J . Phys. Chem. 1989, 93, 3170-3174
3170
Kinettc Study of NH(a) by Emission F. Freitag,+ F. Rohrer,*and F. Stuhl* Physikalische Chemie I , Ruhr-Uniuersitat, 0-4630 Bochum, Federal Republic of Germany (Received: August 1, 1988; In Final Form: October 31 1988) ~
-
Metastable NH(a) radicals were generated in the ArF laser (193 nm) photolysis of HN3 and were monitored by their (alA X3Z7 phosphorescence emission. Rate constants for a number of quenching gases were obtained. These rate data are compared with that previously reported in the literature for NH(a) and related species. While the NH(a)-N, system encounters a significant barrier in the entrance channel, the NH(a)-CO system does not. Metastable O,(b) molecules were observed to be a major product of the quenching by 0,.
to monitor the decay of NH(a). This emission was dispersed by a 0.25-m monochromator to select the wavelength range 786-792 Time-resolved detection of NH(a’A) has been performed in nm. In some experiments, wavelengths from 791 to 797 nm were previous work by (a) absorption,’ (b_) chemiluminescence via the chosen. The time-resolved signal was stored and accumulated in reaction NH(a) HN3 NH2(A2Al) + N 3 and subsequent a multichannel analyzer using a dwell time of 10 p s per channel. monitoring of NHz(A2Al) f l u o r e ~ c e n c e , ~(c) * ~ laser-induced Usually, the signal in the time range 50-1000 p s was evaluated fluorescence (LIF) by means of the singlet NH(c,u’=O-a,u”=O) to obtain a decay rate. The counts in ‘‘late’’ channels (after about transition,” and (d) LIF of the possible product NH(X) detected five lifetimes) were used to determine the background signal which through the triplet transition NH(A-X).4s7 The most specific subsequently was subtracted from the signal. Measured decay of these methods is direct (singlet) LIF, since it allows the detection rates ranged from 700 to 23 500 s-’. All kinetic measurements of single rotational/vibrational states with high temporal and were performed twice on two different days. For a determination spectral resolution. This specificity can be of advantage in many of a rate constant at least 11 decay rate measurements were cases but has to be carefully considered if collisional redistribution performed. The temperature was 296 f 3 K. in internal degrees of freedom occurs during the reaction or The gases were mixed in a flow and photolyzed under slow flow quenching process under investigation. conditions. The flow of HN, premixed with Ar was measured Because of their radiative lifetime (1.7 s,* 11.9 s9) metastable by the drop of pressure in the storage bulb and by a flow controller NH(alA) radicals are potentially long-living energy carriers of (MKS 1259 BG). The flows of the quenching gases and diluent 1.57 eV.lOJi In practice, their lifetime is determined by the Ar were determined by flow controllers (Tylan FC 260) too. In quenching efficiency in collisions with the other molecules present, case of very efficient quenchers, a part of the total mixture had in particular, with the parent molecules. In the case of removal to be discarded to the open air by use of a rotary pump. Pressures by a (near) resonance energy-transfer process, the energy can be were measured by capacitance manometers (MKS, ranging from conserved as a part or whole by a species possibly less sensitive 133 Pa to 133 kPa full scale). All flow controllers were calibrated to collisional quenching. This study uses a recently introduced by measuring pressure increases in the photolysis cell. Because detection methodl2 for NH(a), namely, phosphorescence from its of adsorption of H N 3 or other species on the walls, the mixtures forbidden transition to the NH(X38-) ground state. This sensitive flowed through the cell for 30 min before an experiment started. technique is of advantage when searching for such energy-transfer After a reactant pressure was changed, 5 min was allowed for processes that result in additional emissions close to the wavelength attaining a steady state before the next run was performed. of the NH(a-X) emission, as has been done in this work for O2 Emission spectra were taken as described previously.I2 For this as a quencher. purpose a multichannel analyzer was used as a digital gated For kinetic studies, NH(a) radicals are commonly generated integrator with typical gate times of 10-1000 ps. The typical in the UV photolyses of HN3,1-5v12HNC0,6*7or N2H4.496 spectral resolution was 0.8 nm. Quadrupled Nd:YAG (266 nm), KrF (248 nm), and ArF (193 H N 3 was produced and stored as described p r e v i o u ~ l y . ~ ~ ~ ~ ~ ~ nm) have been found to be convenient laser light sources for this purpose. The photolytically produced radicals are k n ~ w n ~ , ’ ~ - ’ ~All quenching gases were from Messer Griesheim except NzO to have the following properties: (a) A large fraction of the energy in excess over the dissociation energy is channeled into translational motion. (b) NH(a) is formed rotatianally hot and (c) with (1) Paur, R. J.; Bair, E. J. In?. J . Chem. Kinet. 1976,8, 139. Paur, R. J.; Bair, E. J. J . Photochem. 1972, 1 , 255. substantial vibrational excitation. It is desirable in kinetic studies (2) McDonald, J. R.; Miller, R. G.; Baronavski, A. P. Chem. Phys. 1978, that the energy content of the reacting species is well-defined and 30, 133. remains unchanged during the course of the investigated process. (3) Piper, L. G.; Krech, R. H.; Taylor, R. L. J . Chem. Phys. 1980,73, 791. Care was therefore taken in this investigation to deal with NH(a) (4) Cox, J. W.; Nelson, H.H.; McDonald, J. R. Chem. Phys. 1985, 96, 175. being as close as possible to equilibrium with a room-temperature (5) Hack, W. Habilitationsschrift, Universitat Gottingen, 1986. distribution for all degrees of freedom. (6) Bower, R. D.; Jacoby, M. T.; Blauer, J. A. J . Chem. Phys. 1987, 86, Introduction
+
-
Experimental Section
The generation of NH(a) from HN3l4.I5and its detection by phosphorescence emissionl2 have been described in detail recently. Briefly, NH(a) was produced in the ArF (and in a few experiments in the KrF) laser photolysis of HN3 with laser fluences of 7-30 (and about 73) mJ cm-2. Emission from the forbidden transition NH(a1A-X32-) was observed at right angles to the laser beam Present address: Max-Planck-Institut fur Strahlenchemie, Stiftstrasse 34-36, D-4330 Miilheim/Ruhr, West Germany. ‘Present address: Institut fur Chemie 3: Atmospharische Chemie der KFA Jiilich, D-5170 Jiilich, West Germany.
0022-3654/89/2093-3170$01 .SO10
1954. (7) Drozdoski, W. S.; Baronavski, A. P.; McDonald, J. R. Chem. Phys. Lett. 1979, 64, 421. (8) Marian, C. M.; Klotz, R. Chem. Phys. 1985, 95, 213. (9) Ramsthaler-Sommer, A.; Eberhardt, K. E.; Schurath, U . J . Chem. Phys. 1986, 85, 3760. (10) Crossart, D. J . Chim. Phys. Phys.-Chim. Biol. 1979, 76, 1045. (11) Ram, R. S.; Bernath, P. F. J . Opr. SOC.Am. B 1986, 3, 1170. (1 2) Rohrer, F.; Stuhl, F. Chem. Phys. Left. 1984, I I I , 234. (13) Baronavski, A. P.; Miller R. G.;McDonald, J. R. Chem. Phys. 1978, 30, 119. (14) Kenner, R. D.; Rohrer F.; Stuhl, F. J . Chem. Phys. 1987.86, 2036. (15) Rohrer, F.; Stuhl, F. J . Chem. Phys. 1988, 88, 4788. (16) Spiglanin, T. A,; Chandler, D. J . Chem. Phys. 1987, 87, 1577.
0 1989 American Chemical Society
Kinetic Study of NH(a) by Emission
The Journal of Physical Chemistry, Vol. 93, No. 8, 1989 3171
TABLE I: Experimental Parameters and Results of Plots of T-I vs P(Q)According to Eq 1 for the Quenching of NH(a)" Q P(HN,)/Pa P(Q) range/Pa P(Ar)/Pa no. of expts ro-~/s-~ k/
HN, 0.033 0.067 0.029 0.029 0.044 0.076 0.064
0.015 0.0 12 0.032 0.028
0.08-0.56 0.3-4.0 30-270 271-1365 0.3-1.8 3-38 1.7-23.7 0.8-20.6 0.22-0.83 0.15-0.33 0.22-1.3 0.085-0.3
1400 590 5450
100 f 270 890 f 25 2870 f 170 1070 f 670 1250 f 100 2060 f 50 3130 f 110 3280 f 240 670 40 710 f 60 1220 f 120 1330 f 100
12 22 12 11 17 15 15 15 17 11 15 16
0 665 1240 5320 1285 1050 860 660 356
___
cm3 s-I
12000 f 600 340 f 20 6.2 f 0.8 6.8 f 0.5 1350 f 70 25 f 2 158 f 11 300 f 20 1 7 0 0 k 140 3100 f 350 2400 f 160 7000 f 800
*
"The error limits represent 3 times the standard deviation. bCyclopropane. (L'Air Liquide). They had the following stated purities: Ar and Nz, 99.999%; CO, 99.997%; COZ, 99.995%; N20,99.99%; CH4, 99.995%; CZH6, C3H8,and C2H4, 99.95%; C3H6(cyclopropane), 99%.
Results and Discussion Formation and Characterization of NH(a). The ArF (193 nm) laser photolysis of HN, was used to generate NH(a). Because NH(a) was recently observed in our laboratory in the A r F laser photolysis of NH3,14we have also used this parent molecule. Upon the photolysis of 8.6 Pa of NH3 mixed with 133 Pa of Ar, the time profile of a weak, long-lasting emission was observed at 794 f 3 nm exhibiting a complex behavior with a relatively fast decaying component in the beginning, which might be due to NH(a). At long times the signal increased again up to a peak intensity at 2 m s . According to known kinetic data,6 this long-lasting signal cannot be due to NH(a). Obviously, the quantum yield (0.00814) is too low for the generation of sufficient emission intensity in the case of ammonia. In a separate experiment, HN, was photolyzed by KrF laser light. With 0.081 Pa of HN, in 5610 Pa of Ar, a biexponential decay was observed. The first fast decay is in agreement with the kinetic data of this work. This fast decaying emission ( T = 0.47 ms) was followed by an emission having a lifetime of 2.7 ms. The long-lasting decay (and the weakness of the signal) prevented us from using the 248-nm photolysis of HN,. Upon addition of He, the decay rate of NH(a) in the reaction with HN, has been previously shown to decrease by about 30%.2 This fact has been attributed to rotational and/or translational relaxation. A recent study has reported no difference in the rate constants when different rotational levels were monitored.6 In order to define and characterize the energy in the NH(a) radical, several NH(a-+X) emission spectra were recorded in the presence of various amounts of Ar, N2, or 02.The results of these experiments can be summarized as follows: (a) A number of additional emissions at around 790 nm, where NH(a) emits, were observed at low pressures of added gas (for example, at 320 Pa of N2). (b) The intensity of these emissions was minimized by large inert gas pressures which quench these interfering emissions much more efficiently than that from NH(a) and by low HN, pressures to suppress secondary reactions such as NH(a) + HN,. A spectrum taken at 0.27 Pa of H N 3 and 1330 Pa of Ar did not show any interfering emission. It should be noted in Table I that all kinetic data were taken at significantly lower HN, pressures. (c) With inert gas added the intensity distribution in the observed NH(a-+X) spectra can be approximated by assuming a roomtemperature Boltzmann distribution for rotation. For the delays (10 F S for the spectra, 50 ws for the kinetic runs) and the Ar pressures (roughly 1000 Pa) used, relaxation to rmm temperature is expected to occur for rotation (and also for translation) according to experience with other N H states.17J8 ~~~
~
(17) Hofzumahaus A.; Stuhl, F. J . Chem. Phys. 1985, 82, 3152. (18) Rohrer, F. Dissertation, Ruhr-Universitat Bochum, 1987. Rohrer, F.; Stuhl, F. Book of Absrrucrs, F7, 9th International Symposium on Gas Kinetics, University of Bordeaux, Bordeaux, France, 1986.
7
v)
E
15
I 0.2
0.L
Pressure of HN31Pa
Figure 1. Decay rate, i-I, of NH(a) vs pressure of HN3.
Vibrational relaxation most likely is not complete in the present photolysis system. NH(a) has been recently observed to be generated with appreciable vibrational excitation in both KrF (-40% of the NH(a) in v = 114) and ArF laser photolysis of HN, (-20% in v = l15). Calculations show that the (1,l) band is shifted with respect to (0,O) by only 4 nm toward short wavelengths and thus overlaps with the (0,O) band. The monitored wavelength interval at around 790 nm is estimated to comprise emissions from both v = 0 and 1 with intensities proportional to the respective population. We therefore conclude that the present study monitors the total removal of both NH(a,v=O) and NH(a,v=l), although the population of v = 1 might be relatively low at the total pressures and after the initial delays used. For the quenching by 02,a number of experiments were performed with detection at about 794 nm. N o difference in the decay rates was observed. At this wavelength, negligible emission intensity may be generated by NH(a,u=l). Kinetics. The decays of NH(a) monitored in the ArF laser photolysis of HN, were always found to be exponential. Hence, a decay rate, T - ~ was , plotted in the usual way vs the pressure, P ( Q ) , of the quenching gas, Q : T-'
=
~0-l
+ kP(Q)
(1)
The slope of such a plot determines the quenching rate constant, k . T ~ is- ~the decay rate in the absence of Q and is mainly determined by quenching by the parent molecule, HN,, and buffer gas and by diffusion. The radiative decay is very low^,^ and is negligible in the present experiments. Care was taken to perform all rate measurements under constant-pressure conditions (with the exception of those for the quenching by N2) to keep constant the removal of NH(a) by diffusion. Two examples of plots 7-l vs P ( Q ) are given in Figures 1 and respectively. Figure 1 displays the data 2 for Q = HN, and 02, of the most precise and Figure 2 of the least precise experiment. The results for the quenching gases ( Q ) HN,, Hz, 02,N2, CO, C o 2 , NzO, CHI, C2H6, C3H8, C3H6(cyclopropane), and C2H4 are summarized in Table I. The stated error limits represent 3u. This table additionally lists some relevant experimental conditions. All values of T ~ (column - ~ 6 of Table I) are consistent with the
3172
The Journal of Physical Chemistry, Vol. 93, No. 8, 1989
Freitag et al. are consistent with these activation energies. It has been previously shown by the diatomic-in-molecule approach and a dynamical treatment including surface hopping that the comparable O( ID) + H2 system apparently is not confined long enough (or not at all) to the deep H-0-H potential minimum to permit randomization of the reaction exoergicity.,, Similarly, the reaction of NH(a) H2 might occur first by insertion of NH(a) into the H-H bond, followed immediately (or simultaneously) by fast abstraction of an H atom. No physical quenching of NH(a) generating NH(X) has been previously observed for Q = H,, CH,, and C3Hs.4 Support for the addition of NH(a) across the carbon-carbon multibond of C2H4 has been previously given by Cox et aL4 The fast reaction and the negative (near zero) activation energy4 support this reaction mechanism. The quenching by 0, and N, is equally inefficient. On account of the markedly different value of the rate constants reported for 02,its quenching will be discussed separately in the next section. Because of crossing of the singlet NH(a)-N2 potential surface with the lower triplet NH(X)-N2 surface23the small value for Q = N2 is surprising. Although recent MCSCF-CI calculations do not exhibit a barrier, some CASSCF calculations suggest a barrier on the singlet surface at large NN-NH separation^.^^ In a recent study of IR multiphoton dissociation of deuterated hydrazoic acid, the formation of ND(a) radicals has been observed.24 These radicals were generated with translational energy but without vibrational and with little rotational energy. An energy barrier in the exit channel of 1460 cm-' was inferred from these experiments for the di~sociation.~~ This barrier height is consistent with the value of the present rate constant and implies the following approximate Arrhenius parameters: A = 7 . 5 X IO-" cm3 s-l and E, = 17.5 kJ mol-'. It is interesting to note that the rate constant for the quenching by the isosteric C O molecule is 200 times larger than that for Q = N2. We therefore conclude that the corresponding NH(a)-CO surface does not exhibit a significant barrier. Whether the different barrier heights have to do with different orbital rearrangements during the d i s s o c i a t i ~ nhas ~~,~~ to await further calculations. Another example of quenching by a pair of isosteric molecules is presented by Q = CO, and N20. It again is observed that the polar molecule quenches more efficiently. However, without further analysis, at least of the products, the discussion has to be highly speculative. Note that the replacement of 0 by NH(a) is energetically possible (but violates conservation of spin) for N 2 0 but not for CO,. In the united atom model, NH(a) (and also NH(b) and NH(c)) is represented by O('D).26 Moreover, CH2(B1Al)is an isoelectronic species and its quenching has been thoroughly studied previously.27 A comparison of the quenching behavior of NH(a), O('D),,* and CH2(B'Al) reveals (a) that the two latter species are quenched much more efficiently (in less than about 100 collisions) by all molecules and (b) that the ranges of rate constants for O(lD) and CH2(5i1A,)are much smaller than that measured here for NH(a). Note that the quenching behavior of NH(c)'* is much closer to that of O(lD) and CH2(H1A,) than that of NH(a).
+
iI
20001
t
200
100
Pressure of %/Pa Figure 2. Decay rate, i-l,of NH(a) vs pressure of 02.The experimental conditions are given in Table I. TABLE 11: Comparison of the Present Rate Data for the Quenching
of NH(a) with Literature Data' Q
this work
literature data
H N , 12000 f 600 9300 f 1000,O 9300 i 900,b 18000,' 10400 f 1000,6 13 100 f 3600,e 1 2 1 0 6 H7 340 f 20 460 f 40,' 290 f 608 0 2 6.2 f 0.8 1550,* 1550,' 4 . 9 N2 6.8 f 0.5 83.', 7.5 f 0.69 CO 1350 f 70 C02 2 5 i 2 23' N2O 158 f 1 1 CH, 300 f 20 1200 i 380 i 40' C2H6 1700 f 140 C3HB 3100 350 4200 i 300' C3H$ 2400 f 160 C z H 4 7000 f 800 3800 i 400: 8800 i 8OOc
*
Reference 1 . Reference 2. Reference 3. dReference 12. 'Reference 4. /Reference 5. ZReference 6. *Reference 7. 'The stated error limits do not have the same meaning in all cases. 'Cyclopropane.
rate data obtained in this work (quenching by the parent and buffer gas), the radiative lifetime,*s9and the expected diffusion of NH(a) from the effective photolysis volume.12 Table I1 shows a comparison of the present data with literature values. To our knowledge, no values have been reported before for the quenching by CO, N20,C2H6,and C3H6. In general, the values from the present work are in reasonable agreement with most of the literature values. It should be noted, however, that the data obtained by chemiluminescent detection* deviate from the present ones to a much larger extent than that measured by LIF of NH(a).3-5 Moreover, LIF measurements of the product NH(X) (Q = 0,) gave values that are distinctly different from those obtained in this study and by LIF of N H ( ~ , U = O ) . ~A, ~ similar discrepancy has been recently observed by Bower et al. for Q = H N C 0 . 6 The most recent values of rate constants for Q = HN34.5312agree with the present value, and minor differences are probably due to uncertainties in the concentration of H N 3 in the photolysis system. The major product of the reaction NH(a) + HN3 is known to be electronically excited NH2(A2A1).I9 Previous work on the reactions of NH(a) with H2 and alkanes has been discussed in detail by Cox et a1.4 and will not be repeated here. Both insertion into the H-H and C-H bonds and H atom abstraction have been considered in gas- and liquid-phase reactions. Earlier calculations indicate no barrier for insertionz0and barriers of 35 and 57 kJ mol-' for H atom abstraction from Hz and CH4, respectively,21 while the reported respective activation energies are 6.4 and 7.8 kJ m01-I.~ The present values at room temperature (19) Baronavski, A. P.; Miller, R. G.; McDonald, J. R. Chem. Phys. 1978, 30, 119. (20) Fueno, T.; BonaEiE-Kouteckp, V.; Kouteckg, J. J . Am. Chem. SOC. 1983. IOS. 5547. (21) Fueno, T.; Kajimoto, 0. J . Am. Chem. SOC.1984, 106, 4061.
(22) Kuntz, P. J.; Niefer, B. I.; Sloan, J. J. J . Chem. Phys. 1988,88, 3629. (23) Alexander, M. H.; Werner, H.-J.; Dagdigian, P. J. J . Chem. Phys. 1988, 89, 1388. (24) Stephenson, J. C.; Casassa, M. P.; King, D. S . J . Chem. Phys. 1988, 89, 1378. (25) Huisken, F.; Krajnovich, D.; Zhang, Z.; Shen, Y. R.; Lee, Y. T. J . Chem. Phys. 1983, 78, 3806. (26) Herzberg, G. Molecular Spectra and Molecular Structure, I . Spectra of Diatomic Molecules; Van Nostrand: New York, 1950. (27) Langford, A. 0.;Petek, H.; Moore, C. B. J . Chem. Phys. 1983, 78, 6650, and references therein. (28) DeMore, W. B.; Molina, M. J.; Sander, S. P.; Hampson, R. F.; Kurylo, M. J.; Golden, D. M.; Howard, C. J.; Ravishankara, A. R. JPL Publication 87-41, 1987. (29) Rohrer, F.; Stuhl, F. J . Chem. Phys. 1987, 86, 226. (30) Browarzik, R. K.; Kaes, A,; Kenner, R. D.; Stuhl, F., unpublished results. (31) Zetzsch, C.; Stuhl, F. Ber. Bunsen-Ges. Phys. Chem. 1976, 80, 1354. (32) Zetzsch, C. Habilitationsschrift, Ruhr-Universitat Bochum, 1978.
The Journal of Physical Chemistry, Vol. 93, No. 8, 1989 3173
Kinetic Study of NH(a) by Emission
TABLE 111: Comparison of Rate Constants, k/10-I2 cm3 s-l, for the Removal of Five Different Electronic NH States by Selected Diatomic Molecules NH(c) NH(N
NH(b)‘ NH(aY
NH(W
H2
0 2
N2
co
154 f 8” 84 f 4c 0.86 3.4 i 0.2 10.0000 18
196 & 15b 44 f 5 c 0.0024 0.062 0.008 0.0085 f 0.0009h
13.4 f 0.7“ 0.049d 0.0006 0.068 f 0.005 10.0000003‘
342 f 25” 52 5d 0.02 13.5 f 0.7 j
*
*
a Reference 18. *Reference 29. ‘Reference 30. dReference 17. CReference 31 (error limits generally within i30%). /This work. ZReference 32. hReference 33. ‘Reference 34. jProbably very small.
Table 111 compares rate data for the removal of the lowest five N H states by some selected diatomic molecules. The following trends can be deduced from this table: (a) The triplet states NH(A) and NH(X) are removed much less efficiently than the corresponding singlet states, Le., NH(c) and NH(a,b). The same tendency has been observed for O(,P) and O(lD) atoms. (b) The singlet N H states are quenched by various diatomic gases in the following increasing order: b < a < c. N H ( a ) 02.The kinetics of the removal of NH(a) by O2has been studied previously by several a ~ t h o r s . ~ vTable ~ * ~ *I1~shows that there is considerable disagreement among the reported rate constants. Although slightly larger, the present value supports the lower value given by HackS rather than the two coinciding large value^.^^^ The first kinetic study on the quenching by O2 has been reported by McDonald et aL2 Using chemiluminescent detection, these authors have obtained a negative slope in a plot 7-l vs P ( 0 2 ) . Therefore, no rate constant could be determined. Among several reaction channels leading to chemiluminescent species, they particularly considered those resulting in a secondary chemiluminescence emission such as formation of excited NO2(A2BI)and reactions o,f vibrationally excited NH(X) with H N 3 to yield excited NH2(A2Al).2 The quenching by O2was reinvestigated thereafter by Drozdoski et aL7 using HNCO as precursor and LIF from the possible triplet product NH(X) for detection. They reported that the reaction of NH(a) with O2produces only nonemissive products’ in variance with the present work as shown below. From the time of growth of the NH(A,v’=O-X,u”=O) LIF intensity, these authors deduced the rate constant for the quenching by O2to be 1.55 X lo-” cm3 s-I. Cox et aL4 have subsequently confirmed this value by applying the same detection method but photolyzing HN,. Drozdoski et aL7 furthermore concluded that the NH(X) production ratio of v = 0 and v = 1 is 8. These products are probably generated in close resonance with O2(a’A,u=3) and 02(alA,u=l), respectively. The above value of the rate constant is about 300 times larger than the values obtained in this work and by Hack, who used LIF detection on the NH(c,u’=O-a,v”=O) t r a n ~ i t i o n . ~ Upon the addition of 02,we have searched for emissions in the 600-880-nm range and observed phosphorescence at around 760 and 865 nm besides that from NH(a) at about 790 nm (and some weaker emissions which were not sufficiently quenched by the inert gas present). Figure 3 shows the most dominant of these emissions. They were identified to be the (0,O) band of the 02(b1Z,+-X32,) transition (with the R and P branches clearly separated), the NH(a+X) phosphorescence, and, at 750 nm, possibly the (4,2) band of the N2(B311,-+A3Z,+) transition which disappeared in spectra taken at longer times and high O2pressure ( P ( 0 , ) = 5330 Pa). The emission at 865 nm was estimated to be about 15 times weaker than that at 762 nm, as expected for the (0,l) band of the 02(b-+X) transiti01-1.~~ To limit the number of possible precursors of metastable 02(b) molecules in this system, the photolysis of H N 3 was additionally performed at 248 nm and O2 phosphorescence was readily detected. (From the weaker intensity, the known different laser fluences, and the assumption of similar quantum yields, we estimated the ratio of the relevant absorption cross sections u( 193
+
(33) Zetzsch, C.; Hansen, I. Ber. Bunsen-Ges. Phys. Chem. 1980,82, 830. (34) Zetzsch, C.; Stuhl, F. Ber. Bunsen-Ges. Phys. Chem. 1981, 85, 564. (35) Burr, J. G.Chemi- and Bioluminescence; Marcel Dekker: New York, 1985.
I
7L0
I
760
I
I
700
800
a20
Wavelength / nm
Figure 3. Emission spectrum from the NH(a) + O2 reaction. The pressures of HN,, 02, and Ar were 0.84,280, and 3870 Pa, respectively. The laser energy was 60 mJ, and the spectral resolution was 2 nm. The gate was open from 10 to 1000 1.1s.
nm)/a(248 nm) to be about 30 for HN,, which is in reasonable agreement with literature value^.'^,^^ Contrary to the 193-nm photolysis of HN,, which yields all energetically accessible N H states, the only detectable N H state formed in the 248-nm photolysis is NH(a) (and very weakly NH(A)).lS In addition, H N 3 is formed with a quantum yield of 0.38 f 0.02.37 These products cannot excite 02(b) directly, because of lack of internal 2N238 energy (but a very fast sequence such as 2N3-N2* followed by N2* + 0 2 ( X ) N 2 + 0 2 ( b ) cannot be excluded completely). We hence conclude that the energy-transfer reaction
+
-
NH(a)
+ 02(X)
+
-
NH(X)
+ 02(b)
(2)
most likely generates 02(b). In order to support this conclusion, (a) the intensity of the 02(b) emission was recorded as a function of time and (b) the relative intensities in Figure 3 were checked for consistency. Reaction 2 is endothermic by 433 cm-’ for the reactants in their lowest internal states. This endothermicity is too small to account for the relatively small quenching rate constant measured in this work. An additional barrier is likely for NH(a,v=O) which might be easily overcome by NH(a,u>O). Hence, future studies should carefully consider the kinetics of internally (such as vibrationally) excited NH(a). A time profile of the 762-nm emission was recorded in the 0.012 Pa of HN,, and 750 Pa of Ar. The presence of 40 Pa of 02, growth of the intensity was roughly estimated to support a rate constant for reaction 2 being about 3 times larger than that measured here for the quenching of NH(a) by 02.The enhanced formation rate might be due to reactions of internally excited NH(a). Although more kinetic experiments are planned for a more definite statement, this experiment clearly indicates that most of the O,(b) is formed from a long-living precursor and not from NH(c) or NH(A) which are known products of the ArF laser photolysis of HN3.15 It should be noted that the decays of 02(b) are found to be much slower than those of NH(a). They were (36) Okabe, H. Photochemistry of Small Molecules; Wiley: New York, 1978. (37) Bersohn, R., private communication. (38) Jourdain, J. L.; LeBras, G.; Poulet, G.; Comborieu, J. J. Combust. Flame 1979.34, 13. Piper, L.G.;Krech, R. H.; Taylor, R. L. J. Chem. Phys. 1979, 71, 2099.
J . Phys. Chem. 1989, 93, 3174-3178
3174
as a precursor of Oz(b), since, in the ArF laser photolysis, its quantum yield is 20 times smaller than that of NH(a).I5
measured at two HN, pressures and gave a rough estimate of 7 X IO-', cm3 s-' on the value for the quenching of Oz(b) by HN,. Because of this less efficient quenching by the parent, HN,, the energy of the originally formed NH(a) will remain 15 times longer in form of 02(b). The relative intensities in Figure 3 were used to obtain a rough estimate on the efficiency of the energy-transfer process (2). With the value of the rate constants for the quenching of NH(a) and and known radiative lifetimes of Oz(b) by both H N 3 and 0239 NH(a)8v9 and 02(b),40 it was calculated that the efficiency of reaction 2 is not far from unity. We therefore exclude NH(b)
-
Note Added in Proof. Recent MCSCF calculations indicate a barrier for the dissociation of HN3 NH(a) Nz(X) of about 2000 cm-I (Alexander, M. H., private communication) and smaller than 3000 cm-' (Meier, U.; Staemmler, V., private communication).
+
Acknowledgment. We thank R. D. Kenner and V. Staemmler for useful discussions, M. H. Alexander, J. C. Stephenson, and W. Hack for sending us copies of ref 23, 24, and 5, respectively, prior to publication, and R. Bersohn for informing us on the quantum yield of HN,. We gratefully acknowledge financial support of the Deutsche Forschungsgemeinschaft. Registry No. N H , 13774-92-0; HN,, 7782-79-8; 02, 7782-44-7; H2,
(39) Slanger, T. G. In Reactions of Small Transient Species, Kinetics and Energetics; Clyne, M. A. A., Fontijn, A,, Eds.; Academic Press: London, 1984, and references therein. (40) Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular Structure. IV. Constants of Diatomic Molecules; Van Nostrand: New York, 1979.
1333-74-0; Nz, 7727-37-9; CO, 630-08-0; C02, 124-38-9; N20, 1002497-2; CH,, 74-82-8; CZH,, 74-84-0; C3H8, 74-98-6; C3H6, 75-19-4; CZH4, 74-85-1.
Investigation by *'Na N M of the Anion Role in the Dissociation Kinetics of Sodhrm-Dibenzo-24-crown-8' In NHromethane Harald D. H. Stovert and Christian Detellier* Ottawa-Carleton Chemistry Institute, Ottawa University Campus, Ottawa, Ontario, K1 N 9B4 Canada (Received: August 17, 1988; In Final Form: November 7 , 1988) The role of the coordinating counteranions thiocyanate and iodide in the bimolecular sodium cation interchange for the complex dibenzo-24-crown4 (DB24C8)-NaX (X- = SCN-, I-) in nitromethane was determined by 23NaNMR. The variation of the 23Na NMR data with the salt concentration (between lo-' and M) shows ion pairing, with an ion-pair formation constant (at 294 K) of lo3 for NaNCS and 80 for NaI. In the presence of the crown ether DB24C8, in the case of NaNCS, small amounts (
