R. Simonaitis and Julian Heicklen
Reactions of CH3, CH30,and CH302Radicals with O3 I?. Slmonaitls and Jullan Heicklen* Department of Chemistry and Ionosphere Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received July 5, 1974) Publication costs assisted by the National Science Foundation, the National Aeronautics and Space Administration, and the Department of Defense
Ozone was photolyzed at 253.7 nm a t 25 and -52' in the presence of CH4 and 0 2 to measure the reactions of O3 with CH3, CH30, and CH3O2. The O(lD) atoms produced in the primary photochemical act react with CH4 to give CH3 radicals which in turn can react with 0 2 to give CH3O2 and CH30 radicals. At very approached 1.0, indicating that O3 high values of [02]/[03],the quantum yield Of 0 3 disappearance, -@{03], reactions with CH3O2 and CH30 are slow. Upper limits to the rate coefficients a t 25' were computed to be 2.4 X and 2 X cm3/sec, respectively. At lower values of [ 0 2 ] / [ 0 3 ] , chain decomposition of O3 occurred which could be explained by the reaction of O3 with CH3 radicals to produce CH20, 0 2 , and H atoms all the time. The two routes to these products are CH3 O3 0 2 CH30* CH2O H (sa) and CH3 O3 CHzO HOz* ---* H 0 2 (9b). The competition between 0 3 and 0 2 for CH3 was measured: CH3 0 2 CH302 (10). It was found that (kg, h g b ) / k l o = 2.2 and 1.2, respectively, at 25 and -52". cm3/sec for klo, the Arrhenius expresWhen this ratio is combined with the literature value of 4.3 X sion becomes 129, k g b = 5.4 X exp(-1050/RT) cm3/sec. The large preexponential factor suggests a linear transition state, and thus reaction 9a is probably the preferred reaction channel.
+
+
--
+
+
-
+
-
+
+
+
+
Introduction The reactions of CH3, CH3O and CH3O2 radicals with 0 3 have never been studied, as far as we know. These radicals are produced in the earth's atmosphere by CH4 oxidation; therefore, their reactions with O3 are of interest. In the study reported here, the CH3 radicals are generated by the photolysis of O3 in the presence of excess CH4. The primary photolytic act produces O(lD) which abstracts an H atom from CHI to produce CH3. If 0 2 is also present in the system, it will add to CH3 to produce CH3O2. These radicals can produce CH30 by mutual interaction. Thus CH3, CH3O2, and CH30 can all be produced in the same system. From the measurement of the O3 removal quantum information regarding the reactions of these yield, -@(03), radicals with O3 is obtained. Experimental Section The experimental procedure is identical with that described in an earlier publication where O3 was photolyzed a t 253.7 nm in the presence of Hz to study the reactions of HO2 radicals.1 In the present study CH4 was used instead of Hz. The CH4 was from the Matheson Co., and it was used without further purification. The O3 removal was monitored photometrically, and initial quantum yields were obtained. Results Irradiation of CH4-03-02 mixture a t 253.7 nm at 25 and -52' leads to the removal of 03. The initial quantum yields for 0 3 removal, -@{03), are presented in Table I. In these experiments, the absorbed light intensity, I,, was varied by a factor of 15, the CH4 and O3 pressures each were varied by a factor of 5, and the 0 2 pressure was varied by a factor of 200. The results show that - @ { O 3 ] decreases regularly as the [ 0 2 ] / [ 0 3 ] ratio increases, but is otherwise relatively insensitive to other variables at a given temperature. In particular -@{O3] is independent of I,. The Journal of Physical Chemistry, Vol. 79, No. 4, 1975
Some experiments were also done at 25' in the "absence" of 0 2 . Actually some 0 2 (-20 mTorr) is always present because of 0 3 decomposition during the gas handling process. Initially -@{Os) is very large, approaching 100 in some of our experiments. However as the run progresses, 0 2 accumulates as a product, and the rate of O3 removal rapidly diminishes. The initial quantum yields are presented in Table 11. The experiments are listed in order of increasing [03]/Ia1/2 since the chain-propagating steps are expected to be proportional to [O,]; the radical concentrations are expected to be proportional to Za1/2. There may be a trend in the data, possibly -@{03] tends to rise along with [03]/Za1/2. However there is considerable scatter, and this probably reflects the variable amount of background 0 2 present a t the beginning of the run.
Discussion The initial act in the photolysis is the decomposition of O 3 by the well-known process 0,
+
hu (253.7 nm)
-+
-
'
O2 ( A)
+
O('D)
(1)
The O&A) will react predominantly with O3 o~(*A + )0,
202 +
o(3~)
(2)
The reactions of 0 2 ( l A ) with 0 2 and CH4 are negligible as can be shown by using the rate coefficients in Table 111. The O(3P) atoms produced in reaction 2 can react with 0 2 , 0 3 , and CH4 o(3m
+ o2 +
M
o ( ~ P+) O3
o(3~) + CH,
o3 +
M
202
QH + CH,
(3) (4)
(5)
The rate coefficients for reactions 3, 4,and 5 are also given in Table 111. The O(lD) will react predominantly (>97%) with CH4 as can be shown by the rate coefficients given in Table 111
Reactions of CH3,
CH30,
and CH302 Radicals with
299
03
TABLE 11: Photolysis of CH4-O3 Mixtures
TABLE I: Photolysis of CH4-02-03 Mixtures at 253.7 nm
at 253.7 nm and 25" I at
[CHdIt mTorr/ Torr min
[02It
[O,]/[O,] mTorr
Torr
-iP{03}
T = 25" 0,00116 0.00153 0.00247 0.004 1 0.0042 0.0057 0.0058 0.014 0.022 0.025 0.043 0.057 0.073 0.083 0.105 0.106 0.114 0.117 0.147 0.165 0.175 0.200 0.235 0.3 08 0.356 0.41 0.059 0.066 0.145 0.256 0.259 0.41
81 107 370 370 350 380 390 313 380 82 268 68 390 245 214 180 320 203 200 340 315 340 296 330 320 290 202 82 216 230 216 285
730 720 600 740 590 720 740 730 720 650 750 650 135 144 760 680 760 175 166 660 690 700 6 80 155 720 650
70 70
150 90 83 67 68 22 17 3.3 6.3 1.2 5.3 2.95 2.03 1.70 2.80 1.74 1.36 2.06 1.80 1.70 1.26 1.07 0.90 0.71 T = -52" 3.45 1.24 1.49 0.90 0.834 0.700
11.0 2.1 6 .O 6 .O 5.3 0.76 0.48 4.5 6 .O 1.8 3.9 1.3 4.4 4 .O 3.4 0.34 4.6 1.09 5.4 8.9 1.42 8.4 0.71 8.3 4.6 6.7
650 650 650 650 650 650
5.35 2.65 5.70 4 -85 5.75 6.75
1.3 1.1 1.4 1.3 1.3 1.05 1.25 1.30 1.55 0.93 1.56 1.70 2.1 3 .O 2.2 2.6 2.3 3.3 3.9 3.4 3.2 3.2 3.4 6.9 5.5 6.7 1.06 1.4 1.75 2.6 2.2 3.3
(64 (6b) According to Lin and DeMore, channel 6a and 6b account for 88 and 9%) respectively.' However, Greenberg and Heicklena report that channel 6a occurs 95 f 5% of the time. Whether the former or latter result is correct will not significantly affect the results of this study. The OH radical produced in reactions 5 and 6a will react with CH4. The reaction with 0 3 is negligible for all [CH4]/ [03] ratios used in the present study. OH + CH, OH + O3
--
H2O HOz
+
CH,
( 7)
+
O2
( 8)
The CH3 radical produced in reactions 5 and 6a can react with O3 as follows:
-
CH, + 0, CH,O* + O2 (94 CH, + 0, H02* + CH,O (9b) where the superscript f designates excess vibrational energy. The energetic CH30* and HOz* may be stabilized or they may decompose
53 89 89 110 188 188 196 285 286 336 3 60
84 233 233 76 180 180 180 ,276 350 248 276
2.5 6.8 6.8 0.48 0.93 0.93 0.80 0.94 1.5 0.59 0.55
700 6 80 680 680 184 680 680 70 0 680 147 6 80
-
CH,O* H + CH,O CH3@ + M CH,O + M HOz* H + 02 HOz* + M -t HOZ + M and in the presence of 0 2 , reaction 10 will occur +
--+
30 43 48 30 91 63 60 48 65 100 68
(sa') (sa") (9b') (9b")
CH302 + M CH3 + 02 + M (10) The HOz radicals produced in 9b" will propagate the chain decomposition of O3 uia the reaction --+
H02 + O3 OH + 20, (11) The H atoms produced in reactions 9a' and 9b' will propagate the chain decomposition of O3 uia the known reactions +
H + 02 H + O3 OH*
+
-+
+
HOz + M OH*((v = 9) + O2
M
O3
---+
H
+
+
O2
(12) (13)
202
(14) HZO + CH3 (15) OH* + CH, Since -@{03] is independent of I , under all experimental conditions, chain termination cannot be by radical-radical reactions. Therefore, all radical-radical reactions which involve chain carriers and do not lead to chain carriers are not important. The CH302 radicals produced in reaction 10 will be removed uia +
2CH302 4 2CH,O
(164
The most recent measurements give k16,/k16 = 0.43.6 The CH3O radical produced in reactions 98" and 16a will be removed by 2CH30
-
and by CH30 + O2
CH,OH
-
+
CH,O
CH20 + H 0 2
(17) (18)
CH3O2 and CH30 may also react with O3 CH302 + 0,
+
products
(19)
products (20) CH30 + 0, To estimate the importance of reactions 19 and 20, consider the results a t high values of [ 0 ~ ] / [ 0 3 ]Under ~ these The Journal of Physical Chemistry, Vol. 79, No. 4 , 1975
300
R. Simonaitis and Julian Heicklen
TABLE 111: Summary of Rate Coefficients Reaction 2. 3. 4. 5. 6a. 6b.
Rate coefficient, cm3/sec
--o(3~) + 203 O(,P) + o(,P) + o2 + - +M o(,P) + 202 a 3 P ) + CH4 - OH + CH, O('D) + CH4 - OH + CH, O('D) + CH, - CH20 + H, O('D) + - O(3P) + 02('A) -I-0, OZ('A) + CH4
O2
0,
M
Ref
5.0 x lomi1exp(-565O/RT) Too slow to measure 6.6 x e~p(1020/RT)~ 1.4 x lo-'' exp(-4600/RT) 3.5 x lo-'' exp(-9100/RT) 4 x 10-10 ha/k8b 2 7 7.5 x 10-1' 5.0 X 10'" 4.8 x lo-" exp(-5000/RT) 1.6 x 10-l2 exp(-2000/RT) 4.3 x 10-136 1 x 1 0 - l ~e x p ( - 2 4 0 0 / ~ ~ ) 6.7 x lom3,exp(660/RT) 2.6 X lo-" 7.7 x 10'12 1.4 X k16 = 2.4 X lomi3 kifla/k18 = 0.43
3
3 3 3 3 3 O2 O2 3 O('Q + O3 O2 -t 02*(or 20(,p)) 3 7. OH + CH, * H20 CH, 3 8. OH + O3 H 0 2 -t O2 3 10 * CH3 + 02 CH,O, 3 11. HOz + O3 OH + 2 0 2 3 1 2 . H + 02 + M H02 + M 3 13. H + O3 OH(v = 9) + O2 3 14. OH' + 0, H + 202 4 15. OH* + CH4 H20 + CH, 4 / 16a. 2CH,02 2CH30 + O2 5 16b. CH3OH + CH2O + 02 6 16c. 6 CH302CH3 + 02 17. ZCH30 CH,OH + CH2O 1-6 X 10"' 3 18. CH,O + O2 CH20 + H02 -3 x l O " 8 C 3 21. CH3O2 + HOz CH302H + 02 -5 x 10'12 9 a For M = Ar. Efficiency for CHI taken as 4 times that for Ar. * High-pressure limit at 298°K. Applicable at >lo0 Torr pressure. At 25", Ea 6 kcalimol. 0,
-
----- - -
-
+
conditions, more CH3O radicals are removed via reaction 18 than reaction 17. With the assumption that the rate of reaction 17 is negligible compared to reaction 18, and the realization that reaction 19 is less important than reaction 16, then a t high values of [ 0 2 ] / [ 0 3 ] , the steady-state rate law is -@m{03} 2 1
+
*
/
/
1
/
+
k i 9 ( ~ ) " ' ~ k6k16
where -@lm(03) represents the limiting value of -@{03} for large [ 0 2 ] / [ 0 3 ] ratios. The inequality sign in eq I comes from three separate considerations. (1)The [ 0 2 ] / [ 0 3 ] ratio might not be sufficiently high to completely supress reactions 4 and 9. (2) Reaction 19 or 20 might lead to a chain decomposition of 03.(3) Reaction 18 leads to H02 which can decompose O3 via reaction 11. At the highest [ 0 ~ ] / [ 0 3ratios ] used in our studies - @ ( 0 3 ) is reduced to below 1.3. Thus both the pentultimate and ultimate terms in eq I must be I600
301
TEMPT 25
25 -52
@"{o,} @'{o~}(I - ~l'~a~!~) Equation I1 can be rearranged to
6''{03}= 4(k9a
kgb)[O,l/k,,[O2]
(IID
Figure 2 shows plots of @"(03} us. [ 0 ~ ] / [ 0 3 at ] both temperatures. There is some scatter in the data. A t -52', the intercept is zero. However for the data at 25', the intercept is 0.1 f 0.1. (We have ignored the inaccurate values at [ O 3 ] / [O,] < 0.02 in drawing the best line.) If the nonzero value of the intercept is real, it indicates that all the assumptions were not exactly fulfilled. However they do not introduce significant errors. The slopes of the two lines give 4(kg, kgb)/klO = 8.7 and 4.7, respectively, at 25 and -52'. From these two points the Arrhenius expression becomes
+
Flgure 2. Plot of @"(OS)vs. 0 2 mixtures at 253.7 nm.
[03]/[02]
In the photolysis of 03-CH4-
4(kga C k9b)/ki,
k 9 ' = 2{[1
+
aia2 ( 11 +- qff2 )~]ksa
50 exp(-1050/RT)
where the activation energy is given in kcal/mole. Since klo = 4.3 X cm3/sec at the high pressure limit with presumably no activation energy +
k9,
+
k9,
5.4 x
exp(-1050/RT) cm3/sec
The large preexponential factor favors a linear over a five-membered ring transition state.
Reaction 9b must proceed through a cyclic transition state. Therefore reaction 9a probably is the major, if not exclusive, pathway by which reaction 9 proceeds. If this is so, then f' must be 1.0 to explain the zero intercept in Figure 1. In computing @,we have used kca/k6 = 1.0 since this gives a more consistent fit among the data than using kca/k6 = 0.9. A plot of @'(O~j-lus. [02]/[03] will be linear iff' and kg' are insensitive to changes in reaction conditions. With this hope in mind, @'(03) was computed from the quantum yields in Table I and the rate coefficients in Table 111. @ ' ( 0 3 ) is approximately equal to -@{Os) - 1 under all conis not very senditions since y -1 and @ 1; thus W(03) sitive to the values of the rate coefficients used in computing y and @. The plot of @'{os}-' us. [ 0 2 ] / [ 0 3 ] is shown in Figure 1. It is apparent that the points for which [CHd = 600-760 Torr lie reasonably on a straight line with an intercept of 0.0 f 0.1 at both temperatures. However, the points for which [CH,] = 135-175 Torr lie well below the line. Thus kg' is larger at lower [MI. This effect could be due to variations in f, f', 011, or 012. The variations in 011 and 012 can be computed directly from the known rate coefficients. With the computed values for c q and 012 the only way to account for the discrepancy is iff = f' = 1 (see Appendix). From the intercept of Figure 1, we find (1 - f')kga/k9'= 0.0 f 0.2. Thus the production of CH30 is not important.
-
-
Acknowledgment. This work was supported by the Atmo-. spheric Sciences Section of the National Science Foundation through Grant No. GA 12385, by the National Aeronautics Space Administration through Grant No. NGL009-003, and by the Department of Transportation Climatic Impact Assessment Program through Contract No. DOT-OS-40051 for which we are grateful. Appendix We now justify the assertion in the text that f = f' = 1. To simplify the analysis we assume that 2 - f = 1.0 in the expression for kg', and then test for consistency. Since (1 f')kg,/kg was shown to be N O , then eq I can be rearranged to
where The values of f were calculated where the calculations are meaningful, Le., for the last 15 entries at 25' in Table I. The average values and mean deviations are 1.15 f 0.34 for The Journal of Physical Chemistry, Vol. 79. No. 4, 7975
Grimley, Wagner, and Castle
302
the five points a t [CH4] = 135-175 Torr and 1.11f 0.19 for the ten points a t [CHd] = 650-760 Torr. The fact that the average values slightly exceed 1.0 can be accounted for by a slight change in the ratio klolkg. More meaningful is the fact that at both CH4 pressures, f is the same, a result which can only hold iff = 1.0. It is interesting to estimate how much f could be below 1.0 and still be consistent with the experimental data. Considering the mean deviations, the largest reasonable spread in f a t the two CH4 pressures might give an f 30% higher a t the lower pressure. To be consistent with the definition for f, the two values for f would be 0.915 and 0.705, respectively, at the lower and higher CH4 pressures.
References and Notes (1) R. Simonaitis and J. Heickien, J. Phys. Chem., 78, 653 (1974). (2) E. Lissi and J. Heicklen, J. Photochem., 1, 39 (1972). (3) D. Garvin, Ed., National Bureau of Standards Interim Report No. NBSiR203, 1973, "Chemical Kinetics Data Survey iV. Preiiminary Tables of Chemical Data for Modelling of the Stratosphere." (4) S. D. Worley, R. N. Coltharp, and A. E. Potter, Jr., J. Phys. Chem., 78, 1511 (1972). (5) D. A. Parkes, D. M. Paul, R. P. Quinn, and R. C. Robison, Chem. Phys. Len., 23,425 (1973). (6) J. Weaver, R. Shortridge, J. Meagher, and J. Heicklen, Center for Air Environment Studies Report No. 363-74, Penn State University, University Park, Pa., 1974. (7) C. L. Lin and W. B. De More, J. Phys. Chem., 77, 863 (1973). (8) R. I. Greenberg and J. Heickien, Int. J. Chem. Kinet., 4, 417 (1973). (9) J. Heickien, Advan. Chem. Ser., No. 76, Part II,23 (1968).
Angular Distributions of Molecular Species Effusing from Near-Ideal Orifices Robert T. Grlmley,*' L. C. Wagner, and Peter M. Castle Department of Chemistry, furdue University, West Lafayette, Indiana 47907 (Received July IO, 1974)
A mass spectrometric investigation has been made of the angular number distributions for the NaC1, KCl, CsCl, and Sm vapor systems effusing from a near-ideal cylindrical orifice, LIR = 0.127. The experimental angular distribution curves and transmission probability of Sm are in excellent agreement with the Clausing values. The angular distribution curves and transmission probabilities for the monomeric alkali halide species show positive deviations from the Clausing values whereas the dimeric vapor species exhibit negative deviations. For near-ideal orifices and the monomer-dimer vapor system, the Voronin approximation equations which were based in part on the assumption of monomer-dimer equilibrium at the orifice wall provide a substantially improved representation of the angular distribution curves over the Clausing theory.
Introduction The Knudsen2s3 effusion method has been used extensively for the determination of the vaporization thermodynamics of slightly volatile substances. The method involves the measurement of the rate of mass loss of vapor from an equilibrium enclosure through an orifice. An orifice the length of which is vanishingly small is termed an ideal orifice. For the ideal orifice the kinetic theory4 predicts that the total number of particles which flow through the orifice per unit area per unit time d N l d t is given by
dS\ildt = nG/4
(1)
where n is the number of particles per unit volume and D is the average molecular speed. Similarly the number of particles ( d N l d t ) @which pass through an orifice of unit area per unit time a t an angle 0 from the normal to the orifice plane and which are contained in the solid angle dw is given by the equation (cW/dt), = (nc/4a) cos 0 d o
( 2)
For an ideal orifice, therefore, the ratio of the number of molecules which pass through the orifice at angle 8 to those which pass along the normal to the orifice is cos 8,the socalled cosine law. The Journal of Physical Chemistry, Voi. 79, No. 4, 1975
For finite or real orifices it is necessary to correct for the effect of nonideality, and it has become standard practice to correct for the nonideality by use of the Clausing transmission factors. The accuracy of the determination of the absolute pressures and related thermodynamic quantities is, therefore, dependent in part on the extent to which flow through an orifice of finite length is described by the Clausing m0de1.~-~ Clauping has shown that the number of particles which pass through a cylindrical orifice of unit area per unit time a t an angle 0 from the normal to the orifice plane and which are contained in the solid angle dw is given by the equation (cw/dt), =
( V L ; / ~ V ) T ( BL/R) , COS
e dw
(3)
The functional form of T is dependent on the relationship between tan 8 and the orifice geometry LIR. At those values of 0 for which tan 0 < 2R/L, the molecular flux arises from two sources. There is a contribution from molecules which flow directly through the channel without collision with the walls and a contribution from those molecules which have first undergone wall collisions. For this case
p = ( L / ~ Rtan ) and
e
