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Rate constants for reactions of CH3O2 in the gas phase
VoIume 65. number 2
CHEBIICAL PHYSICS LJ3-TERS
15 Auqst 1979
RATE CONSTANTS FOR REACTIONS OF CH,O, IN THE GAS PHASE RA_ COX Enrironnrenml and dledical Sciences Dil ision. A.E_R_E_ Ilanr ell. Oxon. UK and GS. TYNDALL Departtnetlt of Ph> sical Chemisrr_b. Unit ersity of Cambridge, Cambridge. UK Receiwd 16 Ma> 1979 MoIecuhr modulation spectrometry has been employed to mesure the rate constants for the reactions ofCHxO2 with HOz. NO2 and X0. A negnthe tcmpcraturc dependence is reported for the rextion nirh HO_r. Product spectra are giwn for the reactions with lIOt and NO?_
I_ lntrcduction
2. Experimental
hfethyi peroxy radicals are important chemical intermediates in the atmospheric oxidation of methane and other hydrocarbons [I], and in low-temperature combustion processes_ TI;e only reaction of CH302 for which absolute measurzzents of the rate constant have been reported is the disproportionationjcombination process CH;O, + CH,O, + CH,OH + HCHO + 02,
(Ia)
+XH,O+O,. (lb) 3 There is now a considerable body of kinetic information concerning this reaction in the gas phase [Z---5] _ We report here measurements of the rdte constants for the reactions
The technique of molecular modulation spectromet;y @MS) was used to monitor CH,O, by its well characterised absorption band in the ultr&iolct [?,a,51 _ The spectrometer and ancillary apparatus have been described previously [6] _ CH;02 \\as generated by modulated photoIysis (0.5-25 Hz), at 350 I 50 nm, of mixtures of Cl, + CH, in the presence of 0,. and monitored at 250 nm with a beam of UV light from a deukrium lamp. The digital lock-in detection provided output in the foml of separate in-phase and in-quadrature COIIIponents of modulated absorption (P aud Q)_
3. Results
CH30, + HO2 + CH300H + 0 , . CHI,O, + NO, --z CH,OOXOI. CH,O, -I- NO --f CH;O + NO, -
1 Hz photolysis of static C12-CH4--OI mihrurcs at 1 atmosphere gave rise to strong modulated absorption signals in the region 280-210 nm, with a ms\imum at 235 urn. After allowing for absorption due to products [7] and for HO2 produced by the reaction CH,O + 0, - HCHO + HO,.
(5)
the spectrum agreed with that reported previously 357
\‘olume 65.
number 2
CHE.\lICAL PHYSICS LElTERS
[2,4,5] for the CH,02 radical. In our system the rate constant for the overall decay of CHi02 via reactions (la) and (lb) was (5.2 + 09) X IO-*’ cm3 moIecuIe-t s-t, in good agreement with the value reported by Parks [3]_ HO, ~3s thcn’generatcd by the addition of H-, to the mixtures until 1 iO1 nnd.CH502 were being produced photoIytic;llly at the same mte. At this point the ratio Q#’ was found to be equal for the two radicals. Since Q/p is ;I measure of the lifetime of a radical, it rnuid be ;latrmed that HO? and Cfi307 were disrtppcxing prcdominrrntly xc&ding to the reaction CH>O, t HO, + CEIjOOH + O,_
12)
The esperimcntal data were well-Gttcd by computed pseudo-secoud-order curves using 1, = 6-O X 10-t? cm: molecuIe-i s-t and o~~~~CH,O,) =3_9 X IO-ts cd_ The esperimental error is & 15%_
Experiments carried out at 274 and 338 K using the appropriate “equal lifetime” mixture gave lO”k, (cm3 molecule-1 s-t) = 8.5 -+ l-2 and 3.5 f 0.5 respect&ely, using the same cross section as at 298 K_ In all these experiments a strongly absorbing product was detected in the 210-280 nm range. This was thought to be methyl hydroperoxide and the spectrum, which is similar to H,O-,, is shown in fig I_ The absorption cross sections were calculated on the basis of computed yields of CH,OOH_
The reaction of CH30z with NO1 has recentiy been studied using Fourier transform infrared spectroscopy [S ] which revealed that a rather unstable product, pcroxymethy1 nitrate. is formed CH;O, •t- KO, - CH30,N0~.
t
7
15 Augast 1979
(3)
In order to reduce the possible effects of thermal decomposition of the product molecule, CHq-Cl?O1--h’Oa-Nz mixtures were photolysed at 275 K_ Photofysis of static mixtures containing 2 IOt3 molecules cm-3 NO, indicated the presence of a product exhibiting moderate absorption at 250 nm and an intense band below 225 nm (see fig- 1). This was ascribed to CH30,NOI_ Absorption cross sections were calculated assuming that CHjOz was lost enrireiy by rcaction (3). For kinetic measurements, mistures containing /I--2)X 10’3 mofecules cm-3 NO, were flowed through the ceil at 5-10 Torr total pressure_ After allowing for the absorption due to product, reduced first-order plots were made [9] _ The experimental data were wvcli-described by computed curves using k3 = 1.6 X IO-11 -3 moleculc~t s-l and azso(CH302) = 5.0 X IO-t8 cm?_ The estimated experimental error for k3 was F 20% , although a systematic error due to the decomposition of CIi30,NOI would lead to an underestimation of k3_ A similar set of experiments were conducted at a total pressure of 50 Torr, mainIy Ar @resent in the CH,-Ar mi\turc)_ These conditions yielded the rcstdts Bj = (I 2 2 0.3) X IO-*” cm3 moIecule-t s-l and u = 4.9 X I O-t* cm”. Thus k; appears to fall off sIightIy with pressure over the range 540-50 Torr, although this may just be an effect of the change of carrier gas from N, to Ar_
Volume 65. number 1
CHEMICAL PMYSICS LEL-I-ERS
3.3. Reaction of CH302 with NO previous studies have provided lower limits of IO-*2 cm3 molecule-1 s-t [S_lO] for the rate constant of this reaction, which appears to proceed entire!y by Oatom transfer: Cll;O, + NO + CH,O + NO,_
(4)
Flowing mixtures containing less than 10” molecule cm-S NO were photolysed and an averaging time of 10 min was used to enhance the low absorptions, which were typically (l-3) X 10-S (compared with a noise level of 5 X 10m6)_ Measured absorptions indicated a strongly absorbing product. identified as CHSONO. After subtraction of the components due to product the data were plotted in reduced form. as shown in fig. 2_ Computer simulation of the concentration gradients down the cell enabled the effective first-order rate constant X-L = k; [NO,] t k, [NO] to be calculated at each point_ This was then related to the overall in-phase and in-quadrature counts to give the curves shown Despite the comples mechanism. the curves are quite
15 Aueust 1929
sensitive to kz_ particularly at low frequency_ We estimate that the best fit to the observations is given by k4 = 6-S X IO-12 cm3 molecule-t s-1 (540 To:r_ 29s K). oxo(CH,02) = 42 X lo-L9 cm2_ with an overall experimentai error * 30%.
4_ Discussion The rate constant for CHjOZ -r HO, at 29s I( esceeds those for the respective disproportionations by factors of 10 and 3_ This fact. along with tha negative temperature dependence su,, O&Tests 3 coniplc~ niechanisni analogous to that recently proposed for the HO, + HO, reaction [ 11 ,I 2]_ The renuhs of this study are consistent with the conclusion that the dominant pathway in the reaction of CH;02 with NO1 is association to fortu CH,OzNO1_ -4 full investigation of the pressure and temperature dependence is required to characterise the reaction conipletely. The rate constant obtained for reaction (4) is comparable with the now well-established rate for HO7 +
15
/
/
/
, Lx 1o-‘2
/
6x
/
/I
“0
A /
x In --
El
70
x 0
P.
x c
e f=:
Q
iO-I2
1
5
~ 0L 01
-z-o 0
n
,
i
I
1 7 x lNOlo
molecule
cm
-3
5 x10
I
i0 -12
Fig. 7. PIor of reduced absorption componcnrs ;1r 250 nm arainst reduced photoI>sls period in photol,sls of Cl-02-_CH4-_NO mi\tures. Qxn points- in-phase; tilled points: inquadraturc. fi.9 = [NOlo = 9 6 and 4.9 X IO” molecules cmm3, respccti\zI~
359
Volume 65. number 2
CHEMICAL PHYSICS LE-l-FERS
NO [i2-14]_ Cur system is not ideal for such fast reactions, although the rate constant falls within fairiy well-defmed limits_ More work is required on this important reaction. especially with regard to its temperature dependenceThe results quoted are ali dependent on the particular v&e of u7&CH302) chosen to optimise the
computer fit_ Despite the range of values obtained for a, atI fall within the rrmce of previously reported reSuits [3.4] ~ (28-5.0) x IO-‘8 cnG_
Acknowfedgement This work was supported by the UK_ Department of the Environment_ The authors would Iike to thank the S-R-C_ for financia1 support for GS_T_ References f
11 J-a_ Kerr_ J.G- Calrert sod K-L_ Demerjiax. s 11972) 15z
360
Chcm_ Britain
15 August 1979
[21 DA. Parkes. D_U. Paul. C_P_ Quinn and R-C- Robson. Chcm_ Phys. Letters 23 < 1973) 425. [3] DA_ Parkes, Intern J.Chiem_ Kinetics9 (1977) 451. [41 CJ. Hochanadel. J.A. Chormley, J-W. Boyleand PJ. Ogrcn, J_ Phys- Chem. 81 (1977) 3. 151 C_ Anxstasi. LW_M_ Smith and D-A_ Parkes. J- Chem. SocFaraday I 74 (1978) 1693_ [6] R.A. Coa. R-G. Denvcnt. A.EJ_ E&eton and H.J. Reid. J. Cbem- Sot- Faraday (1979). to be published. [7] D.A. Parkes. DM_ Paul and C-P_ Quinn. J_ Chem_ Soti-xaday 177 (1976) 193% [S] II_ Niki. P-D. Maker. CM. Savage and L.P. Breitenbach. Chem- Phys_ Letters 61 (1979) lOO_ 191 R;\. Cos and R. Lewis. J. Chem- Sot. Faraday I. to be published_ [ 101 R-4- Cox, R-G. Derrvent. P.M. Holt and J.A. Kerr, J. Chcm. Sot. ramday I72 (1976) 3044. [ 111 RA Cox and J-P. Burro\\s. Kinetics and Mechanism of the Disproportionntion of HO2 in the Gas Phase. J. Phys. Chcm-, submitted for publication1171 J.P. Burrows. D-I. Cliff, G.W. Iiarris. B.A. Thrush and J-P-T- Wdkinson. Proc_ Roy_ Sot_ A (1979). to be published_ [ 131 CJ. Hooard and K_ Etenson. Geophys_ Res_ Letters 4 (1977) 43-z. 1141 Ming-Tzmn Lcu, J_ Chcm. Phgr 70 (1979) 1661.
CHEBIICAL PHYSICS LJ3-TERS
15 Auqst 1979
RATE CONSTANTS FOR REACTIONS OF CH,O, IN THE GAS PHASE RA_ COX Enrironnrenml and dledical Sciences Dil ision. A.E_R_E_ Ilanr ell. Oxon. UK and GS. TYNDALL Departtnetlt of Ph> sical Chemisrr_b. Unit ersity of Cambridge, Cambridge. UK Receiwd 16 Ma> 1979 MoIecuhr modulation spectrometry has been employed to mesure the rate constants for the reactions ofCHxO2 with HOz. NO2 and X0. A negnthe tcmpcraturc dependence is reported for the rextion nirh HO_r. Product spectra are giwn for the reactions with lIOt and NO?_
I_ lntrcduction
2. Experimental
hfethyi peroxy radicals are important chemical intermediates in the atmospheric oxidation of methane and other hydrocarbons [I], and in low-temperature combustion processes_ TI;e only reaction of CH302 for which absolute measurzzents of the rate constant have been reported is the disproportionationjcombination process CH;O, + CH,O, + CH,OH + HCHO + 02,
(Ia)
+XH,O+O,. (lb) 3 There is now a considerable body of kinetic information concerning this reaction in the gas phase [Z---5] _ We report here measurements of the rdte constants for the reactions
The technique of molecular modulation spectromet;y @MS) was used to monitor CH,O, by its well characterised absorption band in the ultr&iolct [?,a,51 _ The spectrometer and ancillary apparatus have been described previously [6] _ CH;02 \\as generated by modulated photoIysis (0.5-25 Hz), at 350 I 50 nm, of mixtures of Cl, + CH, in the presence of 0,. and monitored at 250 nm with a beam of UV light from a deukrium lamp. The digital lock-in detection provided output in the foml of separate in-phase and in-quadrature COIIIponents of modulated absorption (P aud Q)_
3. Results
CH30, + HO2 + CH300H + 0 , . CHI,O, + NO, --z CH,OOXOI. CH,O, -I- NO --f CH;O + NO, -
1 Hz photolysis of static C12-CH4--OI mihrurcs at 1 atmosphere gave rise to strong modulated absorption signals in the region 280-210 nm, with a ms\imum at 235 urn. After allowing for absorption due to products [7] and for HO2 produced by the reaction CH,O + 0, - HCHO + HO,.
(5)
the spectrum agreed with that reported previously 357
\‘olume 65.
number 2
CHE.\lICAL PHYSICS LElTERS
[2,4,5] for the CH,02 radical. In our system the rate constant for the overall decay of CHi02 via reactions (la) and (lb) was (5.2 + 09) X IO-*’ cm3 moIecuIe-t s-t, in good agreement with the value reported by Parks [3]_ HO, ~3s thcn’generatcd by the addition of H-, to the mixtures until 1 iO1 nnd.CH502 were being produced photoIytic;llly at the same mte. At this point the ratio Q#’ was found to be equal for the two radicals. Since Q/p is ;I measure of the lifetime of a radical, it rnuid be ;latrmed that HO? and Cfi307 were disrtppcxing prcdominrrntly xc&ding to the reaction CH>O, t HO, + CEIjOOH + O,_
12)
The esperimcntal data were well-Gttcd by computed pseudo-secoud-order curves using 1, = 6-O X 10-t? cm: molecuIe-i s-t and o~~~~CH,O,) =3_9 X IO-ts cd_ The esperimental error is & 15%_
Experiments carried out at 274 and 338 K using the appropriate “equal lifetime” mixture gave lO”k, (cm3 molecule-1 s-t) = 8.5 -+ l-2 and 3.5 f 0.5 respect&ely, using the same cross section as at 298 K_ In all these experiments a strongly absorbing product was detected in the 210-280 nm range. This was thought to be methyl hydroperoxide and the spectrum, which is similar to H,O-,, is shown in fig I_ The absorption cross sections were calculated on the basis of computed yields of CH,OOH_
The reaction of CH30z with NO1 has recentiy been studied using Fourier transform infrared spectroscopy [S ] which revealed that a rather unstable product, pcroxymethy1 nitrate. is formed CH;O, •t- KO, - CH30,N0~.
t
7
15 Augast 1979
(3)
In order to reduce the possible effects of thermal decomposition of the product molecule, CHq-Cl?O1--h’Oa-Nz mixtures were photolysed at 275 K_ Photofysis of static mixtures containing 2 IOt3 molecules cm-3 NO, indicated the presence of a product exhibiting moderate absorption at 250 nm and an intense band below 225 nm (see fig- 1). This was ascribed to CH30,NOI_ Absorption cross sections were calculated assuming that CHjOz was lost enrireiy by rcaction (3). For kinetic measurements, mistures containing /I--2)X 10’3 mofecules cm-3 NO, were flowed through the ceil at 5-10 Torr total pressure_ After allowing for the absorption due to product, reduced first-order plots were made [9] _ The experimental data were wvcli-described by computed curves using k3 = 1.6 X IO-11 -3 moleculc~t s-l and azso(CH302) = 5.0 X IO-t8 cm?_ The estimated experimental error for k3 was F 20% , although a systematic error due to the decomposition of CIi30,NOI would lead to an underestimation of k3_ A similar set of experiments were conducted at a total pressure of 50 Torr, mainIy Ar @resent in the CH,-Ar mi\turc)_ These conditions yielded the rcstdts Bj = (I 2 2 0.3) X IO-*” cm3 moIecule-t s-l and u = 4.9 X I O-t* cm”. Thus k; appears to fall off sIightIy with pressure over the range 540-50 Torr, although this may just be an effect of the change of carrier gas from N, to Ar_
Volume 65. number 1
CHEMICAL PMYSICS LEL-I-ERS
3.3. Reaction of CH302 with NO previous studies have provided lower limits of IO-*2 cm3 molecule-1 s-t [S_lO] for the rate constant of this reaction, which appears to proceed entire!y by Oatom transfer: Cll;O, + NO + CH,O + NO,_
(4)
Flowing mixtures containing less than 10” molecule cm-S NO were photolysed and an averaging time of 10 min was used to enhance the low absorptions, which were typically (l-3) X 10-S (compared with a noise level of 5 X 10m6)_ Measured absorptions indicated a strongly absorbing product. identified as CHSONO. After subtraction of the components due to product the data were plotted in reduced form. as shown in fig. 2_ Computer simulation of the concentration gradients down the cell enabled the effective first-order rate constant X-L = k; [NO,] t k, [NO] to be calculated at each point_ This was then related to the overall in-phase and in-quadrature counts to give the curves shown Despite the comples mechanism. the curves are quite
15 Aueust 1929
sensitive to kz_ particularly at low frequency_ We estimate that the best fit to the observations is given by k4 = 6-S X IO-12 cm3 molecule-t s-1 (540 To:r_ 29s K). oxo(CH,02) = 42 X lo-L9 cm2_ with an overall experimentai error * 30%.
4_ Discussion The rate constant for CHjOZ -r HO, at 29s I( esceeds those for the respective disproportionations by factors of 10 and 3_ This fact. along with tha negative temperature dependence su,, O&Tests 3 coniplc~ niechanisni analogous to that recently proposed for the HO, + HO, reaction [ 11 ,I 2]_ The renuhs of this study are consistent with the conclusion that the dominant pathway in the reaction of CH;02 with NO1 is association to fortu CH,OzNO1_ -4 full investigation of the pressure and temperature dependence is required to characterise the reaction conipletely. The rate constant obtained for reaction (4) is comparable with the now well-established rate for HO7 +
15
/
/
/
, Lx 1o-‘2
/
6x
/
/I
“0
A /
x In --
El
70
x 0
P.
x c
e f=:
Q
iO-I2
1
5
~ 0L 01
-z-o 0
n
,
i
I
1 7 x lNOlo
molecule
cm
-3
5 x10
I
i0 -12
Fig. 7. PIor of reduced absorption componcnrs ;1r 250 nm arainst reduced photoI>sls period in photol,sls of Cl-02-_CH4-_NO mi\tures. Qxn points- in-phase; tilled points: inquadraturc. fi.9 = [NOlo = 9 6 and 4.9 X IO” molecules cmm3, respccti\zI~
359
Volume 65. number 2
CHEMICAL PHYSICS LE-l-FERS
NO [i2-14]_ Cur system is not ideal for such fast reactions, although the rate constant falls within fairiy well-defmed limits_ More work is required on this important reaction. especially with regard to its temperature dependenceThe results quoted are ali dependent on the particular v&e of u7&CH302) chosen to optimise the
computer fit_ Despite the range of values obtained for a, atI fall within the rrmce of previously reported reSuits [3.4] ~ (28-5.0) x IO-‘8 cnG_
Acknowfedgement This work was supported by the UK_ Department of the Environment_ The authors would Iike to thank the S-R-C_ for financia1 support for GS_T_ References f
11 J-a_ Kerr_ J.G- Calrert sod K-L_ Demerjiax. s 11972) 15z
360
Chcm_ Britain
15 August 1979
[21 DA. Parkes. D_U. Paul. C_P_ Quinn and R-C- Robson. Chcm_ Phys. Letters 23 < 1973) 425. [3] DA_ Parkes, Intern J.Chiem_ Kinetics9 (1977) 451. [41 CJ. Hochanadel. J.A. Chormley, J-W. Boyleand PJ. Ogrcn, J_ Phys- Chem. 81 (1977) 3. 151 C_ Anxstasi. LW_M_ Smith and D-A_ Parkes. J- Chem. SocFaraday I 74 (1978) 1693_ [6] R.A. Coa. R-G. Denvcnt. A.EJ_ E&eton and H.J. Reid. J. Cbem- Sot- Faraday (1979). to be published. [7] D.A. Parkes. DM_ Paul and C-P_ Quinn. J_ Chem_ Soti-xaday 177 (1976) 193% [S] II_ Niki. P-D. Maker. CM. Savage and L.P. Breitenbach. Chem- Phys_ Letters 61 (1979) lOO_ 191 R;\. Cos and R. Lewis. J. Chem- Sot. Faraday I. to be published_ [ 101 R-4- Cox, R-G. Derrvent. P.M. Holt and J.A. Kerr, J. Chcm. Sot. ramday I72 (1976) 3044. [ 111 RA Cox and J-P. Burro\\s. Kinetics and Mechanism of the Disproportionntion of HO2 in the Gas Phase. J. Phys. Chcm-, submitted for publication1171 J.P. Burrows. D-I. Cliff, G.W. Iiarris. B.A. Thrush and J-P-T- Wdkinson. Proc_ Roy_ Sot_ A (1979). to be published_ [ 131 CJ. Hooard and K_ Etenson. Geophys_ Res_ Letters 4 (1977) 43-z. 1141 Ming-Tzmn Lcu, J_ Chcm. Phgr 70 (1979) 1661.