J . Phys. Chem. 1985,89, 5353-5358 the 0 2 ( a ) concentration, with simultaneous incorporation of a means to suppress the 02(b) and H 2 0concentrations, by chemical generation of 02(a). However, the very small Oz(a) quenching rate constants with the hydrogen halides, CO, and N O make state-testate studies with molecules amenable to detection by IR emission intrinsically very difficult. The trend in the rate constants for CH3C1, CH3Br, and CH31 suggests that spin inversion is an important restriction to 02(a) quenching, and the magnitude of the rate constants for quenching by reagents containing heavy atoms will generally increase, other factors being equal. A strong intermolecular interaction probably aids quenching, and this may explain why kHF> kHa. According to our results, the rate constant increases in the HC1, HBr, HI series, and this probably is a consequence of the heavy atom effect. Since the intermolecular forces between 0 2 ( a ) and most of the molecules studied here are rather weak, one can expect both energy defect and Franck-Condon restrictions to be important in governing the magnitude of the quenching rate constant. The much lower vibrational frequencies associated with the C-F bond relative to the C-H bond presumably are why quenching by CF3X molecules have smaller rate constants than the CH3X molecules. Clearly, no simple model will suffice for explaining E-V transfer with 0 2 ( a ) . Since E-V transfer is a slow process, if alternative quenching channels exist, such as chemical reaction with unsaturated molecule^,^^^^^ other quenching mechanisms will dominate
5353
over E-V transfer for the deactivation of 02(a).
Acknowledgment. This work was supported by the U.S. Air Force Weapons Laboratory (AFWL) and, in part, by the National Science Foundation (CHE-8217051). We thank Dr. Heidner of Aerospace Corp. for performing the [Oz(a)] calibration for our detector and for a preprint of ref 22. Registry No. HF, 7664-39-3; HCI, 7647-01-0; HBr, 10035-10-6; HI, 10034-85-2; HCN, 74-90-8; HZS, 7783-06-4; Cl2, 7782-50-5; NO, 10102-43-9; CO, 630-08-0; COS, 463-58-1; CO2, 124-38-9; CZNI, 46019-5; NH,, 7664-41-7; CH,CI, 74-87-3; CH,Br, 74-83-9; CH31,74-88-4; CF,Br, 75-63-8; CFJ, 2314-97-8; CF,NO, 334-99-6; C4H6, 106-99-0; cis-C4H8,590.18-1; (CH,),N, 75-50-3; CF,COOH, 76-05-1; Fe(CO)5, 13463-40-6; N2F4, 10036-47-2; NFZ, 3744-07-8; 0 2 , 7782-44-7. (26) Bogan, D. J.; Durant, J. L.; Sheinson, R. S.; Williams, F. W. Photochem. Photobiol. 1979, 30, 3. (27) Bogan, D. J. “Chemical and Biochemical Generation of Excited States”; Adam, W., Cilento, G., Eds.; Academic Press: New York, 1982; p 37. (28) In this paper the periodic group notation is in accord with recent actions by IUPAC and ACS nomenclature committees. A and B notation is eliminated because of wide confusion. Groups IA and 1IA become groups 1 and 2. The d-transition elements comprise groups 3 through 12, and the p-block elements comprise groups 13 through 18. (Note that the former Roman number designation is preserved in the last digit of the new numbering: e.g., 111 3 and 13.)
-
Electronic-to-Vibrational Energy-Transfer Studies of Singlet Molecular Oxygen. 2. O,(b’Z,+) J. P. Singht and D. W. Setser* Department of Chemistry, Kansas State University, Manhattan, Kansas 66506 (Received: March 15, 1985; I n Final Form: August 1, 1985)
A flow reactor was used to measure the total quenching rate constants of 02(b1ZB+) by several small molecules at 300 K and to observe infrared emission from HF, HCl, HBr, HCN, and C 0 2 products. The preferred product state is u = 2 from HBr quenching, but in all other cases the main product emission was from v = 1. For each of these cases the product distribution strongly implies that the final oxygen state is 02(a’A,) and not Oz(X’Z;). Vibrational relaxation may have affected the observed distributions for HCN and C02, but the infrared emissions from the other cases are thought to reflect initial product distributions.
Introduction The quenching rate constants for electronic-to-vibrational energy than transfer of 02(b1Zg+)are 3-4 orders of magnitude those for 02(a1A,). Consequently, in our attempt3 to observe the products from E-V quenching of 02(a), it first was necessary to characterize the 02(b) reactions. In this paper we describe the state-to-state results for 0 2 ( b ) interacting with HF, HCl, HBr, HCN, NO, and C 0 2 . The [Oz(b)] was monitored by the 02(b-X) emission intensity, and the products were observed from their infrared emission by using a Fourier transform spectrometer. The experiments were done in the discharge-flow apparatus described in the preceding paper. Although there are numerous rate constant studies for 02(b), the rate constants for several molecules of interest to our work, especially HF, were not available. Therefore, experiments were done to measure those quenching rate constants for 02(b). In order to calibrate our method, measurements were made for some well-established cases, as well as for the hydrogen halides, HCN, CH3Cl, and H2S. Since the 0 2 ( b ) concentration reaches a steady state with the 0 2 ( a ) concentration, quenching ‘Present address: MHD Energy Center, Mississippi State University, Starkville. MS 39762.
rate constant measurements can be done in two ways. The decay
of the [02(b)J can be studied in the transition regime as the initial steady-state concentration relaxes to the lower value in the presence of the reagent: or the initial and final steady-state concentrations can be related to obtain rate constant^.^ We opted for the time-dependent approach and observed the decline in the 02(b) concentration vs. time for a constant reagent concentration, or vs. reagent concentration for a fixed time in the flow reactor. Unfortunately, the system did not behave ideally and our quenching rate constants have limited reliability. ( 1 ) (a) Ogryzlo, E. A. In ‘Singlet Oxygen”; Wasserman, H. H., Murray, R. W., Eds.; Academic Press: New York. 1979. (b) Ogryzlo, E. A. In “Singlet Oxygen Reactions with Organic Compounds and Polymers”; Ranby, R., Rabek, J. F., Eds.; Wiley: New York, 1978. (2) Thomas, R. G. 0.;Thrush, B. A. Proc. R. SOC. London, A 1977,356, 287, 295, 307. (3) Singh, J. P.; Bachar, J. J.; Setser, D. W.; Rosenwaks, S. J. P h y s Chem., preceding paper in this issue. (4) O’Brien, R. J.; Myers, G. H. J . Chem. Phys. 1970, 53, 3832. (5) (a) Boodaghians, R. B.; Borrell, P. M.; Borrell, P. Chem. Phys. Lett. 1983, 97, 193. (b) Borrell, P. M.; Borrell, P.; Grant, K. R. J. Chem. Phys. 1983, 78, 748.
0022-3654/85/2089-5353$01.50/00 1985 American Chemical Society
5354
The Journal of Physical Chemistry, Vol. 89, No. 25, 1985
The quenching of 0 2 ( b ) also can be studied in real time6,’ by using laser photolysis to excite molecules from 0 2 ( X ) to 02(b), 160 nm), by photolysis of O2 with a vacuum-UV H, laser* (A or by photolysis of 03.9The last two methods depend upon excitation transfer from O(ID) to 0 2 ( X ) to give 0 2 ( b ) . The temperature d e p e n d e n ~ eof ~ ,the ~ HBr and HC1 quenching rate constants for O,(b) studied by these techniques is of direct interest to our work. The E-V transfer to CO, recently has been observed6 in real time following laser excitation of 02(b) by monitoring both reactants and products. However, the initial CO, vibrational distribution was not identified. Our main objective was to establish the product-state distriV quenching of 02(b). The only comprehensive bution for E study of this type is the work of Thomas and Thrush,2 who observed the infrared emission from a 10-L integrating sphere. Assuming that the IR emission can be observed, the critical problem is the degree of vibrational relaxation. We argue that relaxation was avoided for HF, HC1, and HBr. However, extensive relaxation is suspected for our experiments with NO and CO,, and initial distributions cannot be reported. The situation for HCN is not entirely clear, since all of the relevant vibrational relaxation rates have not been measured. We claim that relaxation probably was not so extensive as to obscure a resonant E-V reaction; Le., the E-V quenching by HCN is not an energy resonant process. In all cases the observed products are consistent with 02(aiA,) being the O2 product state, and all discussion of O,(b) quenching will be given in terms of the following reaction.
Singh and Setser
-
40
-
+
O2(b’Zgf,0’=0) HX O,(a’A,,u”=O)
+
+ HX(U)
AH,’ = -5239 cm-l (1)
The possibility of 02(aiAg,u”=1) being a product is considered in the Discussion section. Since experimental details were given in the preceding paper, only results and discussion will be presented here.
Experimental Results a. Quenching Rate Constants for O2(b’ZB+).Following the generation of 0 2 ( a ) and 0 2 ( b ) by the microwave discharge, the 0 2 ( b ) concentration will reach a steady state because the quenching rate by impurities (mainly H 2 0 ) and by the wall tends to be more rapid than the rate of formation by energy pooling from 0 2 ( a ) . In the absence of reagent the steady state is given by
Upon the introduction of a reagent, Q, the original steady state will be altered and a second steady state will be reached.
In writing eq 3, we assume k , does not change upon introduction of Q; however, this must be experimentally checked for each Q. For the range of [Q] and the reaction times used for studying the quenching of 02(b), the [02(a)] concentration remains unchanged. Since the formation term is very small, kp = 2.0 X lo-’’ cm3 molecule-’ s-I, the relaxation between the two steady-state 02(b) (6) (a) Muller, D. F.; Houston, P. L. J . Phys. Chem. 1981, 85, 3563. (b) Aviles, R. G.; Muller, D. F.; Houston, P. L. Appl. Phys. Lett. 1980, 37, 358. (7) Lawton, S. A.; Novick, S. E.; Broida, H. P.; Phelps, A. V. J . Chem. Phys. 1977, 66, 1381. (8) (a) Kohse-Hoinghaus, K.; Stuhl, F. J . Chem. Phys. 1980,72,3720. (b) Borrell, P.; Borrell, P. M.; Richards, D. S.; Boodaghians, R. B. J . Photochem. 1984, 25, 399. (9) (a) Braithwaite, M.; Ogryzlo, E. A.; Davidson, J. A,; Schiff, H. I. Chem. Phys. Leu. 1976,42, 158; J . Chem. Soc., Faraday Trans. 2 1976, 72, 2075. (b) Braithwaite, M.; Davidson, J. A,; Ogryglo, E. A. J . Chem. Phys. 1976, 65, 771. (10) Gauthier, M. .I.E.; Snelling, D. R. J . Photochem. 1975, 4 , 2 7 .
h
n
v
H IO
4t 1
0
I
8
1
I
16
1
I
24
Time (m s e d
Figure 1. (a) Decay of [02(b)] vs. distance (time) for various [CO,] in the halocarbon wax coated 7-cm-diameter reactor. The O,(b-X) emission was observed with a photomultiplier tube plus filter. The flow speed for this experiment was 12.5 m s - I . The [CO,] are (a) 0.0, (*) 1.5 X (0) 4.1 X and (m) 6.8 X I O i 4 molecules cm-3. The apparent increase in [02(b)] with distance in the absence of reagent was reproducible (see text). (B)Plot of In ([O,(b)], - [02(b)lS2)vs. time for the two highest CO, concentrations, (0) 4.1 X 1014 and (B)6.8 X I O L 4 molecules
concentrations should follow first-order kinetics with a decay constant given by the denominator of eq 3, which will be written as B + kQ[Q]. The integrated rate equation for the time dependence of [02(b)] in the presence of [Q] is given by [O2(b)l, - [02(b)lS2 = ([O,(b)lsi - [O,(b)ls,) exP{-[B -k ~ Q [ Q I I(4) ~) The data of Figure lA, which were obtained in the same 70mm-diameter fluorocarbon-coated reactor that was used for the O,(a) quenching study, show that this description is qualitatively valid. With no added reagent, the relative [02(b)] measured from the 02(b-X) emission intensity with a movable photomultiplier tube plus interference filter was approximately constant. Upon addition of Q, the [O,(b)] does decay to a new steady state that depends on [Q] in the expected way. The data in Figure 1 were taken in the halocarbon wax coated tube, and k , should be small; also, the discharge was located rather close to the entrance of the reactor. The small apparent increase of [02(b)] with time for [Q] = 0 may be a consequence of slightly different transmission properties of the halocarbon wax coated wall or to a real increase in [02(b)]. The latter could result, if most of the [02(b)]observed at short time was generated in the discharge and if the denominator of eq 2 was quite small so that the [O2(b)lS,would be larger than the 0 2 ( b ) concentration entering the reactor. We extrapolated the data of Figure 1A in order t o assign [02(b)lS2a n d made the plots of In ([O,(b)], - [O2(b)lS2)vs. time for various [Q] shown in Figure 1B. These plots suggest difficulties in interpretation at the quantitative level because [O,(b)] decays too rapidly at early the curves appear to reach constant [O,time and, for low [Q], (b)lS2too quickly. The higher [Q] data seem to be the more useful, and the low [Q] results were ignored. The slopes of these lines should be B + kQ[Q], and plots of these slopes vs. [Q] would give values for B and k,. If the t = 0 data point is ignored, the plots in Figure 1B are linear, and the slopes of the two highest [CO,] plots give kCO2== 3 X cm3 molecule-’ 5-l with [B] being small. Unfortunately, it was necessary to terminate this project before these experiments could be refined, and preliminary results, such as shown in Figure 1 , were collected only for HBr and CO,. In principle, measurements such a s shown in Figure
The Journal of Physical Chemistry, Vol. 89, No. 25, 1985 5355
Energy-Transfer Studies of 02(b1Zg+) 101
-
IL
16 24 8 [HF] ; IO 13 molecules cm-3
L. ’
1
8
1
1
16
I
1
24
1
1
[HCN]; IO Is molecules ~ m ‘ ~
Figure 2. Plots of In ([O,(b)], - [O2(b)lS2)vs. [Q] for two fixed observation times for H F and HC1 as reactants. The times were 6.3 and 11.3 ms for H F and 6.7 and 14.7 ms for HCI. The [02]and [Ar] were 2.2 X 10I6and 6.4 X 10l6molecules ~ m - respectively. ~, Experiments were done in the 4-cm-diameter reactor with the prereactor configuration.
Figure 3. Plots of In ([02(b)], - [OZ(b)lS2)vs. [Q] for two fixed observation times (6.7 and 12 ms) for HBr and HCN reagents. The [O,] and [Ar] were 2.2 X 10I6 and 6.4 X 10I6molecules cmW3,respectively. Experiments were done in the 4-cm-diameter reactor with the prereactor configuration.
1 should be the best way to measure quenching rate constants of 0 2 ( b ) in a flow reactor. However, the delay time between the discharge and the introduction of Q and the magnitude of k, k ~ ~ o [ Hshould ~ o ] be sufficiently large to ensure the development of [O2(b)lSl before Q is added to the reactor. Another approach to measuring k , is the fixed-point method in which the decay of 0 2 ( b ) is observed vs. [Q] for a fixed time. In the initial phase of this work, fixed-point measurements were made in the 40-mm-diameter flow reactor, that was used for infrared emission measurements, to obtain estimates of kq. The configuration of the flow reactor for these experiments included the prereactor (see Figure 1 of ref 3), and the [02(b)] had sufficient time to reach steady state relative to [Oz(a)]. In these experiments k, was considerably larger than in the coated reactor, and [O,(b)] was a factor of 8 smaller than in the coated reactor. Since the Roots blower was not used, the flow velocity was 0.8 m s-l and the typical [02(a)] was 3 X 1015 molecules ~ m - ~In. spection of eq 4 shows that the fixed-point technique is feasible, but one must identify [ O , ( b ) l ~for ~ each value of [Q]. The problem of assigning [O2(b)lSzwas reduced by working at a high degree of quenching so that [O,(b)lsl - [ O ~ ( b ) l s ~[02(b)ls,. The reagents were added 5-10 cm in front of the observation window, which was viewed by the monochromator to record the O2(bX) intensity. The distance from the window to the addition point was variable, and longer distances were used for reagents with smaller k,. These experiments were done by adding [Q] until Z(b) was reduced by a factor of 3, and then [Q] was doubled or tripled to obtain a limiting z(b)Q. Subtracting the limiting I(b)Q from the other intensities gave linear first-order plots. The subtraction, in fact, only affected the larger [Q] data. Plots of some typical data are shown in Figures 2 and 3. We first measured rate constants for H2, C 0 2 , and NH3and found that our kq values were in moderately good agreement with published values. Therefore, we extended the measurements to the hydrogen halides, HCN, COS, CH3Cl, and H2S. We consider these rate constants to be reliable to within a factor of 2; the quoted uncertainty is the deviation from the mean of several experiments. The rate constants measured here are compared to other measurements in Table I. The large spread in reported values for HCl suggests that the k, may be dependent upon [HCl]. Our value for kHCl tends to support the lower range of values. Our results seem to COnfiMl that kHF > kHBr > kHa; the kHI (2.7 x Cm3
TABLE I: Rate Constants for Quenching of 02(b*Z.+) at 300 K kQ,(l.b reagent cm3 molecule-’ s-l ref HF 110 f 30 this work HC1 4.0 f 2.0 this work 7.3 f 0.2 2 2.7 f 0.4 5a 6.7 3.5 10 13.0 f 4.0 8a HBr 20 f 6c this work 37 & 7 9a HCN 8.0 f 2.0 this work cos 2.3 1.0 this work 5.5 f 3.5 9b CH3C1 80 f 4OC this work this work H2S 20 f 10 49 quoted in 13a 60 f 10 12 co2 30 f 10 this work 35 f 5 5a 30 f 9 11 41 12 45 f 3 6b 50 f 3 6a NH3 130 f 20 this work 117 f 17 5a 180 f 50 11 199 f 50 2 170 f 20 8a H2 45 f 20 this work 4 0 k 12 11 82 f 10 8a 92 f 5 10 83 f 4 9 CF3NO 400 f 200 this work
+
* *
The uncertainties quoted here are the standard deviations from multiple experiments. The absolute uncertainty is estimated as a factor of 2. bThe rate constants for quenching 0 2 ( b ) by H20I1and 02 have been reported as ( 5 f 1) X lo-’* and 3.9 X lo-’’ cm3 molecule-’ SKI, respectively. ‘The recommended rate for CH31 is 30 X cm3 molecule-I s-l; the rate constant for H I was reported13bas (2.7 f 1.0) X cm3 molecule-’ s-I. s-l) is the smallest of the series. The H F rate constant is the largest and is similar to that for N H 3 but still smaller than the value for
5356
The Journal of Physical Chemistry, Vol. 89, No. 25, 1985 3060
2980
Singh and Setser
2900
I
'
[HF] X 1013 molecules cm3 1 2 3---
P
I
1
/H
10
4
CI x 0 . 5
X
I
a
\ X
I
- 5
I
I
I
2630
2530
2430
3350
3290
3230
Figure 4. Emission spectra from reactions O,(b) with HCI, HBr, and HCN. The spectra were taken in the 4-cm-diameter reactor with a reaction time of 0.67 ms for [02(a)] = 1.4 X lo'', [O,] = 1.0 X 10l6, and [Ar] = 1.3 X 10l6molecules cm-3. The [HCI], [HBr], and [HCN] concentrations were 2.0 X lOI4, 2.5 X loL4,and 2.0 X lOI4 molecules ~ m - ~For . our apparatus the [O,(b)] was proportional to the O2 flow passed through the discharge: thus, high O2 flows were used to enhance the product emission.
HzO (5.0 X cm3 s-l).ll The large kHF is consistent with the correlation of k, with the magnitude of the highest vibrational frequency in a reagent molecule.'*11,12The C F 3 N 0 rate constant is anomalously large, which may indicate an electronic excitation-transfer mechanism3 rather than E-V quenching. b. Infrared Emission from O,(b'Z,+)Reactions. These experiments were done without the prereactor, and the singlet oxygen from the discharge was added directly to the 40-mm-diameter flow reactor (see Figure 1 of the preceding paper). This-geometry maximized the [02(b)]/[02(a)]ratio and ensured that the contribution to the observed infrared emission from quenching of O,(a) was negligible. Relative to the configuration with the prereactor, the O,(b)/O,(a) ratio was increased by a factor of 10. The absolute [Oz(a)] was -2 X lOI5 molecules ~ m - and ~, from the previously estimated3 [02(b)lsl, the absolute [O,(b)] must have been approximately 2 X 10l2molecules ~ m - ~In. our apparatus, [O,(b)] was generated by the microwave discharge, and the highest [O,(b)] was for the shortest flow time between the discharge and observation point, the highest 0, flow, and the highest microwave discharge power (we used two 100-W microwave discharges in series). In order to minimize product vibrational relaxation, the 0, and reagent flow rates were reduced as much as possible, consistent with obtaining satisfactory emission spectra. Experiments were done at constant pressure, 0.8 torr, with the smallest fraction of O2that was possible. The pumping speed was adjusted from the maximum speed with the blower plus mechanical p u m p for H F to slightly throttled conditions for other reagents. The infrared emission spectra observed from the HCI, HBr, and H C N reactions are shown in Figure 4. The spectrum from HF, which consists only of HF(u= 1) emission, is not shown. The infrared emission intensity from the hydrogen halides and H C N was first-order in [O,(b)] and [HX] (see Figure 5). Furthermore, the relative product concentrations (relative HX(U) intensities
-
Figure 5. First-order dependence of the [HX,o] product upon [HX] from the reaction with 02(b). The experimental conditions were the same as for Figure 4. The factors denote the reduction in the ,,Z values prior to making the plot; note the different concentration scale for [HF].
divided by the Einstein coefficients14for the same [O,(b)]) were in qualitative agreement with the relative quenching rate constants measured in the preceding section; HF:HCl:HBr:HCN 10:0.4:2.0:0.8, with HC1 having the lowest product formation rate. The match with the relative rate constants implies that formation of HX(u=O) is not a major product channel for reaction 1; however, our rate constants are not sufficiently reliable to state this with certainty. Infrared emission could be observed from the NO and CO, reactions; however, the N O and C 0 2 concentrations were so high that vibrational relaxation was suspected to be extensive and these systems will not be discussed in detail. Attempts to observe emission from the quenching of 02(b) by NH3, COS, and H2S were unsuccessful. (i) Reaction with HF. Since H F has the largest Einstein coefficient and the largest quenching rate constant, the H F reaction could be studied at the lowest concentration and shortest contact time of any reagent. For a contact time of 0.25 ms and for [HF] = 1012-10'3 and [O,] = 5 X lOI5 molecules ~ m - ~ , emission from only HF(u= 1) could be observed. Not even trace amounts of HF(u=2) emission were present. Modeling of possible HF(2) formation and subsequent relaxation for these flow conditions (including vibrational relaxation by both H F and 02)15,16 suggested that HF(2) vibrational relaxation should be modest. The degree of HF(2) relaxation was experimentally checked by adding I2 to the singlet 0, system to generate I(zPli2)atoms. The I(,PIi2) reaction with HF generated a significant HF(2) concentration. Changing the H F and O2concentrations in the 12/02 system over the same range as for the 0, system alone did affect the HF(2)/HF(I) ratio; thus, if HF(2) were formed in reaction 1, it should have been observed. We conclude that quenching of 02(b) by H F gives mainly HF( 1) with perhaps some contribution from HF(0).
-
02(b1Zg+)+ H F
-
-
02(a1A,)
02(a'Ag) + HF(1) Moo= -1280cm-I (sa)
+ HF(0)
AH," = -5239 cm-I
(5b)
(ii) Reaction with HCI. The smaller rate constant and Einstein coefficient for HC1 relative to H F required both longer contact time and higher [HCl] and [O,]. The HCl emission was studied for [O,] = 1.1 X 10l6and [HCl] = 10'3-1014molecules cm-l with a contact time of 0.67 ms (see Figure 4). The emission was mainly from HCl( 1); although, some HCl(2) could be observed with a HC1(2)/HC1(1) ratio of 0.11. This ratio was independent of [HCI] over the 10'3-10'4 molecules cm-3 range. The invariance of the HC1(2)/HC1(1) ratio with change of [HCI] suggests that
~~
(1 1) Becker, K. H.; Groth, W.; Schurath, U. Chem. Phys. Lett. 1971, 8,
259. (12) (a) Kear, K.; Abrahamson, E. W. J . Photochem. 1974, 3, 409. (b) Davidson, J. A.; Kear, K. E.; Abrahamson, E. W. J . Photochem. 1973,1,307. (13) (a) Schofield, K., private communication, 1984. (b) Koffend, J. B.; Gardner, C E.: Heidner, R. F. I11 J . Chem Phys. 1984, 80, 1861
(14) (a) Oba, D.; Agrawalla, B. S . ; Setser, D. W. J . Quanr. Spectrosc. Radiat. Transfer, in press. (b) Malins, R. J.; Setser, D. W. J . Chem. Phys. 1980, 73, 5666. (15) Dzelzkalns, L.S.;Kaufman, F. J . Chem. Phys. 1983, 79, 3363, 3836. (16) Poole, P. R.; Smith, I. W. M. J . Chem. Soc., Faraday Trans. 2 1977. 73, 1434, 1447.
The Journal of Physical Chemistry, Vol. 89, No. 25, 1985 5357
Energy-Transfer Studies of 02(b'Zg+) HCl(2) is not the major product channel. The HCl(u=2) quenching rate constant by HC1'7a (3.4 X lo-', cm3 molecule-' s-') is a factor of 6 smaller than for the HF system; quenching has a rate constant of -4 X cm3 molecule-' s-'. by 0217b According to these rate constants, vibrational relaxation by 0, and by HCl (for the l O I 3 molecules cm-3 experiment) should not have been very serious. Thus, the following reactions describe the quenching of 02(b), with reaction 6b being dominant at 300 K. 02(b'Zg+) + HCI
-
HCI(1)
-
HCl(2)
+ O2(a'Ah,) AHo' = 429 cm-' (6a)
+ 02(a'A,)
AH,' = -2353 cm-'
(6b)
+
The results in the next section for HBr O,(b) support the claim that if formation of HCl(2) had been comparable to that of HCl( l ) , the variation in the HCl(Z)/HCl( 1) ratio with [HCl] would have been easily observable for our operating conditions. Our conclusions differ from those of Thomas and Thrush: who assigned HCl(2) as the dominant exit channel. Our work was done with approximately the same HCl concentration range as Thomas and Thrush but for shorter contact times. The explanation of the discrepancy in the experimental observations is not obvious, but one possibility may be self-absorption of the HCl(1-0) emission in the 10-L reaction vesseL2 If our results are accepted, then interpretation of the 5.0 f 1.1 kT mol-' activation energ9 as being equivalent to the endoergicity for HCl(2) formation needs to be revised. As noted by Kohse-Hoinghaus and Stuhl,8" many of the measured activation energies for 02(b) quenching do not correlate well with the thermochemistry of the presently assigned product states. Reactions with two product channels, such as for reaction 6, are likely candidates for non-Arrhenius behavior and in fact Borrell et a1.8breport a virtually constant rate constant from 600 to 1000 K. Extrapolation of the Borrell et aLSbresult gives a 300 K rate constant that agrees with our measurement rather than the larger value of Kohse-Hoinghaus and StuhLSa More work is required to settle the temperature dependence of reaction 6. The quenching of O,(b) by DCl would be an especially interesting study, since the energy level pattern for DCI would resemble that for HBr and DCl(2) would be the expected product state. (iii) Reaction with HBr. The HBr quenching rate constant is 3-4 times larger than for HCl. Since theEinstein coeffi~ient'~ for HBr is about 5 times smaller than that of HC1, the HBr emission (see Figure 4) from the HBr reaction had about the same intensity as that from HCl for similar concentrations. For most experiments we used a pumping speed of 30 m s-' (the contact time was 0.67 ms) with [HBr] = (0.4-2.0) X lOI4 and [ 0 2 ] = 1.2 X 10l6molecules ~ m - ~The . HBr(2)/HBr(l) ratio exhibited a distinct increase with reduction in [HBr] (see Figure 6), and at 0.4 X 1014molecules cm-3 the ratio was -2.5. Extrapolation of the curve to zero [HBr] suggests an initial HBr(2)/HBr(l) ~, ratio of -3.5. For higher [O,], 1.9 X 10l6 molecules ~ m - and the same contact time, the extrapolated HBr(2)/HBr( 1) ratio was 2, suggesting that 0, causes some relaxation. We finally studied the HBr/02(b) system with a contact time of 0.4 ms and [O,] = 8 X 1015 molecules ~ m - ~Although . the spectra were of poor quality, they could be analyzed, and extrapolation to zero [HBr] (Figure 6) gave a HBr(2)/HBr(l) ratio of -4. We conclue that HBr(2) and O2(a'Ag,u"=0) are the main product states from quenching of 02(b) by HBr. O2(b'Z,+)
+ HBr
-
HBr(1)
-
HBr(2)
+ 02(a'A,)
+ 02(alA,) AH,'
= -210 cm-' (74
AHo' = -2679 cm-'
I
I
1
I
[HBr] 1 X16w2molacules3cni3
1
4
Figure 6. Dependence of the HBr(2)/HBr(l) ratio upon [HBr] for two sets of conditions: ( 0 )[O,], [Ar], and [02(a)] = 1.2 X 1.3 X 10l6, and 2.4 X lOI5 molecules cm-3 with At = 0.67 ms; (a) [O,], [Ar], [02(a)] = 8 X 1.6 X 10l6, and 1.3 X lOI5 molecules cm-' with At = 0.4 ms. The vertical arrows denote the uncertainty in the ratio associated with the noise in the emission spectra.
was varied from 0.2 X 1014 to 1.4 X 1014 molecules cm-3 for a contact time of 0.67 ms. For these experimental conditions onZy the HCN(001-000) transition was observed. There is sufficient energy to excite the HCN(O11) and HCN(O21) levels which require 4005 and 4724 cm-' of energy, respectively. However, careful searches failed to identify the HCN(021-020) or HCN(01 1-010) transitions.I8 Since the energy match between 2v2 with v3 or v1 is not close, it is likely that r e l a ~ a t i o n ' ~of - ~HCN(021) ' by collisions with H C N or O2 proceeds by Av2 = 1 transitions. Thus, some HCN(O11) should have been observed if HCN(O21) and HCN(O11) were important primary states. The selfquenching rate constant for HCN(Ol1) has been reported19 as 4.1 X lo-'' cm3 molecule-' s-', and there would be significant, but not total, relaxation of the populations in the v2 levels under our experimental conditions. From the present data, we deduce that the main primary quenching process must be (8a). 02(b'Zg+) + H C N
-
-+
02(a'A,)
+ HCN(001) AHo' = -1927 cm-] (8a)
02(a'A,)
+ HCN(O21) or HCN(Ol1) AHo' = -500 or -1215 cm-' (8b)
However, some v2 excitation via (8b) cannot be excluded. The infrared emission from the v I mode, 2089 cm-', is very difficult to observe, and excitation of v 1 or v1 + v2 combinations would not have been observed. Excitation of HCN(101) is 160 cm-' endoergic. Relaxation of this state probably would proceed via HCN(O21) as an intermediate state. As already noted, HCN(021) was not observed. (u) Reaction with NO. We observed N O emission only for long contact times and high [NO]. For a flow velocity of 10 m s-', which corresponds to a contact time of 2 ms, and [NO] = 1 X 10l5molecules ~ m - the ~ , emission mainly was from NO(u=l) with a trace from NO(u=2). For these experimental conditions the initial NO(u) distribution almost certainly was relaxed, and no conclusion can be reached about the initial NO(u) product states. However, we strongly suspect that 02(a'A,) was the oxygen product state. (vi) Reaction with CO,. Since the quenching rate constant by C 0 2 is large, 3 X lo-'' cm3 molecule-' s-', and the Einstein
(7b)
The quenching by HBr has a small temperature coefficient, which is consistent with an energy resonant process.9a (iu) Reaction with HCN. The Av3 = 1 emission spectrum from the reaction of 02(b) with H C N is shown in Figure 4. The [HCN] (17) (a) Berquist, B. M.; Dzelzkalns, L. S.; Kaufman, F. J . Chem. Phys. 1982,76,2984. (b) Zittel, P. F.; Moore, C. B. J . Chem. Phys. 1973.59.6636.
(18) Agrawalla, B. S.; Wategaonkar, S.; Setser, D. W., to be submitted for publication. In independent work we have observed HCN(O21) and HCN(O11) emission in analysis of the Au, = 1 band. The HCN infrared emission was generated by the reactions of C N with hydrogen donor molecules. (19) Hariri, A.; Petersen, A. B.; Wittig, C. J . Chem. Phys. 1976,65, 1872. (20) Hastings, P. W.; Osborn, M. K.; Sadowski, C. M.; Smith, I. W. M. J . Chem. Phys. 1983,78, 3893. (21) Cannon, B. D.; Smith, I. W. M. Chem. Phys. 1984.83,429.
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The Journal of Physical Chemistry, Vol. 89, No. 25, 1985
coefficients are favorable, we purged the spectrometer with N, to reduce the absorption by atmospheric COz and attempted to record the emission spectrum from C 0 2 with 02(b). Emission from the Av3 = -1 band easily could be observed, but self-absorption from the COz rotational levels that are highly populated at 300 K gave badly distorted spectra in spite of the purging of the spectrometer. The emission of the P- and R-branch lines from the very high and very low rotational levels of the 12C’602(O01000) band was strong. Emission from the ‘2C1602(01 1-010) and ‘2C1602(021-020)transitions and the ‘3C1602(OOl-OOO) transition were much weaker, but they could be identified in the recorded spectrum. The shortest contact time and lowest concentrations were 0.67 ms, [CO,] = 3.6 X lOI3, and [ 0 2 ] = 1.2 X 10I6 molecules but the CO,(OOl), CO,(Ol l ) , and CO2(O21) states were still the only ones that could be identified. As for HCN, enough energy exists to produce simultaneous excitation of other modes and co,(o41), C02(121), CO2(2O1),and CO2(O02) are close to energy resonance. However, no emission from these resonant energy levels could be observed in the Av, = -1 band. Our observations may be compared with those of Thomas and Thrush,* who observed stronger emission from (021) (000) and (101) (000) than from the Av3 = -1 transition. Self-absorption may have been more serious in the Thomas and Thrush work, but we are puzzled by our failure to observe the other two transitions because our detector sensitivities for the 3700- and 2350-cm-’ regions are comparable. Inspection of vibrational relaxation dataz2 for C0,(021) and CO2(lOl) by C 0 2 and 0, suggests that the relaxation times would be kHCl 5 kHI and enough to obtain state-to-state data from E-V transfer from O,(b) since the rate constants for the H X series are rather small, we to H C N and CO,. The N H 3 and C2H2molecules from which seriously question the general applicability of the long-range, emission was observed by Thomas and Thrush2 also are prime multipole quenching m 0 d e 1 ~ ~that ’ ~ has been used to correlate candidates for more extensive study. It seems certain that detailed trends in 02(b) quenching rate constants. From the small magproduct-state data, including the vibrational state for 02(a1Ag), nitude of the rate constants and the activation e n e r g i e ~ , ~ , ’ ~ must be collected before models can be constructed to describe short-range forces seem to be responsible for the E V coupling. the E-V quenching processes for O,(b). The reactions of O,(b) with HF, HC1, HBr, and H C N were Acknowledgment. This work was supported by the U S . Air successfully observed at the state-to-state level. Product emission Force Weapons Laboratory and, in part, by the National Science was observed for N O and CO,, but relaxation precluded stateFoundation (CHE-8217051). Dr. Rosenwaks and Mr. Bachar to-state analysis. We also attempted to observe product infrared (Ben Gurion University, Israel) participated in the design of the emission from quenching of O,(b) by COS, H,S, and NH,; experiments described in this paper. We thank Dr. Schofield for however, the small Einstein coefficients for the transitions from a preprint of ref 13. these polyatomic molecules that were within the range of our InSb detector precluded the detection of any infrared emission (but see Registry No. HF, 7664-39-3; HC1, 7647-01-0; HBr, 10035-10-6; HCN, 74-90-8; CO,, 124-38-9; CHQ, 74-87-3; HZS, 7783-06-4; NHS, ref 2). Based upon the observed H X product states, we conclude 7664-41-7; Hz, 1333-74-0; CFSNO, 334-99-6; 0 2 , 7782-44-1. that in each case the quenching of O,(b) gives Oz(a) and not
-
-
-
+
-
-
(22) Finzi, J.; Moore, C. B. J . Chem. Phys. 1975, 63, 2285.
(23) Downey, G. D.; Robinson, D. W. J . Phys. Chem. 1976, 80, 1234.
