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The ultraviolet absorption spectra of the acetyl radical and the kinetics of the CH3 + CO reaction at room temperature
Volume 77, number 3
CHEMICAL
THE ULTRAVIOLET AND THE KINETICS
1 Fcbruxry
LETTERS
1981
ABSORPTION SPECTRA OF THE ACETYL RADICAL OF THE CH, + CO REACTION AT ROOM TEMPERATURE
DA PARKES Shell Research Lrd
Thowtot~
Received
1980,
19 August
PHYSICS
Rrsearclr
m linal
term
Corrre
Clresrrr
7 Novcmbcr
CHI
3SH
UK
1980
A contmuum-absorption spectrum bctwcen 700 and 240 nm IS asqned to the ace&I rodlcal Klnetlc measurcmcnts usnf moleculru modulntlon spectroscopy sho\r for the rcnctlon CH3 + CO (+kl) + CH,CO + 31 the rate constants JTC at 750 Torr The rate constant tar ncctj I comb](1 8 I 0 2) X IO-” cm3 molecule-’ s-’ at 100 Torr and (6 r 1) X lo-‘*
nation 2CH3CO - (CH3C0)2 IS (3 0 + 1 0) X IO-”
it 25°C
1. Introduction
The acetyl radical plays an unportant role m hydrocarbon oudatlon The competltlon between Its d~ssoclatlon and the reactlon with oxygen which leads ultimately to the degenerate branchmg agent acetyl hydroperoxlde provides the baas of many models [I]
of the acetyl radlcai obtamed usmg molecular moduiatlon spectrometry [4] and show that direct morutormg of the rad1ca.l can be used to measure the rates of acetyl radical reactlons directly The spectrum has also been obtamed recently usmg flash photolysls by Adachl et al. [S]
of cool flame behavlour III acetaldehyde, CH,CO
(+M) -+ CH, + CO (+M),
(1)
CH3C0
+ 0,
(2)
CH,C002 CH,COO,H
The
+ CH3COOZ,
+ RH + CH,C002H
+ R
(3)
-+ CH, + CO? + OH’.
The rate of addltlon
and Its mverse reaction times [2,3] by product
(4)
of Cd to CH,.
CH, + CO (+M) - CH,CO
2. Experimental
(+M),
(5)
(l), have been measured several analysts usually the yield of
apparatus
and
experimental
method
are de-
scribed m detad elsewhere [4] The method uses perIodIcally mterrupted photolysls (X = 300%400 nm or 354 nm, where stated) to produce reactive mtermediates whose concentrations are modulated and are hence detectable by I&t absorption using averaghe m the rage mg techru ues even when the absorptlons IS measured both of 1 m 10 ‘J to 1 m IO3 Absorption III phase and m quadrature with the photolytrc !Ighht and the ratlo of these measurements may be used to obtam kmetlc mformatlon
acetone 1s compared with the yield of ethane m the photolysls of azomethane III the presence of CO, and kg and kl are deduced usutg the wellestabhshed rate constant for methyl radical combmation Unfortunately, results from these studies are widely dtiferent. Here we present the ultraviolet absorption spectrum
3. Results
* Present
Fig. la shows tne absorption from the photolysis of acetone
address Laboratonum,
Shell Research BV, Koninklqke/ShelI1003 AA Amsterdam, The Netherlands
0 009-2614/81/0000-0000/S
02.50
0 North-Holland
3 1 Spectroscopic
Publishmg
Company
spectrum obtruned usmg the 254 MI Hg 527
Volume
17 number 3
CHEhlICAL
1 I-ebruary 1981
PHYSICS LETTERS
_L
I OL
’ .?I:
I IlO
220 WA’JELENGTii
230
240
f om
FIN 1. The uttravtol”t absorption spectrltm of ncctj 1radtcals (a) 3 0 Torr acetone I ntm N=. 40 W photo&% (k = 254 nm), period T = I s. signal predommnntls m-phase. (b) 7 6 Torr azomethane, 736 Torr CO, 60 W photo11 SIS (A = 300-400 nm), r = 1 s, sgnal m-phase, (L) 10 1 Torr blacetyl, 1 atm N2 60 W photoI\ SIS (A = 154 nm) r = 0 06 s, m-quad signal from slow component not shotin - m-phase only
resonance lute Two features are apparent, a broad contmuum ektendmg from 200 to 240 run on top of whtch IS supertmposed :he relatively narrow CH3 absorptton at 2 16 nm The methyl radtcai IS produced dtrectly and from the dtssoctatton of the other prtmary photolyttc product, the acetyl mdtcal, CH,CO, KH,)$O
2
Both the CH,
CH,
f CN3CO
(66)
dbsorptton and the contmuum absorption are proportional to the square root of the light mtenstty. We asstgn the contmuum to CH,CO It IS not possrble to estrrnate the absorptton cross sectton from the experunent because the phototysls rate cannot be measured directly. acetone being partially reformed by the combmation between CH3 and CH3C0 It is also not posstble to assIgn spectra from the shape of the continua, and other sources of the radrcals must be found The change wtth temperature m the absorp-
uon spectrum produced on acetone photolysts IS mterestmg. as the results m fig 2 show. Although the nl~~~u~l cross sectton stays at the same wavelength as the temperature mcreases, the absorption IS lost more rapidly at the longer-wavelength end We have asslgned earlier [6] the growth of a sundnr absorption produced on the continued photolysls of dl-r-butyt peroude m an Inert atmosphere to the aceryt radical It IS formed from reactlons (6), (7) and (8) C(CH3)302C(CH3)3 C(CH,),O
+ (CH&O
k
XXCH, + CHg
)3 0,
(7) (S)
The strongest evidence for the absorptron betng due to acetyl 1s provided by photolysls of azomethane III the presence of CO. Ftg. lb shows the absorption obtained_ Such a result could only be obtained after strurgent
Volume 77, number
3
CHEMICAL
PHYSICS
LETTERS
1 February
1981
, I’
Rg 2 The change u1 modulated
absorption alth tcrnpersture photol! SIS 3 3 Torr acetone, I atm N2 20 W photolysls (A = 254 nm). + = 1 s, sg,na.l predommantly mphase, A 303 K, 0,323 K, 6. 373 ?i;, 4.423 Ii w 3 9 Torr acetone, 348 K
ad
acetone
purlflcrltlon of the CO. The strength of the contmuum absorptron IS a functron of CO pressure The absorpbon at 739 Torr CO LSshown m fig 3. At 100 Torr it LSat the lunlt of detectability We therefore assume t!lat acetyl IS formed m reactwn (5) and disappears prmclpalIy by reacllon wth CH3 or at the hrghest CO pressures by reaction wth Itself, CH,CO
+ CH, + CH,COCH,
_
(9)
rug 3 The absorption at 333 and 216 5 nm on the photoIysls of xzomethanc m the presence of CO. sholvn 3s 3 iuncfxm of photol) tic mtenslt) 2 6 Torr xzomethhane, 739 Torr CO, l, 216 5 nm. *, 233 nm, (a) calculated CH3 absorptron at 7-16 5 rn the absence of CO, 7” = c=, (d) absorption at 216 5 m presence of CO by difference T= 1 s
3 2 Krjretzcs The followmg (CH,)2N,
XH,CO
+ (CH,CO),
XH&O
are unportant
[2,3] (12)
-+ 2 CH, + N,,
W)
A fourth source of the absorption IS the photolysls of blaceryl at 254 nm. The absorption here IS agam shown m fig. 1 where It IS compared with that from acetone The continuum at Ionger wavelengths than that of methyl absorption IS reproduced by the absorptlon by the substrate itself makes measurements at shorter wavelengths difficult_ At the photolysls frequency of the experiment, the in-phase absorption should reflect that of the radical and compares well. Agam methyl radrcals are produced by decomposition of the acetyl ra&cals produced m the imtlal photolysis. (CH,CO)+
reactlons
(11)
XH,
-+ C2H6,
(13)
together wth (l), its mverse (.5), and combmatlon and (10) The rate equations are. d[CH,]
/dt = 2P - 31q3 [CH,]”
+ kt tCH,CO] d[CH,CO]
4,
[M] - kg [CH,CO]
/ar = k, [CH,CO]
- kg [CH,COI
- k, [CH3] [CH3],
(9)
[CO] [M] (I)
K&l WI WI [M] - %,,[CH,CO]’
[CQ
I,
(u) 529
Voiunir
where
71. number
CHCXIICAL
3
P IS the photolysls
rate
At 25OC. k,
may
PHYSICS
be lg-
nored [? 31 At low CO pressures (
condltlons
steady
given
state
[CHjCO] and CH, d[CH,]
/dr = Y
w111 be in close
to a
of the CH, plot against (photolysls rate)’ ’ reduced because reactlon (9) 1s now In parallel to The CHjCO density mcreases with mcreasmg CO seen m fig 3. at a slope lndrcatlng between 0 and dope
1981
IS (13) as
0 5
order dependence upon I&t tntenslty The data may I,P .,rr~I\rrcz.A hnrusxror af tlxn rr.n+ m-n~....--a vu “l.LYJ .I-%&, Ifi”..b.CL, il LLIbr”“L-l,,Lhllr-aqjua,r -..I,“IEi
by
= k, [CO] kmetlcs
CH,CO
1 February
LII-I-ERS
[hi] /k,
descr,bzd - 7k,3
b> [CH#
- ?k5 [CH;]
[CO]
[hl]
Followmg the analysts of the effect of a small firstorder contrlbutlon on second-order hlnetlcs published sarller [4] I[ may be sho\r I-I that a plot of CH, absorption at Intinlce photolysls perlods Lsrsus the square root of the photoiytlc miens~ly w:ii be a straldit ilne
for recombmatlon IS assumed, 1.e k, = z(/~,~x-,~)O 5 Ignonng reactlon (I). and cons.dermg only long photolysls periods where a steady state may be assumed for the modulated signal, eq (1) becomes
dlCH311dt
=Y-
- I;, [CH,CO]
2k,3[CH3]' -k,[CH,] [CHJ
[CO]
[M]
(111)
= 0
slope “!/X-t? with a negatlvc mtercept u/h5 [CO] [hl] / 2k13 wllere u IS the absorption cross section of CH3 and I the cell length At a relatlv=zty long period of I s there 1s still 3 negative Intercept tllat ansrs from the tinlre lengtll of the prrlod and the data may be corrected for this as shown earlier [4] After the ccrrecnon has been applied to the dsra II-It!le absence of CO the lllle passes through the ongln (fig 8 ot ref [-I] ) __I_ __.. -1.. ..--ri_._-^_I and tii? SLOpt: iS IclaLlvcIy uIiilticuLcu PiOViCied iiiC first-order component IS small, the correction w~li be tile SLILI~K s.Ee m the presence of CO and we have t&en the difference u-t Intercept as our measure of/,-,/k,, Thus difference of 1 4 X lo-‘. together with the pubhshcd [A] values of k13 and CJ(effecclve at the partxular slit \\ldth) 21ves k, [Xl] = (1 S 2 0 5) X lo-” cm’ molecule-1 s-l at LOO Torr CO Kerr and Caljvert [2] measured a somewhat lower valur of IO-” cm molecule-1 s-l which rises only slowly 11th pressure from 50 to 200 Torr at the same temperature The accuracy of the current measurement can be estimated from the fact the presence of 35 Torr CO ~3s from that m the dbsenLe of of that measured represents of accuracy
methyl absorption In tile not slgmficantiy alfferent CO. I e a rate one-quarter somethlng Ike tile range
That the rate constant is In tile fall-off resme IS confirmed by addmg an atmosphere of nitrogen to the same pressure of CO ds a result of wtuch the ab-
sorption
at 220 nm which IS barely detectable m CO alone, becomes measurable At hg,her pressures of CO the kmetlcs are less smple. CH, IS still governed largely by second-order kmetlcs but it can be seen from figs 3 and 4 that the 530
SOUCRE
FlOGT OF PHOTOLYSIS
RCTE
X 106/fAC)LECULEt’ZcmMs”Z
Trg 4 The nbsorptlon by CH, III the presence of 100 Torr CO 2 8 Torr azomethane, T = 1 s, absorption at 223 nm unmensurnbly small. (a) CH, absorption (T = -) no CO, X, experunental pomts, (b) expenmenta pomts corrected to T = m
Volume
77, number
3
CHEMICAL
W-i31 = V’/43)o’ - (k,,/k,,)u5
[CH,CO].
PHYSICS
+ 2k13
Pi31
[GcH3)1
(VI
The effect of the finite length of the period
IS relatively small now, both because the change m [CH3] IS larger and because reactlon (5) has Increased the rate of the CH3 kmetlcs. consequently it has been neglected k, can be obtained duectly From eq. (v) and the data m fig 3 and kg [M] at 739 Torr CO w found to be 6 0 X 10mL8 cm3 molecule-’ s-l Thus value suggests a somewhat Iilgner hrmtmg rate constant than that of Watkms and Word [3] who found IO-l7 cm3 molecule-* s--I at the h&pressure lurut m the presence of SF, and azomethane To obtam a value of either kg or X-t, and separately (7 a measurement of the acetyl reactlon time 1s needed The acetyl dbsorptlon measured directly from graphs such as fig 3 gives a u/rate constant ratlo. from [A(CH,)] and [CH,CO] usmg (iv) uacepL723nm
= 12
x lo-l7
4. DIscussion 4 I 771~ absorptlotl
(kto/klp
/dt = k, [CH3] [CO] [hl] - 2k,,[CH,CO]”
- (X9 [CH,CO]
[CHj]
or
2x-,; [CH3]
[a(CH,)]
spectra
of acet\*l
The ulrravlolet absorption spectrum between 2 10 and 240 run IS obtamed from four different starting materials. all of which should produce acetyl radicals and methyl radicals The relatively sharp methyl absorption can be clearly dtiferentlated from the contlnuum Ada&l e: al. [5] observed a sunllar spectrum Ln the same wavelength region, although there are d!fferences m detail The absolute values of the absorpuon cross sectlons at the peak near 2 10 nm are sumiar 1 1 X lo-l7 cm’ here compared with 7 X lo-l8 cm2 from Adachl et ai The absolute value here depends ultimately on the rate constant for mutual combmatlon of acetyl radicals m both cases and there are here m facr slgmficantly larger differences which need clanficatlon (q v ) Sunons [7] has also observed a slmtlar absorption m the flash photolysls of azomethane m the presence of CO.
It can be shown that the mutual renctlon dommates m determmmg the CH,CO kmetrcs at the lowest photolytlc mtensltles If the tune constant for [CH,CO] kmetlcs IS measured here It can be used to find k10
d[CH,CO]
1981
3 X lo-l1
[CO] [hf]
+k5[CH3]
1 remwry
and k, [CH3] [CO] [M] = 7 S X 1012 leading to k, o = cm3 molecule-’ s-l From the assumed crosscombmatlon rule kg = 7.0 X lo-l1 cm3 molecule-’ s-l u is therefore 1 0 X lo-l8 cm2 at 723 nm and 1 4 X IO-l7 cm2 at the maumum. If these values of kt0 and kg are used to calculate 7. at the fughest mtenslty, agam assunung that acetyl beha\lour IS close to sunple second-order kmetlcs, good agreement IS obtamed kvlth the observed value 0 07 s, cf 0.09 s The value of k10 can be further checked by substltutmg back m eq (VI) and checkmg that d[CH,CO] /dt = 0 when low photolysls frequency steady-state concenrratlons are used
(iv)
I e (“lo/1;13)o ’ [CH3CO] = n(CH3) where A(CH3) IS the difference between the measurements of [CH,] at a given photolytic rate III the presence and absence of CO. as shown m fig 3 Therefore 2P= 2X-,,[CH,]’
LETTERS
)
42
?ke kmetm
of the CH, f- CO feactloiz
(VI) 4t the lowest intensity where rreasuremenrs were made, the fist term on the r&t-hand side IS 4 t~nles the third, mdlcatmg that near steady-state condltlons the second IS 3 tunes larger than the third The in-phase and quadrature signals are equal at 7. = 0 2 s Therefore_ to a good approummatlon, assummg CH,CO 1~ governed by second-order kmetlcs. I- = 2.2(; k, [CH3] [CO] [Ml k 1o)- 05
These results show that the experimental data of Kerr and Calvert [2] and Watkms and Word [3] are both lower by a slmllar amount from tilt- present wcrk and that the differences III the extrapolated secondorder llmlts stem from the very long extrapolation m the former work The present analysis 1s being unproved by detailed computer modellmg as m the CH, + 0, system [8] The ranges of pressure and temperature over wbch measurements have been made IS also bemg extended 531
Volume
77, number
CHEhllC4L
3
4 3 Rathcal reco~nbmztto~~
PHYSICS LETTERS
kulehcs
If the cross-combtnatton rule IS assumed, then the recomblnatlon kmetlcs of the two radicals CH, and CH,CO can be separated Furthermore the rate constants obtnmed usmg the present analysts are not drssunder to those obtamed by fittmg the In-phase and mquadrature signals ustng the abovementtoned computed solutton to the klrettc equntlons From their data. Watkms and Word [3! dertve a cross-comblnatton rate constant kg of 4 8 X lo-l1 cm3 molecule-’ s-l at temperatures above amblent which agrees rzasonabl} well ,vvlt_hthis work as does Kerr and Calvert‘s nssumptlon that kg = k t3. the meth) I radical recombmatlon rate constant Adach et al 151, who photolysed acetone wtth a rclattvely I~I$ flash snersy but wt:h a pyre\ filter. find a very much more rapld dlsappcarancc of acetyl rndlcals than would be elpectcd on the basis of our data Tlus leads to \alues of 7 5 X lo-” for Lzg and 13X 10-‘Ofork 1o Althotl& one would not elpect exited acetyl radlc& at the wavelength employed [9], the formatloll of such radlcais could offer a posstblltty of reconcdmg tile data It IS difficult to see how m the present system the acetyl radicals would react apparently too slowI>
1 February
1981
Acknowledgement ‘The author IS mdebted to Mr. W N Satles who carried out the elpertmental work, and to Dr. C Anastnsi for comments on the analysts of the results
References M P Halstcnd. 4 Prothero and C P Qumn, Proc Rq SOL A 377 (197 1) 377, and references thereln J A Kerr and J G Calvcrt. J Phys Chcm 69 (1965) 1022 K W Wathms and W iv Word. Intern J Chem limetics 6 (1974) 85.5 D A P.uhes, D hl Paul and C P Quinn, J Chem Sot I-nraday 172 (1976) 1935 II: Adachl N Basco and D G L James, Chem Phls Letters 59 (1978) 502 D A Parks and C P Qumn. Chem Phls Letters 33 (1975) 483 J P Sunons, Unwcrs!t> of Bummghrun, prwate communecanon D 4 Parhes, Intern J Chem Kmetlcs 9 (1977) 1977 A Gandml znd PA Hackett. J Am Chem Sot 99 (19i7) 6 195
CHEMICAL
THE ULTRAVIOLET AND THE KINETICS
1 Fcbruxry
LETTERS
1981
ABSORPTION SPECTRA OF THE ACETYL RADICAL OF THE CH, + CO REACTION AT ROOM TEMPERATURE
DA PARKES Shell Research Lrd
Thowtot~
Received
1980,
19 August
PHYSICS
Rrsearclr
m linal
term
Corrre
Clresrrr
7 Novcmbcr
CHI
3SH
UK
1980
A contmuum-absorption spectrum bctwcen 700 and 240 nm IS asqned to the ace&I rodlcal Klnetlc measurcmcnts usnf moleculru modulntlon spectroscopy sho\r for the rcnctlon CH3 + CO (+kl) + CH,CO + 31 the rate constants JTC at 750 Torr The rate constant tar ncctj I comb](1 8 I 0 2) X IO-” cm3 molecule-’ s-’ at 100 Torr and (6 r 1) X lo-‘*
nation 2CH3CO - (CH3C0)2 IS (3 0 + 1 0) X IO-”
it 25°C
1. Introduction
The acetyl radical plays an unportant role m hydrocarbon oudatlon The competltlon between Its d~ssoclatlon and the reactlon with oxygen which leads ultimately to the degenerate branchmg agent acetyl hydroperoxlde provides the baas of many models [I]
of the acetyl radlcai obtamed usmg molecular moduiatlon spectrometry [4] and show that direct morutormg of the rad1ca.l can be used to measure the rates of acetyl radical reactlons directly The spectrum has also been obtamed recently usmg flash photolysls by Adachl et al. [S]
of cool flame behavlour III acetaldehyde, CH,CO
(+M) -+ CH, + CO (+M),
(1)
CH3C0
+ 0,
(2)
CH,C002 CH,COO,H
The
+ CH3COOZ,
+ RH + CH,C002H
+ R
(3)
-+ CH, + CO? + OH’.
The rate of addltlon
and Its mverse reaction times [2,3] by product
(4)
of Cd to CH,.
CH, + CO (+M) - CH,CO
2. Experimental
(+M),
(5)
(l), have been measured several analysts usually the yield of
apparatus
and
experimental
method
are de-
scribed m detad elsewhere [4] The method uses perIodIcally mterrupted photolysls (X = 300%400 nm or 354 nm, where stated) to produce reactive mtermediates whose concentrations are modulated and are hence detectable by I&t absorption using averaghe m the rage mg techru ues even when the absorptlons IS measured both of 1 m 10 ‘J to 1 m IO3 Absorption III phase and m quadrature with the photolytrc !Ighht and the ratlo of these measurements may be used to obtam kmetlc mformatlon
acetone 1s compared with the yield of ethane m the photolysls of azomethane III the presence of CO, and kg and kl are deduced usutg the wellestabhshed rate constant for methyl radical combmation Unfortunately, results from these studies are widely dtiferent. Here we present the ultraviolet absorption spectrum
3. Results
* Present
Fig. la shows tne absorption from the photolysis of acetone
address Laboratonum,
Shell Research BV, Koninklqke/ShelI1003 AA Amsterdam, The Netherlands
0 009-2614/81/0000-0000/S
02.50
0 North-Holland
3 1 Spectroscopic
Publishmg
Company
spectrum obtruned usmg the 254 MI Hg 527
Volume
17 number 3
CHEhlICAL
1 I-ebruary 1981
PHYSICS LETTERS
_L
I OL
’ .?I:
I IlO
220 WA’JELENGTii
230
240
f om
FIN 1. The uttravtol”t absorption spectrltm of ncctj 1radtcals (a) 3 0 Torr acetone I ntm N=. 40 W photo&% (k = 254 nm), period T = I s. signal predommnntls m-phase. (b) 7 6 Torr azomethane, 736 Torr CO, 60 W photo11 SIS (A = 300-400 nm), r = 1 s, sgnal m-phase, (L) 10 1 Torr blacetyl, 1 atm N2 60 W photoI\ SIS (A = 154 nm) r = 0 06 s, m-quad signal from slow component not shotin - m-phase only
resonance lute Two features are apparent, a broad contmuum ektendmg from 200 to 240 run on top of whtch IS supertmposed :he relatively narrow CH3 absorptton at 2 16 nm The methyl radtcai IS produced dtrectly and from the dtssoctatton of the other prtmary photolyttc product, the acetyl mdtcal, CH,CO, KH,)$O
2
Both the CH,
CH,
f CN3CO
(66)
dbsorptton and the contmuum absorption are proportional to the square root of the light mtenstty. We asstgn the contmuum to CH,CO It IS not possrble to estrrnate the absorptton cross sectton from the experunent because the phototysls rate cannot be measured directly. acetone being partially reformed by the combmation between CH3 and CH3C0 It is also not posstble to assIgn spectra from the shape of the continua, and other sources of the radrcals must be found The change wtth temperature m the absorp-
uon spectrum produced on acetone photolysts IS mterestmg. as the results m fig 2 show. Although the nl~~~u~l cross sectton stays at the same wavelength as the temperature mcreases, the absorption IS lost more rapidly at the longer-wavelength end We have asslgned earlier [6] the growth of a sundnr absorption produced on the continued photolysls of dl-r-butyt peroude m an Inert atmosphere to the aceryt radical It IS formed from reactlons (6), (7) and (8) C(CH3)302C(CH3)3 C(CH,),O
+ (CH&O
k
XXCH, + CHg
)3 0,
(7) (S)
The strongest evidence for the absorptron betng due to acetyl 1s provided by photolysls of azomethane III the presence of CO. Ftg. lb shows the absorption obtained_ Such a result could only be obtained after strurgent
Volume 77, number
3
CHEMICAL
PHYSICS
LETTERS
1 February
1981
, I’
Rg 2 The change u1 modulated
absorption alth tcrnpersture photol! SIS 3 3 Torr acetone, I atm N2 20 W photolysls (A = 254 nm). + = 1 s, sg,na.l predommantly mphase, A 303 K, 0,323 K, 6. 373 ?i;, 4.423 Ii w 3 9 Torr acetone, 348 K
ad
acetone
purlflcrltlon of the CO. The strength of the contmuum absorptron IS a functron of CO pressure The absorpbon at 739 Torr CO LSshown m fig 3. At 100 Torr it LSat the lunlt of detectability We therefore assume t!lat acetyl IS formed m reactwn (5) and disappears prmclpalIy by reacllon wth CH3 or at the hrghest CO pressures by reaction wth Itself, CH,CO
+ CH, + CH,COCH,
_
(9)
rug 3 The absorption at 333 and 216 5 nm on the photoIysls of xzomethanc m the presence of CO. sholvn 3s 3 iuncfxm of photol) tic mtenslt) 2 6 Torr xzomethhane, 739 Torr CO, l, 216 5 nm. *, 233 nm, (a) calculated CH3 absorptron at 7-16 5 rn the absence of CO, 7” = c=, (d) absorption at 216 5 m presence of CO by difference T= 1 s
3 2 Krjretzcs The followmg (CH,)2N,
XH,CO
+ (CH,CO),
XH&O
are unportant
[2,3] (12)
-+ 2 CH, + N,,
W)
A fourth source of the absorption IS the photolysls of blaceryl at 254 nm. The absorption here IS agam shown m fig. 1 where It IS compared with that from acetone The continuum at Ionger wavelengths than that of methyl absorption IS reproduced by the absorptlon by the substrate itself makes measurements at shorter wavelengths difficult_ At the photolysls frequency of the experiment, the in-phase absorption should reflect that of the radical and compares well. Agam methyl radrcals are produced by decomposition of the acetyl ra&cals produced m the imtlal photolysis. (CH,CO)+
reactlons
(11)
XH,
-+ C2H6,
(13)
together wth (l), its mverse (.5), and combmatlon and (10) The rate equations are. d[CH,]
/dt = 2P - 31q3 [CH,]”
+ kt tCH,CO] d[CH,CO]
4,
[M] - kg [CH,CO]
/ar = k, [CH,CO]
- kg [CH,COI
- k, [CH3] [CH3],
(9)
[CO] [M] (I)
K&l WI WI [M] - %,,[CH,CO]’
[CQ
I,
(u) 529
Voiunir
where
71. number
CHCXIICAL
3
P IS the photolysls
rate
At 25OC. k,
may
PHYSICS
be lg-
nored [? 31 At low CO pressures (
condltlons
steady
given
state
[CHjCO] and CH, d[CH,]
/dr = Y
w111 be in close
to a
of the CH, plot against (photolysls rate)’ ’ reduced because reactlon (9) 1s now In parallel to The CHjCO density mcreases with mcreasmg CO seen m fig 3. at a slope lndrcatlng between 0 and dope
1981
IS (13) as
0 5
order dependence upon I&t tntenslty The data may I,P .,rr~I\rrcz.A hnrusxror af tlxn rr.n+ m-n~....--a vu “l.LYJ .I-%&, Ifi”..b.CL, il LLIbr”“L-l,,Lhllr-aqjua,r -..I,“IEi
by
= k, [CO] kmetlcs
CH,CO
1 February
LII-I-ERS
[hi] /k,
descr,bzd - 7k,3
b> [CH#
- ?k5 [CH;]
[CO]
[hl]
Followmg the analysts of the effect of a small firstorder contrlbutlon on second-order hlnetlcs published sarller [4] I[ may be sho\r I-I that a plot of CH, absorption at Intinlce photolysls perlods Lsrsus the square root of the photoiytlc miens~ly w:ii be a straldit ilne
for recombmatlon IS assumed, 1.e k, = z(/~,~x-,~)O 5 Ignonng reactlon (I). and cons.dermg only long photolysls periods where a steady state may be assumed for the modulated signal, eq (1) becomes
dlCH311dt
=Y-
- I;, [CH,CO]
2k,3[CH3]' -k,[CH,] [CHJ
[CO]
[M]
(111)
= 0
slope “!/X-t? with a negatlvc mtercept u/h5 [CO] [hl] / 2k13 wllere u IS the absorption cross section of CH3 and I the cell length At a relatlv=zty long period of I s there 1s still 3 negative Intercept tllat ansrs from the tinlre lengtll of the prrlod and the data may be corrected for this as shown earlier [4] After the ccrrecnon has been applied to the dsra II-It!le absence of CO the lllle passes through the ongln (fig 8 ot ref [-I] ) __I_ __.. -1.. ..--ri_._-^_I and tii? SLOpt: iS IclaLlvcIy uIiilticuLcu PiOViCied iiiC first-order component IS small, the correction w~li be tile SLILI~K s.Ee m the presence of CO and we have t&en the difference u-t Intercept as our measure of/,-,/k,, Thus difference of 1 4 X lo-‘. together with the pubhshcd [A] values of k13 and CJ(effecclve at the partxular slit \\ldth) 21ves k, [Xl] = (1 S 2 0 5) X lo-” cm’ molecule-1 s-l at LOO Torr CO Kerr and Caljvert [2] measured a somewhat lower valur of IO-” cm molecule-1 s-l which rises only slowly 11th pressure from 50 to 200 Torr at the same temperature The accuracy of the current measurement can be estimated from the fact the presence of 35 Torr CO ~3s from that m the dbsenLe of of that measured represents of accuracy
methyl absorption In tile not slgmficantiy alfferent CO. I e a rate one-quarter somethlng Ike tile range
That the rate constant is In tile fall-off resme IS confirmed by addmg an atmosphere of nitrogen to the same pressure of CO ds a result of wtuch the ab-
sorption
at 220 nm which IS barely detectable m CO alone, becomes measurable At hg,her pressures of CO the kmetlcs are less smple. CH, IS still governed largely by second-order kmetlcs but it can be seen from figs 3 and 4 that the 530
SOUCRE
FlOGT OF PHOTOLYSIS
RCTE
X 106/fAC)LECULEt’ZcmMs”Z
Trg 4 The nbsorptlon by CH, III the presence of 100 Torr CO 2 8 Torr azomethane, T = 1 s, absorption at 223 nm unmensurnbly small. (a) CH, absorption (T = -) no CO, X, experunental pomts, (b) expenmenta pomts corrected to T = m
Volume
77, number
3
CHEMICAL
W-i31 = V’/43)o’ - (k,,/k,,)u5
[CH,CO].
PHYSICS
+ 2k13
Pi31
[GcH3)1
(VI
The effect of the finite length of the period
IS relatively small now, both because the change m [CH3] IS larger and because reactlon (5) has Increased the rate of the CH3 kmetlcs. consequently it has been neglected k, can be obtained duectly From eq. (v) and the data m fig 3 and kg [M] at 739 Torr CO w found to be 6 0 X 10mL8 cm3 molecule-’ s-l Thus value suggests a somewhat Iilgner hrmtmg rate constant than that of Watkms and Word [3] who found IO-l7 cm3 molecule-* s--I at the h&pressure lurut m the presence of SF, and azomethane To obtam a value of either kg or X-t, and separately (7 a measurement of the acetyl reactlon time 1s needed The acetyl dbsorptlon measured directly from graphs such as fig 3 gives a u/rate constant ratlo. from [A(CH,)] and [CH,CO] usmg (iv) uacepL723nm
= 12
x lo-l7
4. DIscussion 4 I 771~ absorptlotl
(kto/klp
/dt = k, [CH3] [CO] [hl] - 2k,,[CH,CO]”
- (X9 [CH,CO]
[CHj]
or
2x-,; [CH3]
[a(CH,)]
spectra
of acet\*l
The ulrravlolet absorption spectrum between 2 10 and 240 run IS obtamed from four different starting materials. all of which should produce acetyl radicals and methyl radicals The relatively sharp methyl absorption can be clearly dtiferentlated from the contlnuum Ada&l e: al. [5] observed a sunllar spectrum Ln the same wavelength region, although there are d!fferences m detail The absolute values of the absorpuon cross sectlons at the peak near 2 10 nm are sumiar 1 1 X lo-l7 cm’ here compared with 7 X lo-l8 cm2 from Adachl et ai The absolute value here depends ultimately on the rate constant for mutual combmatlon of acetyl radicals m both cases and there are here m facr slgmficantly larger differences which need clanficatlon (q v ) Sunons [7] has also observed a slmtlar absorption m the flash photolysls of azomethane m the presence of CO.
It can be shown that the mutual renctlon dommates m determmmg the CH,CO kmetrcs at the lowest photolytlc mtensltles If the tune constant for [CH,CO] kmetlcs IS measured here It can be used to find k10
d[CH,CO]
1981
3 X lo-l1
[CO] [hf]
+k5[CH3]
1 remwry
and k, [CH3] [CO] [M] = 7 S X 1012 leading to k, o = cm3 molecule-’ s-l From the assumed crosscombmatlon rule kg = 7.0 X lo-l1 cm3 molecule-’ s-l u is therefore 1 0 X lo-l8 cm2 at 723 nm and 1 4 X IO-l7 cm2 at the maumum. If these values of kt0 and kg are used to calculate 7. at the fughest mtenslty, agam assunung that acetyl beha\lour IS close to sunple second-order kmetlcs, good agreement IS obtamed kvlth the observed value 0 07 s, cf 0.09 s The value of k10 can be further checked by substltutmg back m eq (VI) and checkmg that d[CH,CO] /dt = 0 when low photolysls frequency steady-state concenrratlons are used
(iv)
I e (“lo/1;13)o ’ [CH3CO] = n(CH3) where A(CH3) IS the difference between the measurements of [CH,] at a given photolytic rate III the presence and absence of CO. as shown m fig 3 Therefore 2P= 2X-,,[CH,]’
LETTERS
)
42
?ke kmetm
of the CH, f- CO feactloiz
(VI) 4t the lowest intensity where rreasuremenrs were made, the fist term on the r&t-hand side IS 4 t~nles the third, mdlcatmg that near steady-state condltlons the second IS 3 tunes larger than the third The in-phase and quadrature signals are equal at 7. = 0 2 s Therefore_ to a good approummatlon, assummg CH,CO 1~ governed by second-order kmetlcs. I- = 2.2(; k, [CH3] [CO] [Ml k 1o)- 05
These results show that the experimental data of Kerr and Calvert [2] and Watkms and Word [3] are both lower by a slmllar amount from tilt- present wcrk and that the differences III the extrapolated secondorder llmlts stem from the very long extrapolation m the former work The present analysis 1s being unproved by detailed computer modellmg as m the CH, + 0, system [8] The ranges of pressure and temperature over wbch measurements have been made IS also bemg extended 531
Volume
77, number
CHEhllC4L
3
4 3 Rathcal reco~nbmztto~~
PHYSICS LETTERS
kulehcs
If the cross-combtnatton rule IS assumed, then the recomblnatlon kmetlcs of the two radicals CH, and CH,CO can be separated Furthermore the rate constants obtnmed usmg the present analysts are not drssunder to those obtamed by fittmg the In-phase and mquadrature signals ustng the abovementtoned computed solutton to the klrettc equntlons From their data. Watkms and Word [3! dertve a cross-comblnatton rate constant kg of 4 8 X lo-l1 cm3 molecule-’ s-l at temperatures above amblent which agrees rzasonabl} well ,vvlt_hthis work as does Kerr and Calvert‘s nssumptlon that kg = k t3. the meth) I radical recombmatlon rate constant Adach et al 151, who photolysed acetone wtth a rclattvely I~I$ flash snersy but wt:h a pyre\ filter. find a very much more rapld dlsappcarancc of acetyl rndlcals than would be elpectcd on the basis of our data Tlus leads to \alues of 7 5 X lo-” for Lzg and 13X 10-‘Ofork 1o Althotl& one would not elpect exited acetyl radlc& at the wavelength employed [9], the formatloll of such radlcais could offer a posstblltty of reconcdmg tile data It IS difficult to see how m the present system the acetyl radicals would react apparently too slowI>
1 February
1981
Acknowledgement ‘The author IS mdebted to Mr. W N Satles who carried out the elpertmental work, and to Dr. C Anastnsi for comments on the analysts of the results
References M P Halstcnd. 4 Prothero and C P Qumn, Proc Rq SOL A 377 (197 1) 377, and references thereln J A Kerr and J G Calvcrt. J Phys Chcm 69 (1965) 1022 K W Wathms and W iv Word. Intern J Chem limetics 6 (1974) 85.5 D A P.uhes, D hl Paul and C P Quinn, J Chem Sot I-nraday 172 (1976) 1935 II: Adachl N Basco and D G L James, Chem Phls Letters 59 (1978) 502 D A Parks and C P Qumn. Chem Phls Letters 33 (1975) 483 J P Sunons, Unwcrs!t> of Bummghrun, prwate communecanon D 4 Parhes, Intern J Chem Kmetlcs 9 (1977) 1977 A Gandml znd PA Hackett. J Am Chem Sot 99 (19i7) 6 195