Rate constants for the reaction of atomic chlorine with dimethyl peroxide from 220 to 330 K

CHEXIIC4L

RATE CONSTANTS

FOR THE REACTION

WITH DIMETHYL

PEROXIDE

IO-‘* cm3 molccuk-’

<;’

FROM

1 January

1951

OF ATOMIC CHLORINE

220 TO 330 K

I Introduction

The reacttons of Cl atoms wtil metholy contaming molecules have been recently conwlered m this Iaborntory [2,3] larly vation tlons

[I] Our Interest m stratospheric ozone depletion by anthropogemc release of chemicals, porucuthe ~lllolo~uoromethartes_ has supplied the mottfor tfus work. As pomted out earher, the ceacuwolved 111the oxldcttlon of CH, _1 e the redcttons with Cl. OH. and O(l D). creare methyl radlcaIs wluch are then scavenged by the Iu& concentra~ons of02 present tn the stratosphere The methylperoxy radicals can then react m n number of ways [3] ultimately gwng H2C0 which subsequently IS degraded both by reaction [4,5] and photodecomposttron to CO [3]. Thus, for a complete understandmg of stratosphertc chemtstry, the carbon cycle and all of the molecules formed m this cycle, have to be consIdered. We have noted earlier also that the H atom abstractlon reactions by Cl atoms with organic molecules (other than CH,) are quite fast [ 1,4]. In modehng calculattons the remporaty reservoir property of the product molecule, WI, is well known and therefore, these

* Vartmg Professor 0fCherntstrq , Cathohc Unwerslry of kmericrt, Washmgton, D C 20064, USA ** AdJunct Professor of Chennstry, Cathohc Untverszty of AmenC3, ~V~~~~ton~D C 20064, USA I10

PHYSICS LKI-l-LRS

reactions offer dddltlonal pathways for HCI productIon. Their speclfIc mlportance depends on wluch organic molecules have the largest concentrations, and this question is only Just now bemg addressed experimentally by m situ and remote measurements. However. oneduncnslonal photochermcal-chemlcai hnettc models do suggest srgntficnnt concentrations, particxdarly of H,CO and CH,OOH From our prevrous work [I ,4] we have developed a simple but adequate theoretical method to allow pre-

dlctlons of rate constants to within a factor of 2 for thus type of reactton If the carbon-hydrogen boad strength IS less than z 93 kcal mole-l, the CI reactton rate constant w11irange from (0 5-2.0) X 10wro cm3 molecuie-’ s-l Any methoxy contammg moiecule, mcluding those of presumed unportance m the stratospheric methane degradatron scheme, will satisfy this crlterlon In order to add Further credence to thisgenera1 predlctlve method, we have elected in GUSwork to study the reaction of Cl atoms 1~1th “‘dlmethoxy” or dunethyl peroxide, Cl+ CH,OOCH, + HCI f CH,OOCN,.

(1)

No earher determmatlons have been performed on reaetlon (l), however our predictive scheme suggests that the rate constant will be temperature mdependent and w*Lllbe witin a factor of two of 1 X lo-10 cm3 molecule-1 s-1 _

Volume 77, number L 2

CHtXlICAL

PHYSICS LETTERS

~~p~~rne~ta~

The flash photolysis-resonance fluorescence techmque (FP-4.F) [6,7] with a newly improved reactlon tell [8], has been prevmusly described C1 atoms were generated by flash photolysts of Ccl, The photolysls wavelengths were those determmed by the absorption coefficient of CCI, above the wavelength cut-off of the LIF wuldow on the flashlamp @ > 105 nm) Chlorme atom resonance rachatlon was produced by a mixture of 0.1% Cl, in hehum flowing through a microwave dlschargeiamp at a pressure of 0.5 Torr The resonantly scattered fluorescent photons were detected at right angles to both the photolyzmg and resonance l&t sources through 760 Torr of dry N, and ;I BaF2 filter A multIchannel analyzer operatmg in the multiscaling mode recorded the signal from the photomulrtpher m repetItIve flashes, the srgnal being directly proportmnal to chlorrne atom concentration. Dunethyl pero,ude was prepared by methylatmn of hydrogen peromde with dunethyl sulfate accordmg to the method described by Hanst and Calvert [9] The condensable products from the preparation renctron were outgassed at 77 K and then carefully bulb-to-bulb dIstilled The middle fraction dlstdhng at 173 K was collected, and a sample was analyzed with a mass spectrometer This analysis mdlcated that the product has n purtty of 95% dune&y1 pero_ude with the prmc~pnl Impurity bemg CO, Stnce this mass spectrum, to our knowledge, has never been previously reported, the present results are given m table 1 Due to the explosive nature of the compound, we eIecred to use thus product wtthout further purification. A substantial number of prehrmnary expenments were performed to assess the long-term stab&ty of CH,OOCH, smce thus compound IS known to be subJect to violent decompositton. partlculariy If pressure shocked. We found that the compound was stable IR the gas phase and could be stored as a gas for mdefinite periods as shown by the invariance of its mass spectrum Reaction expenrnents were also performed as a functlon of flow rate through the metal dehvery hnes mto the teflon coated fluorescence cell. We found at any temperature that decay constants at lower flow rates were generally lower than at higher flow rates. Systematic mvestigatlons yrelded the v&e of the flow rate above whch no further perturbahon occurred for a partlcular conditmn. At T> 350 K, a IunIting fiow rate

Tabfc 1 70 eV mass

I January 1981

spectrum 0fCH~00Cf-l~ Ion

Rclattvc abundance

12

c+

2

13

CHf

3

I.4

aI;

1s

CH;

7 63

I6

0+

2

28

CO*

6

7-9

HCd

100

30

H2CO+

20

31

CHxO+

98

32

0:

6

43

CH30CH:

6

-16

CH&CH;

3

61

CH302CH;

-I

62 (p)

CFi30,CHf

54

63

J

1

condition could not be attamed, presumably due to heterogeneous pero.ude decomponhon. All of the rate measurmg experunents were then obtamed under these tigher flow condmons Lastly, experunents were performed as a function of flash energy No energy dependence m the decay constants was noted. Thus, except for loss from dlffuslon, the temporal behavior of Cl atoms IS comp1eteIy determmecl by reactIon (I), and secondary reactions are not perturbmg m the present case. The observed pseudo-first-order decay constants are composed of contnbutmns from Cl atom reactIon and d~fusIona1 10s~.The dlffuslona~ correctton from the reactlon zone vrewed by the photomultlpher was derer-

mined independently under exactly Identical expenmental conditions as those fork, determmatlon except that [CH,OOCH,] = 0 At 220 K, kd never exceeded 5% of the observed pseudo-fist-order decay constant. The correction at 298 and 330 K was generally less than 10% except for the experrments at 10 Torr for which kd ranged from 20-30%. Argon ~a~eson, 99_999~%) and helium (AIXO, 99.9999%) were used wrthout further pur&catron. B&b-to-bulb dIstd.latlon at 195 R was used to further purify chlorme tmatheson, 99.5%), the middle tfurd bemg retained. Purlficatlon of CC& (Matheson, Cole111

Volume

77, number

1

CHEhlICAL

PHYSICS

3 Results Absolute rate constants were measured under pseudofust-order condttlons, and table 2 hsts the condlrrons of temperature, [CH300CH, ] _and total pressure. Smce Cl atom decays are ryp~caIly measured with [Cl] o 2 I X IOtt cm-x, [Cl], 4 [CHj00CH3j0 for ali reported experunents Then Cl atom attenuation IS given by

Table 7Rate data for the ilnk T Ki)

t-20

‘ohs

fluorescence

[CH300CH3] (mTorr)

wxl I (mTorr)

stud)

= k,

[CH, OOCH,

(3)

] -c k, .

ic, IS the fist-order rate constant for dlffusronal loss of Cl atoms. The hnear least-squares values of kobs and k, can then be used to unambiguously obtam kI through eq. (3), provided that, as shown by the prehmmary experiments dlscussed above, no secondary reactions arlse

)

photo15 sls-resonance

of the renctlon

IArl

Flab

Uorr)

(J)

energy

Cl + CHsOOCH3 N ‘)

b) kb, (lO-‘O cm3 molecule’

0 397

7-5 0

50

7-8.56

140

0 154

125

50

1-8,56

131-+001

% 0 03

135-c-007

0 215

17 5

35

28,56

0 108

88

35

3-8 56

1 34 + 0 09

0 17-3

10 0

20

18-95

I 34 i 0 04

006l

50

XI

7-8,56

l35iOOl

0061

50

10

28-95

f 28 z 0 04 1 34 ? 0 05 c)

298

0445

75 0

50

26-144

3

120%0

0 315

75 0

50

36,56

2

119+001

0 307

7-5 0

50

36 - 144

3

134fO13

0 154

12 5

5@

30-56

3

119+009

0312

17 5

3.5

44-144

3

119~005

0 178

100

20

42-144

3

1 16 *_ 0 10

0 089

50

10

32-144

3

108+0

13

118k!l

10’)

ii 330

0 37.5

1.5 0

50

36-81

3

1 10 + 0 09

17 5

35

36-81

3

105

0 150

100

20

36-95

3

1.04 + 0 07

0 075

50

if!

68.95

2

116+004

i-i-

112

1.5

0 262

N = number of experunents b, In l@’ cm3 molecuid’ S’ ‘) Average at that temperature.

, error

IS one smndxd

devmuon.

1981

where [Cl] IS proportlonai to fluorescent counts. Typical examples of hnear plots of lnfCi] agamst tune are given M fig. 1 and demonstrate that eq. (2) is strictly obeyed KI these experiments. The decay constants derived from such plots are related to the b~o~ecular rate constant, II-, , through

man, and Bell, 99%) was achieved by bulb-to-bulb dlstillatlon at 233 K with coilectlon of the middle thud

ln[Cl) = --kobst + !n[Cl]

1 January

LETTERS

f 0 06

1.08 + 0.07

c)


Volume

77, number

1

CHEMICAL

PHYSICS

LETTERS

1 January

1981

4. Discussion

0

I

2

3

4

5

6

TIME/msec

Fig 1 Typical fnst-order decay plots for the Cl + CH300CH3 reactlon at 7-10 K 0 PT = 7_0 Torr.PCCb = 10 mTorr, = 0 17-3 mTorr, flash ener,y = 56 J, and klst = kHzOOCH, 730 -C 3 ~1 p fi = 35 Torr, PCCb = 17 5 mTorr, PCH300CH, = 0 715 mTorr, flash energy = 56 J andklst = 1758 f 36 ct o PT = 50 Torr. PcC4 = 25 mTorr, PCH,OO~-H~ = 0.307 mTorr, flash energ = 56 J and k Ist = 1839 c 5 I ~1

whrch can perturb Cl temporal profiles and no loss of

CH300CHJ

occurs.

The rate data at 220,295, and 330 K are summarrzed III table 2. The rndrcated errors are at the one standard deviatron level as determmed from repeated determinations. It IS probable that there is a shght negatrve temperature dependence, being descnbable by either k, = (7.37 + 1.68) X 10-l 1 exp(132 t 58/T) cm3 molecule-l s-r (2~ error) or k, = (2.07 f 1.36) X 10-g Tqo 51s-u ol) cm3 Ku 51 molecule-l s-1 (lo error). We also pomt out that the results m table 2, when considered at the 20 or 95% confidence level,

nearly overlap. Thus, a wnple average of the results IS also an adequate representation: k, = (1.20 f 0.26) X lO-‘O cm3 molecule-1 s-l (20 error) for 220 < T <

330 K.

The fast reaction between Cl atoms and CH,OOCH, , kl = (1.20 + 0.26) X lo-r0 cm3 molecule-l s-1 for 220 < T < 330 K, is, by now, not suprrsing. Even though thrs 1s the first determination of the absolute rate constant for this reactron, an empirical correlatron wtth related Cl atom abstraction reactrons suggests the strong possrbtlrty of a temperature-mdependent rate constant approachmg the colhs~on rate constant In earher work from this laboratory, values of (1.76 f 0.15) X 10-10, (0.63 + 0.07) X lo-lo, and (0.75 f 0 05) X lo-10 cm3 molecule- 1 s-l (200 G TG 500 K) were reported for the reactrons of Cl atoms with dimethylether [l] , methanol [l] , and formaldehyde [4] , respectrvely. Other Cl atom rate constants have recently been reported [lo] whrch also indrcate values wrthm a factor of about two of lo-10 cm3 molecule-l s-1. In all of these cases erther temperature independence or a shght negatrve temperature coefficient 1s indicated. It 1s probable that the temperature dependences should not be interpreted in terms of energy barrrers but rather as thermalrzed effects of the dynamrcs of the interactrons. Thus, there is very hkely no potential barrier between reactants and the mtermediate cotiguratrons. The same is not true for the Cl atom with CHq reactron where a define barrier 1s mdrcated [3,1 l] . The Cl + C7Hg reactron appears to be an mtermedrate case, as described by Johnston [12], and a small barrier (a 120-l 50 cal mole-l) has been measured [10,13] _ Johnston has rationalized the Cl + RH abstraction reactions in terms of the bond energy-bond order (BEBO) method [ 121. In an earher paper [l] we detailed the theoretIcal imphcatrons and suggested that d the C-H bond strength was less than that m C,H, ( i.e., * 98 kcal mole-*), then no T dependence should be observed. It is hkely that the C-H bond strength in CH300CH3 will be the same as in dimethylether or methanol (a 94-95 kcal mole-l). Therefore, we expect little or no T dependence in the present case. A theoretical estimate of the absolute value for the present rate constant can be made with the kinehc theory of gases. We have earber suggested that hvo factors totally determine the absolute rate constant: (1) the mteraction rate constant, and (2) the steric factor for the interaction [l] _ The interaction rate constant can be estimated from transport property data [14] . A recent value for ‘&e diffusion coefficient of Cl atoms 113

Volume

77, number

CHE\fICAL

1

PHYSICS

tn Ar at 2cf5 K [ 151 ytelds Lenndrd-Jones parameters for Cl a tams of act = 2 34 ,% and eci,‘k = 119 K Unfortunately no Lennard-Jones parameters have ever been measured t-or CHJOOCH~, however, mspecrion of vaiues for motecuies of strmlar comple\tty [ 141 sug= 5 00 a and ectt300cR3/k = gests that ocH,OOCCI, 410 K wtfl be representattve The Lennard-Jones rate constant can then be calculated through ZLI = o; $2’(2,2)

(SakT,‘~) ‘j’_

14)

the combmmg rules dre u i 7 = +(G i i- CJT)and El 2 = @IQ ‘I2 5’?,*(2.2) IS a rab\ylated mtegr~l and IS d funct!orl of 2”” =x-r/e t 7 [i4]. We fmd that Z,, = (3 06 + 0 02) X lO-1u cm3 molecule-1 s-l for the temperature range 220-330 K_ Since Lennard-Jones prtrameters for CH,OOCH, are only estmlates, thts value IS probably accur;Ite to *iS% The sterlc factor for the present symmetrical mterxt~on might naively be considered to be umty smce tmpmgemenr of Cl on e&her end of the molecule can lead to reaction However, ‘;i e have cnrher suggested, m the case of C1+ CH,OCH, [l ] _ timt ;t colhslon wtth the central 0 atom may be elsst~c, and therefore the narve stew factor would be 1-13 With the same crrtermn for the present case we would suggest a stew factor of I/2 teadmg to an es&mate of I 53 X IOMto cm3 molecule-1 s-l The vaiue obtamed. (1 26 + 0 26) X lo-i0 cm3 molecule-1 s-l, IS tn excellent agreement with thrs estmlate Thus study and the earher one frcm tius laboratory [I J have demonstrated that Cl atoms react rapidly with methoxy contammg molecules. Thus, we expect that me reactron, Cl T CH30,I-I --f HCI + CH202H, ~111 have a large rate constant (w i‘O_‘6 cm3 molecule-1 s-l ) Constdermg the much siower rate For Cl f H20~ 1161, we suggest that abstractron of rhe peroudlc hydrogen, and thus CH,O, f HCI productron, wrll be small by comparison. In the stratosphenc methane dcgradatton scheme, CH30,H for-matron from CH,O, f HO, has been suggested as a product at certam altttudes [ 171, and thus compound may butld to apprecrable concentratrons smce the solar photoche~c~ Ioss rate 1s Iow. We have earlrer discussed the possrble stratospherrc lmplrcstions [1,4]. The present results confirm our claim that Cl

LETTERS

atom with methoxy m the stratosphere,

containing molecules, wll be raprd.

if they exrst

We express our gratitude to Dr D lndrrtz for advice on proparatton and handhng of dunethyl peroxrde Jvhl acknowledges support by NASA under grant NSG 5 I73 to Cathohc Untversity of Amerrca

where

114

References [I]

J V Xltchnet D I- Narct, W A Paynednd Chcln Phys 70 (1979) 3652

[ 71 F S Rowtad [5]

L J Stlef, J

Jnd hf J Ifoltnci Rev Ceophys 13 (197.5) 1 R D ~ludson .md c: I Reed, cds , NASA Rcf

Space Phys Pub1 (1979)

1039 (41 J V M~rh.xcl, D r Nwa, W A Payne and L J Sttef, J Chem Phys 70 (1979) 1147 [Zt L J Stxcf J V hf!chrtei,W A Payne D I- Nd\a. D \I Butler and R S Stotnrshl, Geophys Rcs Letters 5 (1978) [6]

819 R B Rlcmm 4900

and L J Stref, J Chem

Ph>s 61 (197-t)

(71 J V XIIchael and J f-i Lee, J Phys Chrm 8s (1979) 10 [Sj L J St&. D I- Naw 1%’X Payne and J V Mitchncl, J Chem Ph) s , to be publtshed

[9] P L Hnnst and J G Cailvert, J Phys Chem 63 (1959) IO4 [ 101 R S Lews, S P Sander, S 6. Wagner and R-T Watson, m preparation [ I 11 W B De&fore et n1, JPL PubhcaUon 79-27 (1979), A R Ravtshnnkara. and P H Wme, J Chem Phys 72 (1980) 25, and references thcrem [ 121 H S Johnston, Gas-phase reaction rate theory (The Ronslld Press, New York, 1964). [ 131 R G. Mannmgand hi J Kurylo, J. Phys Chem 81 (1977) 291 [ 141 J 0 HusclrfeIdcr, C F Curtiss and R B Bud, hfolccular theory of gases and hqutds (W&y. New York, 1964) f 151 H S Judelkls and bl Wun, J. Chem Phys 68 (1978) 4123. [ 161 J V. hlxhael, D A Whytock, J-H Lee, W A Payne and L J Stlef, J. Chem Phys 67 (1977) 3533. L F Keyser, J Phys Chem 84 (1980) 11 1171 R D Rundel, D hl Butler and R S. Stolarskl, J Gcophys Res 83 (1978) 3063