J. Phys. Chem. 1981, 85,2873-2877
oxidizing IrC163-(Figure 4B), on the other hand, opens reaction 1 as a new pathway for oxidation of Fe(CN)$-. Transport of charge for Fe(CN)$- oxidation by reaction 1 is a t this point more efficient31 than transport by Fe(CN),” diffusion or by Fe(CN)63-/4-electron self-exchange because a t the prepeak potential in Figure 3C, the concentration of IrCl$ sites produced near the electrode and the dCIrcls?/dx gradient near the electrode are both large relative to those of Fe(CN),4- as illustrated in Figure 4B. Not all of the remaining Fe(CN)64-necessarily becomes oxidized via reaction l , but more is oxidized within the time span of the potential sweep than would be oxidized in the absence of the IrC1,3- oxidation reaction. Results in parts B and C of Figure 3 directly illustrate the occurrence and importance of electron exchange reactions in charge transport in these films. It is important to realize that, in thin films of redox materials, the effects in parts B and C of Figure 3 depend on charge transport diffusional control of the currents. In contrast, with a thinner film or (low concentration) fast charge transport, which promotes fast equilibration of redox sites in the film with the electrode potential during a potential scan, a Fe(CN),’ oxidation rate enhancement cannot be observed at the IrCb3- oxidation potential since the Fe(CN)&-in the film is already completely oxidized. This voltammetric behavior is illustrated in Figure 1B for a low concentration mixture of IrCb3- and Fe(CN)63-,where the reactions occur independently, in the same manner as they do in solutions (Figure 1A). Finally, note that the appearance of the (asterisk) prepeaks in the voltammograms of Figure 3C is qualitatively reminiscent of prepeaks in voltammetry of spatially segregated bilayers of redox polymer^.^^^^^ In bilayers, electron exchange cross reactions between inner and outer film layers cause sharp prepeaks; D, between the electrode and the outer film layer is (effectively) zero due to the spatial (.e-)
(31) Transport rate enhancements as large as ca. 38% can be anticipated from eq 14 and 17 of ref 2. (32) Abrufia, H. D.; Denisevich, P.; Umai5a, M.; Meyer, T. J.; Murray, R. W. J. Am. Chem. SOC.1981,103, 1. (33) Denisevich, P.; William, K. W.; Murray. R. W. J. Am. Chem. SOC. 1981,103, 4727.
2873
arrangement. Special attention was accordingly paid in the present study to the possibility of IrCb3- and Fe(CN)6* somehow spatially segregating themselves in the MPSPVPyH+ film, since this would compromise the charge transport-based interpretation of Figure 3 which assumes uniform mixing of the two ions. Pertinent observation are as follows: (a) The manner of mixed film preparation (simultaneous partitioning of the two ions) is not, by itself, conducive to layer making for Fe(CN)63-and IrCb3-. Indeed, Figure 1B shows various stages of the partitioning, and the waves for IrC&2-/3-and Fe(CN),*l4- increase together, in proportion to the relative ion concentrations in the partitioning bath. (b) If a film containing only IrCb3- is contacted with a Fe(CN):- solution, IrCe3- is expelled from and Fe(CN)$incorporated into the film to just the extent anticipated by the knownz4partition coefficients of the two ions. (c) X-ray photoelectron spectroscopy was carried out at near normal (87O), intermediate (35O), and grazing (go) electron emission angles, on films containing both IrC1:and Fe(CN)e3-. The ratio of intensities of the Ir 4f and Fe 2p bands did not change with emission angle being 0.40, 0.35, and 0.38, respectively. Since the grazing angle favors surface atoms more than does the near normal angle, this observation shows that no gradient exists in the relative Fe(CN)63-and IrC163-populations a t the outermost film surface. The above results do not rule out spontaneously formed, randomly located, “pockets” of the redox ions, some containing only one ion and some containing only the other. We think this is in fact an implausible eventuality, and are accordingly inclined to accept the idea that the IrClaand Fe(CN)63-in these films are homogeneously mixed. We maintain therefore that the phenomena of Figure 3 demonstrate, for the first time, a role of electron exchange cross reactions (reaction 1)in charge transport in redox mixtures. Acknowledgment. This research was supported in part by a grant from the National Science Foundation. The authors are indebted to Professor A. J. Bard for pointing out the prior work of Ruff. This is paper 40 in a series on chemically modified electrodes.
Studies of H and 0 Atom Reactions by OH Infrared Chemiluminescence B. S. Agrawalla, A. S. Manocha, and D. W. Setser” Department of Chemistry, Kansas State University, Manhattan, Kansas 66506 (Received: June 24, 198 1; In Final Form: August 17, 198 I )
The vibrational energy disposal to OH and relative rate constants have been measured for the 0 + HI and GeH, reactions and for the H + NOz and CIOz reactions. The experiments were done in a flowing afterglow apparatus which gives arrested vibrational distributions as shown by comparisons of the OH(u) distribution from H + NOz with other data in the literature. The energy disposal pattern for the 0 atom reactions closely resembles that for F (or C1) atom reactions. Comparison of the relative emission intensities from H + NOz and H + Clz and the accepted rate constants for the reactions permits a selection to be made for the better vibrational OH u’--* u’- 1 Einstein coefficients. Emission from NO was observed from the H + NOz reaction confirming predictions that NO is involved in the energy disposal. Introduction Infrared chemiluminescencestudies of HF, DF, HC1, and DC1 from H, F, and C1 atom reactions have been extensively investigated in our lab by means of the cold-wallarrested vibrational-rotational relaxation techniqueliz and
the flowing-afterglow-arrested vibrational relaxation technique.3,4 Recently, the flowing-afterglow technique (1) K. Tamagake, D. W. Setser, and J. P. Sung, J. Chem. Phys., 73, 2203 (1980).
0022-3654/81/2085-2873$01.25/00 1981 American Chemical Society
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The Journal of Physical Chemistry, Vol. 85, No. 20, 1981
was extended to HBr chemiluminescencefrom the reaction of H atoms with bromine-containing compound^.^ In the present work we have extended the flowing-afterglow technique to the study of OH, produced from 0 + RH (R = I, GeH3) and by H + X02 (X = N, Cl). The initial OH vibrational distribution for each reaction has been established by selecting suitable concentration ranges for each reactant. The enthalpies of the reactions are as follows:
+ HI
0
-
OH
moo = -30.9 0 + GeH4
-
+ I(2P3,2)
(1)
kcal mol-' OH
+ GeH3
(2)
AH,' = -27.2 kcal mol-'
+ NO2'OH
+ NO
moo = -28.8
kcal mol-'
H
H
+ Cl02
+
OH
(3)
+ C10
(4)
The vibrational energy disposal to OH offers some insight to the reaction dynamics of the two sets of reactions. Our OH vibrational distribution from H + NOz is the same as the well-established distribution deduced from molecular beam which supports the claim for observation of initial OH vibrational distributions. The OH rotational and the 2111j2and 2113 populations were Boltzmann for reactions 1-4. We wili show that the OH vibrational energy disposal from 0 RH reactions resembles that of HF from the analogous F + HR reactions. A further interest was to test the OH Einstein coefficients. The rotationless Einstein coefficients for the lower u levels reported by MieslO are about a factor of 2 greater than the values calculated (see footnote in Table 11) based on the dipole moment function of Meyer and Rosmus.",12 Our method involves comparing the infrared emission intensities from H + NOz with that from H C12for known reagent concentrations. Since the rate constants are well-known, and since the HC1 Einstein coefficients are also establi~hed,'~ the comparison gives the OH Einstein coefficients. The relative rate constants for 0 HI and GeH4 and for H NO2 and C102 were also measured.
+
+
+
Experimental Section The experimental technique for observing the OH emission is similar to that described in ref 4. A microwave discharge (60 W, 2450 MHz) in mixtures of Ar/Hz and Ar/02 was used for H and 0 atom generation. Careful poisoning of the quartz discharge tube toward atom re(2) D. J. Bogan and D. W. Setser, J. Chem. Phys., 64, 586 (1976). (3) J. P. Sung and D. W. Setser, Chem. Phys. Lett., 58,98 (1978). (4)J. P. Sung, R. J. Malins, and D. W. Setser, J. Phys. Chem., 83,1007
(1979). ~ -.-,. -
H + CLO,
L
I
AH," = -43.3 kcal mol-'
+
Letters
(5) R. J. Malins and D. W. Setser, J. Chem. Phys., 73, 5666 (1980). (6) R. P. Mariella, Jr., B. Lantzsch, V. T. Maxson, and A. C. Luntz, J. Chem. Phys., 69, 5411 (1978). (7) J. A. Silver, W. L. Dimpfl, J. H. Brophy, and J. L. Kinsey, J.Chem. Phys., 65, 1811 (1976). (8) E. J. Murphy, J. H. Brophy, G. S. Arnold, W. L. Dimpfl, and J. L. Kinsey, J . Chem. Phys., 74, 324 (1981). (9) E. J. Murphy, . . J. H. Brophy, and J. L. Kinsey, J. Chem. Phys., 74, 331 (1981). (10) F. H. Mies, J. Mol. Spectrosc., 53, 150 (1974). (11) W. Meyer and P. Rosmus, J. Chem. Phys., 63,2356 (1975). (12) P. Rosmus and H. J. Werner, J.Mol. Structure, 60,405 (1980). (13) K. Tamagake and D. W. Setser, J.Phys. Chem., 83,1000(1979).
4
OH++CLO
I
'
3doo
3dOO
'
32'00
'
30bO
28b0
I
+
Flgure 1. A typical OH( v - ( Y -1)) emission spectrum from the H C102 reaction recorded at 1-cm-' resolution. Only the P branch lines for each vibrational level have been labeled in this figure. The 2113/2 and '111,2 spin doublets are clearly resolved; the upper labels are for the state and the lower row labels are for the 2111,2state.
combination was done by coating the tube with phosphoric acid for each set of experiments. The NO2, CIOz, GeH4, and HI were introduced through small holes in a glass ring concentric with the flow axis. The reactions were carried out in a Ar carrier with a flow velocity of about 80 m s-'. The total pressure at the reaction zone was maintained at 0.7 torr. Emission was observed perpendicular to the flow axis through a NaCl window situated about 1.5 cm downstream from the reagent inlet. Thus, the reaction time was =0.45 ms. The reagent gases, Le., HI, GeH4, and NOz, were obtained from Matheson Gas Products. Chlorine dioxide was synthesized in our lab following the procedure of Arnold et al.14 Dilute mixtures (1-10%) of the reagents were made in Ar. The reagent flow rates were measured by a calibrated capillary flow meter. All reagents were vacuum distilled prior to use. The OH infrared emission was recorded with a Fourier transform spectrometer (Digilab) equipped with a liquid nitrogen cooled In-Sb detector. Figure 1 shows a typical spectrum of the emission obtained from H + C102reaction at 1-cm-l resolution. The Einstein coefficients of MieslO were used to obtain relative vibrational-rotational populations. We calculated, see footnote d of Table 11, the Einstein coefficients from the Meyer and Rosmusl' dipole function. The relative Ai, A:, and A i are in modest accord and, using values from Meyer and Rosmus' dipole function, will not change the ul, v2, and u3 relative populations significantly. The rotational 2111 and 2113/2 distributions were 300 K Boltzmann distrihution for each reaction. Therefore, only the strong P branch lines of the OH(2113/2) state were used to obtain relative vibrational populations. The OH(21132)/OH(21112) 300 K population ratio, 2.0 & 0.1, was inciuded in obtaining the total OH relative emission intensity for comparison to the HC1 intensity from H Clz. In order to observe the NO emission, an interference filter with 60% transmission in the range 2075-1875 cm-' was placed in front of the detector and the quartz plate filter, which normally is used to block radiation below 2000 cm-', was removed. We were successful in observing the NO(1-0) band from H + NO,; Figure 2 shows the spectrum, mainly the R and Q branches of the 1-0 band, re-
+
(14) s.J. Arnold, K. D. Foster, D. R. Snelling, and R. D. Suart, IEEE
J. Quantum Electron., 14, 293 (1978).
The Journal of Physical Chemlstty, Vol. 85, No. 20, 1981 2875
Letters
TABLE I: OH and HF Vibrational Distributionse reaction ( ( E ) ,kcal/mol) u, Vl u,
u4
u3
Cfv)
u,
US
technique
ref
flowing afterglow this work 0.11 0.37 0.52 flowing afterglow this work 0.67 0.51 0.02Q 0.11 0.36 1 cold-wall reactor 0.16 0.20 0.24 0.59 0.10 0.11 0.13 F + HI (67.2) 0.06= this work flowing afterglow 0.59 0.05 0.36 0 + GeH, (30.5) this work flowing afterglow 0.45 0.48 0.04 0.19Q 0.29 this work flowing afterglow 0.54 0.05 0.35 0.57 0.03b flowing afterglow 2 0.39 0.02 0.57 0.10 0.17 0.28 F t GeH, (64.2) 0.04a flowing afterglow 2 0.40 0.02 0.58 0.10 0.17 0.28 0.03b this work flowing afterglow 0.34 0.05 H iNO, (31.6) 0.61 this work flowing afterglow 0.25 0.46c 0.33 0.18 0.03 6 LIF/crossbeam 0.58 0.42 LIF/crossbeam O.lgd 0.03d 0.25 8 0.44 0.35 this work flowing afterglow H + C10, (46.1) 0.25 0.27 0.28 0.20 this work 0.25 0.49 flowing afterglow 0.24 0.18 0.22 0.12c 6 LIF/crossbeam 0.59 0.41 The uo population was estimated from a The uo population was estimated from a surprisal plot by using three-body prior. a surprisal plot by using a prior that includes the rotational degree of freedom of the polyatomic fragment. u, population Assigned by a surprisal plot with reference t o D + NO, data. e The OH(v) distributaken from Mariella’s paper (ref 6). tions were obtained by use of Mies’ Einstein coefficients (see footnote d of Table 11). The use of Einstein coefficients calculated from Rosmus’ dipole function would significantly change only the relative population for OH(u = 4 ) of H + C10, (0.14:0.21:0.25:0.23 :0.17).
0
~
i-
HI (34.4)
H+NO,+NO+
I
R-
A
I
+p
“14
* *H+N02
b) I
,
I
19’sO
1900
19‘40
lb0
1a120
+
Flgure 2. A NO( v - ( v - 1)) emission spectrum from the H NOz reaction recorded at 1-cm-’ resolution through an interference filter. The Q( v = 1 v = 0) branch at 1875 crn-’ is reduced in intensity 1875 cm-’ and only a few P branch because the filter cuts off at 0 band can be seen. The NO(X211)spin doublets are lines of the 1 not resolved.
-
-+
00
o,61
* H+C102
0
=
Vl
pv
-
corded at 1-cm-l resolution. In order to obtain the full NO(u) distribution a special filter selected to pass the NO(v - (u - 1))emission, but to reject the black-body radiation that would saturate the detector, will be required.
Results Our primary goals were to obtain initial OH vibrational distributions and to measure relative rate constants for OH formation. The first step was to establish the reagent concentration ranges that could be used without OH(u) relaxation. Hydrogen atom concentrations of 0.2 X 1013-2 X 1013molecules cm-3 were used without any sign of OH(u) relaxation. The oxygen atom reactions are slower and higher [O] was needed; however, there was no apparent OH(u) relaxation for [O] = 0.4 X 1013-4 X 1013molecules ~ m - These ~ . are estimated concentrations based upon an assumed 50% dissociation by the discharge. The independence of the OH(u) distributions from the concentration of HI, GeH,, NOz, and CIOz is demonstrated in Figure 3, a and b. Below 2.5 X 1013molecules cm-3 P,(OH) was invariant to [HI] and [GeH,]. For C102and NOz there was no relaxation up to a concentration of 7 x 10l2molecules The best OH(v) distributions for the four reactions
r
&
1
I
1
t 1 v3
[FH]x 1 0 ~ 1 2 ~ o l e c u l e5:tn-3 s Figure 3. Plot of the OH vibratlonal energy dlstributlons vs. reagent concentrations: (a) 0 GeH, and 0 -k HI and (b) H NO, reactions. In (a) [O]= 2 X l O I 3 atom cm-3 and in (b) [HI = 1.5 X l O I 3 atom ~ r n assuming -~ 50% dissociation of both O2and Hz.
+
+
are summarized in Table I. Before doing the rate constant measurements, the first-order kinetics of the OH emission intensity was demonstrated for the [O] and [HI mentioned above and for [HI] and [GeH,] < 3 X 1013and for [NO,] and [ClO,] < 7 X 10l2molecules ~ m - ~The . rate constant of 0 + GeH4 was measured relative to that of 0 + HI.15 The rate constants of H + NO, and C102 were measured relative to H + C1,. For this measurement we included the intensity of H3’Cl (relative isotopic abundance) and the contribution of OH(2n112).In making the comparison to H + C12,the OH formation rate constant depends directly on the values of the OH Einstein coefficient; our calculations were done with the Einstein coefficients of both (15) D. L. Singleton and R. J. Cvetanovic, Can. J . Chem., 56, 2934 (1978).
2876
The Journal of Physical Chemistry, Vol. 85, No. 20, 1981
TABLE 11: Rate Constants rate constant, c k , abs ( u > 0), cm3 reaction re1 ( u > I ) molecule-'s-l 0 t HI 1.0 0 t GeH, 2.2 * 0.3 4.3 x F t HI 1.0 7.2 X lo-'' F t GeH, 8* 1 5.8 X lo-'' H t NO, 7.2 X lo-" 12.5 X lo-"
* 0.3) X lo-'' (13.0 * 0.2) X lo-" (11.0
12.2 x
H t C10,
lo-"
(14.1 * 2.6) X 2.2 x 3.6 X lo-''
lo-"
*
loF1'
(5.7
1.2) X
Letters
technique flowing afterglow flowing afterglow flowing afterglow flowing afterglow flowing afterglow flowing afterglow decay of [HI, flow tube decay of [HI, flow tube decay of [ H I , flow tube flash photolysis flowing afterglow flowing afterglow decay of [HI, flow tube
ref this work this work
remarks
1
17 this work this work
using Einstein coefficient of Mies using rotationless Einstein coefficient of Rosmusd
20 21 22 23 this work this work
using Einstein coefficients of Mies using rotationless Einstein coefficients of Rosmusd
24
a Obtained from absolute value of 0 t HI, k S o 0=~1.6 X lo-'' cm3 molecule-' sbl, ref 15. Obtained from absolute value of F t HI, ref 17. Obtained from absolute value of H t Cl,. The rotationless Einstein coefficients based on the dipole moment function of Meyer and Rosmus" were c a l c ~ l a t e das ~ +A~i % ~ ; = 11.2, Ai:: = 14.6, AEZi = 12.8, AEZ: = 8.51 s-'; Mies': values are 21.6, 25.7, 19.5, and 9.6 s-',
Mies'O and Meyer and R0smus.l' All of the rate constants are summarized in Table 11. The relative OH(u) and NO(u) emission intensities were observed for [H + H,] = 2 X 1013and [NO,] = 1 X 1013 molecules ~ m -the ~ ; NVE (OH) Nu,, (NO) ratio was found to be -6. We used AiZi(NO)i, = 10.8 s-' and Ai1h(OH)l2 = 11.5 s-' for this calculation and corrected for the relative detector response that was measured with the band pass filter (for NO) and the quartz cutoff filter (for OH).
Discussion Oxygen Atom Reactions. The 300 K rate constant for 0 + HI is thought to be reliable.15 The 0 + HI reaction is 50 times slower than F + HI' and 0 + GeH4is 100 times slower than F + GeH4.17 However, the trend of,,k > kHI is similar in both sets. The trend is also seen in the analogous C1 atom reactions.18 The initial vibrational distributions from 0 + HI and GeH4 along with those of F + HI and GeH4 are listed in Table I. The vibrational energy disposal from 0 + HI and F + HI are very similar; (fv) is high (10.60) for both reactions. Both reactions give negligible Nu=,and the relative populations increase with increasing u. No I(?Pl,z) emission was observed from the 0 + HI reaction. (Somewhat surprisingly we did not even observe I(2P112) from O,(alA) + I(2P3,2)unless the seasoning on the discharge tube deteriorated and [O]was low.) Figure 4a shows surprisal plots for 0 + HI and F + HBr and HI. Although the F + HBr and HI plots are not truly linear,' whereas the 0 + HI plot appears quite linear, the three reactions certainly are very similar in the energy disposal despite the slower rate for 0 + HI. This similarity in energy disposal probably is a consequence of the mass combination H + L-H, which can override small differences in potential energy for direct transfer reactions. The F + GeH, and 0 + GeH, reactions also seem to have similar energy disposal, as shown in Figure 4b. The
-
7
-2.0
I
4.0c _, - - -
-
(16) F. P. Billingsley, 11, J. Mol. Spectrosc., 61, 53 (1976). (17) D. J. Smith, D. W. Setser, K. C. Kim, and D. J. Bogan, J.Phys. Chem., 81, 898 (1977). (18) M. M . A. Wickramaaratchi and D. W. Setser, submitted to J.Phys. Chem.
*/.*---
._---
-4.OC
'V
Flgure 4. Vibrational surprisal and vibrational distribution plots. (a) 0 4- HI, F H I and HBr reactions. The distributions from the F atom reactions are taken from ref 1. (b) 0 GeH, and F GeH4 reactions. The distribution from the F atom reaction is taken from ref 2. The two upper surprisal plots are for a prior that Includes the rotational degrees of freedom of the polyatomic fragment. The lower two surprisal plots are for a three-body prior.
+
+
+
vibrational surprisal plots are linear for both reactions. The u = 0 population from F GeH, is negligible. The estimate of OH(u=O) can be made from the model I surprisal which yields Po = 0.19, whereas model I1 gives Po = 0.03. These can be considered upper and lower limits. The (fv) values for F + GeH, and 0 + GeH, are similar, especially for the model I1 estimate of Po(OH). The observed rotational distributions for 0 + HI and 0 + GeH, were 300 K for all u levels, and high J levels could not be observed in contrast to F + GeH, and F + HI.1J9 This may be just a consequence of the reduced
+
The Journal of Physical Chemistry, Vol. 85, No. 20, 198 1 2077
Letters
available energy for the OH formation reactions so that for the same (or similar) (fR(0H)) the high J OH states are never formed. Hydrogen Atom Reactions. The H NO2 reaction has been widely studied. Table I1 lists some recent values of rate constants; they are in good agreement. The OH formation rate constant obtained in our laboratory depends on the u = 0 contribution and the Einstein coefficients. We assigned the u = 0 contribution from the molecular beam data, Pl/P0 = 0.7, which should be quite reliable.7-9 If we use Rosmus’s Einstein coefficients, our rate constant becomes very similar to the recent direct measurements of Clyne et al.,Q21 Wagner et and Michael et al.23 Our measurements thus support Rosmus’s Einstein coefficients. We draw the same conclusion based on the comparison of our H C102 rate constants (see Table 11) with that of Bemand, Clyne, and Watson.% Both Mies’s and Rosmus’s Einstein coefficients are based upon calculated dipole moment functions. We believe further experimental and theoretical work is needed to definitively establish the true OH(X211)Einstein coefficients and their dependence on u and J . However, at present the Rosmus values are preferred. The H + NOz and CIOz reactions have similar large thermal rate constants, but the energy disposal is apparently different. The H + NO2 reaction has been given much recent attention and there is agreement for the u = 0 and 1 relative populations. Our work provides firm Pl:P2:P3ratios and the entire OH distribution is well established. Murphy et a1.8 estimated the higher OH and OD vibrational distributions by using linear surprisal extrapolations and obtained for OH uo:u1:u2:u3 = 0.44:0.350.19:0.03,which is identical with our experimental result. The combination of laser fluorescence (for u = 0 and 1)and infrared emission for u 2 2 is thus a powerful way to obtain complete OH vibrational distributions. Smith and Fisher25also used a flow technique to measure the OH(u) distribution; they report P2/Pl= 0.35 compared to our value of 0.55. Their distributions may be partially
+
+
(19) J. P. Sung and D. W. Setser, J . Chem. Phys., 69, 3868 (1978). (20) M. A. A. Clyne and P. B. Monkhouse, J. Chem. SOC.,Faraday Trans. 2, 73, 298 (1977). (21) P. P. Bemand and M. A. A. Clyne, J. Chem. SOC.,Faraday Trans. 2, 73, 394 (1977). (22) H. G. Wagner, U. Welzbacher, and R. Zellner, Ber. Bunsenges. Phys. Chem., 80,1023 (1976). (23) J. V. Michael, D. F. Nava, W. A. Payne, J. H. Lee, and L. J. Stief, J. Phvs. Chem.. 83. 2818 (1979). ( 2 4 P. P. Bemand, M. A. A. Clyne, and R. T. Watson, J. Chem. SOC., Faraday Trans. 1, 69, 1356 (1976). (25) G. K. Smith and E. R. Fisher, J. Phys. Chem., 82, 2139 (1978).
relaxed because of the high reagent concentrations: [HI = 1014atoms ~ m - [NO,] ~ , = 9 X 1013molecules ~ m - When ~. obtaining vibrational distributions by infrared chemiluminescence in a fast flow reactor, care must be taken to establish the reagent concentrations that avoid relaxati~n.~ Polanyi and S10an~~ interpreted their chemiluminescence OH(u) distribution (cold-wall, arrested relaxation) as having been affected by fast OH vibrational relaxation with AJ = 0. In fact the observed distribution (0.70:0.28:0.02) is not so different from our distribution. Based upon their EPR measurements of OH(u = 0-3) in a flow reactor and considerations of relaxation, Spencer and Glassz6 also concluded that the cold-walled dataz7were not seriously relaxed. From our value of (fv(OH)) = 0.25, (fR(0H)) = 0.208 and ( f T ) = 0.259p28it is predicted that (fv+R(NO))= 0.30. The NO rotational excitation is not expected to be too high and appreciable NO vibrational excitation is implied. If the NO vibrational energy disposal resembles that for OH, then the population will be mainly in levels u = 0-3. Our estimate for Nu,l(OH)/Nu,l(NO) = 6 depends on the ratio of the NO and OH Einstein coefficients, which are not well established. Thus, our observation of NO(u = 1)should be viewed as confirmation of the importance of NO in the energy disposal of H + NOz with further work, which will be attempted, needed to assign the NO(u) distribution. For the H + C102reaction, (fv(OH)) is nearly 2 times larger than that for H NOz, but the vibrational distribution still is rather flat. Mariella6 reported (fR(OH))= 0.26, which implies that 25% of the energy must be shared between the translational and internal energies of C10. Either ( f T ) or (fvR(CIO))must be lower than the corresponding values for H + NO2. The H + NO2 reaction proceeds by a bound intermediatemand a bound HO-C10 species also is expected. However, the HO-NO and HOC10 potential surfaces will not be identical and that part of the energy disposal associated with the separation of the products can be quite different.29 Acknowledgment. This work was supported by the National Science Foundation under Grant 77-21380. We thank Dr. Wickramaaratchi for discussions about the OH and NO Einstein coefficients.
+
(26) J. E. Spencer and G. P. Glass, Chem. Phys., 15, 35 (1976). (27) J. C. Polanyi and J. J. Sloan, Int. J. Chem. Kinet. Symp., 1, 51 (1975). ‘ (28) H. Haberland, W. Lucadou, and P. Rohwer, Ber. Bunsenges. Phys. Chem., 84, 507 (1980). (29) B. E. Holmes and D. W. Setser, “Energy Disposal in Chemical Reactions” in “Physical Chemistry of Fast Reactions”, Vol. 3, I. W. M. Smith, Ed., Plenum, New York, 1980.
