The reactions of alkyl radicals with oxygen atoms: Identification of primary products at low pressure

T H E REACTIONS O F ALKYL RADICALS WITH OXYGEN ATOMS: I D E N T I F I C A T I O N O F PRIMARY P R O D U C T S AT LOW P R E S S U R E K. HOYERMANN AND R. SIEVERT

Institut fiir Physikalische Chemie der Universitdt, Go'ttingen, W. Germany

The primary products formed in the reaction of oxygen atoms with CH3, C2H5, C a R 7 and iso-C4H9 at low pressure, in the absence of wall and secondary reactions, are studied, Alkyl radicals are produced in an isothermal flow reactor and are transferred into a Laval nozzle reactor, where the oxygen atoms are injected into the flow in the divergent part of the nozzle at low pressure. Samples are withdrawn by a molecular beam sampling system into the ion source of a mass spectrometer for analysis of stable and labile species. The measurements established the following reaction mechanisms: O+CH a--~HCHO+ H O + C2H~---> C H 3 C H O + H

(at)

H C H O + CH a

(az)

(branching ratio (a t ):(%) = (5 + 1): 1) O + n-CaR7-"-* C2H~CHO + H

(a~)

HCHO + CzH 5

(%)

O + iso-CaHT--~ C H a C O C H a + H C H 3 C H O + CH 3

(b 1) (bz)

(branching ratio (at):(a2) = (6 _+ 1.5):1, (b~):(b2) = (1 +__0.2):1) O + iso-CaHg---~ iso-CaH7CHO + H

(al)

---* H C H O + C~H r

(az)

O + tert-C4Hg--~ C H a C O C H a + C H 3

(c)

(branching ratio (al):(a2) = (7 _+ 2):1) No abstraction reactions, yielding O H radicals, have been observed.

Introduction

Similarly, attack may occur by Oz in initiation reactions during induction processes 4'5'6

Alkyl radicals are the essential radicals in the mechanism of hydrocarbon oxidation: Primary attack of alkanes by atoms and radicals proceeds via abstraction reactions to radicals, t.2~

02 + R H ~ HO~ + R Alkenes like C~H 4, C 3 H , , C4H 8 also react with atoms and radicals to form alkyl radicals: z.7,8,9

(O,H,OH) + RH(~.~_CH4,C2H~,C3H 8 ...) --* (OH,H2,H~O) + R(~---CH~,C2Hs,C3H7 ...) 517

H + C2H4---~ C2H~*, H + CzH~--* 2CH~

518

KINETICS

Production of Alkyl Radicals

O + C~H4--* HCO + CH3 OH + CzHo"* H~CO + C~H~ (in addition to C3Hs,CH~O,C2H 4) In this context, the secondary reactions of alkyl radicals are of special interest. In a combustion process, the concentration of oxygen atoms is often below that of oxygen molecules, but the rate constant k (0 + R) may exceed that of k (O2 + R) by several orders of magnitudeJ2 For example, the reaction of CH3 with O proceeds with a rate coefficient k ~ 23 3 tO It = ( 5 - 11)" 10 cm /tools, ' whereas k ~ (O2 + CHa'-'> HCHO + OH) is only 2 9 108 crn~a~mol s and k~s (O2 + CH3 + M ~ CHaO s + M) is 8 9 10 ~ cma/mol s at 1 atm. ~'~3 Thus the O + R reactions have to be included in the reaction scheme, as already demonstrated for the methaneoxygen flame) ~ As the O + alkyl reactions are very fast and highly exothermic and thereby offering a large number of open reaction channels, it is difficult to deduce the mechanism conclusively from a complex reaction system such as a flame or an isothermal flow reactor. In the present paper, we report the direct identification of the primary products formed in the reaction of oxygen atoms with CH~, C~H~, Call 7 and iso-C4H9 at low pressure, in the absence of wall and secondary reactions.

Experimental Alkyl radicals are produced in an isothermal flow reactor and are transferred into a Laval nozzle reactor, where the oxygen atoms are injected into the flow in the divergent part of the nozzle at low pressure. Samples are withdrawn by a molecular beam sampling system into the ion source of a mass spectrometer for analysis of stable and labile species (Fig. 1, 2). Part of the experimental set up is described in an earlier paper. /(}\.

~'~l~

~"~" . . . . . . . . . ~

\/'"

~

...... o.E,~

The alkyl radicals reactions: 2.~,,~,J6.~7

are

produced

via

the

H + C2H4--* C~H~', H + C2H5--* 2CHa OH + C~Ho---, C2H ~ + H~O OH + C a H s ~ C3H7+ H~O OH + iso-C~H ~o---~C4H ~ + H~O F + CH4---, CH3 + HF F + C~H6---, C~H~ + H F F + Calls---, C~H 7 + H F F + iso-C4H to---) C4H 9 + HF The OH/OD radicals are generated by the wellknown titration reaction H / D + NO~---, OH/OD + NO. The H / D atoms are first produced from highly diluted H 2/D~--He mixtures in a discharge flow reactor (0.1-1 tort), and NO~ is then added. The appropriate parent hydrocarbon is added further downstream, at the point where the maximum OH/OD concentration is reached, and the alkyl radicals are formed. The alternative method of generating alkyl radicals, which is simpler and easier to control, employs H-atom abstraction by F-atoms, as shown schematically in Fig. 1. Highly diluted F 2 in He (0.01-0.05 mole %) is passed through a liquid nitrogen trap to remove HF, ~6 and is dissociated 95-99% by a microwave discharge. The quartz discharge tube is coated internally with a 0.5 mm thick film of AI~Oa ceramic in order to reduce formation of SiF3 and SiF4, which could be detected by the mass spectrometer. (In any case, these species are inert in the O + alkyl reaction.) An excess (>10:1) of hydrocarbon, diluted in He (1:5) for rapid mixing, is admixed to the F-atom/He flow at the entrance of the nozzle. The hydrocarbon radical concentrations are kept small and flow velocities are high (up to 80 m/s, maintained by a 110 l / s rotary pump); thus the F + alkyl and the radical disproportionation reactions that lead to alkenes are suppressed, as verified by the mass spectrometer. 1 8 ' 2 9 '2~)

~ =',~

Nozzle Reactor ~

..

FIG. I. Experimental setup.

The reactors for O + alkyl are convergent-divergent (60~, 30 ~) teflon nozzles, with throat diameters of 2-4.6 mm (Fig. 9.). Oxygen atoms, formed in a second discharge flow reactor (pumped by a 10 l / s rotary pump) from 0 2/He mixtures, are present in a 6 • 8 mm annulus around the outer part of the nozzle. From this reservoir the atoms are injected uniformly into the main alkyl/He flow at the divergent part of the nozzle, where the pressure (0.01-0.2

ALKYL RADICALS

FLOW REACTOR FOR 0 ATOM PRODUCTION

.~.j,

F 9 I"~, NC

I---4

FLOW REACTOR

lcm

FOR ALKYL PRODUCTION

FIG. 2. N o z z l e r e a c t o r .

torr) is 0.1-0.2 times less than in the entrance part (0.1-1 torr). Three types of arrangement were tested: An annular orifice under real effusion flow condition (Knudsen number -> 1); a concentric array of 20 flow-directing channels (0.9 mm diameter, 3 mm long); and a concentric channel ring of 0.9 mm width, 8 mm length. This last arrangement minimizes boundary layer effects and was used for the results reported here. Figure 2 gives the details. The surrounding pressures of 10-3-10 -4 torr are maintained by pumping with two 2000 l / s mercury diffusion pumps.

519

fcrent ionization energies, in order to find their specific "finger prints" for the definitive identification of the reaction products. Discrimination of the formed products, aldehydes and ketones, exclusively by determination of their appearance potentials is difficult, as these values are very similar 2~ (e.g. AP (C2HsCHO) = 9.94-9.98 eV, AP (CH 3COCH 3) = 9.68-9.71 eV). Oxygen-containing species and their oxygen-containing fragments after ionization are well suited for product identification. However, the cracking pattern of these species are sufficiently different (particularly for the O-containing fragments) to allow quantitative discrimination. Large ion signals (>106 ions/s) are recorded via electron multiplier and dc amplification. Sensitivity is improved by modulation techniques and synchronous ion counting. This has been achieved in two devices, shown schematically in Fig. 1: (i) At high intensities (up to 10~ ions/s) the ions are counted (after amplification and discrimination) by a 2-channel counter (2-CC, 85 MHz). The discharge, either that for F-atom or O-atom production, is switched on/off, timed by a frequency generator. The delay gates the 2-channel counter for the A (reaction "on") and B (reaction "off") position. This technique makes it possible to measure small intensity differences on high intensive mass lines. (ii) At low intensities a double modulation tech-

- O ATOM

§

- O ATOM

1500

IONS . . . A

Specific and Sensitive Detection Samples are withdrawn from the reacting mixture by a molecular beam system and analyzed with a mass spectrometer. In order to maintain a high intensity beam in the ion source at low surrounding pressures (5 9 10-6-10 -7 tort), differential pumping is applied. The background in the ion source is pumped directly by a 550 l / s mercury diffusion pump (liquid N 2 baffle) and the core of the beam, after having passed the ion source, is destroyed in a second vacuum chamber. This is evacuated by a 600 l / s mercury diffusion pump. The mass spectrometer (VARIAN MAT CH5) can be operated at high resolution (e.g. separation of CH3CHO/C3Hs on mass peak 44) and at different ionization energies via electron impact (4.5-29.5 eV, 70 eV). Spectra of the different species (aldehydes, ketones, epoxides) are measured separately at dif-

' t I

B

y f }

t I I

t Lf"

,

I I

''

J

FIG. 3. Double modulation technique (formation of C4HsO in the reaction O + C4H ~. Mass spectrometer: electron energy: 70 eV, high resolution operation. Multi channel averager: 2.9 ms/channel, 128 cycles).

520

KINETICS

nique is used. The oxygen atom discharge is switched on/off with a frequency f, and the discharge that produces the alkyl radical R is switched on/off synchronously with twice that frequency (2f). This double modulation corresponds to the condition in the reaction center: +R + O / - R + O/ + R - O/ - R - O which is recorded by ion counting via a multichannel averager (Tracor Northern NS 575 A, 15 MHz). Figure 3 gives an example for low modulation frequencies such as were used in the detection of mass C 4 H 8 0 in the O + C4H9 reaction. Part A gives the recorded ions per channel (2.9 ms) after 128 cycles. Part B gives the sum of ions registered, from which the averaged ion signals can be extracted for an appropriate duty cycle. This chemical modulation reduces the "chemical noise," as the one reaction signal (+R + O) can be corrected for the build-up of background signal e.g. acetone, unsaturated hydrocarbons, as well as minimizing the "electronic noise" by allowing averaging of enough cycles to reduce the experimental uncertainty to the required level.

hydrocarbon background from C~H 4. The main product of CH 3 + O is HCHO as identified by mass peaks 29 (HCO) and 30 (HCHO). Detection of the corresponding H is difficult, as cracking at mass peak 1 is found for all other hydrogen containing substances. Also, the mass spectrometric sensitivity is low, and the light Hatoms tend to be scattered out of the reaction center. An absolute calibration of HCHO better than +20% was not possible, and a minor pathway to HCO + H 2 (<20%) cannot be excluded. A search was made for the abstraction reaction leading to CH ~and OH. In some runs, small amounts of CH~ were found, but no OH. Test of the mass spectrometric sensitivity for OH radicals was carried out using the reaction F + HaO ~ HF + OH for OH production. Based on these measurements, the reaction pathway to CH 2 + OH is excluded, but the origin of CH~ in some experiments remains unknown. No trace of the addition complex CH30 is ever observed and the reaction scheme is concluded to be: O + CHa"-* HCHO + H

Results and Discussion CH2 + OH

1. Test of the Procedure The experimental setup and conditions are tested by investigatingthe well-known reaction O + C a H6. Propylene is injected at high dilution via the convergent part of the nozzle and the reaction products are found to be the same as reported in the literature. ~.~2.~No C a H ~O is found, indicating wallfree reaction conditions. The measurement of the conversion rate (A [C~H6] / [C~H,] o) and the known rate coefficient ~ indicate that reactions of O + alkyl R can be observed if k(O + R) > 10~a cma/mol s. For the generation of hydrocarbon radicals, the high reactivity of the F-atom makes it a rather non-specific source agent, t7 as opposed to the OH radical which preferentially attacks H-atoms in the tertiary (>secondary > primary) position,a Most of the measurements reported here are obtained using F-atom attack on the parent hydrocarbon, in order to boost the production of the primary alkyl radicals whose reactions are of most interest here. This also ~voids the problems which arise with OH-attack, where persistence of the original OH confuses product identification in the O + alkyl reaction.

2. The Reactions of Alkyl Radicals with Oxygen Atoms The CH 3 + O Reaction The CH~ radical is produced via H + C2H 4 or, preferably, via F + CH 4 which avoids a high

"-* HCO + Ha

(?)

CHaO These findings are in excellent agreement with the measurements of Bayes et al.Y TM Gutman et al., 2z Niki et al. ~6'27 and of former experiments of this laboratory by Gehring.~

The CaH~ + O Reaction The products of this reaction show signals at mass peaks characteristic of CHaCHO and HCHO. The corresponding reaction products, H and CHa, are masked by fragments of reactants (C2H8, C2H~) and products. In no experiment is the abstraction reaction (OH + C2H4) or the addition reaction (C2H50) observed. These measurements establish the reaction mechanism: O + C 2H5---~ CHaCHO + H HCHO + CH~

(a~) (a~)

-~ OH + C a l l 4 C~H.~O The intensities at mass peaks 44 (CHaCHO), 43 (CHACO), 30 (HCHO), 29 (CHO), and 16 (CH 4 from CHaCHO), combined with the independent calibration of CHaCHO and HCHO (via 0 + CHa), give the branching ratio of (a~):(a2) = (5 _+ 1):1. There are no other direct measurements of 0 +

521

ALKYL RADICALS C~H~ to our knowledge. Ashmore et al., 29 studying the reaction O + C~H6 ~ OH + C2H5, found CH3CHO and H C H O in the ratio of about 1:10. As discussed in their paper and the comment, H C H O is not thought to be solely a primary product of O + C2H5 9 A reaction mechanism consistent with their observations would be (a~), (a2), O + CH 3, and H + C~Hs --~ 2CH~, which is known to be fast, 3~ and O + aldehydes. The C 3 H r + 0 Reaction

When F-atoms react with propane, two isomers of propyl radicals (n-C3H 7, iso-C3Hz) are formed in the concentration ratio 3:1. These different propyl radicals can produce different reaction products in the C3Hv + O reaction. Figure 4 gives the measured relative ion signals for some key mass peaks. The cracking patterns of the pure substances C 2H~CHO, C H , COCH3, CH~CHO, and H C H O are recorded under the same experimental conditions. The absolute mass spectrometric sensitivities are determined from analyses of equimolar gas samples, prepared from their mixture in double stage saturators. These measurements make it possible to split up the total ion counts at the different mass peaks, giving the identity of the reaction products and their relative production rate. H C H O formed directly from the reaction is

o

o

o

o

o

o

o

"I-

-I-

-r e~

"Ieq

:I~ t~

It~

.I. e~

29

30

AA

57

58

59

43

FIc. 4. Relative ion signals as observed in the reaction O + C3H v ( ~ : contributions from C z H s C H O ; B: contributions from C H 3 C O C H 3, []]: contributions from CH 3CHO, []: contribution from HCHO).

difficult to estimate, first because of the calibration error margin of +20%, and secondly because of the error progression due to contributions from all other species at mass 29. Nevertheless, the ratio of the remaining ion signals at 29 and 30 is the same as that for the H C H O produced in O + CH 3. The signal at mass 59 is fully explained by the isotopic contribution from 58, leaving no evidence for an addition complex C3HrO. No formation of OH is observed. The reaction mechanism follows as O + (n, iso)-CaHT---~ C 2 H ~ C H O + H

(a~)

H C H O + C2H 5

(a2)

--~ C H 3 C O C H 3 + H

(bL)

CH~ CHO + CH 3

(b2)

-~ OH + C ~ H , C~HTO non-deuterated propyl radicals, especially the ratio of (a I + a2):(b ~ + b2) ~ 6:1.5. These experiments with specific labelled propyl radicals verify (i) the assignment of the reaction products due to primand sec-C3H 7 radical reaction and (ii) the quantitative determination of the different reaction paths. The C4H9 + O Reaction The F + iso-C4 H ~oreaction produces the primary butyl radical (iso-C4Hg) and the tertiary butyl radical (tert-C4H9) in the ratio of 9:1. Thus in the O + C4H 9 reaction the products of O + primary alkyl are mainly to be expected. The branching ratios for the reactions are found to be: (a~):(b~) = (7.3 + 1):1, (a~):(b2) = (7.2 ___ 1):1, and (at):(a2) = (6 + 1.5):1. The ratio (a~ + a2):(bt + b2) = 6:1.5, which corresponds fairly well to the abundance ratio [n-CaHv]: [iso-C3H7]. This suggests that a~ and a 2 are the product channels resulting from attack on the primary radical: (n-propyl), which is thus characterized by formation of (H + aldehyde) and (formaldehyde + alkyl radical), exactly as in the case of O + CH~ and O + C2H 5. The channels b~ (ketone + H) and b z (higher aldehyde + alkyl radical) are consistent with attack on the secondary radical, and are formed in the ratio (b~):(b2) = 1:1. This interpretation has been confirmed by an independent experiment. The use of 2.2-dideuteropropane, CH 3CD 2CH 3, as source for propyl radicals yields prim-CH2CD2CH 3 (mass 45) and secC H 3 C D C H 3 (mass 44) in the concentration ratio of 6:2. The reactions of these propyl radicals with O atoms lead to the primary products, represented in the reaction scheme:

CH2CD2CH31 + 0 --~ C H 3 C D z C H O + H (al) CH3CDCH:~ J

~

H C H O + C H 3 C D z (a2)

522

KINETICS --* CHaCOCH a + D

(bt)

--* CHaCDO + CH~

('02)

The precursor for the products of channels (at) and (a2) is the prim-CaHsD= (2 D atoms in propionaldehyde, no D atom in formaldehyde), the precursor for the products of channels (bt) and (b2) is the sec-CaH6D (no D atom in acetone, D atom in the aeetyl group of acetaldehyde). Calibration of the products formed give the efficieneies of the different reaction channels: (a~):(bt) = 5.4:1; (at):(b~)= (9 + 1.5):1; (at):(b=) = (5.4 + 1):1. These branching ratios agree with those of the experiments with Figure 5 gives the relative ion intensities for some masses, representative of the main reaction products: isobutanal, formaldehyde, and acetone. There is also some formation of an oxygen-containing product C4H60, but for which no structure (epoxide, dimethyl ketene) and no conceivable reaction path could be found. No OH and no addition complex C4HoO are detected; the registered ion signal at 73 is due to the isotopic contribution of C4HaO. The reaction scheme is given by O + (iso, tert)-C,Ho--* iso-CaHrCHO + H HCHO + Call r

(a,) (%)

--* CHaCOCHa + CHa ~r OH +

(e)

C~H~

C4H90

Calibration procedures give the branching ratios (at):(c) = (9 + 2):1, and (at):(a=) = (7 + 2):1. These findings are explicable if (at) and (a~) are the channels available to the primary alkyl radical (iso-C4H a) + O and (c) is the channel for the tertiary alkyl radical tert-C4Hg + O. (The isomerization of alkoxy radicals is thought to be slowa~ under our conditions)

3. Discussion In none of the four reactions is an abstraction reaction resulting in OH + alkene observed, although this route is energetically accessible e.g. O + C, H5 ~ OH + C2H4:270 kJ/mole exothermal. The addition complex RO is not found, although the RO radicals are a stable entity and occur in many reacting systems. This does not necessarily mean that the O + alkyl reaction proceeds directly to the observed products because, under our conditions, it is an intermediate. Any RO will be so highly excited by chemical activation that it will decompose within the transfer time from the reaction center to the detection point. The lifetimes of RO can be estimated via AH~ (RO), AH~ (aldehyde + R')3~ and the activation energies for thermal decomposition of RO using unimoleeular reaction theory. ~'~' This leads to lifetimes on the order of I 0 - " - I0 -~~ sec. It can be argued that, for the O + alkyl reaction, the RRK calculation favors the split of the C--C bond over the C - - H bond because of the lower bond energy of the former. This is true for thermal decomposition of RO, where the energies involved (activation energy, difference in reaction enthalpy for different reaction channels) are similar. However, for chemical activation of the complex, (e.g. O + C, H~ ~ C= H 5O, AH* = -380 kJ/mole) the available energy greatly exceeds the activation energy for thermal decomposition (53 kJ/mol) to either of the product states. In the decomposition of photochemically produced C,H~O, with high internal energy, there is evidence for reactions (at) and (a=).a~ Moreover it is reported that iso-propoxy radicals (CHa)2CHO, formed in the reaction CH a + CHa CHO, decompose to CH3COCH 3 + H via the mechanisma5 CH 3 + CHaCHO ~ (CHa)~CHO

-I-

--* CH3COCH ~ + H (blt)

u

~7

,Y

,.7

u"

J

.~

29

43

44

58

71

72

73

FzG. 5: Relative ion signals as observed in the reation O + C4H a (C4H 9 prepared via F + isoC4Hto ) ( , : contribution from iso-CaI"ITCHO, B: contribution from CHaCOCHa, I"1: contribution from HCHO).

In the O + iso-CaH 7 reaction reported here both reaction paths (bt, b2) are observed. The primary products of the reactions of OH radicals with alkenes ~ have been found to proceed via a complex of the RO type. These product distributions differ from those of the O + R reactions, indicating different energy situations and structures, which is

ALKYL RADICALS to be expected from the higher n u m b e r of states available in the O + alkyl reactions.

Acknowledgment The continuous interest and encouragement of Prof. H. Gg. Wagner is gratefully acknowledged.

REFERENCES 1. HUIE, R. E., AND HEBRON, J. T.: Progr. Reaction Kinetics, Vol 8, p. 1 (1975). 2. JONES, W. E., MACKNIGHT, S. D., AND TENG, L.: Chem. Bey. 73, p. 412 (1972). 3. GREINER, N. R.: J. Chem. Phys. 53, 1070 (1970). 4. LEWIS, B., AND VON ELBE, G.: Combustion, Flames, and Explosions of Gases, Academic Press 1961, p. 90 (New York). 5. Josr, W. (ed.): Low Temperature Oxidation, Gordon and Breach, Science Publishers, New York (1965), p. 191. 96. POLLAnD,R. T.: Comprehensive Chemical Kinetics (ed. C. H. Bamford, C. F. H. Tipper), Vol 17, p. 249 (1977), Elsevier Publishing Comp. Amsterdam. 7. l~oss, F. SLAGLE,I. R., AND GtrrMAN, D.: J. Phys. Chem. 78, 662 (1974). 8. BLUMENBERG,B., HOYERMANN,K. AND S1EVERT, R.: Sixteenth Symposium (International) on Combustion, p. 841, The Combustion Institute, 1977. 9. SLAGLE, I. R., GILBERT, J. R., GRAHAM,R. E., AND GtrrMAN, D.: Int. J. Chem. Kinet. Syrup. 1, 317 (1975). 10. WAsmnA, N., ANDBAYES,K. D.: Chem. Phys. Letts. 23,373 (1973); Int. J. Chem. Kinet. 8, 777 (1976). 11. SLAOLE,I. R., Pnuss, F., ANn GUTMAN, D.: Int. J. Chem. Kinet. 6, 111 (1974). 12. BALDWIN,R. R., BENNETT,J. p., ANDWALKER,R. W.: Sixteenth Symposium (International) on Combustion, p. 1041, The Combustion Institute, 1977. 13. HAMPSON, JR, R. F., ANn GARWIN, D.: Chemical Kinetic and Photochemical Data for Modelling Atmospheric Chemistry, NBS Technical Note, 866 (1975). 14. PE~rEas, J., ANDMAHNEN, G.: Fourteenth Symposium (International) on Combustion, p. 133, T h e Combustion Institute, 1973.

523

15. PEARSON, R. K., COWLES, J. O., HERMANN, G. L., GBEGG, D. W., AND CREIGHTON, J. R." I E E E J. Quant. Elec. 9, 879 (1973). 16. WAGNEn, H. GG., WARNATZ, J., AND ZETSCH, C.: Anals. Assoc. Quim. Argentina 59, 169 (1971). 17. FOON, R., AND I~UFMAN, M.: Progr. Reaction Kinetics, Vol. 8, p. 81 (1975). 18. GOLDEN, D. M., CHOP, K. Y., PERONA, M. J., AND PISZKmWCZ, L. W.: Int. J. Chem. Kinetics 8, 381 (1976). 19. GOLDEN, D. M., PiszmEvncz, L. W., PERONA, M. J., AND BEADLE, P. C.: J. Am. Chem. Soc. 96, 1645 (1974). 20. CHOO,K. Y., BEADLE,P. C., PISZK1EWICZ,L. W., AND GOLDEN, D. M.: Int. J. Chem. Kinetics 8, 45 (1976). 21. ROSENSTOCK,H. M., DI~XL, K., STEINER,B. W., AND HEnaON, J. T.: Energetics of Gaseous Ions, J. Phys. Chem. Ref. Data 6, Suppl. 1, p. 284 (1977). 22. KANOFSKY,J. R., LUCAS, 0., GUTMAN, D.: Fourteenth Symposium (International) on Combustion, p. 285, The Combustion Institute, 1973. 23. JONES, I. T. N., AND BAYES, K. D.: J. Am. Chem. Soc. 94, 6869 (1972). 24. WASHIDA,N., AND BAYES,K. D.: Chem. Phys. Letters 23, 373 (1973). 25. SLAGLE,I. R., PRUSS,JR, F. J., ANDGUTMAN,D.: Int. J. Chem. Kinetics 6, 111 (1974). 26. Monms, E. D., ANDNIm, H.: Int. J. Chem. Kinetics 5, 47 (1972). 27. NIKI, H., DABY, E., AND WEINSTOCK, B.: J. Chem. Phys. 48, 5729 (1968). 28. GEHmNG, M.: Ph.D. Thesis, GiSttingen (1971) 29. PAPADOeOULOS,C., ASHOMOaE, P. G., AND TYLER, B. J.: Thirteenth Symposium (International) on Combustion, p. 281, The Combustion Institute, 1971. 30. TENG, L., ANDJONES, W. E.: J. Chem. Soc., Faraday I 68, 1267 (1972). 31. FISH, A.: Quart. Rev. 18, 243 (1964). 32. BENSON, S. W.: Thermoehemical Kinetics, 2ed. Wiley, New York (1976). 33. O'NEAL, H. E., AND BENSON, S. W.: Kinetic Data on Gas Phase Unimolecular Reactions, NSRDSNBS 2I, Washington, D.C., 1970. 34. GRAY, P., SHAW, R., AND THYNN, J. C. J.: Progr. Reaction Kinetics, Vol. 4, p. 6,3 (1967). 35. Lw, M. T. H., AND LAIDLER, K. J.: Can. J. Chem. 40, 479 (1968).

COMMENTS W. C. Gardiner, University of Texas, USA. Your results for O + CH a suggest an upper limit for the H C O + H I channel of < 20%. Since the O + C H 3 reaction is expected to be an important pathway

in hydrocarbon flames, it would be useful for modeling purposes to know whether the H C O + H z channel exists at all. Are you aware of any experimental evidence which bears on this point?

524

KINETICS

Authors" Reply. T h e u p p e r limit of 20% for c h a n n e l C H a + O ~ H C O + H a is the m a x i m a l error, c o n s i d e r i n g a b s o l u t e calibration of H C H O . T h e d e t e r m i n a t i o n of H a c o n c e n t r a t i o n at r e a s o n a b l e m e a s u r i n g time (i. e. w o r k i n g at i o n i s a t i o n energies, w h e r e already c o n t r i b u t i o n from f r a g m e n t a t i o n from o t h e r h y d r o g e n c o n t a i n i n g s p e c i e s is observed) gives a n u p p e r limit of a r o u n d 5%. T h i s is c o n s i s t e n t w i t h the results of B a y e s et al. (ref. 23,24). Experim e n t s d i r e c t e d to a m o r e accurate d e t e r m i n a t i o n of t h e b r a n c h i n g ratio are u n d e r w a y in this laboratory.

s t u d y t h e s e e n d o t h e r m i c r e a c t i o n s in o u r p r e s e n t system.

1. Peeters, University of Leuven, Belgium. I n your i n t r o d u c t i o n , y o u s t a t e d that t h e reaction o f C~H 4 w i t h O H y i e l d s m a i n l y C H a + I-ICHO (and H + C H a C H O ). I feel o n e s h o u l d be careful in interpreti n g t h e r e s u l t s o f " c r o s s e d m o l e c u l a r b e a m " or Laval-nozzle w o r k for r e a c t i o n s s u c h as the above w h e r e t h e m e c h a n i s m is p r o b a b l y of the type C2H 4 + O H ~-- C 2 H 4 O H *

D. B. Smith, British Gas, U. K. A q u e s t i o n conc e r n i n g the a p p l i c a t i o n of your r e s u l t s to h i g h t e m p e r a t u r e flames: do y o u t h i n k y o u r r e s u l t s can be u s e d at h i g h t e m p e r a t u r e s , or c o u l d a d d i t i o n a l p r o d u c t c h a n n e l s b e c o m e active?

Authors'Reply. T h i s q u e s t i o n c a n n o t be a n s w e r e d in general, s i n c e in s o m e c a s e s new c h a n n e l s m i g h t o p e n u p energetically at h i g h e r t e m p e r a t u r e s . H o w e v e r , if the c h a n n e l s are h i g h l y e x o t h e r m i c (as (a), (b), (c)) one w o u l d expect only a s m a l l t e m p e r a ture d e p e n d e n c e of t h e b r a n c h i n g ratios in m o s t eases.

V. V. Azatyan, Academy of Science, USSR. Are y o u s t u d y i n g the kinetics o f r e a c t i o n s b e t w e e n atomic oxygen and saturated hydrocarbon: O+RH~

RO+H,

w h i c h is similar to O + C2H~ = C z H 4 0 + H (a) ?

Authors" Reply. T h e m o r e likely p r o d u c t s from O + R H are R + O H , b u t it is n o t p o s s i b l e to

9

[~CHa + C H 2 0

C2HH4OH --~tH~ + C H 3 C H O C2HH,OH ~ + M ~ C , H 4 O H I n t h i s case, CI.I a + H C H O (and H + CH~CHHO) m i g h t be t h e major p r o d u c t s at t h e very low p r e s s u r e s of c r o s s e d m o l e c u l a r b e a m work, b u t at p r e s s u r e s above s a y a f e w torr, t h e a d d u c t w o u l d be the d o m i n a n t p r o d u c t . T h i s in fact b o r n e o u t b y the overall rate b e i n g linearly d e p e n d e n t o n p r e s s u r e in the r a n g e of a few tort to several h u n d r e d torr.

Authors" Reply. It is true that the O H + olefin reactions p r o c e e d m a i n l y via an a d d i t i o n m e c h a n ism, f o r m i n g a n i n t e r m e d i a t e (olefin 9 O H ) , w h i c h h a s b e e n d e t e c t e d e v e n at l o w p r e s s u r e s ( a r o u n d 1 torr). O u r s t u d i e s o n t h e p r i m a r y p r o d u c t s o f O H + olefin will be p u b l i s h e d e l s e w h e r e . I n t h e e x t r e m e l y fast alkyl + O reaction, w h i c h is t h e topic of this paper, t h e a d d i t i o n c o m p l e x is n e i t h e r e x p e c t e d (short lifetime) n o r detected. It m i g h t b e n o t e w o r t h y that in o u r nozzle reactor c o n t i n u u m flow is realized w i t h s p e e i f i e d temperature a n d p r e s s u r e . T h e r e f o r e t h e r e s u l t s p r e s e n t e d are t h o u g h t to b e o f direct r e l e v a n c e to c o m b u s t i o n systems.