J. Phys. Chem. 1995, 99, 16296- 16300
16296
Quenching of NH(alA) by Molecular Halogens and Interhalogens S. M. Singletont and R. D. Coombe" Department of Chemistry, University of Denver, Denver, Colorado 80208 Received: June 16, 1995; In Final Form: August 15, 1995'
Rate constants have been determined for the quenching of NH(a'A,v=O) by a number of molecular halogens and interhalogens. The values measured are (1.0 f 0.5) x lo-'?, (3.7 5 0.4) x lo-", (6.8 0.4) x lo-", (6.0 f 0.3) x lo-", and (1.6 f 0.3) x cm3 s-I for quenching by Fl, ClF, Cll, Br?, and ICl, respectively. For quenching by ClF, Clz, and Br2, excited singlet halogen nitrenes and vibrationally excited hydrogen halides were detected as products of the quenching processes. This result is interpreted as indicating NH(alA) insertion into these molecules. For NH(a' A) quenching by ClF, production of NF(a' A) predominates over production of NCl(a'A).
Introduction The reactions of hydrogen atoms with halogen aminyl radicals such as NF2, NC4, or NFCl have been studied by a number of research g r ~ u p s . l - These ~ reactions are thought to proceed on singlet potential energy surfaces of the halogen amine intermediates, forming hydrogen halides and excited singlet halogen nitrene products by spin-allowed elimination from the amine. For example, the H N F 2 reaction is known5 to produce NF(ala) with a yield exceeding 90%. The singlet halogen amine intermediates of these reactions also correlate adiabatically to molecular halogens or interhalogens and excited imidogen radicals, NH(a'A), but formation of these products is endothermic. In principle, the exothermic reverse of such processes, reaction of NH(alA) with the molecular halogen, could proceed through the halogen amine intermediate via spin-allowed processes to produce hydrogen atoms and halogen aminyl radicals (two doublets), halogen atoms and HNX radicals (two doublets), or hydrogen halides and excited singlet halogen nitrenes (two singlets):
+
+ X z ( ' 2 1 ) - HNX,*(singlet) HNX,*(singlet) - H(2S) + NX2('BI)
NH(a'A)
-HX('2+)
+ NX(alA)
(la)
(IC)
Such reactions would involve insertion of the imidogen into the bond of the molecular halogen. Direct, nonreactive energy exchange via spin-allowed processes would require excitation of the molecular halogen to excited triplet states. Given the energy carried by NH(a'A), 12 593 cm-', such processes would be endothermic for most halogens. We have recently investigated the possibility of insertion processes such as (1) in a preliminary study6 of the reaction between NH(a'A) and Clz. This reaction was found to be very rapid (k = 6.8 x lo-" cm3 s-'), and NCl(a'A) and vibrationally excited HCI were observed as products, strongly suggesting the operation of the insertion mechanism. In the present work, we extend this investigation to reactions between NH(a' A) and a number of molecular halogens and interhalogens. Rate constants
' Current address:
U.S. Air Force Phillips Laboratory. Hanscom AFB.
MA. @
Abstract published in Aduance ACS Abstracrs, October 1, 1995.
0022-365419512099- 16296$09.0010
for these processes and observations of reaction products are presented. The data offer insight into the reaction mechanisms, particularly for NH(a' A) reactions with interhalogens in which branching to different products was observed. The results are compared with data from analogous H atom reactions with halogen aminyls.
Experimental Details The experimental methods and apparatus employed in the present experiments were similar to those used in our previous work6 with the NH(a'A) Clz system. The reactions were initiated by photolysis of mixtures of gaseous HN3, halogen, and diluent which were flowed continuously through a Pyrex cell. The photolysis was accomplished with a pulsed excimer laser (Questek Impulse) operated at either 193 or 248 nm. Typical laser fluences were in the range from 10 to 50 mJ/cm'. The NH(a'A) produced' by photodissociation of the HN1 was cll7 probed by laser-induced fluorescence (LIF) on the aIA transition of this radical.* The probe pulse, from a frequency doubled excimer-pumped dye laser (Lambda Physik F12001/ LPXlOO), was triggered by the excimer laser pulse through a variable delay generator (SRS DG535) and passed through the center of the volume excited by the excimer laser beam along the axis of the cell. The resulting LIF signal was detected with a cooled GaAs photomultiplier tube equipped with a band-pass filter, at 90' to the laser beams. The LIF signal was amplified. digitized, and stored using a gated boxcar integrator (SRS 250/ 245/240) triggered by the probe pulse. Near-infrared chemiluminescence from excited species produced by reactions subsequent to the photolysis laser pulse was sought using an intrinsic Ge detector (North Coast Scientific) cooled to 77 K. in combination with various interference filters. Time profiles of the emissions observed were amplified. digitized, and averaged with a Nicolet 1270 data acquisition system. Chemiluminescence in the visible region was observed by using appropriate filters and a cooled GaAs photomultiplier tube. Similarly, mid-infrared emission was observed with filters and a cooled InSb detector (EG&G Judson). In every case, the flowing gases were passed through calibrated mass flow meters (Tylan) and mixed just upstream of the photolyis cell. There was no evidence of prereaction of the reagents during the mixing time or during passage through the cell. Argon (General Air research grade) was used as a diluent at a partial pressure of several Torr. The argon flow increased the linear flow velocity of the gases passing through
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0 1995 American Chemical Society
Quenching of NH(a' A)
J. Phys. Chem., Vol. 99, No. 44, 1995 16297
the cell, minimized the effects of diffusion, and ensured thermalized rotational and translation distributions among the photolysis products. Chlorine (Matheson), fluorine (Spectra Gases), and ClF (Ozark-Mahoning, 97%) wercdiluted in helium (General Air, UHP) to give 5-20% mixtures. IC1 (Kodak) was purified with two freeze-thaw cycles at 273 K prior to dilution with helium to give 5-15% mixtures. The C12/He and IC1/He mixtures were kept in Pyrex bulbs. The F2/He and ClF/He mixtures were kept in stainless steel vessels. The concentrations and purities of the gases in the mixtures were determined by W and (where appropriate) IR absorption spectroscopies.
15.44 0
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9
mmes 10E15) 45
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P
Results Rate Constants for NH(alA) Quenching. Rate constants for quenching of NH(a'A) by various molecular halogens and interhalogens were determined by tuning the wavelength of the probe laser to the P(2) line of the 0,O band of the aIA cIrI transition in NH (325.8 nm) and manually varying the delay time betweens the photolysis and probe laser pulses. A set of such experiments gave the time profile of the NH(a) decay for fixed densities of HN3 and halogen. Under such conditions, the decay rate is given by
-
0
100
200
300
400 500 600 (rimes 1OEl2)
700
I
800
900
40 I
35
where X2 represents the halogen or interhalogen, and decay by radiation and by diffusion out of the observation zone are both too slow to be significant. The term ~ Q [ Qrepresents ] the rate of quenching by species other than HN3 or halogen, Le., impurities that may be present. The rate constant kxZ was determined by measuring ;Idecay in several sets of experiments in which [X2] was varied while [HN3] was held fixed. These data were plotted as ;Idecay vs [X2], such that the slope gives the value of kx2. Data obtained for the quenching of NH(a'A) by Cl2, Fz, Brz, ClF, and IC1 are shown in Figure 1, a, b, and c. In every case, the observed decays were well fitted to single exponentials, and the Stem-Volmer plots (Figure 1) yielded good linear relationships as in eq 2. The intercepts of these plots at [X2] = 0 correspond to quenching of NH(a) by residual HN3 and impurities (denoted Q in eq 2). For all the measurements except for quenching by F2, the intercept was in good agreement with the expected rate8 of quenching of NH(a) by residual HN3. In the F2 case, the intercept was significantly larger, indicating the presence of a possible impurity in the HN3 sample. Even in this case, a good linear Stem-Volmer plot was obtained, and the larger intercept should not affect the validity of the rate constant determined. Previously,6 we reported a measurement of the rate constant for NH(a) quenching by C12. In these previous experiments, HN3/Cl2 mixtures were photolyzed at 249 nm, and the LIF probe clll was tuned to the Q(3) line of the 0,O band of the aIA transition in NH. These experiments were repeated, using photolysis at 193 nm and the probe laser tuned to the P(2) line of the transition in NH. HN3 has a significantly larger absorption cross section9 at 193 nm relative to 249 nm, and given the laser fluences of our experiments (typically 50 mJ/ cm2), the change in wavelength resulted in dissociation of 3 3 % of the HN3 present, a significant improvement. The data obtained in the new experiments are shown in Figure 1, and the rate constant determined from the slope of the plot, k12= (6.8 f 0.4) x lo-" cm3 s-l, is virtually identical to the result previously reported.6 This gratifying result is important, as Bonn and Stuhl' have shown that photolysis of H N 3 at 193 and
-
I
30 25
P
g
20
A
15
10 5
OJ,
io
40
60
lbo 120 (Times 10E12)
60
I40
160
180
2 )O
Density ( ~ m - ~ ) Figure 1. (a) Plot of the exponential decay rate (s-l) of NH(alA,u=O) vs the density of Fz. (b) Plots of the NH(a'A,u=O) decay rate vs the densities of Br2 (*) and ClF (0).(c) Plots of the NH (alA,v=O) decay rate vs the densities of IC1 (W) and C12 (A).Lines through the data are least-squares fits. In each case, the initial density of HN3 was on the order of lOI4 ~ m - ~ .
249 nm produces NH(a) with distinctly different vibrational population distributions. Clearly, these different populations have no effect on the observed decay of NH(a,u=O), suggesting that vibrational relaxation of the NH(a) does not occur to any great extent during the time frame of electronic quenching by Cl2. The results of measurements of NH(a'A) quenching by F2 are also shown in Figure 1. The slope of this plot indicates a rate constant k~~ = (1.0 f 0.5) x cm3 s-I. Clearly, quenching by F:! is very much slower than is quenching by Cl2. The small slope of the plot leads to significant uncertainty in the value of the rate constant (the uncertainty reported represents 20). As indicated in the plot, much larger densities of F2 (relative to the other quenchers tested) were needed in order to observe any quenching at all. Attempts to further increase the F2 density in these experiments led to significant suppression of the H N 3 flow, resulting in lower NH(a) densities and hence an overestimation of the rate constant. For this reason, we
Singleton and Coombe
16298 J. Phys. Chem., Vol. 99, No. 44, 1995 TABLE 1. Quenching of NH(a*A) by Molecular Halogens and Interhalogens ko (em's-')
P (collision-')"
(1.0 i 0.5) x lo-" (3.7 i 0.4) x lo-''
6.7 x lo-' 1.8 x l o - '
(6.8 i 0.4) x l o - ' ' ( 6 . 0 i 0.3) x lo-'' (1.6 i 0.3) x lo-'"
3.2 x l o - ' 2.7 x lo-' 7.1 x lo-'
quencher
Fz C1F C1. Brz
IC1
2500
obsd products
a .'
2000
NF(a'A), HCl(u) NCl(a'A),H F k ) NCl(a'A,b'E+), H C k ) NBr(a'A.b'E-)
\
1500
h
.-c
NBr(alA)
1 .
loo0
4-
2
500 I'
P = kq/khdrd,phrie, = quenching probability per collision. x
.-
believe the rate constant found for quenching by F? to be an upper limit to the true value. The rate constant determined for quenching of NH(a) by C1F lies between those for quenching by F? and Cl?, at (3.7 f 0.4) x lo-" cm3 s-'. The rate constant for quenching by Br? is cm3 s-', nearly equal to that for quenching (6.0 i 0.3) x by C12. The largest rate constant was obtained for quenching by ICI, at 1.6C! 0.3 x cm3 s-', near the gas kinetic limit. All of the rate constants measured are collected together in Table 1. Observation of Reaction Products. Products of the quenching reactions noted in Table 1 were sought by monitoring visible and infrared chemiluminescence from these systems. For the excited singlet states of the nitrenes this is a rather insensitive method. since transitions to the ground ? - states are forbidden and have very small radiative rates.','0,' Regrettably, MPI and LIF techniques are not available for these species. Radiative rates are considerably greater, however, for the vibrationally excited hydrogen halides expected to be the coproducts of the reactions. As noted above, emissions from different species were observed using different detectors and a variety of interference filters. This method was used successfully in our Cl? reaction, in which preliminary studyh of the NH(a) emissions from excited NCl(a' A) and vibrationally excited HCI were observed. These measurements were repeated in the present set of experiments, and the data were found to be quite reproducible. We note here that NX(a'A) might also be produced by secondary reactions, to the extent that they occur. Gericke and co-workers'? have shown that photolysis of HN; at 193 or 249 nm produces a small yield (-158) of H atoms and Ni radicals. In principle, the Ns radicals might react with any halogen atoms present to generate excited NX(alA). Halogen atoms might be generated by photodissociation of the molecular halogens (surely a minor channel at the wavelengths used) or by reaction lb. The X N3 reactions should be second-order processes with rates limited by the very small densities of N; radicals and X . NX(a'A) generated atoms present (surely 10" ~ m - ~ )Any in this manner would be produced over a time frame of many milliseconds. As noted below, NX(a' A) products observed in the present experiments are observed in the time frame of hundreds of microseconds after the photolysis pulse. Hence we are confident that production of NX(a'A) by N3 reactions is unimportant in the present work. Experiments directed toward observation of the products of a reaction between NH(a) and F2 were not successful. As described above, Fz is a very inefficient quencher of NH(a), and for all but the most extreme conditions the loss of NH(a) from the system is dominated by quenching by the parent HNi, a process8 which produces a small yield of electronically excited NH?. We sought emission from both NF(a'A) near 874 nm and vibrationally excited HF near 2.5 pm. Both a cooled GaAs photomultiplier tube and the Ge detector were used in the search for NF(a). The vibrationally excited HF is a much stronger radiator'? and, we believe, should have been detected in our
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w
3000
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2500
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2000
1500 1000 500 0
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10
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35
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Time (psec)
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Figure 2. (a) Time profile of NBr a X emission produced by the NH(a'A) Br? reaction. (b) Time profile of NBr b X emission from this reaction. The initial densities of HNI and Br! were 2.6 x 10l4 and 2.4 x IO" cm-',respectively.
+
-
experiments (which used a cooled InSb detector and appropriate filter) if the reaction were occurring at all. The absence of any observable emission from HF(v) in this system certainly suggests that the very small proportion of NH(a) quenching caused by the F? probably does not proceed by the insertion mechanism. Emission from NBr(a'A) was readily observed from the NH(a) Br? reaction using an interference filter with a band-pass near 1.08 ,um and the cooled Ge detector. A time profile of this emission is shown in Figure 2a. For the density of Br? used in these experiments (-10'' cm-j)), the NH(a) quenching process occurs in a few microseconds, comparable to the time constant of the Ge detector. The combination of the NH(a) decay rate and the detector time constant is seen as the rise of the emission signal. The decay corresponds to the loss of NBr(a) by collisional relaxation, probably quenching by Brl. A small part of the emission is associated with excited NH? generated by the NH(a) HN3 reaction.8 which still accounts for about 15% of the NH(a) quenching at the conditions of the experiment. Emission was also observed from the b'C- state of NBr, in the visible near 674 nm, using a filter and a cooled GaAs photomultiplier tube. The time profile of this emission is shown in Figure 2b. In this case, the decay of the emission corresponds well with the rate of NH(a) quenching by Br2. The rise corresponds to collisional quenching of NBr(b), likely by Brz. W e note here that the emission was well bracketed using other filters in the visible; for example, no emission was observed using a filter with a band-pass at 665 nm. In the NH(a'A) C12 experiments, infrared emission from vibrationally excited HCI was readily observed, and analogous emission from HBr(v) was sought from the NH(a'A) Br? reaction. Using a cooled InSb detector with a band-pass filter for the appropriate region (transmission maximum at 4.13 ,um), a feeble emission was observed sporadically. The signal was not reproducible under conditions similar to those employed in the measurements of NH(a) quenching rates or the observations of NBria) emission. Given the much smaller radiative rate from HBr(u) relative to HCI(u), and the possibility of more efficient quenching. this result is not surprising.
+
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Quenching of NH(a' A)
J. Phys. Chem., Vol. 99, No. 44, 1995 16299 detector, emissions with identical time profiles (but different intensities) were also observed at 0.874, 1.08, and 1.3 pm, suggesting a very broad-band emission spectrum that may or may not be attributed to excited NI.
Discussion
~~~~
50
150
100
200
250
lime (psec)
Figure 3. Time profiles of emission from NF(a'A) (broken line) and NCl(alA) (solid line) produced by the reaction of NH(a'A) with ClF. For the data shown, the initial densities of HN3, ClF, and C02 were 2.6 x 5.3 x IOl5, and 6.9 x 1OI6 ~ m - respectively. ~,
Similar experiments were performed in an effort to observe the products of the reaction of NH(a) with C1F. In this case, the insertion reaction might proceed to NF(a) HCl(u) or to NCl(a) HF(u). We looked for both sets of products, using a filter which transmits at 1.08 p m for the NC1 aIA XIEtransition and a filter with a band-pass near 874 nm for the analogous transition in NF. The emissions were detected with the Ge detector. A complication was interference in the 874 u = 0 overtone emission. The nm region by HF v = 3 radiative ratel4 of this transition is about 5 times greater than that for the a X transition in NF. COz was added to the flow stream to selectively quench the HF emission. The rate constantI5 for HF vibrational relaxation by C02 (which varies with v) is near lo-" cm3 s-l, whereas the rate constantI6 for electronic quenching of NF(a) by CO;! is 6 x cm3 s-l. Hence, the HF(u) could be completely relaxed without significantly effecting the NF(a). Figure 3 shows time profiles of the emission signals from NF(a) and NCl(a) obtained from the NH(a) ClF reaction. The branching fraction between the production of NF(a) and NCl(a) in the system was estimated by comparison of these intensities, taking into account the radiative rates of the a X transitionsl0S' and the response of the detectodfilter combinations at 1.08 and 0.874 pm. Further, the time decays of the emissions (corresponding to loss of NF(a) and NCl(a) from the system) were extrapolated to time = 0 using a single-exponential function. Again, C02 was added to the system at densities which resulted in quenching of the HF(u) in the system within a few microseconds. These measurements were made for 13 different C1F densities, the results yielding an average NF(a)/ NCl(a) intensity ratio of 1.3 & 0.3. Correction of this factor by the detector1 filter response function and the ratio of the radiative rates yields a density ratio [NF(a)]/[NCl(a)] = 18 & 6 . This result implies that the branching fraction to NF(a) HCl(u) in the reaction is greater than 90%. Chemiluminescence from products of the NH(a) IC1 reaction was examined using methods similar to those described above. To our knowledge, the emission spectrum of gaseous NI(a'A) has not been reported, but it has been measured in lowtemperature rare gas matrices." Assuming a matrix shift the same as that found'* for the a X transition in NC1, the analogous transition in gaseous NI is expected to lie at about X emission 1.17 pm. From the matrix spectrum, the NI a feature is expected to be quite broad and hence relatively insensitive to the matrix shift. Although emission was readily observed using a filter transmitting in this region and the Ge
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The data presented in Table 1 show that the rate constants for NH(a' A) quenching by halogens and interhalogens increase in the series F2 -= C1F < C12 Br2 < IC1. On first glance, this result correlates with molecular properties of the halogens that one might expect to be associated with the efficiency of the insertion process. The NH(a'A) molecule has filled and unoccupied n nonbonding orbitals in its outermost subshell. Presumably, insertion proceeds by electrophilic attack by NH(a'A) on sites which can donate electrons to the unoccupied orbital. Hence, insertion into the molecular halogens might depend in some sense on the lability of the electrons in the bonds of these molecules. Such lability would correlate with the ionization potentials of the electrons in the bonding orbitals of the halogens or perhaps the perpendicular component of their molecular polarizabilities. Such properties do indeed follow the series noted above, at least in a qualitative sense. A similar trend has been observed by Du and Setser l9 for the quenching of NF(a'A) by molecular halogens and interhalogens. In this case, the quenching rate constants were correlated with the ionization potentials, and the result was taken as support for the insertion mechanism. The pattern becomes somewhat different if one considers the quenching efficiency per collision corresponding to the rate constants in Table 1. These numbers are also given in the table for collisions defined as hard-sphere encounters. The efficiencies are quite comparable for quenching by ClF, Cl2, and Br2, all within 50% of 0.25 per collision. Excited products of the insertion processes were observed for these three reactions. The upper limit of the efficiency for quenching by FZ is a factor of 35 smaller than this value, however, suggesting that the mechanism operative in quenching by ClF, Clz, and BrZ (presumably insertion) simply does not happen in this case. It seems likely that the electrons in the F2 bond are simply not labile enough to be drawn into the unoccupied n orbital of the excited NH. The efficiency for NH(aIA) quenching by IC1 is nearly a factor of 3 greater than that for quenching by ClF, C12, or Brz, at 0.71 per collision. This result suggests that some additional reaction channel may be open in this case, an inference supported by the fact that emission from the products of the reaction is observed over a very broad wavelength range in the visible and near-IR. In this particular case among those studied, it may be that the lower lying states in the excited 311 manifold of IC1 are accessible via direct electronic-to-electronic energy transfer from NH(a' A): NH(a'A)
+ ICl(X'X+)
-
NH(X3Z)
+ ICl(3112,1,0)(3)
The 0,O band of the X3E- -A3111 transition in IC1 lies at 13 658 cm-', to be compared with the 12 593 cm-' carried by NH(a'A). This-1065 cm-' energy defect might well be overcome by vibrational excitation of the NH(a), as is known to occur when this species is produced by photodissociation7 of HN3. The A'31T2 state of IC1 should lie below the A 3 n l state and might be accessible in collisions with NH(a' A,u=O). Although such processes are clearly spin allowed, spin-orbit coupling in the collision complex should be substantial because of the presence of the iodine atom, and correlations involving only the total angular momentum quantum numbers (Q) of the
16300 J. Phys. Chem., Vol. 99, No. 44, 1995
Singleton and Coombe
collision partners are expected to have an effect (if any). A collinear collision complex would have Q = 2 , such that the resultant excited IC1 might have Q = 2 or Q = 1, Le., the A’3112 and A 3 n 1states. As noted above, these are the states that are likely to be energetically accessible. An alternative possibility is the opening of the doublet product channels reactions l a and Ib. In particular, channel l b (which would generate I HNCl in this case) might be important because of the relative weakness of the N-I bond. The data from the NH(a’A) CIF experiments indicate that the branching fraction between NF(a’A) and NCl(a’A) is on the order of 10, favoring the excited NF. This preference for the NF channel agrees with observations of the analogous H NFCl reaction in discharge-flow reactors and certainly supports our inference that both processes occur on the singlet HNFCl potential energy surface. As pointed out by Setser and c o - ~ o r k e r showever, ,~ this result is difficult to understand since. for both reactions, production of NCl(a’A) HF is much more exothermic, and dissociation of the HNFCl intermediate to these products would be expected to have a smaller barrier. It may be that transmission over the barriers on the HNFCl surface is affected by the presence of nearby excited state potential surfaces, as in the case of the “non-Born-Oppenheimer” processes discussed by Butler and co-workers.’O In any case, computations of the HNFCl surface would surely be of help in resolving this issue.
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Acknowledgment. This work was supported by the US. Air Force Office of Scientific Research under Grant F4962093-1-063 1.
References and Notes ( 1 ) Herbelin. J. M.: Cohen. N . Chern. Ph!,.s. Lerr. 1973. 20, 605. Heidner. R. F.: Helvajian. J.: Holloway. J. S.: Koffend. J. B. J. Phys. Chern. 1989. 93. 7818. ( 2 ) Cheah. C. T.: Clqne. hl. A. A,: Whitefield. P. D. J. Chern. Soc., Fnmrkiy Trari.\. 2 1980, 76. 71 1, 13j Arunan. E.: Liu. C. P.: Setser. D. W.: Gilben. J. V.: Coombe. R. D. J . Phys. Cherri. 1994. 98. 494. (3) Exton. D.B.: Gilbert. J. V.: Coombe. R. D. J. Phys. Chern. 1991. 95. 2692: 1991. Y5. 7758. (-5) Malins. R. J.: Setser. D. W. J. Phy.\. Chem. 1981. 85. 1342. (6) Singleton. S . M.: Coombe. R. D. Chern. P h ~ sLerr. . 1993. 215. 237. ( 7 ) Bohn. B.: Stuhl. F. J. Pizys. Chern. 1993, 97. 4891, 7234. (8) McDonald. J . R.: Miller. R. G.: Baronalski. .4. P. Chem. Pl7y.s. Let?. 1977. 51. 5 7 : Chert P/iy.s. 1978. 30. 119. ( 9 ) McDonald. J. R.: Rabalai\. J . W: McGlqnn. S. P. J. Chem. Pky.\. 1970. 52. 1332. (10) Benard. D. J.: Choudhury. M. A,: Winker. B. K.: Sedar. T. A,: Michels, H. H. J . PIiyc. Chem. 1990. 94. 7507. Yarkony. D. R. J. Chern. Phys. 1987. 86.1642. ( 1 1 J Bradburn. G. R.: Lilenfeld. H. V. J. Phy.\. Ciiern. 1991. 95, 555, (12) Gericke. K.-H.: Lock. M.:Comes. F. J. Chew?. Phys. Leu. 1991. 186. 327. (13) Arunan. E.: Setser. D. W.: Ogilvie. J. F. J. Chenz. Phys. 1992, Y7. 1734. 114) See for example: Herbelin. J. M.:Emanuel. G. .I. Chern. Phys. 1974. 60, 689. (15) Bott. J. F. J . Cherri. Phys. 1976. 65. 3239. Arunan. E.; Raybone. D.: Setser. D. W. J. Chrrn. PI i 16) Du. K. Y.: Setser. D. ( 17) Becker. A. C.: Oberhoffer. M. H.: Langen. J.: Schurath. U. J . Chern. Phy.s. 1986. 84.2907. (18) Becker. A. C.. Schurath. U.Chern. Phyx. Leu. 1989. 160. 586. (19) Du. K. Y.: Setser. D. W . J . Phys. Chem. 1992. 96. 2553. (20) Person. >I. D.: Kaih. P. W.: Butler. L. J. J. Chrrn. Phys. 1992. 97. 355.
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