7043
J. Phys. Chem. 1992, 96,7043-7048 Scaiano, J. C.; Abuin, E. B.; Stewart, L. C. J . Am. Chem. SOC.1982, 104, 5673-5679. (b) Evans. C.: Innold. K. U.: Scaiano. J. C. J. Phvs. Chem. 1988. " 92, 1257-1262 (32) From the expression l/T1= 1/T2 = (bgX2+ bg; + 6g!2)/97,, a rotational correlation time i n= 2 X IO-" s and bg, __ = 0.2 yields a lifetime of 45 ps (see ref 30). (33) Symons, M. C. R. J. Chem. Soc., Perkin Trans. 2 1974, 1618. (34) Akasaka, K. J . Chem. Phys. 1965, 43, 1182. (35) Ferris, K. F.; Franz, J. A.; Sosa, C. P.; Bartlett, R. J. Chem. Phys. Lett. 1991, 185, 251-255. (36) Wallace, W. L.; Van Duyne, R. P.; Lewis, F. D. 1976,98,5319-5326. (37) Carmichael, I.; Hug, G. L. J. Phys. Chem. Ref.Data 1986, I S , 1-250. (38) Franz, J. A.; Alnajjar, M. S.;Barrows, R. D.; Kaisaki, D. L.; Camaioni, D. M.; Suleman, N. K. J. Org. Chem. 1986, 51, 1446-1456. (39) (a) Janousek, B. K.; Reed, K. J.; Brauman, J. I. J . Am. Chem. SOC. 1980, 102, 3125-3129. (b) Sugeta, H. Spectrochim. Acta 1975, 31A, 1729-1737. (c) May, I. W.; Pace, E. L. Spectrochim. Acta 1968, 24A, 1605-1615. (d) Kojima, T. J . Phys. Chem. Jpn. 1960, 15, 1284-1290. ( e ) Benson, S.W. Chem. Rev. 1978, 78, 23-35. (40) Estimation of the activation barrier for hydrogen transfer using the BEBO method as modified by Gilliomsbfor CH3S' + CH3SH yields a reference S-H bond strength of EM = 91.83 kcal/mol, a sum of stretched S-H bond energies = -94.82 kcal/mol, and a triplet S-S interaction term of 6.32
kcal/mol, for a barrier of 3.3 kcal/mol. BEBO predicts an S-H transition bond distance of 1.529 A and an S-S distance of 3.06 A, in good agreement with ab initio geometries. (41) The Zavitsas method*' employs an assigned three-center resonance term, 10.6 kcal/mol, a reference bond potential of Ebnd= 91.83 kcal/mol, an energy term for S-H-S bonding of -87.33 kcal/mol, and a triplet S-S repulsion term of 15.45 kcal/mol, yielding a barrier of 9.3 kcal/mol. Characteristic of this method, the S-H transition bond distance (1.47 A) and the S-S separation (2.94 A) are significantly shorter than predicted by BEBO and ab initio calculations. The equibonding barrier is decreased to 7.7 kcal/mol if the CH,S-H BDE is reduced to 86 kcal/mol, and the CH3S-SCH3 BDE is correspondingly reduced to 62.8 kcal/mol. (42) AMl/UHF calculations were performed with the MOPAC v. 6.1 set of programs: Stewart, J. J. P.QCPE 566. Dewar, M. J. S.;Zoebisch, E. F.; Healy, E. F.; Stewart, J. J. P. J. Am. Chem. SOC.1985, 107, 3902-3909. (43) For example, we have measured an equilibrium constant, K = 132 (298 K), for the reaction OctS' OctSSOct s OctS(.)(SOct),, using kinetic laserflash spectroscopy, by observing the rate of addition of OctS' to 1,ldiphenylethylene as a function of [OctSSOct]. Thus, AGOa8 = -2.9 kcal/mol for sulfuranyl radical formation, corresponding to a bond strength of about 8 kcal/mol. This radical may be more stable than R2S(')SR, R = alkyl, due to the availability of 3p, electrons from substitution of an RS group for an alkyl group: unpublished work of M. S. Alnajjar and J. A. Franz.
+
A Kinetic and Mechanistic Study of the Reaction of Neopentylperoxy Radicals with H02 David M. Rowley? Robert Lesclaux, Phillip D. Lightfoot,* Laboratoire de Photophysique et Photochimie MolZculaire, UniversitZ de Bordeaux I, 33405 Talence Cedex, France
Kevin Hughes, S,*hoolof Chemistry, University of Leeds, k e d s LS2 9JT, U.K.
Michael D. Hurley, Sara Rudy, and Timothy J. Wallington* Research Stafl, SRL-E3083, Ford Motor Company, Dearborn, Michigan 481 21 -2053 (Received: April 30, 1992)
-
The kinetics and mechanism of the reaction between neopentylperoxy radicals and hydroperoxy radicals: neo-C5Hl1O2+ H 0 2 neo-C5Hl100H+ O2(l), have been studied between 248 and 365 K, using the flash photolysis/time-resolvedUV absorption and the blacklamp photolysis/FTIR product analysis techniques. The rate constant for reaction 1, kl, is significantly exp((1380 larger than that for smaller primary alkylperoxy radicals, with kl/cm3 molecule-' s-l = (1.43 f 0.46) X f 100) K/O, giving kl/cm3 molecule-' s-I = (1.5 f 0.4) X 10-" at 298 K. Consideration of possible random and systematic errors results in absolute uncertainties of 25% in kl over the experimental temperature range. Product analysis studies at 296 f 2 K show that the yield of hydroperoxide from reaction 1 is (92 2)%; possible systematic errors may add an additional 15% uncertainty. It is concluded that the rate constant for the generic reaction R 0 2 + H02used in the modeling of non-methane hydrocarbon atmospheric chemistry should be increased. The rate constant for the reaction of chlorine atoms with neopentane, C1 + neo-C5Hlz HCI + neo-C5HlI (3), was also measured relative to that for the reaction of chlorine atoms with methanol, C1+ CH30H HCl + CH20H ( 5 ) . Within experimental error, k3 is independent of temperature over the range 248-366 K, with k3/cm3 molecule-' s-l = (1.16 f 0.05) X using kJ/cm3 molecule-' s-I = 5.7 X lo-''. The error in k3 does not include uncertainties in k5. Errors are 1u.
*
--
Introduction
Although the reactions of alkylperoxy radicals (ROJ with the hydroperoxy radical (HOz) are important pathways in the tropospheric degradation of hydrocarbons under low NO, conditions, the information available on their rate constants is still rather sparse.' As a consequence, modeling studies have used generic rate constants based on the reasonably well-studied CH3O2 + H02 and C2H5O2 + H02reactions? The currently recommended room temperature (298 K) rate constants for these reactions are very similar, 4.8 X and 5.8 X cm3 molecule-] s-',~respectively, and extrapolation to other alkylperoxy radicals would thus *Authors to whom correspondence should be addressed. 'Current address: Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 lEW, U.K.
seem reasonable. However, we have recently studied the reactions of the cyclopentylperoxy and cyclohexylperoxy radicals with H02 and have shown that their room temperature rate constants are much larger: 1.8 X lo-" and 1.7 X lo-" cm3 molecule-' s-I, respectively! In the absence of other direct studies of ROz + H02 reactions, it was not clear whether these differences arise from the greater size of the alkyl groups in the cyclic radicals or from the fact that they are based on secondary alkyl radicals. The present study of the reaction of the neopentylperoxy radical, (CH3)3CCH20z,with HOZ neo-C5Hl1O2+ H 0 2
-
n e 0 - C ~ H ~ ~ 0+0 O2 H
(1)
was undertaken to address this question and as such represents the first study of the reaction of a large (>c2) Primary a h ' b r o x y radical with H02.
0022-3654/92/2096-7043$03.00/0 0 1992 American Chemical Society
7044 The Journal of Physical Chemistry, Vol. 96, No. 17, 195'2
Experimental Section The FT'IR product analysis (Ford) and flash photolysis/ time-resolved UV absorption (Bordeaux) experiments used in this study have both been described in detail p r e v i o ~ s l y ~and - ~ are therefore discussed only briefly here. For both sets of experiments, neopentylperoxy and hydroperoxy radicals were generated from the photolysis of C12/neo-CSH12/CH30H/02/N2 mixtures, according to the reaction scheme (2-6) below
+ + + + + +
hv (A
> 280 nm) + C12
C1+ neo-CSH12 neo-C5Hl1 O2
M
HCl
2C1
(2)
neo-CSHl1
(3)
neo-C5Hl102+ M
C1+ CH30H
HCl
CH20H
HCHO
0 2
CH20H
(4)
(5)
HOz
The concentrations of neopentane, methanol, and oxygen were chosen such that stoichiometric conversion of chlorine atoms to peroxy radicals was very rapid (99% and were used without further purification. Experiments were performed using three different O2partial pressures; 15, 150, or 700 Torr with N2used as appropriate to bring the total pressure up to 700 Torr. Unless otherwise stated, errors are 1IJ and represent experimental uncertainty only.
Results After the rapid production of the peroxy radicals, as described above, the following reactions can take place:
-
neo-C5Hl102+ H 0 2
-
Rowley et al.
neo-CSHl100H+ O2
+
2neo-C5HlIO2 2neo-C5HI10 O2 2neo-CSHI1OH+ t-C4H9CH0+ O2
(1)
(74 (7b)
We have recently studied the UV absorption spectrum and self-reaction kinetics of neopentylperoxy radicals in our laboratories.",l2 The neopentyl radicals produced via channel 7a decompose rapidly to give tert-butyl radicals which are converted to tert-butylperoxy radicals in the presence of oxygen
-+ -
neo-C5Hl10+ M t-C,H,
+0 2
t-C4H9 + HCHO
M
t-C&902
+M
+M
(9) (10)
Although the self-reaction of tert-butylperoxy radicals and their reaction with neopentylperoxy radicals are very slow compared to reaction 7," it is possible that the reaction with H 0 2 t-C4H902 H 0 2 products (11)
+
-
is fast and care must be taken to ensure that the secondary chemistry arising from reaction 7a does not complicate the measurement of k l . This point is returned to below. Flash Photolysis Study of Reaction 1. The neopentylperoxy radical displays a UV absorption spectrum typical of alkylperoxy radicals, peaking near 250 nm.I1J2 The H 0 2 UV spectrum is similarly broad and structureless but is displaced to shorter wavelength^.^ The differences between the spectra of the neopentylperoxy and hydroperoxy radicals enable the absolute initial concentrations of both radicals to be determined from pairs of experiments at different wavelengths, as was done in earlier experiments on other R 0 2 + H02 reaction^.^,'^ Values for the cross-reaction rate constant, k l , were extracted from pairs of flash photolysis experiments that differed only in the analysis wavelength. In general, the wavelengths chosen were 220 nm, where u(H0,) > u(neo-C5Hl102),and 260 nm, where u(H02) rl
-
0 -
8
4
4.01
I/ 12
U
-
\
Y
10-12
5 4 1000 K / T Figure 2. Arrhenius plot for kl. The line represents a best weighted fit to k = A exp(B/T). 2
16
1 Time/ ms
Figure 1. Typical decay traces and fits, 365 K (a) monitoring wavelength 220 nm; (b) monitoring wavelength 250 nm. [H02],.,, = 4.91 X 10') molecules ~ m - [ne~-C.,H,,0~],,~ ~; = 4.90 X I O l 3 molecules cm-'; kl = 6.07 X cm3molecule-' s-I. The individual contribution of each significantly absorbing species is shown.
3
of the radicals were removed during the experimental dead time at 365 K, resulting in a much shorter extrapolation to t = 0 and consequently less experimental scatter. A weighted Arrhenius fit to the data in Table I1 was therefore used, with the individual data points weighted by a factor of l / a 2 , giving kl/cm3 molecule-' s-l = (1.43 f 0.46)
no. of determinations" 6
*
365
E
m
X
exp((1380 f 100) K/T)
The corresponding plot is shown in Figure 2. Consideration of the covariance between the Arrhenius parameters gives average experimental uncertainties of 17% at 248 K, which drop to 8% at 365 K,reflecting the increased experimental scatter at the lower temperatures. As kl was extracted from a relatively complex chemical system, it is essential to quantify its sensitivity to the analysis parameters. Sensitivity parameters were derived as described p r e v i o ~ s l y . ~ * l ~ The value of k l l , which has not been measured experimentally, was generally taken to be equal to that of k l . Varying kll between zero and 1 X cm3molecule-' s-I changed the derived value of kl by less than 10%. Detailed numerical simulation of the complete reaction system, shown in Table 111, using the Acuchem kinetic modeling program19 demonstrated that, owing to the fact that kl is approximately an order of magnitude greater than k7 over the experimental temperature range, less than 2% of the H02 initially produced reacts with t-C4H902under all conditions. Only the values (both relative and absolute) of the cross sections of H02 and neo-C5HI'02had a significant effect on k l . Using average uncertainties of 10% for kll/u(H02),13 10% for klo/a(neoC5Hl1O2),"and 10%for the relative absorption cross sections of H 0 2 and neo-C5Hl1O2,and including additional absolute uncertainties of 10% and 15% for the effect of reaction 11 and uncertainty in the absolute radical cross sections, respectively, the additional uncertainty in kl is 19%. Combining this uncertainty with the experimental scatter results in total estimated absolute uncertainties in kl of 25% or below over the entire experimental temperature range. FIlR Study of Reaction 1. In the continuous photolysis reactor there is a competition between reactions 1 and 7 for the available neopentylperoxy radicals. Fortunately, the rate constant for reaction 1 is over an order of magnitude larger than for reaction 7 a t room temperature. Thus, by suitable choice of initial conditions it is possible to conduct experiments in which the neopentylperoxy radicals are lost by reaction with H 0 2 rather than by self-reaction. To choose the initial conditions, the chemistry within the chamber was simulated using the Acuchemlg kinetic modeling program and the chemical mechanism given in Table 111. In selecting the optimal initial [neo-C5H12]/[CH30H] concentration ratio there is a compromise between working under conditions where the largest fraction of neopentylperoxy radicals react with H 0 2 and the need to measure appreciable neopentane
7046 The Journal of Physical Chemistry, Vol. 96, No. 17, 1992
Rowley et al.
1
TABLE III: Reaction Mechanism for Detailed Simulations at Room Temperature
-- --
reaction (2) CI2 + hu 2CI (3) CI + (CH3)4C HCI + NPT” (4) NPT + 02 NPTOz (5) C1 + CH3OH CH2OH + HC1 (6) CH2OH + 0 2 --c HCHO + H02 (7a) 2NPT02 2NPTO + 02 (7b) stable products ( 9 ) NPTO + M t-C4H9+ O2 + M (10) t-CdH9 + 02 t-C4H902 (1) NPTOl + H02 products (8) H02 + H02 products 4
-4
NPT = (CH3)$CH2.
rate constant/cm’ molecule-I s-’
ref
L
+0
1.2 X
1.6 X 5.7 X IO-” 9.1 X 0.68 X 0.32 X IO-’’ fast 2.3
1.6 2.8
X X X
lo-”
this work 9 3
E
v
6
I
I
/e
3
11, 12 11, 12 11
18 this work 13
* Pseudo-second-order rate constant.
D a A : BEFORE IRRADIATION
-
I
?--
W C 0
+
c 1.6 u a
/I
-
0 0
>-
f, 1 . 2
:I E
C : RESIDUAL
W
I0
800
: 2.2-DIMETHYL
900
PROPANAL
1000
E : ETHYLHYDROPEROXIDE
3500
-
-
E
0.8
-=
0.0 0.0
I
3600
WAVENUMBER ( c m - ’ )
Figure 3. Infrared spectra taken before (A) and after (B) a 70-s irra-
diation of a mixture of 59.2 mTorr of neopentane, 298 mTorr of methanol, and 110 mTorr of chlorine in 700 Torr of air. Spectrum C shows the residual after subtraction of neopentane and methanol features from (B). Spectrum D is a reference spectrum of 2,2-dimethylpropanaland spectrum E is a reference spectrum of ethyl hydroperoxide. consumption for a given chlorine atom flux. In these experiments the concentration ratios of [neeC5H12]/[CH30H]were 0.20-0.51, under such conditions numerical integration of the reaction system shows that 84-97% of the loss of neopentylperoxy radicals will be via reaction 1. Figure 3 shows typical spectra taken before (A) and after (B) irradiation of a mixture of 59.2 mTorr of neopentane, 298 mTorr of methanol, and 110 mTorr of chlorine in 700 Torr of air. Spectrum C is the residual obtained by subtracting features attributable to neopentane and CH30H from spectrum B. Spectra 2D and 2E are reference spectra of 10.2 mTorr of 2,2-dimethylpropanal and 31 mTorr of ethyl hydroperoxide, respectively. The loss of neopentane is 6.5 mTorr (1 1% of the initial concentration). The yield of hydroperoxide is 6.5 mTorr; the upper limit for the yield of 2,2-dimethylpropanal is 0.6 mTorr. The total yield of hydroperoxide in the system was derived by comparing the integrated absorption features in the region 3575-3625 cm-’in our product spectrum with that in our reference spectrum for C2HSOOH.’ If alkyl hydroperoxides other than the neopentyl hydroperoxide are formed, then the measured yield represents an upper limit. One potential complication is the formation of HOCH200Hvia the reaction of HOz radicals with formaldehyde, a secondary product in many peroxy radical sys-
0.4
0.6
0.8
1 .o
0.2
Figure 5. Plot of the loss of neopentane as a function of the loss of methanol following the photolysis of neopentane/CH30H/C12mixtures in air (circles) or N2 (triangles) diluent. The solid line represents a linear least-squares fit.
tems. Tests for this complication have been performed previously and formation of HOCH200H was shown to be of negligible importance under the conditions of the present study.4 Thus, the observed product features a t around 3600 cm-’ following the irradiation of neopentane/Clz/CH3OH/O2 mixtures are attributed to the formation of neopentyl hydroperoxide. Figure 4 shows a plot of the observed increase in neopentyl hydroperoxide as a function of the loss of neopentane, corrected for that fraction of neopentylperoxy radicals which react via the self-reaction (3-14%, see above). As seen in Figure 4, within experimental errors, the yield of the hydroperoxide is independent of the oxygen partial pressure over the range 15-700 Torr. A linear least-squares fit to the data shown in Figure 4 gives a yield of (92 f 2)% for neopentyl hydroperoxide in reaction 1, where the quoted errors are l a and refer to statistical uncertainties in the fit. We estimate that potential systematic errors could add an additional 15% uncertainty (comprised of a 5% uncertainty in the absorption cross section of ethyl hydroperoxide and a 10% uncertainty in the use of this value to quantify neopentyl hydroperoxide. No evidence for the formation of 2,2-dimethylpropanal was observed; the upper limit for the yield of this species is 10%. Reaction 3: CI n e ~ - C & ~ .The constant ratio k 3 / k 5is a critical parameter in the product analysis experiments as, together with the concentration ratio [neo-C5H12]/[CH30H], it determines the relative production rates of neopentylperoxy and HOz radicals. Relative techniques were used to measure k 3 / k 5as part of the present work, using both the FTIR/continuous photolysis and the
+
Reaction of Neopentylperoxy Radicals with H 0 2
TABLE Iv: E x ~ r i ~ V~ ~ tWOdf kr T/K 296
k3/10-lo cm3 kdks molecule-l s-l a FTIR Experiments 2.03 f 0.04 1.16 f 0.02
Flash Photolysis Experiments 248 273 294 326 348 366
average
2.15 f 0.26 2.30 f 0.18 1.92 f 0.15 2.09 i 0.1 1 1.62 A 0.15 2.03 f 0.12 2.02 0.09
1.22 f 0.15 1.31 0.11 1.09 f 0.09 1.19 f 0.06 0.92 f 0.09 1.16 f 0.07 1.15 f 0.05
aUsing ks/cm3 molecule-l s-I = 5.7 X lo-".' flash photolysis/UV absorption apparatus. The experimental technique for determining relative rate constants by FTIR spectrometry has been described previou~ly.~ Figure 5 shows the Observed decay of neopentane as a function of that of methanol following the irradiation of neopentane/ methanol/C12 mixtures in 700 Torr of either air or N 2 diluent at 296 K. Linear least-squares analysis of the data in Figure 5 gives k3/k, = 2.03 f 0.04. Combining this with the " m e n d e d value of k, (=5.7 x lo-" cm3 molecule-' s - ' ) ~ gives k3/cm3 molecule-l s-' = (1.16 f 0.02) X lo-''. As the analysis of the flash photolysis experiments extracts values of the initial H 0 2 and neo-C5Hl102concentrations for each pair of experiments, the ratios k 3 / k 5can be calculated at each temperature, using the known concentrations of methanol and neopentane. The results are summarized in Table IV, where a value of ks/cm3 molecule-I s-I = 5.7 X lo-", independent of temperature, was again used.3 Within experimental error, k! is independent of temperature over the range 248-366 K, with k3/cm3 molecule-] s-' = (1.15 f 0.05) X lo-''. The errors in our measurements of k3 do not include uncertainties in k,.
Discussion Rate coasbnt k3: CI + ne!iK$IlP The present measurements of k3,using two different techniques, are in excellent agreement. The room temperature rate constants agree well with that obtained cm3 molecule-' s-l)* by Atkinson and Aschmann (1.10 X and the negligible temperature dependence found in the flash photolysis experiments confirms the early findings of Knox and Nelson.20 The good agreement between the present values of k3 derived from the flash photolysis experiments and the literature values gives us further confidence in our radical generation system and data analysis used in the flash photolysis experiments. Mechanism of Reaction 1: neo-C&102 + H02. In the reactions of alkylperoxy radicals with H02, two reaction channels have been postulated, both involving the reversible formation of a tetraoxide association complex. One involves transfer of a hydrogen atom via a four-membered cyclic transition state to form an alkyl hydroperoxide and oxygen, and the other involves the formation of a six-membered transition state to yield a carbonyl compound, water, and oxygen. In the present work, the yield of hydroperoxide is indistinguishable from 100% with an upper limit of 10% for the carbonyl product. Our product data suggest that the reaction of the neopentylperoxy radical with H 0 2 proceeds via one channel to give the hydroperoxide. This conclusion is consistent with our previous observations that the yields of ~yclopentyl,~ and cyclohexy14 hymethyl,21metl~yl-d,,~~ droperoxides from the reactions of the corresponding alkylperoxy radicals with H 0 2 are, within the experimental errors, unity. At the present time, we believe that reactions of the general type R 0 2 + H 0 2 , (where R 0 2 represents an unsubstituted alkylperoxy radical) in computer models of atmospheric chemistry should use a single reaction channel leading to ROOH and 02. Rate Constant k,: neo-C$I1102+ H02. The present work is the first determination of k,;no direct comparison with literature values is thus possible. We are, however, able to compare kl with rate data for other R 0 2 + H 0 2 reactions, given in Table V. As
The Journal of Physical Chemistry, Vol. 96, No. 17, 1992 7041
TABLE V Commriron of RO, + HO, Rate C ~ ~ t . l l t s k,,nwa EIRb ref RO, ____.A' -800 3 CH302 4.8 0.33 5.8 0.65 -650 3. C~HGO, cH~C@)O~ 13 0.43 -1040 24 HOCH102 12 0.0056 -2300 25, 26 HOCH2CH202 10 27, 28 c-CsHg02 18 0.21 -1320 4 c-C6H1102 17 0.26 -1250 4 neo-C5H1102 15 0.14 -1380 this work 'Units of 10-l' cm3 molecule-l s-I. bunits of K. ~~~
can be seen from the table, the dependence of the rate constant on temperature, where measured, is negative; this suggests that the mechanisms for such reactions are analogous, with the initial reversible formation of the corresponding tetraoxide association complex. The room temperature value of kl is comparable in magnitude to the rate constants for the reactions of cyclopentylperoxy and cyclohexylperoxy radicals with H02.4 These reactions have rate constants considerably greater than those for the reactions of methylperoxy and ethylperoxy radicals with H02. As explained in the introduction, the present study was partly undertaken to determine whether this increase in rate constant could be attributed to the fact that the cyclopentyl and cyclohexyl radicals are secondary alkyl radicals, or whether there was a general trend of increased room temperature rate constant with increasing radical size. As the neopentyl radical is a primary radical, the present work would tend to support the latter possibility, although there is no obvious reason why this should be the case. The possibility of incomplete thermalization of the tetraoxide intermediate in the case of the smaller radicals can be discarded as, unlike for the H 0 2 self-reaction, no significant dependence of any of the measured R 0 2 + H 0 2 rate constants on pressure has been observed. Similarly, the room temperature value of the rate constant for the HOCH202+ H 0 2 reaction is faster than that for the C2HSO2 H 0 2 reaction. The absolute values of the rate constants for these reactions will of course depend sensitively on the details of their potential energy surfaces, such as the dissociation energy of the tetraoxide and the difference in energy of the barriers to redissociation and isomerization; slight changes in these energies could significantly affect the relative importance of the redissociation and isomerization pathways for the tetraoxide and hence the overall rate constant. Further information, such as high-level ab initio calculations, would be mast useful in this context. Atmospheric Implications. The data presented here for reaction 1 have interesting ramifications concerning our understanding of the role of R 0 2 + H 0 2 reactions in atmospheric chemistry. As discussed previ~usly,~ in the absence of experimental data save that for reactions of CH3O2 and C2H502 with H 0 2 , atmospheric modelers have used values of (3-5) X cm3 molecule-' s-' for the generic reaction of R 0 2 + H02. In the light of our recent work, a value closer to 1.5 x cm3molecule-I s-I may be more appropriate for large peroxy radicals derived from non-methane hydrocarbons. In the atmosphere the reaction of R 0 2 with H 0 2 competes with the H 0 2 self-reaction for available H 0 2 radicals
+
+ HO2 HO2 + HO2
RO2
+
+ 02 H202 + 0 2
ROOH
+
(12)
(8)
Increasing the rate of reaction 12 leads to a decrease in the predicted H202levels and an increase in the predicted ROOH levels. Hydrogen peroxide is an important oxidant species involved in the aqueous phase oxidation of SO2into sulfuric acid.29 Alkyl hydroperoxides are also atmospheric oxidants although, in general, they are less effective than hydrogen peroxide because of their lower solubility, faster homogeneous gas phase removal, and lower reactivity in the aqueous phase. To provide a quantitative measure of the impact of the use of enhanced rate constants for reaction 12 on the predicted H 2 0 2formation, we have performed urban air shed simulations using the OZIPM-4 modePo with a modified LCC3I chemical reaction mechanism. The initial conditions em-
J . Phys. Chem. 1992,96, 7048-7051
7048
ployed in the model are described in detail elsewhere.32 Reaction 12 was included in the mechanism with a rate constant of either 3.0 X or 1.5 X lo-" cm3 molecule-l s-'. The operator ROz in the LCC chemical mechanism represents both large and small peroxy radicals. The use of kI2= 1.5 X lo-" cm3 molecule"' s-' thus probably represents an upper limit on the importance of reaction 12. The impact of k 1 2on the predicted H202 levels depends upon the initial concentration ratio of non-methane organic compounds (NMOC) to the sum of N O and NO2 (NO,) used in the simulation). For NMOC/NO, ratios of 10-20 (typical of the rural Eastern US) increasing k12over the range stated reduces the predicted H202formation by 25-30%. The increase in k12had little effect on the predicted ozone concentrations (
