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Pressure dependence of chemical branching in the oxygen-atom reaction with allyl chloride
Volume 103,number 5
CHEMICAL PHYSICS LE-I-PERS
13 January 1984
PRESSURE DEPENDENCE OF CHEMICAL BRANCHING IN THE OXYGEN-ATOM REACTION WITH ALLYL CHLORIDE Jong-Yoon
PARK, Paul F. SAWYER
Michael C. HEAVEN and David GUTMAN
Department of Chemistty, Rhois hstitute of Technology, Chicago. Illinois 60616, USA Received 28 July 1983; in fmaJ form 20 October 1983
The pressure dependence of the rate constant of the O-atom reaction with ally1 chloride and its branching fraction to produce HCO + CH&1CH2 were measured over a seven-fold pressure range (0.5-3.5 Torr) at 299 K. Both are independent of pressuref<20% change). The results suggest that the rearrangements in the energy-rich adduct which are required to yield the products of the route studied are not collision induced.
1. Introduction The gaseous reactions of oxygen atoms with olefins involve the formation of energy-rich adducts which are capable of decomposing in a variety of ways [I ,2] _ Recent studies directed at identifying the overall pathways of these reactions and at measuring their importances have obtained significantly different results [3-81. In particular, molecular-beam studies conducted under collision-free conditions have generally observed the dominance of reactive routes which involve simple loss of atoms or pre-existing functional groups from the adduct [7,8], while those conducted under homogeneous conditions (at pressures in the range l-800 Torr) have also observed important routes which must involve internal rearrangements of the energy-rich 0.olefm adduct prior to its decomposition [3-5]_ Hunziker et al. [4] have presented a more detailed mechanism for these reactions which incorporates new knowledge of their potential-energy surfaces. An important feature of the new mechanism is that it offers an explanation of the apparent differences in the reactive routes observed under collision-free and under homogeneous conditions. The explanation involves the recognition that two distinct energy-rich adducts can be formed in the initial addition step, e.g. q3P)
408
+ C,H,
+ [3C,H,0]*.
(1)
The first is a n,o complex which has a large energy barrier in the exit channel preventing product formation on the triplet surface, while the other, the K,II complex, has a much lower barrier to decomposition (via H-atom loss in the case of the 0 + C2H, reaction). The energy-rich n,s complex decomposes essentially immediately via loss of an atom or functional group. The rr,,o complex must either redissociate to reactants or, under suitable conditions, undergo collision-induced processes such as intersystem crossing to the singlet ground state. Once on this surface, rearrangement of the still energy-rich adduct by H-atom migration is likely due to the low barrier for l,‘>-H-atom migration in this electronic state, and subsequent decomposition to yield other products would be expected (e.g. CH3 + HCO in the 0 + C2Hq reaction). Support for this more detailed model of 0 + olefm reactions would come from direct observation of an increasing importance of a collision-induced route with increasing pressure. For this reason we have
begun a series of studies to directly measure these dependences. One such observation has been reported already, that of Temps and Wagner 191, who report a small increase in the branching fraction of the route 0 + C,H,
+ RCO + CH,
(2)
from =0.35 to ~0.6 between 0.75 and 3 Torr total pressure. Hunziker et al. [4] observed no pressure dependence in this same branching fraction at higher pressures, 80-760 Torr. 0 009-2614/84/S 03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
Volume 103.
CHEMICAL
number 5
It is not apparent
whether
the potential
PHYSICS
energy
surfaces of other O-atom reactions with olefins (other than the 0 + C2Hq reaction) have the same features which would result in the coexistence of both colli-
sion-induced and collision-independent reactive routes. The most likely collision-induced processes in 0 + olefin reactions taken as a group are those which must involve 1,2-H-atom migrations in the adduct because of the likelihood that such rearrangements generally involve significant energy barriers in the triplet adduct [4]. We are therefore initially studying the reactive routes of O-atom reactions with terminal olefins which yield HCO since these routes must involve such H-atom migrations. We report here our study of such a route in the 0 + ally1 chloride reaction,
0 + CHzCICHCHz
-+ HCO + CH,CICH,.
(3)
Collision-induced effects at modest pressures (
2. Experimental The studies were performed using a fast-flow reactor coupled to a photoionization mass spectrometer detector. Gas emerging from the end of the reactor is formed into a beam by a conical skimmer before entering the mass spectrometer vacuum chamber where it is photoionized using intense atomic resonance lamps before mass analysis with a quadrupole mass filter. Details of the construction and performance of the apparatus, the gas handling procedures, and the data reduction methods have been described in earlier pubications [5,10,11]. One major change was made in the apparatus in
13 January 1984
LETTERS
those experiments which were performed to measure the branching fraction of reaction (3). It involved the use of laser photolysis of NO, as a pulsed source of oxygen atoms. The NO2 was photodissociated by the 355 MI radiation from a frequency-tripled Nd/YAG laser (Quanta-Ray DCR-2). Three prisms directed the unfocused 0.6 cm diameter beam along the axis of the 1.27 cm inner diameter tubular Pyrex flow reactor.
The annular beam of the laser is partly absorbed on the walls of a cone located at the far end of the reactor, which contains the gas-sampling orifice at its apex. The remaining portion of the beam is reflected
in the direction of the gas which is flowing out of the reactor. Typically the laser was operated at 11 Hz at an output of 35 mJ/pulse. To measure the branching fraction of route (3). HCO and NO concentration profiles were measured during the course of the 0 + ally1 chloride reaction. The laser photolysis of NO, decomposed 1% of the NO2 and initiated the following sequence of reactions which affected the HCO concentration during the experiment: NO, +kt(355
nm)+
0 + CH,CICHCH,
NO + O(‘P).
+ HCO + CH,ClCH,.
0+NOz+NO+02, HCO + first-order
(4) (3) (5)
losses.
(6)
Radical-radical processes are entirely negligible due to the extremely low free-radical concentrations compared to those of CH,ClCHCH, and NO?. Reaction (6) represents both kinetically first-order processes undergone by HCO. a reaction with NO2 and heterogeneous wall loss. Under these conditions
I(HCO+),
the HCO+ ion signal.
is given by
I(HCO+) = I(HCO+), X [exp(-At)
[A/@ - A)]
- esp(-Bt)].
(-4)
where A = k3t [CH-,CICHCH2] + kS [NOz] and B = k,. k,,, the overall rate constant of the 0 + ally1 chloride reaction, was measured in separate experiments and the well-established value of k, was taken from the literature (9.3 X 1O-12 cm3 molecule-’
Volume 103, number 5 s:!)
[12].
The values of $-and
CHEMICAL the important
PHYSICS
13 Jariuary 1984
LETTERS
scal-
I(HO+),; were obtained from a nonlinear least-squares fit of the HCO* ion signal profies to eq. (A)_ I(HCO+)e~is also the ultimate HCO+ ion signal which would have been recorded had HCO not been consumed by any reactions after being produced by reaction (3) and is therefore the measure of the total amount of HCO produced by this reaction. The branching fraction of route (3) of the 0 + CH,ClCHCH, reaction, F3, is defined as F3 = -A[HCO] /A[01 f, the total yield of HCO from route (3) (A[HCO]) per oxygen atom consumed by the overall reaction (-A[O] 3. In each experiment to measure F,, both HCO+ and NO+ ion-signal profiles were recorded (alternately to ensure constant relative sensitivity during an experiment). The HCO+ profiles yielded 1(HCO+),-,, via eq. (A), and the NO+ profiles yielded I(NOtl)o, the initial NO+ ion signal at t = 0, which was the measure of the oxygen atom concentration produced by the NO2 photolysis. NO continues to be produced during the reaction due to secondary reactions involving the free radicals produced by the 0 + ally1 chloride reaction and NO*. It is possible that some of these secondary reactions produce additional HCO as products. Kinetic modeling of both the primary and the likely secondary processes which could occur in this system indicates that additional HCO production by secondary reactions would be noticeable only after the HCO produced by_ the primary process had decayed to half its maximum value. This late additional production of HCO would not alter significantly the value of I(HCO?, obtained by the fitting procedure described above. In future studies we hope to have improved detection sensitivity. which will permit conducting these studies at lower NO, concentrations. Under these improved conditions, the primary reaction will be more isolated from passible interfering secondary processes. Fig. 1 shows both the HCO’ and the NO+ ion-signal profiles recorded during one set of experiments. Both-HCO and NO were monitored using photoionization at 10.2 eV. At this ionizing energy HCO+ is not produced by fragmentation reactions of other possible products of the 0 + ally1 chloride reaction *_
m/u
ing constant,
* Additional details of the experimental procedures used in this study as well as experiments to determine the observable fragmentation reactions of the vinoxy radical at 10.2 eV will be published elsewhere [ 131.
410
I
I
2250
I
m/e = 30
1 TIME
= 29
( HCO’)
I
I
( NO*)
1
I
I
I
0
5
10
15
AFTER
LASER
I
I
PULSE
1 I 20
(m set)
Fig 1. Temporal ion-signal profdes recorded during the 0 + CHaClCHCHs reaction Mass 29 signal (HCO’) was accumulated for twice the duration as that of the mass 30 ion signal (NO?. Fitted lines are obsaincd from eq. (A) for mass 29 and the function A +B[l - exp(-Ct)] for mass 30 to obtain the ion signals at f = 0. Conditions of the experiments: P = 3.92 Torr (He d&rent), T = 297 K, u (gas flow velocity) = 11.7 m/s. [NOa] = 557 X 10” cmm3, [CHaClCHCHa ] = 8.64 X 1014 cm-s, [O Jo = 5.59 x 10’0 cm-s_
F3 was obtained from the ratio of the two extrapolated ion signals, the ratio of photoionization sensitivities for the two species [S(NO)/s(HCO)] under our experimental conditions **, and the fraction of the original oxygen atoms which reacted with ally1 chloride (obtained from the rate constants of the 0 + NO, and 0 + CH,C!lCHCH, reactions) *_ The results of four experiments, which covered a IO-fold range of pressure (0.37-3.9 Torr), are given in table 1. The overall rate constant was measured over nearly the same pressure range as were the branching fractions_ These additional experiments were conduct-
** Underour experimentalconditions S(NO)/S(HCO) f 0.04.
See also ref. [13].
= 0.28
Vblume 103. number 5
CHEMICAL
PHYSICS LEI-IERS
13 January 1984
Table 1 Conditions and results of experiments to measure the branching fraction of 0 + CH2 CHCH2 Cl - CHsCH,Cl 297:
-t HCO
reaction at
I#
P
”
(Torr)
(m s-l)
1 O-l2 [NO2 ] (molecules cmm3)
lo-l4 [C3Hs Cl] (molecules cmS3)
-lO-‘“AIO]o (molecules cmg)
-lo-“A[O]f (molecules cmb3)
lo-“A[HCO] (molecules cmS3)
(-Ail-JCO]/A]D]f)
0.37 0.96 195 3.92
10.0 11.9 11.4 11.7.
4.84 5.28 5.16 5.57
6.90 8.25 7.87 8.64
3.14 5.76 7.76 5.59
2.95 5.44 7.32 5.28
0.42 0.79 1.14 0.79
0.14 0.15 0.16 0.15
a) Estimated absolute accuracy ?35%, relative accuracy [i.e. F3(P2)/Fs(Pr)] ed using a continuous microwave
source of oxygen
atoms (a
discharge through an 02-He
gas mix-
ture). In these experiments the O-atom concentration was always in great excess over that of ally1 chloride_ The rate constants were obtained from the measured exponential
decay profiles of the ally1 chloride
the reactor and the measured The apparatus
O-atom
and experimental
along
concentrations_
procedures
for this
portion of the study are the same as were used in prior measurements of C&atom reaction rate conTable 2 Conditions and results of experiments
stants in table
F3
GO%. [6,10]
*-
The results
of this study
T* Observed first-order rate constants (kobs) Deere corrected for axial diffusion using k = kobs(l + kobpfv2), where D is the binary (reactant-carrier gas) diffusion coefficient and u is the flow velocity. Diffusion coefficients were calculated from Lennard-Jones (6-12) potential parameters from ref. [ 141. Parameters of CHaClCHCHa were taken as those of a similar compound (CHaCICHs). The corrections for axial diffusion were 4-14% at 0.6 Torr and 1-X at 3.5 T0l-L
to measure the rate constant of 0 + CHaCHCH2Ci
reaction at 299 i 4 I;
IO-t2 ]C3HSCl]o (molecttles cmm3)
1013 k (cm3 molecule-’
15.1 14.1 13.2
1.82 2.35 3.48
0.62
13.4
0.58 1.58
13.0 13.3
1.60
18.9
1.60 3.53 3.53 3.45 3.55
11.5 13.1 16.3 14.0 13.2
3.96 4.15 5.45 099 4.61 4.69 4.34 5.72 6.25 1.23 2.00 2.18 4.36
2.40 5.54 3.95 7.81 3.81 7.34 6.18 2.38 8.36 4.63 2.66 4.98 5.31 2.80 2.13 4.31 5.24 9.38
9.9 10.3 10.3 9.2 10.0 105 9.3 9.7 9.8 10.0 10.3 10.0 9.8 11.5 11.5 12.6 9.6 9.8
(Tom)
b
0.54 0.60 0.60
s-r)
are given
2.
10-14 [O]e (molecules cmm3)
P
a) a) a) =)
s-r)
-c 0.1 a) I 0.2 * 0.6 f 0.7 L 0.3 f 0.2 f 0.9 z 0.3 2 0.1 -c 0.3 f 0.3 i 0.1 i 0.4 i 0.7 f 0.4 i 0.3 * 0.2 f 0.6
avg. 10.2 -c 0.6 b, (10) a) Error limit is one standard deviation of fitted slope of ln c versus t. b, Error limit is one standard deviation of averaged values. Estimated absolute accuracy
is *20$%.
411
CHEMICAL PHYSICS LETTERS
Volume 103, number 5
(3) of the 0 + ally1 chloride reaction with increasing pressure in a pressure range where collisional processes are not expected to dominate the fate of the energy-rich O*allyl chloride adduct, suggests that this route is not collision induced. The internal rearrangements which are required in the adduct to yield the
All experiments were conducted at ambient temperature, 299 f 4 K.
3. Results and discussion The measured branching
fractions
of route (3) are
independent of pressure over the range of conditions covered, O-37-3.9 Torr (mostly He). The route is a
minor one, having a branching fraction of 0.15 + 0.05 in this pressure range. Since komparisons of branching fractions at different pressures do not re-
quire the use of mass spectrometer sensitivity measurements, changes in the branching fraction can be determined with more accuracy than can their absolute values. In these bperiments, the branching fraction did not change by more than 20% between the highest and lowest pressures of this study. No significant pressure dependence in the overall rate constant of the 0 + CH,CICHCH, reaction was detected. The average value of the 18 determinations of k3t is (1.02 f 0.09) X lo- ” ‘_ At the three pressures studied the average values are (0.99 + 0.05) X 10-l’ at 0.6 Torr, (0.99 + 0.02) X lo-l2 at 1.6
Studies of the similar 0 + propene reaction indicate that both the collisional and the unimolecular processes involving the energy-rich adduct have about the same rates near 100 Torr [ 1,2,4] _Near this pres-
sure, Cvetanovid [ 1,2] has observed a marked increase with pressure of products which are stabilized adducts, and Hunziker et al. [4] have observed a comparable major decrease with pressure of the branching fraction of one of the reactive routes, (7)
Unfortunately neither of these observations indicates whether there are collisiorrinduced routes other than stabilization. Only the observation of a route whose impoaance increases with increasing pressure would provide such evidence.
The absence of an increasing ’ All units are cm3 molecule-’
importance
of route
s-l, and the error limits indicated in parentheses are the standard deviations of the several determinations. The estimated accuracy in k,t is +20%.
412
products of route (3) must therefore occur without the intervention of collisions_ The energy barriers which appear to prevent such rearrangements in the
OC,H, adduct must be significantly lower in the 0CH2CICHCH2 adduct. Additional studies of the pressure dependence of chemical branching in other 0 + olefin reactions are in progress. Acknowledgement
This research was supported by the National Science Foundation under Grant CHE-80-06573.
References R_J. CvetanoviC, Advan. Photochem.
Torr, and (1.10 f 0.05) X lo-l2 at 3.5 Torr. There have been no prior reported studies of this reaction.
0 + CH3CHCH2 + CH3 + CH2CH0.
13 January 1984
1 (1963)
115.
R-l. CvetanoviC,Can. J. Chem. 36 (1958) 623. B. Blumenberg, K. Hoyermann and R. Sieve& Proc. Intern Symp. Combust. 16 (1976) 841. H.E. Huruiker, H. Kneppe and H.R. Wend& J. Photothem. 17 (1981) 377. J.R. Kanofsky, D. Lucas and D. Gutman, Proc. Intern.
Symp. Combust. 14 (1973) 285. 1-R. Slagle, D. Gutman and J.R. Gilbert, Proc. Intern. Symp. Cornbust. 15 (1974) 785. R.J. Buss, RJ. Baseman, G. He and Y.T. Lee, J. Photothem. 17 (1981) 389. [8] G. He, R-I. Buss, R.J. Baseman, R. Tse and Y-T. Lee, J. Phys. Chem. 86 (198213547. [9] F. Temps and H.Gg. Wagner, Max-Plan&Institut fir Strsmungsforschung, Report 18 (December 1982). [lo] LR. Slagle, F.J. Pruss and D. Gutman, Intern. J. Chem Kinetics6 (1974) 111. [ll] J.-Y. Park and D. Gutman, J. Phys. Chem. 87 (1983) 1844. 1121 D.L. Baulch, RA. Cox, RF. Hampson, J-k Kerr, J. Tree and RT. watson, J. Phys. Chem. Ref. Data 9 (1980) 295. 1131 J.-Y. Park, P.F. Sawyer, M.C. Heaven and D. Gutman, 1. Phys. Chem.. submitted for publication. [14] RM. Fristrom and AA_ Westenberg, Flame structure (McGraw-Hill, New York, 1965) ch. 12.
CHEMICAL PHYSICS LE-I-PERS
13 January 1984
PRESSURE DEPENDENCE OF CHEMICAL BRANCHING IN THE OXYGEN-ATOM REACTION WITH ALLYL CHLORIDE Jong-Yoon
PARK, Paul F. SAWYER
Michael C. HEAVEN and David GUTMAN
Department of Chemistty, Rhois hstitute of Technology, Chicago. Illinois 60616, USA Received 28 July 1983; in fmaJ form 20 October 1983
The pressure dependence of the rate constant of the O-atom reaction with ally1 chloride and its branching fraction to produce HCO + CH&1CH2 were measured over a seven-fold pressure range (0.5-3.5 Torr) at 299 K. Both are independent of pressuref<20% change). The results suggest that the rearrangements in the energy-rich adduct which are required to yield the products of the route studied are not collision induced.
1. Introduction The gaseous reactions of oxygen atoms with olefins involve the formation of energy-rich adducts which are capable of decomposing in a variety of ways [I ,2] _ Recent studies directed at identifying the overall pathways of these reactions and at measuring their importances have obtained significantly different results [3-81. In particular, molecular-beam studies conducted under collision-free conditions have generally observed the dominance of reactive routes which involve simple loss of atoms or pre-existing functional groups from the adduct [7,8], while those conducted under homogeneous conditions (at pressures in the range l-800 Torr) have also observed important routes which must involve internal rearrangements of the energy-rich 0.olefm adduct prior to its decomposition [3-5]_ Hunziker et al. [4] have presented a more detailed mechanism for these reactions which incorporates new knowledge of their potential-energy surfaces. An important feature of the new mechanism is that it offers an explanation of the apparent differences in the reactive routes observed under collision-free and under homogeneous conditions. The explanation involves the recognition that two distinct energy-rich adducts can be formed in the initial addition step, e.g. q3P)
408
+ C,H,
+ [3C,H,0]*.
(1)
The first is a n,o complex which has a large energy barrier in the exit channel preventing product formation on the triplet surface, while the other, the K,II complex, has a much lower barrier to decomposition (via H-atom loss in the case of the 0 + C2H, reaction). The energy-rich n,s complex decomposes essentially immediately via loss of an atom or functional group. The rr,,o complex must either redissociate to reactants or, under suitable conditions, undergo collision-induced processes such as intersystem crossing to the singlet ground state. Once on this surface, rearrangement of the still energy-rich adduct by H-atom migration is likely due to the low barrier for l,‘>-H-atom migration in this electronic state, and subsequent decomposition to yield other products would be expected (e.g. CH3 + HCO in the 0 + C2Hq reaction). Support for this more detailed model of 0 + olefm reactions would come from direct observation of an increasing importance of a collision-induced route with increasing pressure. For this reason we have
begun a series of studies to directly measure these dependences. One such observation has been reported already, that of Temps and Wagner 191, who report a small increase in the branching fraction of the route 0 + C,H,
+ RCO + CH,
(2)
from =0.35 to ~0.6 between 0.75 and 3 Torr total pressure. Hunziker et al. [4] observed no pressure dependence in this same branching fraction at higher pressures, 80-760 Torr. 0 009-2614/84/S 03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
Volume 103.
CHEMICAL
number 5
It is not apparent
whether
the potential
PHYSICS
energy
surfaces of other O-atom reactions with olefins (other than the 0 + C2Hq reaction) have the same features which would result in the coexistence of both colli-
sion-induced and collision-independent reactive routes. The most likely collision-induced processes in 0 + olefin reactions taken as a group are those which must involve 1,2-H-atom migrations in the adduct because of the likelihood that such rearrangements generally involve significant energy barriers in the triplet adduct [4]. We are therefore initially studying the reactive routes of O-atom reactions with terminal olefins which yield HCO since these routes must involve such H-atom migrations. We report here our study of such a route in the 0 + ally1 chloride reaction,
0 + CHzCICHCHz
-+ HCO + CH,CICH,.
(3)
Collision-induced effects at modest pressures (
2. Experimental The studies were performed using a fast-flow reactor coupled to a photoionization mass spectrometer detector. Gas emerging from the end of the reactor is formed into a beam by a conical skimmer before entering the mass spectrometer vacuum chamber where it is photoionized using intense atomic resonance lamps before mass analysis with a quadrupole mass filter. Details of the construction and performance of the apparatus, the gas handling procedures, and the data reduction methods have been described in earlier pubications [5,10,11]. One major change was made in the apparatus in
13 January 1984
LETTERS
those experiments which were performed to measure the branching fraction of reaction (3). It involved the use of laser photolysis of NO, as a pulsed source of oxygen atoms. The NO2 was photodissociated by the 355 MI radiation from a frequency-tripled Nd/YAG laser (Quanta-Ray DCR-2). Three prisms directed the unfocused 0.6 cm diameter beam along the axis of the 1.27 cm inner diameter tubular Pyrex flow reactor.
The annular beam of the laser is partly absorbed on the walls of a cone located at the far end of the reactor, which contains the gas-sampling orifice at its apex. The remaining portion of the beam is reflected
in the direction of the gas which is flowing out of the reactor. Typically the laser was operated at 11 Hz at an output of 35 mJ/pulse. To measure the branching fraction of route (3). HCO and NO concentration profiles were measured during the course of the 0 + ally1 chloride reaction. The laser photolysis of NO, decomposed 1% of the NO2 and initiated the following sequence of reactions which affected the HCO concentration during the experiment: NO, +kt(355
nm)+
0 + CH,CICHCH,
NO + O(‘P).
+ HCO + CH,ClCH,.
0+NOz+NO+02, HCO + first-order
(4) (3) (5)
losses.
(6)
Radical-radical processes are entirely negligible due to the extremely low free-radical concentrations compared to those of CH,ClCHCH, and NO?. Reaction (6) represents both kinetically first-order processes undergone by HCO. a reaction with NO2 and heterogeneous wall loss. Under these conditions
I(HCO+),
the HCO+ ion signal.
is given by
I(HCO+) = I(HCO+), X [exp(-At)
[A/@ - A)]
- esp(-Bt)].
(-4)
where A = k3t [CH-,CICHCH2] + kS [NOz] and B = k,. k,,, the overall rate constant of the 0 + ally1 chloride reaction, was measured in separate experiments and the well-established value of k, was taken from the literature (9.3 X 1O-12 cm3 molecule-’
Volume 103, number 5 s:!)
[12].
The values of $-and
CHEMICAL the important
PHYSICS
13 Jariuary 1984
LETTERS
scal-
I(HO+),; were obtained from a nonlinear least-squares fit of the HCO* ion signal profies to eq. (A)_ I(HCO+)e~is also the ultimate HCO+ ion signal which would have been recorded had HCO not been consumed by any reactions after being produced by reaction (3) and is therefore the measure of the total amount of HCO produced by this reaction. The branching fraction of route (3) of the 0 + CH,ClCHCH, reaction, F3, is defined as F3 = -A[HCO] /A[01 f, the total yield of HCO from route (3) (A[HCO]) per oxygen atom consumed by the overall reaction (-A[O] 3. In each experiment to measure F,, both HCO+ and NO+ ion-signal profiles were recorded (alternately to ensure constant relative sensitivity during an experiment). The HCO+ profiles yielded 1(HCO+),-,, via eq. (A), and the NO+ profiles yielded I(NOtl)o, the initial NO+ ion signal at t = 0, which was the measure of the oxygen atom concentration produced by the NO2 photolysis. NO continues to be produced during the reaction due to secondary reactions involving the free radicals produced by the 0 + ally1 chloride reaction and NO*. It is possible that some of these secondary reactions produce additional HCO as products. Kinetic modeling of both the primary and the likely secondary processes which could occur in this system indicates that additional HCO production by secondary reactions would be noticeable only after the HCO produced by_ the primary process had decayed to half its maximum value. This late additional production of HCO would not alter significantly the value of I(HCO?, obtained by the fitting procedure described above. In future studies we hope to have improved detection sensitivity. which will permit conducting these studies at lower NO, concentrations. Under these improved conditions, the primary reaction will be more isolated from passible interfering secondary processes. Fig. 1 shows both the HCO’ and the NO+ ion-signal profiles recorded during one set of experiments. Both-HCO and NO were monitored using photoionization at 10.2 eV. At this ionizing energy HCO+ is not produced by fragmentation reactions of other possible products of the 0 + ally1 chloride reaction *_
m/u
ing constant,
* Additional details of the experimental procedures used in this study as well as experiments to determine the observable fragmentation reactions of the vinoxy radical at 10.2 eV will be published elsewhere [ 131.
410
I
I
2250
I
m/e = 30
1 TIME
= 29
( HCO’)
I
I
( NO*)
1
I
I
I
0
5
10
15
AFTER
LASER
I
I
PULSE
1 I 20
(m set)
Fig 1. Temporal ion-signal profdes recorded during the 0 + CHaClCHCHs reaction Mass 29 signal (HCO’) was accumulated for twice the duration as that of the mass 30 ion signal (NO?. Fitted lines are obsaincd from eq. (A) for mass 29 and the function A +B[l - exp(-Ct)] for mass 30 to obtain the ion signals at f = 0. Conditions of the experiments: P = 3.92 Torr (He d&rent), T = 297 K, u (gas flow velocity) = 11.7 m/s. [NOa] = 557 X 10” cmm3, [CHaClCHCHa ] = 8.64 X 1014 cm-s, [O Jo = 5.59 x 10’0 cm-s_
F3 was obtained from the ratio of the two extrapolated ion signals, the ratio of photoionization sensitivities for the two species [S(NO)/s(HCO)] under our experimental conditions **, and the fraction of the original oxygen atoms which reacted with ally1 chloride (obtained from the rate constants of the 0 + NO, and 0 + CH,C!lCHCH, reactions) *_ The results of four experiments, which covered a IO-fold range of pressure (0.37-3.9 Torr), are given in table 1. The overall rate constant was measured over nearly the same pressure range as were the branching fractions_ These additional experiments were conduct-
** Underour experimentalconditions S(NO)/S(HCO) f 0.04.
See also ref. [13].
= 0.28
Vblume 103. number 5
CHEMICAL
PHYSICS LEI-IERS
13 January 1984
Table 1 Conditions and results of experiments to measure the branching fraction of 0 + CH2 CHCH2 Cl - CHsCH,Cl 297:
-t HCO
reaction at
I#
P
”
(Torr)
(m s-l)
1 O-l2 [NO2 ] (molecules cmm3)
lo-l4 [C3Hs Cl] (molecules cmS3)
-lO-‘“AIO]o (molecules cmg)
-lo-“A[O]f (molecules cmb3)
lo-“A[HCO] (molecules cmS3)
(-Ail-JCO]/A]D]f)
0.37 0.96 195 3.92
10.0 11.9 11.4 11.7.
4.84 5.28 5.16 5.57
6.90 8.25 7.87 8.64
3.14 5.76 7.76 5.59
2.95 5.44 7.32 5.28
0.42 0.79 1.14 0.79
0.14 0.15 0.16 0.15
a) Estimated absolute accuracy ?35%, relative accuracy [i.e. F3(P2)/Fs(Pr)] ed using a continuous microwave
source of oxygen
atoms (a
discharge through an 02-He
gas mix-
ture). In these experiments the O-atom concentration was always in great excess over that of ally1 chloride_ The rate constants were obtained from the measured exponential
decay profiles of the ally1 chloride
the reactor and the measured The apparatus
O-atom
and experimental
along
concentrations_
procedures
for this
portion of the study are the same as were used in prior measurements of C&atom reaction rate conTable 2 Conditions and results of experiments
stants in table
F3
GO%. [6,10]
*-
The results
of this study
T* Observed first-order rate constants (kobs) Deere corrected for axial diffusion using k = kobs(l + kobpfv2), where D is the binary (reactant-carrier gas) diffusion coefficient and u is the flow velocity. Diffusion coefficients were calculated from Lennard-Jones (6-12) potential parameters from ref. [ 141. Parameters of CHaClCHCHa were taken as those of a similar compound (CHaCICHs). The corrections for axial diffusion were 4-14% at 0.6 Torr and 1-X at 3.5 T0l-L
to measure the rate constant of 0 + CHaCHCH2Ci
reaction at 299 i 4 I;
IO-t2 ]C3HSCl]o (molecttles cmm3)
1013 k (cm3 molecule-’
15.1 14.1 13.2
1.82 2.35 3.48
0.62
13.4
0.58 1.58
13.0 13.3
1.60
18.9
1.60 3.53 3.53 3.45 3.55
11.5 13.1 16.3 14.0 13.2
3.96 4.15 5.45 099 4.61 4.69 4.34 5.72 6.25 1.23 2.00 2.18 4.36
2.40 5.54 3.95 7.81 3.81 7.34 6.18 2.38 8.36 4.63 2.66 4.98 5.31 2.80 2.13 4.31 5.24 9.38
9.9 10.3 10.3 9.2 10.0 105 9.3 9.7 9.8 10.0 10.3 10.0 9.8 11.5 11.5 12.6 9.6 9.8
(Tom)
b
0.54 0.60 0.60
s-r)
are given
2.
10-14 [O]e (molecules cmm3)
P
a) a) a) =)
s-r)
-c 0.1 a) I 0.2 * 0.6 f 0.7 L 0.3 f 0.2 f 0.9 z 0.3 2 0.1 -c 0.3 f 0.3 i 0.1 i 0.4 i 0.7 f 0.4 i 0.3 * 0.2 f 0.6
avg. 10.2 -c 0.6 b, (10) a) Error limit is one standard deviation of fitted slope of ln c versus t. b, Error limit is one standard deviation of averaged values. Estimated absolute accuracy
is *20$%.
411
CHEMICAL PHYSICS LETTERS
Volume 103, number 5
(3) of the 0 + ally1 chloride reaction with increasing pressure in a pressure range where collisional processes are not expected to dominate the fate of the energy-rich O*allyl chloride adduct, suggests that this route is not collision induced. The internal rearrangements which are required in the adduct to yield the
All experiments were conducted at ambient temperature, 299 f 4 K.
3. Results and discussion The measured branching
fractions
of route (3) are
independent of pressure over the range of conditions covered, O-37-3.9 Torr (mostly He). The route is a
minor one, having a branching fraction of 0.15 + 0.05 in this pressure range. Since komparisons of branching fractions at different pressures do not re-
quire the use of mass spectrometer sensitivity measurements, changes in the branching fraction can be determined with more accuracy than can their absolute values. In these bperiments, the branching fraction did not change by more than 20% between the highest and lowest pressures of this study. No significant pressure dependence in the overall rate constant of the 0 + CH,CICHCH, reaction was detected. The average value of the 18 determinations of k3t is (1.02 f 0.09) X lo- ” ‘_ At the three pressures studied the average values are (0.99 + 0.05) X 10-l’ at 0.6 Torr, (0.99 + 0.02) X lo-l2 at 1.6
Studies of the similar 0 + propene reaction indicate that both the collisional and the unimolecular processes involving the energy-rich adduct have about the same rates near 100 Torr [ 1,2,4] _Near this pres-
sure, Cvetanovid [ 1,2] has observed a marked increase with pressure of products which are stabilized adducts, and Hunziker et al. [4] have observed a comparable major decrease with pressure of the branching fraction of one of the reactive routes, (7)
Unfortunately neither of these observations indicates whether there are collisiorrinduced routes other than stabilization. Only the observation of a route whose impoaance increases with increasing pressure would provide such evidence.
The absence of an increasing ’ All units are cm3 molecule-’
importance
of route
s-l, and the error limits indicated in parentheses are the standard deviations of the several determinations. The estimated accuracy in k,t is +20%.
412
products of route (3) must therefore occur without the intervention of collisions_ The energy barriers which appear to prevent such rearrangements in the
OC,H, adduct must be significantly lower in the 0CH2CICHCH2 adduct. Additional studies of the pressure dependence of chemical branching in other 0 + olefin reactions are in progress. Acknowledgement
This research was supported by the National Science Foundation under Grant CHE-80-06573.
References R_J. CvetanoviC, Advan. Photochem.
Torr, and (1.10 f 0.05) X lo-l2 at 3.5 Torr. There have been no prior reported studies of this reaction.
0 + CH3CHCH2 + CH3 + CH2CH0.
13 January 1984
1 (1963)
115.
R-l. CvetanoviC,Can. J. Chem. 36 (1958) 623. B. Blumenberg, K. Hoyermann and R. Sieve& Proc. Intern Symp. Combust. 16 (1976) 841. H.E. Huruiker, H. Kneppe and H.R. Wend& J. Photothem. 17 (1981) 377. J.R. Kanofsky, D. Lucas and D. Gutman, Proc. Intern.
Symp. Combust. 14 (1973) 285. 1-R. Slagle, D. Gutman and J.R. Gilbert, Proc. Intern. Symp. Cornbust. 15 (1974) 785. R.J. Buss, RJ. Baseman, G. He and Y.T. Lee, J. Photothem. 17 (1981) 389. [8] G. He, R-I. Buss, R.J. Baseman, R. Tse and Y-T. Lee, J. Phys. Chem. 86 (198213547. [9] F. Temps and H.Gg. Wagner, Max-Plan&Institut fir Strsmungsforschung, Report 18 (December 1982). [lo] LR. Slagle, F.J. Pruss and D. Gutman, Intern. J. Chem Kinetics6 (1974) 111. [ll] J.-Y. Park and D. Gutman, J. Phys. Chem. 87 (1983) 1844. 1121 D.L. Baulch, RA. Cox, RF. Hampson, J-k Kerr, J. Tree and RT. watson, J. Phys. Chem. Ref. Data 9 (1980) 295. 1131 J.-Y. Park, P.F. Sawyer, M.C. Heaven and D. Gutman, 1. Phys. Chem.. submitted for publication. [14] RM. Fristrom and AA_ Westenberg, Flame structure (McGraw-Hill, New York, 1965) ch. 12.