A Fourier transform infrared study of the kinetics ... - ACS Publications

J. Phys. Chem. 1983, 87, 2190-2193

2190

limation of 31.4 and 34.4 kcal/mol13 and an experimental activation energy of only 21.5 f 1.5 kcal/mol. This effect might conceivably arise due to the larger surface/volume ratios expected for the absorbed samples as opposed to a bulk crystal, although no systematic effect of sample size upon activation energy was noted in the present work. (13) S. Seki and K. Suzuki, Bull. Chem. SOC.Jpn., 26,63 (1953); R. S. Bradley and S. Cotson, J . Chem. SOC.,1684 (1953).

Acknowledgment. This research was carried out at Brookhaven National Laboratory under contract with the U.S. Department of Energy and supported by its Office of Basic Energy Sciences. Registry No. AcOCH3, 79-20-9; A~(Ala)~oMe, 30802-26-7; Ac(Ala),OMe,26910-17-8;Ac(Ala),OMe,30802-29-0;Ac(Ala)50Me, 85083-58-5;glycerol, 56-81-5; meso-erythritol,149-32-6; pentaerythritol, 115-77-5; D-mannose, 3458-28-4;D-glucose, 50-99-7; sucrose, 57-50-1;maltose, 69-79-4.

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A Fourier Transform Infrared Study of the Kinetics and Mechanism for the Reaction HO -t CH,OOH H. Nikl", P. D. Maker, C. M. Savage, and L. P. Breltenbach Research and Engineering Staff, Ford Motor Company, Dearborn, Michigan 48 12 1 (Received: October 14, 1982)

The analysis of products in the photolysis of mixtures containing C2H60N0,NO, and CH300H in ppm concentrations in 700 torr of O2-N2diluent by FT IR is consistent with a value of (kl,/klb) = 0.77 (Et20%)for the competitive H-abstraction channels HO + CH300H CHzOOH + H20 (la) and CH300 + H20 (lb). An absolute value of (kla + k l b ) = 1.0 X lo-'' cm3molecule-' s-' was derived from the decay rates of CH300H relative to those of C2H4 and CH3CHO.

-

-

Introduction Methyl hydroperoxide (CH,OOH) is a potentially important atmospheric constituent formed in the photooxidation of CHI and other organic compounds, i.e., CH300 + HOO CH300H + Oz.' The assessment of its subsequent fate is, therefore, of considerable interest. However, no quantitative kinetic or mechanistic studies of the gas-phase bimolecular reactions of CH300H have been made to date, primarily because of numerous experimental difficulties associated with sample preparation, gas handling, identification, and quantification of the CH,00H.2 In the present long-path FT IR work, these problems have been largely overcome and product studies have been made in the HO-radical initiated oxidation of CH300H. The major mechanistic features to be addressed are the relative importance of the two possible H-abstraction channels, in one instance from the CH, group and in the other from the HO group, i.e., reactions l a and Ib,

-

and the subsequent reactions of the ensuing radical CH,OOH, e.g., reaction 2, the relevant reactions of CH300 CHzOOH CHzO + HO (2) being reasonably well understood as discussed later. For comparative purposes, product studies were also carried out for the C1-atom initiated oxidation of CH,OOH to examine the relative importance of the competitive H-abstraction reactions 3a and 3b analogous to reactions

-

CH200H f

HCI

(3a) (3b)

l a and lb. According to an empirical kinetic correlation among related C1-atom reactions? reaction 3a is expected to be fast, -1 X 10-lo cm3 molecule-' s-', and dominant over reaction 3b. Interestingly, in our earlier semiquantitative study of the C1-CH300H reaction in the presence of NOz, a substantial yield of CH300NOz formed via CH,OO + NOz CH300NOZwas observed.2 There are two possible sources for the CH,OO radical in this system, i.e., reaction 3b and, alternatively, reaction 3a followed by reactions 2 and lb. The results obtained in the present study are consistent with the latter mechanism.

-

Experimental Section General features of the long-path FT IR method used were described p r e v i ~ u s l y . ~A~3-m ~ long Pyrex IR cell (180-m path length, multipassed, KBr windows, and internal mirrors) equipped with UV fluorescent lamps (GE F4OBLB) served as the photochemical reactor. Reference absorbance spectra for the reactants and products were generated and calibrated by using a 50-cm long quartz cell (1-m path length, double passed, KBr windows, and external mirrors), directly monitoring partial pressures of pure samples in the range of 0.1-1 torr. The interferometer was operated with either a KBr beam splitter-CuGe detector or a CaFz beam splittel-InSb detector. Spectra with peak-to-peak signal-to-noise ratios exceeding 1OO:l throughout the 600-4000-cm-' region at 1/ 16 cm-' theo-

~

(1) See, for example, H. Levy, Planet, Space Sci., 20, 919 (1972); P. J. Crutzen, Annu. Reu. Earth Planet. Sci., 7,443 (1979); 3. A. Logan, M. J. Prather, S. C. Wofsy, and M. B. McElroy, J. Geophys. Res., 86,7210 (1981), and references therein. (2) H. Niki, P. D. Maker, C. M. Savage, and L. P. Breitenbach, Chem. Phys. Lett., 55, 289 (1978). 0022-3654183J2087-2 190$0 1.5010

(3) J. V. Michael, D. F. Nava, W. A. Payne, and L. J. Stief, Chem. Phys. Lett., 77, 110 (1981). (4) P. D. Maker, H. Niki, C. M. Savage, and L. P. Breitenbach, ACS Symp. Ser., 94, 161 (1979). ( 5 ) H. Niki. P. D. Maker, C. M. Savaae. and L. P. Breitenbach, J.Mol. Struct. 59, 1 (1980).

0 1983 American Chemical Society

IR Study of the Reaction HO

+ CH,OOH

The Journal of Physicel Chemistry, Vol. 87, No. 12, 1983 2191

CH, 00 H SPECTRUM

2963.8 1320.7 1332.4

/


I

(6)t = I m i n

C H 3 0 0 H (-1.98) I

(C) RESIDUAL ( X 5 ) X C = 1.87

I

I

3000

2000

I

IO00

3000

I / x (cm" Flgure 1.

Absorbance spectrum of CH,OOH in the frequency region = 0.95 torr; L = 1 m; and log ( I o / I )=

000-3700 cm-'; [CH,OOH] 0.44 at 2963.8 cm-'.

+

+

-

+

+

-

-

(6) H. Niki, P. D. Maker, C. M. Savage, and L. P. Breitenbach, Chem. Phys. Lett., 80, 499 (1981). (7) See, for example, J. C . Calvert and J. N. Pitts, Jr., 'Photochemistry", Wiley, New York, 1969; J. T. Herron, R. E. Huie, and J. A. Hodgeson, Eds., Natl. Bur. Stand., Spec. Publ., No. 557 (1979); R. Atkineon and A. C. Lloyd, J.Phys. Chem. Ref. Data, in press. (8) A. Baeyer and V. Villiger, Berichte, 34, 738 (1901).

1000

I / X (cm-') Figure 2. Spectral data from the Cl-atom initiated oxidation of CH,OOH.

[CH300H]o and [Cl,l0

retical resolution were obtained with a 1.5-min data acquisition period (16 signal averaging scans). Data were accumulated, processed, and archieved with a dedicated minicomputer having a 256K byte memory, two 13M byte disks, and a magnetic tape drive, using extensive in-house software. Transformation time for the 64K-point interferograms was 30 s. Several hundreds of carefully calibrated reference spectra are available for use in the quantitative (&5%)measurement of reactant and product concentrations. HO radicals were generated photochemically from RONO (R = CH, or C2Hs) as in a previous study of the reaction HO C2H4,6i.e., RONO + hv (300-400 nm) RO NO; RO + O2 HOz + carbonyl product; and HOP NO HO NO2. The kinetics and mechanisms of all the relevant elementary reactions involved in this process are sufficiently well-known for the present purpose.' Methyl hydroperoxide was prepared from (CH3)2S04and H202in solution at 20 "C according to procedures described by Baeyer and Villiger.8 The reaction products were extracted with diethyl ether, and subsequently, the CH,OOH was extracted from the ether with 40% KOH. The resulting solution was further extracted with diethyl ether after neutralizing it with HC1, and was dried with anhydrous Na2S04. The CH3OOH was fractionated by trap-to-trap distillation at -55 "C in vacuo. Impurities in the CH300H samples thus prepared, including the solvent ether and previously observed by-products,2 were below the detection limit of the FT IR analysis, i.e., I 1% , In

2000

N

20 ppm in 700 torr of air.

freshly cleaned and preconditioned IR cells, the CH,OOH samples were stable in the dark and decayed at most 5% in 30 min. Figure 1 shows an IR absorbance spectrum of pure CH300H in the frequency region 600-3700 cm-'. The sharp spectral features in the C-H stretch (-3000 cm-l) and the C-H bend (-1350 cm-') bands, as seen in the scale-expanded display, were particularly useful for the quantitative analysis of CH300H. For instance, the height of the peak at 2963.8 cm-' corresponded to an absorption cross section t of 3.2 X em2. In order to examine the accuracy of this absorption cross section, the carbon balance was measured in the C1-atom initiated oxidation of CH,OOH, as illustrated in Figure 2. Namely, spectra A and B in Figure 2 were recorded before and after 1 min of UV irradiation of a mixture initially containing C1, and CH300H at about 20 ppm each in 700 torr of air (1ppm = 0.76 mtorr). Without the added Cl,, the CH300H alone did not undergo observable decay under these conditions. Thus, the photolysis of C12 caused the conversion of CH,OOH (1.98 ppm) to product CH20 (1.32 ppm), CO (0.34 ppm), and COP(0.03 ppm). Figure 2C results from the spectral subtraction of CH300H and CH20 from Figure 2B followed by scale-expansion (X5). This spectrum reveals the formation of CH30H (0.15 ppm) and HCOOH (0.03 ppm) as additional products. Thus, in terms of carbon balance, the observed products accounted for 94% of the CH300h reacted, a result judged to be within the overall precision of the analysis. Mechanistic discussion of these data are presented later. Results and Discussion The rate constant ~ C H ~ O Ofor H the reaction HO + CH3OOH was determined by the competitive decay method using the integrated relative rate eq I, where (ref), refers

2192

The Journal of Physical Chemistry, Vol. 87, No. 12, 1983

--

- In [(CH,OOH),/(CH~OOH)OI

~CH~OOH

In [(ref),/(reflol

kref

to the concentration of a reference compound at t min of UV irradiation. C2H4 served as a convenient reference compound, especially since the relative decay rates of CH300H and C2H4 were found to be comparable. CH,CHO also could be used as the reference when HO radicals were generated from C H 3 0 N 0 but not from C2H50NOthe carbonyl product from the latter nitrite is CH3CH0. Throughout this experiment, relatively short irradiation times and low reactant conversionswere used together with the addition of excess NO to the reactant mixtures, all as precautionary measures to prevent either the formation or consumption of the CH,OOH and the reference compound by other possible reactions, e.g., CH300H + hv products and C H 3 0 0 + H 0 2 CH300H.7 Results are summarized in Table I. Values of k c ~ ~ o o H / k C ~inH the , last column of this table give an average of 1.20 f 0.09(6), and similarly, k C H 3 0 0 H / k C H 3 C H 0 = 0.68 f 0.07(6). An absolute value of k C H W H = LO x IO-'' cm3molecule-' s-' can be derived from tkese relative rate constants in combination with the literature values of ~ c =~ 8.0H X ~ and k C H C H O = 1.5 X lo-" cm3 molecule-' s -'.*''This value of ~ C H O O Hcorresponds to the overall removal of the CH,OdH by HO, Le., ha+ k l b . It is interesting to note that a previous estimate for k C H 3 W H , (= kq,),based on an assumed similarity to the HO + H202 and HO + C2H6 reactions is a factor of 20 smaller than the present value.13 Clearly, much more extensive kinetic data are needed for the establishment of such empirical relationships. In attempts to determine the mechanism(s) for the reaction HO + CH300H, product studies were made in the photolysis of mixtures containing C2H50N0, NO, and C H 3 0 0 H at approximately 10 ppm each in 700 torr of 02-N2diluent. The O2 pressures were varied from 20 to 140 torr. The major carbon-containing products detected in the early stages of the photolysis were CH20 and C H 3 0 N 0 as well as the expected photooxidation product of the reactant C2H50N0,Le., CH3CH0. CO and CH30NO2 were also observed as minor products. The elementary reactions responsible for the formation of C H 3 0 N 0 and CH30N02can be readily identified as the addition reactions of CH30 radical with NO and with NO2, respectively. CH,O + NO (+M) C H 3 0 N 0 (+MI (4)

-

-


(1)

CH30

+ NO'

-

(+M)

+

CH3ON02 (+M)

(5)

The CH30 radical is, in turn, produced from the C H 3 0 0 radical, i.e., reaction l b followed by CH,OO + NO CH30 + N02.7The CH,O can also react with O2in competition with NO depending on the relative concentrations of O2 and NO used. -+

-

CH30 + O2 CH20 + H 0 2 (6) Literature values for k6/k4 derived at 25 "C and 1 atm of air appear to be consistent at approximately 5 X 10-5.14-17

Niki et at.

Thus, the relative rates of reactions 4 and 6, i.e., k6[02]/ k4[NO], can vary from K0.2 to -1 with [NO] = 10 ppm and the O2 pressure ranging from 20 to 140 torr. In any case, the combined yield of CH,ONO and CH30N02represents a lower limit for the formation of the CH,OO radical by reaction lb. On the other hand, an upper limit for reaction l a followed by reaction 2 can be derived from the combined yield of CH20 and its secondary product CO. The pertinent experimental results are summarized in Table 11. The percentage yields of CHzOOH and C H 3 0 0 derived from A([CH20] + [C0])/A[CH300H] and A([CH30NO] + [CH20NOz])/A[CH,00H] are listed in the last two columns of this table. These values do not exhibit strong O2 dependence partly because of the associated experimental uncertainties. In particular, values for A[CH300H] were difficult to measure accurately due to the small fractional changes employed. As a result, some of the total percentage yields determined for the carboncontaining products are seen to exceed 100%. Nevertheless, the formation of CH20 is undoubtedly substantial even at O2 pressures low enough to greatly reduce the contribution of reaction 6. The most likely precursor for this CH20 is the CH200H formed by reaction la. Thus, these results suggest the significant occurrence of both reactions l a and l b as the primary processes. The ratio kla/klb can be derived from the relative product yield R = A([CH20] + [CO])/A([CH,ONO] + [CH30N02])at zero O2 pressure. Values of R exhibit slight but definite O2 dependence and range from 1.4 a t 140 torr of O2 to 0.8 at 20 torr of 02. The value of R extrapolated to zero O2 pressure falls within 0.66 to 0.88. On this basis, kla/klb was estimated to be 0.77 with a conservative error limit of f20%. Note that this value of kla/k1b derived from R does not suffer from the uncertainties associated with the measurements of A[CH300H]. In contrast to the HO-CH,OOH reaction discussed above, the C1-CH300H reaction appears to undergo predominantly an H abstraction from the CH3group, reaction 3a, rather than from the HO group, reaction 3b. Product distributions in the C1-atom initiated oxidation of CH,OOH indicate that the CHzOOH radical formed in reaction 3a dissociates exclusively to CH20 and HO via reaction 2. Thus, the HO-CH300H reaction should play a key role in the secondary reactions of the C1-CH300H reaction. To illustrate, Figure 2, described in the preceding section, represents typical spectral data from the photolysis of C12-CH300Hmixtures in 700 torr of air. Perhaps the most significant observation to be made here is the presence of CH30H as a minor product with a yield equal to 7.6% of the CH300H consumed. This CH,OH can be taken as an unique product arising from the self-reaction of C H 3 0 0 radicals1*J9

2CH,OO

-

CHBOH+ CH20

(74

Notably, reaction 7a is accompanied by another reaction channel leading to the formation of two CH,O radi~als'~J'

2CH300

2CH30 + O2

(7b)

Under the present experimental conditions, the ensuing (9) A. C. Lloyd, K. R. Darnall, A. M. Winer, and J. N. Pitts, Jr., J. Phys. Chem., 80, 789 (1979). (10)R. Atkinson, R. A. Perry, and J. N. Pitta, Jr., J. Chem. Phys., 66, 1197 (1977). (11) R. Atkinson and J. N. Pitts, Jr., J. Chem. Phys., 68,3581 (1978). (12) H. Niki, P. D. Maker, C. M. Savage, and L. P. Breitenbach, J. Phys. Chem., 82, 132 (1978). (13) NASA Panel for Data Evaluation, "Chemical Kinetic and Photochemical Data for Use in Stratospheric Modelling", Jet Propulsion Laboratory, Pasadena, CA, 1979, JPL Publication No. 79-27. (14) H. A. Wiebe, A. Villa, T.M. Hellman, and J. Heicklen, J . Am. Chem. Soc., 95, 7 (1973). ~~

(15) R. A. Cox, R. G . Dement, S. V. Kearsey, L. Batt, and K. G. Patrick, J. Photochem., 13, 149 (1980). (16) L. Batt, R. T.Milne, and R. D. McCulloch, Int. J . Chem. Kinet., 9, 567 (1977). (17) D. Gutman, N. Sanders, and J. E. Butler, J. Phys. Chem., 86,66 (1982). (18) C. S. Kan, J. G . Calvert, and J. H. Shaw, J.Phys. Chem., 84,3411 (1980). (19) H. Niki, P. D. Maker, C. M. Savage, and L. P. Breitenbach, J . Phys. Chem. 85, 877 (1981).

+ CH,OOH

I R Study of the Reaction HO

The Journal of Physical Chemistry, Vol. 87, No. 12, 1983 2193

TABLE I : Kinetic Data on HO Reactions: hCH,OOH/hC,H, and ~ C H , O O H / ~ C H , C H O concny ppm

conversn,b %

(RONO)

(NO)

(CH,OOH)

13 16 17

10 12 10

10 17

10 18 11 11

11 9

19 10

10 11

10

(ref)

9

9

CH,OOH

ref

hCH,OOH/hef

R = CH,;ref = C,H, 11 10.5 8 19.5 5 20.3

16.7 16.1 16.7

1.26 1.12 1.24

R = C,H,; ref = C,H, 9 17.9 13 20.7 10 19.1 18 18.3

15.2 20.0 16.0 14.4

1.20 1.04 1.21 1.20

1.20 2 0.09


16 17 18

10 10

12 15

11

8

R = CH,; ref = CH,CHO 11 18.0 9 13.5 17 13.0

0.76 0.63 0.64

21.8 20.6 19.5

0.68

" Diluent air a t 700 torr.

Irradiation time was < 4 min for all runs.

TABLE 11: Calculated Yields of CH,OOH and CH,OO Radicals in the Reaction HO concn, ppm (C,H,ONO) 10 13 11 11 11

* 0.07

(NO) 11

11 5 11 11

+ CH,OOH yield"

-A(CH,OOH), (CH,OOH) 14 10 13 11 11

02,b

torr

140 140 140 40 20

%

CH,OOH

CH,OO

13 14 15 5 5

76 56 73 71 49

53 42 67 60

" Yield (CH,OOH) = A [(CH,O) + (CO)l/A(CH,OOH); yield (CH,OO) = A[(CH,ONO) + (CH,ONO,)]/a(CH,OOH). Total pressure of 0, + N, = 700 torr. CH30 reacts with O2via reaction 6 to yield CH20 and HOP The H02, in turn, reacts with the CH300 to regenerate the reactant CH300H.18J9 The overall product ratio [CHOH]/[CH20]in reactions 7a and 7b was previously determined to be 0.48 f 0.03(6).19 Thus, up to 22% of the CH300H consumed in Figure 2 can be attributed to the self-reaction of the CH300 radicals. If all the CH,OO radicals produced were assumed to originate from reaction 3b, the remaining CH20 and its secondary product CO must be formed in a series of reactions triggered by reaction 3a, i.e., reactions 2, la, and lb. Note that reaction l b is an additional source of CH300 radicals which should be taken into account for the observed yield of CH30H. Thus, the contribution of reaction 3b should be significantly smaller than the 22% figure stated above. In fact, the product distribution in Figure 2 is entirely consistent with a mechanism in which all the CH300 radicals giving rise to the CH30H are exclusively formed in the HO-CH300H reaction. Also, as reported earlier,2CH300N02 was formed upon addition of small amounts of NO2to this system. This observation clearly serves as more direct evidence for the formation of the CH300 in the C1-atom initiated reaction of CH300H. However, the results were inconclusive concerning the mechanism of its formation. A large excess [NO,] over [CH300H]would be required to effectively suppress the HO-CH300H reaction by the HO-NO, reaction, since the rate constants for these two HO reactions are nearly the same at 700 torr of air.20

Finally, the relative decay rates of CH300H and C2H6were measured in the photolysis of C12-CH300H-C2& mixtures in 700 torr of air. However, a reliable value of the rate constant for the C1-CH300H reaction could not be derived from these data due to mechanistic complications arising from the secondary reactions. Namely, in addition to an excess consumption of CH300H by the secondary HO radicals, the HO, formed via C1 + C2H6 C2H5+ HC1 followed by C2H6 O2 H 0 2 + CH3CH0 reacted with the CH300 radical to regenerate the reactant CH300H. These as well as other mechanistic problems mentioned throughout this paper should be addressed further in future studies of the HO and C1 reactions of CH300H. In conclusion, the present results point to the potentially significant role of the HO-CH300H reaction in the atmosphere. Namely, the HO radicals can serve as a controlling factor for the atmospheric lifetime of CH300H. Furthermore, this reaction provides a mechanism for the production of CH20 and eventually CO that does not involve CH30 radicals via reaction 6.' Registry No. HO, 3352-57-6; CH300H, 3031-73-0;CzH,ONO,

+

-

-

109-95-5;CH,ONO,624-91-9; NO, 10102-43-9;CH20,50-00-0;CO, 630-08-0; CH&HO, 75-07-0; CHBON02,598-58-3;C1,22537-15-1; CH300NOz, 42829-59-4; NOz, 10102-44-0; CH,OO, 2143-58-0; CHZOOH, 74087-87-9. (20)See, for example, J. S. Robertahaw and I. N. M. Smith, J.Phys. Chem., 86,785 (1982).

A Fourier transform infrared study of the kinetics ... - ACS Publications

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