J . Phys. Chem. 1994, 98, 8940-8945
8940
Collisional Quenching of NCl(alA,v=O) and the Chain Decomposition of ClNj A. J. Ray and R. D. Coombe' Department of Chemistry, University of Denver, Denver, Colorado 80208 Received: March 8, 1994; In Final Form: May 31, 1994'
ClN3 was photodissociated at 193 nm to produce NCl(alA) in the presence of various chaperone gases. Room temperature rate constants for the collisional quenching of NCl(alA) by these gases were determined from the time decay of the excited NCI. These rate constants were found to be 2.5 X 10-12, 1.8 X 6.8 X 10-13, 5.3 X 10-13, 4.9 X 10-12, 8.0 X 10-12, 8.2 X lo-", I1 X and 5 1 X for quenching by 02,Cl2, H2, Dz, HC1, DCl, HF, He, and Ar, respectively. The magnitudes of these rate constants are discussed in terms of crossings among the potential energy surfaces of bound amine-like intermediates. Photolysis of ClN3 at 249 nm produces NCl(alA) which exhibits a time decay suggestive of chain decomposition of the azide parent. It is postulated that the chain is carried by vibrationally excited N2.
Introduction A large body of information is present in the literature concerning the rates of energy transfer processes involving the excited singlet nitrenes NH(a1A)lVZ and NF(alA).3-5 Interest in excited NH(a1A) often stems from the importance of this radical incombustion processesand fromitschemicalsimilarityto O(lD), a significant atmospheric species. NF(a1A) metastables are important as energy storage agents in a number of existing and potential chemical laser systems.6 Similarly, much information is available' concerning energy transfer from the isoelectronic species Oz(alAg). OZ(alA,) is an important energy carrier in many chemical systems,one of which is its interaction with iodine atoms in the chemical oxygen-iodine laser, or "COIL".*.9 Recently, a number of reports have appearedlOJ1concerning an analogous energy transfer process between iodine atoms and excitedNCl(a1A). This process offers thepossibilityofsubstantial improvement over the two-phase chemistry currently used for the generation of Oz(alAg)in COIL devices. Evaluation of the NCl(a'A)/I energy transfer system requires knowledge of the interactions of NCl(alA) with a number of other species present in the chemical environment of its generation. Little or no such information exists, however, other than reports by Clyne and co-workersL2J3of NCl(aIA) quenching by Clz and 02.These data were obtained via indirect tracking of excited NC1 produced in a discharge flow reactor. In this paper, we present the results of experiments in which NCl(a1A) was generated directly from the photodissociationof ClN3and rate constants for its quenching by a number of species determined directly from observations of the time decay of the NC1 alA X3c- emission near 1 .OS pm. Apart from offering insight into the viability of the NCl(alA)/I laser system, these data, taken with rate constants for analogous processesinvolving theNF(aIA) and02(a1A&species noted above, exhibit clear evidence of different energy transfer mechanisms at work in a group of theoretically tractable chemical systems. In addition, the experiments provided information concerning energy transfer processes involved in the chain decomposition of the ClN3 parent.
-
Experimental Methods The experimentswere performed by pulsed photolysis of flowing mixtures of ClN3, diluent (typically He), and quenching agents with the output of an ArF laser at 193 nm. Coombe and Van Benthem14 have shown that photodissociation of ClN3 at this wavelength produces NCl(alA). Minor amounts of NCl(blC+) and excited triplet metastables of NZ are also generated. The Abstract published in Aduance ACS Abstracts. August 1, 1994.
synthesis of gas phase ClN3 has been described previo~sly.~~ In the present experiments, the effluent of the azide generator was collected in a passivated 5 L Pyrex bulb. UV and IR absorption spectroscopieswere employed to characterize the absolutedensity and purity of the ClN3in the bulb. These mixtures were typically 2.0-2.5%ClN3in He, with the principal impurity being Clz present at 1.0-1.5%. Quenching gases were obtained from commercial sources and in general were used without further purification. In all cases, the stated impurities were present at less than 0.1%and typically were comprised of species (e.g., COZor hydrocarbons) which are not expected to be efficient NCl(alA) quenchers. An exception occurred in the case of DCl. Despite a high quoted isotopic purity, IR absorption spectra showed the actual samples used in our experiments to have a DC1:HCl ratio of 5:l. Since HCI and DCl were found to have very similar rate constants for NCl(aIA) quenching, no further purification steps were taken. In the experiments with HZ and Dz, a number of tests were performed using a commercial gas purifier (Supelco) in the gas flow line because of unexpectedly large rate constants obtained for NCl(a1A) quenching by these species. The presence of the gas purifier was found to have no effect on the data. The gas purifier was also used in some of the experiments involving NCl(ala) quenching by Ar and He. The ClN3/He and quenching gas flows were mixed upstream of a Pyrex photolysis cell. The flow rates of the gas streams were measured with calibrated Tylan mass flow meters. The total pressure in the cell was monitored with a capacitance manometer (MKS). The photolysis laser beam was passed down the long axis of the cell via UV grade fused silica windows. The fluence of the laser varied from 35 to 92 mJ/cm2, as measured with a Scientech energy meter. Although most of the experimentswere performed with photolysis at 193 nm using an ArF laser, a few experiments were performed with a KrF laser at 249 nm. Photolysis of the ClN3 in the flowing gas stream produced NCl alA+ X3C-emission near 1 .OS pm. This emission was monitored at 90' to the axis of the photolysislaser beam with an interference filter and an intrinsic Ge detector cooled to 77 K (North Coast Optics). The interference filter had an fwhm bandwidth of 10 nm and passed only the0,O band of the NCl aIA- X3C-emission. Although photolysis of ClN3 at 193 nm also produces excited Nz(B3n,), emission from this species in the near-IR (on the B311g A3&+ first positive bands) does not interfere with the NCl a' A- X3c-emission near 1.08pm. This issue has been discussed by Coombe and Van Benthem,14who recorded the spectrum of emission in this region from 193 nm photolysis of C1N3. Although photolysis of CIN3at 249 nmproduces a small yield of N2(A3c,+), no emission from higher lying states of N2 has been observed in
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0022-3654/94/2098-S940$04.50/0 0 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No. 36, 1994 8941
Chain Decomposition of ClN3 1203
I
fluence a t fixed ClN3 density or with increasing ClN3 density at fixed fluence. In light of the large rate constant reported below for NCl(aIA) electronic quenching by Cl2, it is unlikely that the long tail reflects simple quenching by the parent ClN3,but rather some chain mechanism in which NCl(alA) is regenerated. For example, the following process might well occur: ClN,
N2(u)
+ hv
+ ClN,
-
-
+ N,(u) + NCl(a’A) + N,(U”.)
NCl(a’A)
N,(u’)
(1)
(2)
Since the initial photolytic event (reaction 1) liberates more than 100 kcal/mol over and above the energy needed for formation l i m e (xlV3 seconds) of NCl(alA), it is quite possible that the N2 fragment will have a significant amount of vibrational excitation. In reaction 2, the Figure 1. Time profiles of NCl alA X31- emission from ClN3 photolysis: (a) photolysis at 249 nm, [ClN3] = 2.4 X loL3~ m -total ~, fragile ClN3 (DO 16 kcal/mol) is simply dissociated by its total pressure 4.3Torr; (b) photolysisat 249 nm, [CINs] = 7 X lOI3,“c collision with this N ~ ( u )which , is quenched to a lower lying state pressure0.43Torr; (c) photolysis at 193 nm, [CINJ] = 4.4 X loL3~ m - ~ , (u’) in the process. Since the ground state of ClN3 correlates total pressure 4.5 Torr; (d) photolysis at 193 nm, [CINs] = 6.3 X l O I 3 adiabatically to NCl(aIA) + N2, the NCl(a1A) is effectively ~ m - total ~ , pressure 0.60Torr. regenerated in this collisional dissociation process. Additional vibrationally excited N~(u”)is produced as a cofragment, such this case.I5 The response of the detector was digitized and that a chain is established which can lead to the decomposition averaged with a Nicolet 1270 signal processing unit and sent to of the bulk sample of ClN3. a microcomputer for data analysis. The absorption cross sectionI6 Benard and co-workers18have shown that this dissociation to of C1N3at 193 nm is 1.38 X lo-’*cm2. The fluences noted above NCl(a1A) occurs thermally, and work in our laboratory19 has correspond to dissociation of 7-17% of the ClN3 present in the shown that it can be stimulated by collisions with vibrationally photolysis cell, generating NCl(a1A) densities on the order of excited species like HF(u). Benard has also suggestedZothat a 1012cm-3. Since quencher densities many times greater than this similar chain mechanism carried by N ~ ( umay ) be involved in the were employed in every experiment, pseudo-first-order conditions dissociation of shock-heated FN3. Such processes seem likely were easily maintained. since the vibrational frequency in N2, 233 1 cm-1, is nearly resonant The possibility of prereaction between ClN3and the quenching with the asymmetric stretching vibration of the azide moiety in gas (Le., during the time between mixing of the flows of these the halogen azides, near 2050 cm-1. Hence, rapid intermolecular species and the photolysis laser pulse) was examined for 0 2 , H2, V to V transfer is possible. This asymmetric stretching vibration and HCl. This was done by adding a large excess of the quenching in the azide is essentially the reaction coordinate for dissociation gas to a cell containing ClN3 and observing the behavior of the to NX(a1A) N~(u’).Hence, the long tail in Figure 1b may well contents over a period of hours using IR spectroscopy. N o reflect the removal of the bulk sample of C1N3by a chain initiated prereaction was observed for any of these three gases. by the N2(u) photoproduct. The observed behavior of the system A brief set of experiments was performed in which NCl(a1A) is in qualitative accord with the proposed chain mechanism. generation was observed in a discharge-flow reactor. The flow Increasing the fluencewould increase the initial amount of Nz(u), reactor employed was of standard design, and its characteristics such that its steady statedensity during thechain would begreater, have been described previously.l7 The linear velocity of flowing hence thegreater NCl(alA) intensity in the tail. Similarly, greater gases in this reactor, which was pumped by a 1500lpm mechanical initial ClN3 densities would result in greater initial Nz(u) and pump, was near 1000 cm/s, and the pressure in the reactor was hence greater intensity in the tail, as observed. monitored with a capacitance manometer. Metered flows of The intensity of the long tail is increased dramatically by the reagents and diluent were measured with Tylan mass flowmeters. addition of a few Torr of Ar diluent, as shown by trace (a) of Emission from NCl(alA) present in the flow was monitored at Figure 1. With the addition of still more Ar, the initial rapid a fixed observation point. The emission was chopped before being decay is completely eliminated. This behavior is consistent with detected by the intereference filter-Ge detector combination slower diffusion rates a t the higher Ar densities. As will be shown described above. The modulated response of the detector was below, loss of both NCl(alA) and Nz(u) by diffusion to the walls amplified by a lock-in amplifier (Stanford Research Systems), competes significantly with collisional quenching processes for with the output displayed on a strip chart recorder. the conditions of Figure 1b. At the higher Ar pressure of Figure la, losses by diffusion are not appreciable, and the chain is Results enhanced. However interesting, this complex scenario is clearly not suitable NCl(a1A) Quenching by the Parent Azide. Before NCl(alA) for measurement of the rates of NCl(alA,u=O) quenching by quenching by added speciescould be investigated, it was necessary added species. Fortunately, ClN3 photolysis at 193nm produces to establish conditions where its interaction with the parent ClN3 a result which is quite different. The intensity in the “tail” of the was well understood and constant. This was not a simple task. NCl(a1 A,u=O) time profile is tiny relative to that for photolysis The initial experiments were performed using photolysis with a at 249 nm, and the observed decay is nearly exponential. This KrF laser at 249 nm. For photolysis of ClNj/He mixtures a t low behavior is shown in Figure 1,traced. In the context of the chain pressures (near 0.4 Torr) with no added diluent stream, the NClmechanism proposed above, the amount of N ~ ( uproduced ) by (a1A) decay exhibited a time profile like that shown in Figure 1b. the initial photodissociation a t this wavelength must be much The interference filter used in the experiment transmits only smaller than that produced by photodissociation at 249 nm, such radiation from NCl(alA,u=O), and hence the time profile that NCl(alA,u=O) regeneration by the chain does not compete represents the behavior of this species. The rise time of the signal with pseudo-first-order lossesby diffusion or collisional quenching. corresponds to the detector time constant, near 13 MS. This rise From the absorption spectrum of ClN3, 193 nm photolysis does is followed by a distinctly double exponential decay, with the excite the molecule to a different state than does photolysis at slow component of the decay lasting for many milliseconds. The 249 nm (i.e., these wavelengths lie within two different absorption intensity of this long “tail” was found to increase with increasing 0
1‘
0
I
04
-
OB
12
+
8942 The Journal of Physical Chemistry, Vol. 98, No. 36, 1994 16
1
I
0
3
6
9
Ray and Coombe 25
I
I 12
15
0
05
1
Figure 2. Stern-Volmer plots for NC1 (alA)u=oquenching by (0)02,
(w) D2, and (A) Hz. Solid lines represent least-squaresfits to the data.
bands). When several Torr of Ar diluent is added, the majority of the time decay is still nicely exponential, as shown by trace c of Figure 1. The longer decay in trace c relative to traced reflects the fact that a smaller amount of the ClN3/He mix is present and also the elimination of a contribution from NCl(alA) diffusion out of the observation zone. These conditions would appear to be appropriate for measurements of NCl(alA,u=O) quenching by added species. NCl(alA,v=O) Quenching by Added Species. Measurements of rate constants were made by recording the exponential decay of NCl alA X3E- emission produced by 193 nm photolysis of samples containing fixed densities of ClN3 and diluent (Ar or He) and variable densities of the added quenchers. In this case, the decay rate (A) is given by
15
2
25
3
35
4
45
Ouencher Density (xlOl5 an0)
Ouencher Density ( ~ 1 0 cm-3) '~
Figure 3. Stern-Volmer plots of NCI (alA),.o quenching by (0) DCl,
(A)HCI, and (0)HF. Solid lines representleast-squaresfits to thedata. For the DCl data ( O ) , a constant (1.1 X 104 s-l) has been added to the value of X for each point to facilitatecomparison with the HCI data (A), 30
24
-
m
-
Decay by spontaneous emission (Xrad) makes a negligible contribution, as recent measurements21922indicate this rate to be on the order of 0.9 s-1. Similarly, decay by diffusion out of the observation zone is negligible for the diluent densities (typically near 1.5 X 10'' cm-') used in the experiments. Hence, for fixed densities of ClN3 and diluent, the total decay rate should vary linearly with the density of the added quencher Q, with the slope of plots of X vs [Q] corresponding to the desired rate constant kQ. Close correspondence with this expected behavior was observed in all of the quenching experiments performed. Measurements were made for NCl(alA,u=O) quenching by the diatomics H2, D2, HCl, DCl, HF, 0 2 , and C12, as well as Ar and He. These data are shown in Figures 2,3,4, and 5. Rate constants obtained from the slopes of the plots are collected in Table 1, along the literature values for the rate constants of analogous quenching of NF(aIA) and 02(a'A,). The slopes were determined by least-squares analysis of the data, and the uncertainties indicated in Table 1 represent 2a values from these fits. The largest contribution to the uncertainties accrues from measurement of the flow rates of the quenching gases and from fluctuations in the density of the C1N3/C12/He parent mixture. The values of X a t [Q] = 0 for thedata shown in Figures 2,3, and 4 aredifferent becausedifferent densities of the ClN3/C12/He parent mixture were used. The values of X at these intercepts are in fact in good agreement with kc1~,[ClN3]+ k~l,[C12],if k c l is~ assumed ~ to be approximately equal to k ~ . Figure 5 shows the results of quenching by added Ar and He. Quenching by these species is clearly inefficient, with rate constants on the order of 10-15 cm3 s-1. This result in fact represents the baseline uncertainty limit of the experiments. The solid lines shown in Figure 5 are the results of a calculation of
Figure 4. Stern-Volmer plot for NCI (alA)u=oquenching by Clz. The solid line represents the least-squares fit to the data.
'> ol
0
05
1
15
2
25
3
35
Rare Gas Density (x1017 cW3)
Figure 5. Stern-Volmer plots illustrating the effects of (D) He and (A) Ar on the rate of NCl(alA) quenching. Solid lines represent decays calculated from diffusion considerationsand C12/CIN3 quenching.
the rate of loss by diff~sion,23.~~ given the dimensions of the photolysis cell and assuming unit efficiency for NCl(alA,u=O) quenching a t the walls. Thecalculation assumed a rate constant for NCl(aIA,u=O) quenching by the ClN3 or Cl2 present of 2 X 10-11 cm3 s-1 (i.e., roughly equal to the value measured for Clz). This rate dominates at the higher He or Ar densities, whereas the effects of diffusion are evident at densities less than 5 X 1016 cm-3.
The Journal of Physical Chemistry, Vol. 98, No. 36, 1994 8943
Chain Decomposition of ClN3
TABLE 1: Rate Constants for Collisional Quenchinp of NCl(alA), NF(alA), and Oz(a1A) (em3 molecule-1 s- ) (2.5 f 0.2) X (1.8 & 0.3) X (6.8 f 0.7) X (5.3 0.6) x Dz HC1 (4.9 f 0.7) X DC1 (8.0 f 1.4) X HF (8.2 f 1.2) x Ar s i x 10-15 He si x 1 0 - 1 5
600
(7.0& 0.7) X 10-15 (1.8 & 0.4) X 1O-I8 (5.8 & 0.6) X lWI3 (6 f 3) X 1O-I8 (7 f 2) X 1O-I’ (4.2 f 0.4) X lO-l*
IO-IZ
1O-1l 10-13 10-13
(1.6 & 0.3)
lO-Iz
X
(4 f 3) X 1O-l8
10-12 10-13
3x
10-15
(1.4 f 0.5) X
The uncertainties listed are 2u of the least-squares fits to the SternReference 7.
Volmer plots. References 3-5.
Chain Decomposition of ClN3/02 Mixtures. In their flow reactor work on NCl(alA), Clyne and c o - ~ o r k e r s ~ discussed ~J~ the possibility that quenching of this species by 0 2 occurs via an E to E process which generates 02(a1A,): NCl(a’A,u=O)
-
+ 02(X3Z,)
NC1(X3Z-)
Indirect indications of the presence of 02(a1A,) in Clyne’s experiments suggested the occurrence of this process, which is fully allowed by conservation of both spin and orbital angular momentum. The process as written in eq 4 is nonresonant, with an exothermicity near 1400 cm-1. Production of 02(a1$) with one quantum of vibrational excitationis nearly resonant, however, with an endothermicity of only 87 cm-l. Production of 02(a1A,) by reaction 4 is particularly interesting in the context of the possible chain decomposition of ClN3 as in eqs 1 and 2. Like N ~ ( u )02(a1A,) , metastables carry sufficient energy to dissociate ClN3, and a chain carried by both species can be envisioned:
NCl(a’A) 02(a1A,)
+ 02(X32-) + ClN,
N2(u’)
-
+ hu
-
+ ClN,
+ N,(u) NCl(X3Z-) + 02(a’A,) NCl(a’A)
02(X3ZJ
-
(1) (4)
+ NCl(a’A) + N,(u? (5)
N,(u”)
+ NCl(a’A) + N,(u’)
(2)
At the steady state of this chain, both NCl(alA) and 02(a1A,) would be present, in amounts governed by the rates of reactions 4 and 5. We have obtained evidence of the operation of this chain from experimentsin which the NCl(a1A)/02energy transfer process is observed with higher initial ClN3densities photolyzed at 249 nm. As noted above, these conditions favor onset of the chain decompositionof ClN3as in Figure lb. Figure 6 shows the time profiles of NCl aIA X3c- emission obtained in these experiments, for several different 0 2 densities. Curve a shows the NCl(a1A) time decay in the absence of 02; it exhibits the double-exponential decay described above. Curves b-e show decays for increasing densities of 0 2 . It is evident that the rate of decay of the long tail does not change appreciably but that its intensity is decreased by increasing amounts of 0 2 . In the context of the mechanism above, some of theNCl(alA) has been converted to Oz(aIA,), and the 02(a1A,):NCl(alA) proportion increases with increasing 0 2 . The reduction in intensity of the initial rapidly decaying component is an artifact of the finite time constant of the Ge detector (about 13 ps). As this fast decay shortens, the area under this initial “peak” is r e d u d proportionately. Analysis of the decay suggested by this reduction in peak area gives a rate constant for initial quenching of NCl(alA) by 0 2 in good agreement with that reported in Table 1.
-
0 25
05
-
0 75
125
1
Time (xIV3 seconds)
Figure 6. Time profiles of NCI aIA X3E- emission produced by 249 nm photolysis of ClN3 in the absenceand presence of 02:(a) NCl(a-X) from the photolysis of [CINs] = 3.8 X lOI4 cm-3; (k) photolysis as in (a) but with [Oz] = 8 X lOI4, 1.5 X lOl5,1.9 X 1015, and 3.8 X 10’5 cm-3,
respectively.
+ 02(a1Ag) (4)
ClN,
I 0
This model for the chain mechanism can be used to estimate the rate constant for reaction 6, the dissociation of ClN3 by Oz(a1A8). At the steady state (in the long tail of the NCl(alA) decay),
where k4 and k5 are the rate constants for reactions 4 and 5. The value of k4 is given in Table 1. It it is assumed that the decline in the NCl(a1A) intensity in the tail is caused by its conversion to O;?(a’A,), then the ratio [02(a)]/[NCl(a)] can be obtained from the ratio of the intensity of the emission with and without the presence of 0 2 . From this ratio and the measured value of k4, k5 can be determined. The average valueobtained from several such experimentsis k5 = 1.7 X 1O-lI cm3 s-l. This energy transfer process would therefore appear to be quite rapid. Further, the efficiency with which it regenerates NCl(a) must be very high; from the duration of the emission in the “tail”, many cycles of such a chain would occur before all of the available ClN3 were consumed. These results suggest that the NCl(aIA) and 02(alAg) systems are interchangeable, in that either one might be used to generate the other. To test this possibility, a number of experiments were performed in which ClN3 was admitted to a stream of 02(a1$) generated in a discharge flow reactor by a microwave discharge through 0 2 . A 20% mixture of 02 in He was passed through a 100 W, 2450 MHz microwave discharge (Evenson cavity) to produce the excited 0 2 . Oxygen atoms were removed from the gas stream by an HgO ring25 deposited just downstream of the discharge. The successful operation of the HgO ring was demonstrated by suppression of the air afterglow normally produced by the discharge through 0 2 . The ClNJHe mixture was admitted through a sliding injector such that the point of mixing of the ClNJHe flow with the 02(a1Ag) flow could be varied with respect to an observation port downstream. The azide was added more than 100 ms downstream of the discharge and HgO ring, such that OZ(alA,) should be the only energy carrier in the flow at this point. As noted above, NCl alA X3cemission near 1.08 pm was sought with the filter/Ge detector combination, with phase sensitive signal processing electronics. Such an experiment is expected to perform in a very limited way, since the proportion of Oz(a1A) in the 0 2 flow is at best between 5% and lo%, and the density of 0 2 in the flow was more than 100 times larger than that of CIN3. Hence, NCl(a1A) quenching by ground state O2will dominate over its production by Oz(alAg) dissociation of ClN3. Nonetheless, NCl(alA) generation was -+
Ray and Coombe
8944 The Journal of Physical Chemistry, Vol. 98, No. 36, 1994
readily observed upon admission of ClN3 to the 02(a1A,) flow. The signal was generated when the ClN3 flow was started and was quenched when it was shut off, and its intensity varied with (and was limited by) the amount of ClN3 in the flow. Discussion Although energy resonances are possible in some cases, the rate constants for NCl(alA) quenching by diatomic molecules show no evidence of strong H/D isotope effects or dependence on dipole moments indicativeof “physical”electronictovibrational energy transfer mechanisms. By physical E to V processes we mean those which are dominated by impulsive interactions involving long range multipolarforces. There is little or no isotope effect evident in NCl(alA) quenching by H2 and HC1 vs D2 and DC1. Further, quenching by HF, with its very large dipole, is slower than quenching by HCl. Clearly, other mechanisms must be at work. Du and Setser3have presented extensive discussions of the possibility of “chemical” mechanisms which may be operative in the collisional quenching of NF(a1A) by diatomics. Such mechanisms are not necessarily chemical in the sense that bonds are broken or made but are dominated by the shorter range attractive interactions associated with bonding potentials. In the case of the present data, combination of NCl(alA) with the XIE+ ground states of any of the diatomics (“AB”) of Table 1 will give rise to a bound singlet potential corresponding to the stable ground state of the ClNAB amine molecule. Quenching of NCl(a1A) by AB then corresponds to crossing to a repulsive triplet potential correlating to NCl(X3c-) AB or to similar repulsive potentials correlating to other possible products of the dissociation of the excited ClNAB intermediate molecule, e.g., NA BCl. The rates of such processes are determined by the presence and height of barriers in the entrance channels and, perhaps more critically, by the actual positions of the seams between the bound singlet and repulsive triplet potentials. If this seam is located outside the barrier in the entrance channel, then energy transfer occurs at long range and evidence of the “physical” E to V processes noted above (frequencyand dipole effects) would be expected. On the other hand, if the seam is located inside the barrier, within the attractive well of the singlet ground state, then the shapes of the potentials and the coupling between them will dominate. In this case, some correlation with the stability of the amine intermediateand with the magnitude of spin-orbit coupling in the molecule might be expected. Such a correlation is evident in the present data, with NCl(aIA) quenching by C12 being faster than quenching by HC1, which is faster than quenching by H2. The fact that the rate constant for quenching by HF is slightly smaller than that for quenching by H2 (both molecules with small spin-orbit coupling) may be associated with differences in the barrier heights in the entrance channels of these processes. On the basis of these trends, we would expect NCl(alA) quenching by iodides or bromides (molecules with much larger spin-orbit coupling than C12) to have rate constants significantly greater than 10-1l cm3 s-1. This trend has in fact been observed by Du and S e t ~ e r for ~ . ~the quenching of NF(alA). Table 1 also shows that the quenching of NCl(alA) by a given diatomic is typically more than 2 orders of magnitude faster than analogous quenching of NF(alA), which is itself much faster than analogous quenching of 02(alAg). The slow quenching of o ~ ( a ~ A is . 4understandable in the sense that it can engage only in “physical”E to V energy transfer processes with these diatomics. The difference between the rates for NCl(alA) and NF(alA) seems to be too large to attribute solely to increased spin-orbit coupling in the chloride. Du and Setser3 noted that the rate constants for NF(alA) quenching by these diatomics (all rather slow processes) are consistent with 5-10 kcal/mol barriers in the entrance channels. It may be that such barriers are substantially smaller for the analogous processes involving NCl(a). The chain decomposition of the azide is an interesting and potentiallyuseful phenomenon. The present data are in qualitative
+
+
agreement with models in which the chain is carried by excited metastable species such as N2(u) or 02(a1Ag),which are capable of collisionaldissociation of C1N3. As noted above, the efficiency ofthechainwhencarriedeither byN2(u) aloneor byacombination of excited N2 and excited 0 2 must be very high since many cycles would occur as the azide is destroyed. The magnitude of the steady state NCl(a1A) density in the chain is a function of two factors. The first of these is the branching fraction between dissociation on the single and triplet potential energy surfaces of the azide. The barrier to dissociation of C1N3on its ground state ‘A’ surface correlating to NCl(alA) and N2(u) is thought to have an energy’* of about 16 kcal/mol. The molecule can be easily excited over this barrier to generate these fragments. Ground state fragments, NCl(X3E-) Nz, correlate with a repulsive triplet surface (3A”) which is thought to cross the ground state singlet surface near the top of the barrier. At the equilibrium geometry of ground state ClN,, the triplet surface lies perhaps 20 kcal/mol above the ground state singlet surface. Hence, both surfaces are accessible by excitation of ClN3 in collisions with N ~ ( u ) .Since dissociation on the triplet surface is also likely to regenerate vibrationally excited N2, both channels would propagate the chain. It seems unlikely that dissociation on the triplet surface would occur to any great extent, however, since in this case collisionalexcitation by the N ~ ( uwould ) be a spin-forbidden V to E process. As noted above, it seems much more likely that the energy exchange is a direct, near-resonant V to V process which excites nuclear motion in the CIN3 along the dissociation channel of the ground state singlet potential. The other factor governing the steady state NCl(a) density is the rate of its quenching by the parent azide (and by the Clz impurity) relative to the rate of its production by collisional dissociation of ClNJ. Since the intensity in the “tail” of the NCl alA X3C-emission (correspondingto the steady state NCl(alA) density in the chain) is in fact comparable to that in the initial “peak” (corresponding to NCl(alA) produced by the photolysis), the chain would appear to sustain an NCl(alA) density close to that initially produced. Collisional dissociation of the azide on the singlet surface must be at least as fast as NCl(alA) quenching. Hence, the chain may well be an efficient way to extract the energy stored in CIN3, e.g., by energy transfer from NCl(alA)/ 02(a1A,) to iodine atoms. The discharge flow experiment described above showed that the chain might be initiated by addition of ClN3 to a stream of 02(a1Ag). It would be interesting to perform such an experiment with a chemical source of excited 0 2 (Le., a basic hydrogen peroxide-Cl2 reactor) which produces a high proportion of Oz(alA,). In this case, large densities of both NCl(a1A) and 02(a1Ag)might be generated, leading to interesting possibilities for energy pooling.
+
-.
Acknowledgment. This work was supported by the U.S.Air Force Office of Scientific Research under Grants AFOSR-900259, F49620-93- 1-0631, and F49620-92-5-0210. References and Notes (1) (2) (3) (4) 5155. (5) (6)
Hack, W.; Wilms, A. J . Phys. Chem. 1989, 93, 3540. Freitag, F.;Rohrer, F.; Stuhl, F. J. Phys. Chem. 1989, 93, 3170. Du, K.Y.; Setser, D. W. J. Phys. Chem. 1990, 94, 2425. Quifiones, E.; Habdas, J.; Setser, D. W. J . Phys. Chem. 1987, 91,
Du, K.Y.; Setser, D. W. J. Phys. Chem. 1992, 96, 2553. Benard, D. J. J . Appl. Phys. 1993, 2900. ( 7 ) Wayne, R. P. Singlet 02;Frimer, A. A., Ed.; CRC Press:Boca Raton, FL, 1985; Vol. I, p 81. (8) Bernard, D. J.; MacDermott, W. E.; Pchelkin, N. R.; Bousek, R. R. Appl. Phys. Lett. 1979 34, 40. (9) MacDermott, W. E.; Pchelkin, N. R.; Benard, D. J.; Bousek, R. R. Appl. Phys. Lett. 1978, 32, 469. (10) Ray, A. J.; Coombe, R. D. J . Phys. Chem. 1993,97, 3475. (11) Bower, R. D.; Yang, T. T. J . Opt. Soc. Am. B 1991, 1583. (12) Clyne, M. A. A.; MacRobert, A. J. J. Chem. Soc.. Faraday Tram. 2 1983, 79, 283.
Chain Decomposition of CIN, (13) Clyne, M. A. A.; MacRobert, A. J.; Brunning, J.; Cheah, C. T. J . Chem. Soc., Faraday Trans. 2 1983, 79, 1515. 114) Coombc. R. D.: Van Benthem. M. H. J . Chem. Phvs. 1984.81.2984. (15) Coombe; R.D.; Patel, D.; Pritt, A.T., Jr.; Wodarizyk, F. J: J . Chem. Phys. 1981, 75, 2177. (16) Clark, T. C.; Clyne, M. A. A. Trans. Faraday SOC.1969,65,2994. (17) Liu, X.;Macdonald, M. A.;Coombe, R. D.J. Phys. Chem. 1992,96,
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The Journal of Physical Chemistry, Vol. 98, No. 36, 1994 8945 Machara, N. P.; Coombe, R. D. Unpublished results. Benard, D. J. J . Appl. Phys. 1993, 74, 2900. Yarkony, D., Jr. Chem. Phys. 1987,86, 1642. Becker, A. C.; Schurath, U. Chem. Phys. Leu. 1989, 160, 586. Husain, D.; Donovan, R. J. Adu. Photochem. 1971,8, 1. Cunningham, R. E.;Williams, R. J. J. Difjusion in Gases and Porous Media; Plenum Press: New York, 1980; p 82. (25) Wayne, R. P. Adu. Photochem. 1969, 7, 3 11. (19) (20) (21) (22) (23) (24)
