also observed experimentally. Subsequent experimcnts using balloon-borne mass spectrometers7determined ion concentration profiles in the 25-40-km region and, although thc idcntification of X as CH$N remained circumstantial, were uscd to infer stratospheric mixing ratios for neutral CH3CN7and evcn for thc O H radical.' The first definitive identification of X as CH3CN came from field and laboratory measurements of Schlager and Arnold,' who used electric-field-induced collisional activation to frugmcnt the NPH clusters and produce mass 15, identified as CH3+, as a product. The totally unexpected observation of NPH and inferred free CH3CN mixing ratios in the ppt rangc has led to a number of efforts to identify the source of the CH,CN and model its atmospheric profiles.8 While it now appears that the only significant sources of CH3CN involve emission at thc Earth's surface," modeling efforts have been hampered by the paucity of data on emission rates, tropospheric conccnLration profilcs, and atmospheric degradation processes. Three major deficiencies identified by Arijs et al., in a recent review of stratosphcric positive ion measurements and acetonitrile detection are (a) accurate measurements of rate constants for the atmospheric reactions of CH3CN, in particular with O H , (b) a detailed study of the emission sources of CH3CN,and (c) a better knowlcdgc of atmosphcric cddy diffusion processes. This work addrcsscs thc first of the above-mentioned data requircmcnts, thc ratc and mechanism of reaction I under atmospheric conditions. Six previous studies of reaction I have O H CH,CN products (1)
+
-
employed the flash photolysis-resonance f l u o r e s ~ e n c e ~and -~~ discharge flow-EPRI4 techniques. While the most recent studies are in reasonable agreement on the 298 K rate constant at pressures in excess of 50 Torr Ar,I0-l3 there are considerable discrepancies in observed temperature and pressure dependencies. There has been no investigation of the effects of isotopic substitution on reaction I . Furthermore, since all studies of reaction I have been performed in either Ar or He buffer gases, the effects of N 2 and O2on the rate constant and hence the effective value for the rate constant under atmospheric conditions have not been delineated. In this work we have employed a pulsed laser photolysis-pulsed laser induced fluorescence (PLP-PLI F) technique to study reaction 1 as a function of pressure and temperature in Nz, N z / 0 2 , and He buffer gases. Studies of the isotopic variants, reactions 2, 3, and 4, have been performed in an attempt to shed light on the detailed reaction mechanism. Our results are inconsistent with O H + CDJN products (2) OD
+ CH,CN
OD
+ CD,CN
--
products
(3)
products
(4)
(4) Arnold, F.; B6hringcr. H.: Henschen. G . Geophys. Res. Left. 1978, 5, 653. ( 5 ) Arijs. E.: Ingcls, J.: Ncvcjans, D. Nature 1978, 271, 642. ( 6 ) Mohncn. V. A . Pure Appl. Geophys. 1971.84, 114. (7) Arijs, E.; Nevejans, D.; Ingels, J. Nature 1983. 303, 314, and references therein. ( 8 ) Arijs, E.: Brnsseur. G.J . Geophys. Res. 1986, 91, 4003. (9) Harris, 6 .W.: Klcindicnst, T. E.: Pitts, J . N.. Jr. Chem. Phys. Left. 1981, 80. 479. (IO) Frit7. B.: Lorenz, K.: Steinert, W.; Zellner, R. In Proceedings of the 2nd European .Symposiuni on the Physico-Chemical Behatiour of Atmospheric Pollutants; Varsino, E.,Angclctti. G..Eds.; D. Reidcl: Boston, MA,
1982: p 192. ( I I ) Zetzsch, C. Bunsckolloquium; Battelle Institut: Frankfurt, 1983. (12) Kurylo. M. J.; Knablc, G . L. J . Phys. Chem. 1984, 88. 3305. (13) Rhara, D.; Zcllner, R. Unpublished work. Rhasa, D. Diplomarbcit, Gottingen. FRG. 1983. (14) Poulct, D.: Lavcrdct. G.:Jourdain. J . L.: Le Bras, G . J . Phys. Chem. 1984. 88, 6259.
9,
'
-1'
?,
M
9 ;
I
1
I
, I
1
.
.
.
- 4
-i
4
EXPERIMENT
w
SIMULATION
0
VI w P= 0
3 k 28'71
2878
2881
2888
28Ql
EXCITATION WAVELENGTH (A) Figure 1. Experimental and simulated laser excitation spectrum of the OD A2Z+-X211( 1-0) transition.
the view that reaction I is pressure independent and proceeds exclusively via a direct hydrogen abstraction mechanism.
Experimental Section The PLP-PLIF apparatus has been described in detail elsewhere.I5 Modifications and a brief review of its operation are given below. Three different reaction cells were used, with most of the experiments being performed in a Pyrex cell which had an internal diameter of 4 cm and a length of 50 cm. Two side arms, 4 cm i.d. and 14 cm long, which terminated in Brewster angle windows were attached to the center of the cell. The photolysis laser passed through these side arms across the direction of gas flow while the probe laser passed along the length of the cell. Fluorescence was detected through a third side arm, 4 cm i.d. and 5 cm long, perpendicular to the photolysis and probe beams. The central 25 cm length of the cell was jacketed to permit the flow of heating or cooling fluid from a thermostated bath. Detailed descriptions of the other two reaction cells, which were constructed of Pyrex and brass, are given elsewhere.ls.'h A copper-constantan thermocouple with a stainless steel jacket was inserted into the reaction zone through a vacuum seal, allowing measurement of the gas temperature under the precise pressure and flow conditions of the experiment. O H and O D were produced by the pulsed laser photolysis of H20,, HNO,, or D N 0 3 using the 193-nm (ArF) and 248-nm (KrF) outputs from an excimer laser or the 266-nm fourth harmonic output from a Nd:YAG lascr. Pulsed laser induced fluorescence using a Nd:YAG laser pumped, frequency-doubled dye laser was used for OH(D) detection. Excitation was via the 2 for O H Q I 1 line of the A2Z+-X21T( 1-0) transition at ~ 2 8 nm and -287 nm for OD. All kinctic studics were performed with a line-narrowing ctalon in the dyc lascr cavity giving an cstimated line width of 0.1 cm-' at 280 nm. The laser could be reproducibly scanncd on and off thc OH(D) line by prcssurc tuning using N 2 gas. Fluorcsccncc in the 0--0 and 1-0 bands was dctectcd by an EM I 98 13QB photomultiplier after passing through collection optics and filters to discriminate against.Rayleigh scattering and Raman scattcring from N, and/or 0,. The photomultiplier output ( 1 5 ) Hyncs, A . J.; Wine, P. H.: Semmes, D. H. J . Phys. Chem. 1986.90,
4148. (16) Hynes. A. J.: Wine. P. H.; Ravishankara. A . R. J . Geophys. Res.
1986, 91, 11815.
Hynes and Wine
1234 The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 was appropriately terminated and fed to a 100-MHz waveform analyser to obtain the peak voltage averaged for (typically) 100 laser shots. To ensure that we were, in fact, producing and monitoring OD, a laser excitation spectrum of the A-X( 1-0) band was obtained by scanning the unnarrowed dye laser. In this case the photomultiplier output was processed by a boxcar averager and the output digitized and stored in a small computer. The excitation spectrum was assigned by comparison with a simulated spcctruni of thc six main branches calculated from the term encrgics of Clync ct al." Intensities were calculated for a 300 K Boltzman population distribution by using the formula of Earls'* and convoluting over the laser bandwidth. The experimental and simulated spectra, shown in Figure I , are in good agreement and confirm that we wcrc producing and monitoring OD. Kinetic information was obtained by varying the delay between the photolysis and probc lasers using a digital delay generator. Signal was collcctcd for 15-20 delays, varying from 0 to 20 ms, in ordcr to map out an O H ( D) decay profile over three 1 / e times. In order to avoid the accumulation of photolysis or reaction products all experiments were carried out under "slow flow" conditions with the linear flow rate through the reactor varied between 1 and 5 cm s-l. In a typical experiment the flows were set in the absence of CH,CN to give the desired pressure. A C H , C N / N 2 mixture was then added incrementally to the flow and a range of pseudo-first-order rate constants measured. The throttlc valvcs in thc flow system were not adjusted to keep the pressure constant during this process and hence there was some variation in total pressure over the course of an experiment. Bulbs acetonitrile in N ? were prepared manometrically containing 4% and the concentration of acetonitrile in the reaction mixture was calculatcd from flow ratcs and reactor total pressure. The flow ratcs and total prcssure were measured by using calibrated mass flow metcrs and a capacitance manometer. The response of the mass flow meter which measured the mixture containing CHICN was corrcctcd for its presence by calibrating with CH3CN/N, and CH,CN/Hc mixtures. For a 5% C H 3 C N / N 2 mixture the Correction was 7%,,and for the 6% CH3CN/He mixture used in the two sets of He buffer experiments the correction was 17%. These corrections were somewhat larger than the 5% and 12% corrcctions calculated by using a formula supplied by the manufacturer.'' In ordcr to asscss the possibility of complications arising from isotopic exchange between HzOzand CD$N the infrared spectra of CH3CN, CD,CN, and CDlCN/H20? mixtures were measured. For the CDlCN/H?02 mixtures, no evidence for isotopic exchange was observed on n time scale of 15 min. Chrniicols. The pure gases used in this study had the following statcd minimum purities: N?. 99.999%; O,, 99.99%. Air was zero grade, < I ppm total hydrocarbons. H 2 0 2(90%) was obtained from FMC Corp.; it was further concentrated and purified by bubbling buffer gas through the sample for several days before usc in cxpcrimcnts. H N 0 3 (70% in H,O) was used in a 50/50 mixture with H ? S 0 4(95%); both were Fisher AR grade. DNOl (99 atom % D. 70%solution in D?O) and D,SO4 (99 atom % D) were obtained from ICN Biomedicals. CH3CN, U V grade (>99.9%) was obtained from Burdick & Jackson. CDICN (>99.9% chemical purity and 99 atom '77) D) was obtained from Aldrich. Acetonitrile samples were degassed and purified by trap to trap distillation bcforc usc. Results and Discussion
Experiments in the Absence of Oxygen. All experiments were performed under pseudo-first-order conditions with [CHJN] >> [OH]. Typical initial O H concentrations were I X 10" to 3 X 10l2 molecules cmW3while CH3CN concentrations were in the range 1 X IOis to 3 X IO" molecules cm-). Under these conditions (17) Clync. M . A . A.; Coxon. J . A,; Woon Fat, A . R. J . Mol. Spectrosc. 1973, 46, 146. ( 1 8 ) Earls, L. T.Phys. Reo. 1935. 48, 423. (19) M K S Inslrumcnls, Inc.. Burlington, M A .
1.0
0.1
\ (d) 0.01
I
I
I
I
I
3
0
I
6
t h o (ms) Figure 2. Typical OH temporal profiles for rcaction I . Experimental conditions: 298 K, 587-623 Torr of N,, [OH], = 2 X IO'* molecules cn-3. CH3CN concentrations in units of 10l6molecules were ( a ) 0, (b) 1.39, (c) 4.03, (d) 9.39. Solid lines are obtaincd from linear least-squares analyses and give the pseudo-first-order rate constants (a) 64 f 5 s-l, (b) 478 f 39 s-l, (c) 916 f 76 SKI. (d) 2386 f 173 SKI(errors arc 28). The plots have been displaced vertically for clarity.
we expect the temporal profile of O H to be governed by the following reactions (for HzOzprecursor): H202 + hu 20H
-
+
OH
-
OH
+ CH3CN
products
loss by diffusion, reaction with H 2 0 2 , and reaction with background impurities (6)
Since [CH3CN] >> [OH],, observed O H decays were expected to rollow simple first-order kinetics: In ([OH],/[OH],) = (k,[CH,CN] k,)t = k't (I)
+
The bimolecular rate constant, k,, is determined from the slope of a k'vs [CH3CN] plot. Observation of O H temporal profiles which are exponential, a linear dependence of k'on [CH3CN] and invariance of k'to variations in laser photolysis energy would serve to confirm the above mechanism. Factor of 5 variations in laser photolysis energy were carried out under a variety of experimental conditions and found to have no effect on observed kinetics; these observations confirmed that radical-radical side rcactions had a negligible effect on O H kinetics. O H decay profiles obtained at different CH3CN concentrations in 600 f 20 Torr of N, are shown in Figure 2. Some scatter and curvature is evident in this data which results from instabilities in the photolysis or probe laser. This scatter, which was particularly prcvelent for high-pressure data where the signal to noise ratio was low, is reflected in the rather low precision of some of the ratc constants obtained, particularly for k , . However, there wcrc no systcmatic deviations from exponential behavior for any O H and O D dccays in the abscncc of oxygen. Figure 6d, for example, shows an O H dccay in 450 Torr of N,; this decay, which was mcasurcd on a day the system was particularly stable, shows excellent linearity for greater t h a n 4 I / r times. All decays were analyzed for at least three I / e times; typical 20 errors in measured decay rates were 298 K confirm the observed pressure dependence at 298 K; these data suggest that the highpressure limit is approached at progressively lower pressures as thc tcmpcraturc decreases. The observed pressure dependences suggest that, over the temperature range of our experiments, k 2 is closc to its high-pressure limit at nitrogen pressures of 600 Torr
1236
The Journal of Physical Chemistry, Vol. 95, No. 3, 1991
Hynes and Wine
100
10
1
'EI
I
I
I
I
3.0
2.4
I
I
0
3.6
10001T(K)
Arrhenius plots. (-0-) OH + C H 3 C N ; (--.--) OH + CD,C N . P > 600 Torr. Solid and dashed lines are obtained from lcastsquares analyses. A factors are in units of cm3 molecule-' s-'. Figure 5.
TABLE IV: Summary of Arrhenius Parameters and 298 K Rate Constants for Reactions 1-4"
reaction
Ah
011+CII,CN 011 + C D J N OD + CHzCN OD + CD3CN
l.l?,~:~
EIH. K 1 1 0 4 f 118 I I75 f 243
0.96:;;
k(298 K)' 2.48f0.38 2.16 f 0.1 I d 3.18 f 0.40 2.25 f 0.28
"Errors are 20. /,Units arc IO I ? cm' molecule-' cm3 molecule-' s-l. d A t =680 Torr.
Units are
s-'.
10
background, mV
2372 f 490 903 f 78
917 f 43 918 f 45
linc
"Thc voltagcs arc thc nicans f l r r of 5
X 100 shot
Measured 298 K rate constants are summarized in Table IV together with the Arrhenius expressions for reactions 1 and 2. The mechanistic implications of these results are discussed below. Experinwnts in the Presence of Oxygen. Although dctermination of the effective rate constant for reaction I in air was a central objective of the present work, initial experiments gave indications of severe complications from secondary chemistry when oxygen was present in the reaction mixture. This took the form of nonexponential decays with the apparent regeneration of OH radicals. A set of typical OH temporal profiles, with [CH,CN] constant and varying amounts of O? present, is shown in Figure 6 . To ensure that the deviations from exponential behavior were due to OH regeneration and not to the detection of a second species, the dye laser was pressure scanned off the OH line and the signal monitored. Table V shows the results of such an experiment for curve b of Figure 6 obtained by scanning =O. I nm off line at a delay of 1.5 ms; the reported voltages are the mcans f l u of five 100 shot averages. This and a number of similar experiments clearly demonstrated that OH regeneration was occurring and suggested that a product of reaction 1 was reacting with O2 to regenerate OH. Two possible routes to OH regeneration could involve the reaction of 0, with the methylcyano radical, CH?CN, or its decomposition products, or with an OH adduct or its decomposition products. CHJCN + OH CHICN + H 2 0 (la)
-
-+
+M
(b)
averages.
or greater. The data for P > 600 Torr give the Arrhenius expression k2(n = exp[(-l 180 f 2 5 0 ) / q cm3 molecule-' S K I (9.4$'i4) X
CHJCN + OH
tlme (mr) temporal profiles for reaction I in N 2 and N 2 / 0 2 mixtures. Experimental conditions: 298 K, 450 Torr total pressure, [ C H 3 C N ] = 1.5 X IO1' molecules ~ m - [OH], ~ , = 5 X IO" molecules cm-', bcffer gas: (a) 50/50 N,/O,, (b) air, (c) 2 70 0, in N,, (d) N 2 . Solid lines are obtained from the simulations described in the text. The plots have been displaced vertically for clarity. Figure 6. OH
100
signal + background, mV off 011 linc
1 4
I
TABLE V: On-Line and Off-Line Signals Observed during OH Reneneration" on OH
I
I
2
CHXCN(0H) + M
( 1 b)
-. .
0
Figure 7. O H ( D ) temporal
10
20
time (ms) profiles in N 2 / 0 2 mixtures. (a) O D
+
+
C D , C N , 298 K, 50 Torr total pressurc, 50/50 N,/O,; (b) OH CD3C N , 256 K, I IO Torr total pressure, air; (c) OD + C H , C N , 298 K, 270 Torr total pressurc, 50/50 N 2 / 0 2 . Solid lines are obtained from the simulations describcd in the text. Dashed lines represent behavior observed i n N,. The plots have been displaced vertically for clarity.
In order to attempt to distinguish between regeneration from the products of ( l a ) or ( I b), we examined reaction 2 in the presence of 02.Trace b in Figure 7 shows an OH temporal profile for reaction 2 in 100 Torr of air at 256 K togcthcr with the temporal profile which would be expected from the rate constant measured in N,. Again the deviation from exponential behavior and the presence of OH regeneration is clcar. Similar results were obtained in a number of experiments at 300 K. In this case the interpretation of the data is relatively unambiguous since any CD2CN formed in reaction 2 cannot react with 0, to regenerate OH. Either massive isotope exchange betwccn CD,CN and H2O2must have taken place or OH regeneration must have occurred via a proccss which involves an OH adduct. The former explanation can be rejccted on two counts. First, as noted above, IR spectra of mixtures of CD,CN and H 2 0 2showed no indications of isotopic exchange taking place. Second, since the concentrations of CD,CN and H,O, were 7.1 and 0.003 X I O l h molecules cm-,, respectively, significant isotopic exchange would have affected only a small fraction of the CDJCN while totally depleting the OH photolytic precursor. A very limited amount of data on rcactions 3 and 4 in thc presence of 0, buffer gas was also obtained and is discussed below.
The Journal of Physical Chemistry, Vol. 95, No. 3, 1991
Reaction of Hydroxyl Radicals with Acetonitrile
I237
TABLE VI: Best-Fit Parameters in OH Regeneration Simulations"
simulation parameters
reaction 011
011
+ CII,CN
+ CD,CN
(i = I )
(i
= 2)
O D t CI1,CN ( i = 3) O D t CD,CN ( i = 4 )
ki: 1.25 1.25
kh+:
kidb
1.0 0.7 0.85 0.55 2.25
1.25 1.25 0.7 0.7 0.65 0.35 0.85
1.1
1.1
"Best fits were obtained by visual
5
*4
5 4 5 f4 5 5 f4 5 5 5
measd rate in N , k! 2.5 2.5 2.5 I .38 I .5 0.9 3.2 2.2
[CH ,CN 1, I O I h molecules em-)
[O,],Torr
4.9-1 I .2 15.0 4.0-1 0.7 7. I I .4-7.3 7.1 4.6 I.6
I20 0.9-225 40 23 28 22 I35 25
press., Torr 600 450 200 I I4 I40 1 IO 270 50
T,K no. of expts 298 298 298 256 298 256 298 298
3 3 2 I 3 I I 1
comparison of experimental and simulated decay profiles. Errors were estimated from the scatter in the cm3 molecules-' s-l. 'Units are cm3 molecules-' s-l.
experimental data and simulation parameters. *Units are
Modeling the Regeneration Process. In order to examine the plausibility of thc abovc mcchanism we have modeled some of our 0, data using a GEAR program*O to numerically integrate the ratc cquations. Thc simulations of rcaction 1 in the presence of O2 involved a siniplc four-rcaction kinetic scheme which consisted of two reactivc channcls, (IC) and (Id), background losses, (6), and a regcncration stcp, (7), which involves the reaction of a product of (Id) with 0, to produce OH. CHjCN + OH ---+ A (IC)
-
CH3CN B
+ O2
+ OH B OH + products
(Id)
(7) In this scheme, which clearly oversimplifies the actual reaction mcchanism, A represents the unidentified products of all reactive channcls which do not lead to OH regeneration, and k , , represents thc sum of the rates of those processes. Similarly reaction Id, which may be a single or multistep process, produces an intermcdiatc B, which rcacts with O2to regenerate OH. While A and B may represent multiple products of (IC) and (Id), this scheme docs not prccludc A and B in fact being the same species. For instancc, if thc rcaction procccdcd solely by adduct formation followcd by adduct rcaction with 0, then A and B would both reprcscnt thc adduct. Thosc channcls which led to OH regeneration would bc rcprcsentcd by (Id), and those which did not would bc rcprcscntcd by (IC). Initially thc sum A , , + k l d was taken as the value of k , obtained in N 2 buffcr, k,, was determined experimentally, and the branching ratio bctwccn ( I C ) and (Id) as wcll as the value of k7 were varied to producc thc bcst fit to the experimental data. The fitting process was qualitntivc with bcst fits being obtained by visual comparison of thc simulatcd and cxpcrimcntal data. While the values of the branching ratio and A , couplc to some cxtcnt, the fits arc very sensitivc to the uppcr limit of the branching ratio kl,/kld. Figure 6 shows simulations obtained with k , , k l d = 1.25 X cm3 molcculc-' s-' and k7 = 5 X cm3 molecule-' SS'; these simulations are ablc to rcproducc thc obscrved OH decay profiles at a singlc [CH,CN] as thc O2 partial prcssure is varicd from 0.9 to 225 Torr at 450 Torr total prcssurc. Table VI shows the values of thc "bcst fit" puramctcrs Tor ninc cxpcrimcnts in which OH rcgcncration was obscrvcd including one cxperimcnt at 256 K . All of thc cxpcrimcntal profiles, with the exception of two expcrimcnts in 200 Torr of air at 298 K , are reproduccd wcll with k , , = k l d= k l / 2 ; for these two experiments the value of k l dwas reduccd slightly to givc thc bcst fits. For all thc cxpcrimcnts a valuc of k7 = (5 f 4) X IO-'h cm3 molecule-' s-I was able to reproduce the observed profiles. The success of this simple kinetic scheme indicates that our data is consistent with reaction 1 proceeding by either of two channels with approximately equal probability. One of these channels leads eventually to the regcncration of OH and the process involves a rate-determining step in which O2 reacts with an unknown intermediate with a bimo-
+
(20) McKinncy, R. J.; Weigcrt, F. J . GEAR: Gear Algorithm Integration of Chemirol Kinetics Equotions; QCMP #022, Department of Chemistry, Indiana Univcrsity. Bloomington. I N .
lecular rate coefficient of (5 f 4) x cm3 molecule-' s-l. As noted above, for reaction I , either an abstraction or an addition channel could lead to OH regeneration. An abstration channel could lead to OH regeneration if the methyl cyano radical, or one of its decomposition products, reacted with 02.It is not possible, therefore, to identify the rate constants k , , and kldwith specific channels. For reaction 2 the situation is less ambiguous since any methylcyano-d, (or its decomposition products) produced cannot react with O2 to regenerate OH. Any OD produced in such a reaction would not be observed by LIF when the laser was tuned to an OH transition. In this case observation of OH regeneration provides almost conclusive evidence for the presence of an addition channel. As was noted above, OH regeneration was observed for reaction 2 in the presence of 0,. Simulations were again performed using a kinetic scheme analogous to the one used for reaction I .
+ OH
CD3CN CD3CN
B(0H)
+ OH
+ 0,
-
-
OH
A
(2c)
B(0H)
(24
+
+ products
(8)
Four experiments at 298 K in 140 Torr of air and one experiment at 256 K in 100 Torr of air were analyzed and the results are summarized in Table VI. The fit to the 256 K experiment with kzc = 5.5 X cm3 molecule-' s-I, k?,, = 3.5 X cm3 cm3 molecule-' s-I is shown molcculc-' s-I, and k , = 5 X in Figure 7 together with the decay profile which would be expected in N?. Again the data is consistent with the reaction proceeding via two channels one of which leads to OH regeneration, and again the rate-determining step in the regeneration process proceeds with a bimolecular rate coefficient of (5 f 4) x IO-'6 cm3 molecule-' s-I. OD regeneration was observed for reactions 3 and 4 in the prcscncc of 0,. Two obscrved decay profiles along with simulated profiles and decays whieh would be expected based on rates measured in N 2 are shown in Figure 7; the rate constants used in the simulation are included in Table VI. CH3CN + OD A (3c)
CH3CN + OD
+ OD
CD,CN
+ OD
+ 0,
+ 0,
-
-
OD
OD
-
B(0D)
CD3CN B(0D)
B
-+
(34
A
(4c)
B
(44
+ products
+ products
(9) (10)
Reactions 9 and IO are the analogous OD regeneration steps. Reaction 4 shows similar behavior to rcactions I and 2; however, for reaction 3 the degree of OD regeneration is less pronounced although still evident when compared with the decay profile expected from the rate constant measured in N2. Ovcrall, thc abovc simulations indicatc that the observed OH(D) regeneration profiles obtained under a variety of conditions can be simulated by using a simple four-step kinetic scheme. One channel leads to OH(D) regcncration and the initial stcp in the rcgencration process appears to involve an addition step to produce
1238 The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 an OH(D)/acetonitrile adduct. The adduct, or one of its decomposition products, reacts with 0, to regenerate OH(D) at a rate of ( 5 f 4) X I 0-l6cm3 molecule-' s-I. A second channel or channels lcad to thc formation of products other than O H . By thcmsclvcs thc simulations give no added information on this channel sincc, in tcrms of the generated O H profiles, all of the possiblc mechanisms arc kinetically equivalent. The observed kinetic isotope effects and pressure dependencies, however, have considerable mechanistic implications for this channel and these are discussed below. Reversible Adduct Formation. Since the observed pressure dependence of reaction 2 and the observation of O H regeneration in reaction 2 and O D regeneration in reaction 3 provide strong evidence for an addition channel, we investigated the possibility that reversible adduct formation was occurring. Such behavior might manifest itself in several ways. I f the forward addition step is proceeding at a rate that is faster than the overall rate of reaction, but sufficiently slow that it is resolvable by the (=IO0 ns) time resolution of this experiment, then nonexponential decays would be observed. An initial fast decay into equilibrium would be followed by a slower decay due to chemical reaction. In cases in which the adduct reacts with oxygen, the presence of oxygen results in an increase in the observed rate of reaction and a tendency toward exponential decay with increasing [O,]. At the highest CH$N concentrations used, we saw no deviation from although we saw O H exponential behavior in N 2 buffer. In 02, regeneration, the initial slopes of the decays were consistent with the reaction rate measured by Nz. I f reversible adduct formation took place on a very fast time scale (i.e.?
