11314
J. Phys. Chem. 1996, 100, 11314-11318
Temperature Dependences of Singlet Methylene Removal Rates Frances Hayes and Warren D. Lawrance Department of Chemistry, Flinders UniVersity, G.P.O. Box 2100, Adelaide, SA 5001, Australia
Warren S. Staker and Keith D. King* Department of Chemical Engineering UniVersity of Adelaide, Australia 5005 ReceiVed: January 2, 1996; In Final Form: April 30, 1996X
The technique of laser flash photolysis/laser absorption has been used to obtain absolute removal rate constants for singlet methylene, 1CH2 (a˜ 1A1), in the temperature range 298-462 K. The temperature dependences of the removal rate constants for N2, CH4, C2H4, C2H6, H2O, H2CO, CH2CO, CH3F, C2F6, and CH3OH were measured and the Arrhenius parameters determined. For the reacting species, CH4, C2H4, C2H6, H2O, H2CO, CH2CO, CH3F, and CH3OH, the rate constants exhibit negative temperature dependences with activation energies covering the range -0.7 ( 0.7 kJ mol-1 for CH3F to -3.5 ( 1.0 kJ mol-1 for CH3OH. In contrast, positive temperature dependences are observed for the nonreacting species, N2 and C2F6, with activation energies of 1.8 ( 0.9 and 1.6 ( 2.0 kJ mol-1, respectively. Good agreement was obtained with previously reported measurements for the N2, CH4, CH2CO, C2H4, and C2H6 temperature dependences. For the nonreacting species, comparison is made with theoretical predicted temperature dependences for collision-induced ˜ 3B1). intersystem crossing from 1CH2 (a˜ 1A1) to 3CH2 (X
Introduction The group IV divalent radical methylene, CH2, is an important reactive intermediate in organic chemistry, and it plays a significant role in chemical combustion systems1-4 and planetary atmospheres.5 Because the processes in combustion and planetary atmospheres occur at extreme temperatures, the temperature dependences of the reactions of CH2 are of major interest. The methylene ground electronic state is the triplet (X ˜ 3B1, 3 abbreviated CH2), and the lowest excited electronic state is the singlet (a˜ 1A1, abbreviated 1CH2). The energy difference between the singlet and triplet states is relatively small (37.65 kJ mol-1),6 so that the collision-induced intersystem crossing (CIISC) between the two states is rapid. Therefore, the mutual interconversion of 1CH2 and 3CH2 in collisions with bath gas molecules must be taken into account for a proper description of the thermal reactions of CH2. Reaction from the singlet state at room temperature is typically 5-6 orders of magnitude faster than from the triplet ground state, and different products are often formed. The rates of CH2 reactions are very fast, mostly near the gas kinetic collision number at room temperature for 1CH . Thus only a weak temperature dependence can reason2 ably be expected for 1CH2, and this has been confirmed by recent experiments.7-10 On the other hand, 3CH2 reactions exhibit strong positive temperature dependences.11 Although there have been many direct measurements of 1CH removal rates, most of these studies have been at ambient 2 temperature. The temperature dependence of 3CH2 kinetics has been extensively investigated over the temperature range 295700 K,11 but only two groups have previously directed experimental research toward the temperature dependence of 1CH2 removal rates. Wagener7 directly measured the rate constants at three temperatures only, 210, 295, and 475 K, for 1CH2 removal by He, Ar, N2, H2, and the four hydrocarbons CH4, C2H6, C2H4, and C6H6. The 1CH2 removal by the four hydrocarbons and H2 exhibited weak negative temperature X
Abstract published in AdVance ACS Abstracts, June 15, 1996.
S0022-3654(96)00002-0 CCC: $12.00
dependences, whereas He, Ar, and N2 removal rate constants were found to increase weakly with temperature. Wagener and Wagner8 also found a weak negative temperature dependence for 1CH2 removal by HF by measuring removal rate constants at the three temperatures 295, 380, and 485 K. Hancock’s group measured removal rate constants for Ar, NO, H2, and CH2CO in the temperature ranges 295-431 K (Ar, NO and H2)9 and 295-859 K (Ar, NO, H2, and CH2CO),10 finding almost no temperature dependence for NO and H2 (in contrast to the results of Wagener7 for H2) but significant positive and negative temperature dependences for Ar and CH2CO, respectively. In this study the technique of laser flash photolysis/laser absorption has been used to obtain absolute removal rate constants for 1CH2 in the temperature range 298-462 K. The temperature dependences of the removal rate constants for N2, CH4, C2H4, C2H6, H2O, H2CO, CH2CO, CH3F, C2F6, and CH3OH were measured and the Arrhenius parameters determined. N2, CH4, C2H4, C2H6, and CH2CO have been the subject of previous studies7-10 and were investigated here in order to (i) provide benchmarks against which the accuracy of our results can be measured and (ii) provide a cross-check of the temperature dependences for these species, which are important in combustion, as earlier work had largely measured the rate constants at only three temperatures (except for CH2CO). With respect to the temperature dependences of the removal rate constants, the remaining species, H2O, H2CO, CH3F, C2F6, and CH3OH have been investigated for the first time. H2O, H2CO, and CH3OH were investigated because of their relevance to combustion processes. The reaction cell used in this work limited the temperature range to below temperatures of combustion interest, but the results obtained provide a guide and a basis for extrapolation to rates at higher temperatures. C2F6 was investigated because our previous studies of halogen-containing species12 had shown that it does not react with 1CH2; it thus offers an insight into the temperature dependence of the CIISC rate constant for a “large molecule” as a comparison for the monatomic and diatomic collision partners studied by other © 1996 American Chemical Society
Singlet Methylene Removal Rates
J. Phys. Chem., Vol. 100, No. 27, 1996 11315
groups. CH3F provides a comparison with CH3OH, where the reactive alcohol group13-15 is replaced by the unreactive C-F.12 Experimental Section The apparatus and techniques used by us have been described in full detail in previous publications describing our ambient temperature studies.13-16 We give here a brief overview and provide details relevant to the temperature dependence experiments. The same stainless steel reaction cell is used for the high-temperature work as was used for the ambient temperature studies. To achieve heating of the cell, the entire length is wound tightly with insulated resistance wire (rated to 475 K) and then wrapped in aluminum foil. The tubing leading into the cell inputs is also wound so as to preheat the gas mixture before it enters the reaction zone. By application of a variable current through the wire, temperatures up to a maximum of ca. 460 K can be reached. The current is supplied by a 0-220 V variable transformer and current controller (Ernst Leitz GmbH, Wetzlar), supplying current between 0 and 90 A. Three thermocouples are inserted along the length of the cell, one at either end and one in the middle, positioned such that the tip of each thermocouple is just outside the photolysis beam. The temperatures measured by the three thermocouples were typically within ca. 5 K of one another and varied by only ca. 3 K as the total pressure was increased/decreased, thus indicating an essentially constant and uniform temperature profile along the length of the reaction zone during the course of the experiments. 1CH2 is prepared by the photodissociation of ketene (CH2CO) using an excimer laser at λ ) 308 nm. The 1CH concentration is followed as a function of time by 2 monitoring its absorption with an argon-ion pumped ring dye laser tuned to the 404(0,14,0) r 414(0,0,0) rovibronic transition at 16 928.79 cm-1 (vacuum).17,18 The photolysis and probe beams are collinear with the cross-sectional area of the former considerably greater than that of the latter. This ensures that the loss of 1CH2 through diffusion from the reaction volume was negligible on the time scale of collisional removal. The decay signal is averaged using a digital storage oscilloscope (256 averages) to enhance the signal/noise ratio, and the averaged signal is stored on a laboratory computer. At each reactant pressure, three to five such averaged traces were collected. After each of these traces was fitted, an average of the pseudo-first-order rate constants was obtained to further improve the reliability of the measurements. Each of the component gases (CH2CO, N2, and the reactant, R) is infused directly from its individual source through a mass flow controller and then into the reaction cell through ports at either end of the cell. The gas mixture is pumped out through a port in the middle of the cell. Pressures were monitored using MKS Baratron capacitance manometers. The liquid reactants, H2O (demineralized) and CH3OH (Ajax, 99.8%), were degassed using several freeze-pump-thaw cycles prior to use. The gaseous reactants, CH4 (Matheson, Research Grade), C2H6 (Matheson, C.P.), C2H4 (Matheson, C.P.), H2CO (prepared), CH3F (Matheson), and C2F6 (Matheson), were used directly as supplied. CH2CO was prepared by pyrolysis of acetic anhydride (CH3COOCOCH3)19 and purified to >99% (determined by IR spectroscopy20,21). Results Rate constants for the removal of 1CH2 were obtained by analysis of the time-dependent absorption data. CH2CO, N2, and R each remove 1CH2. Under pseudo-first-order conditions (when R, CH2CO, and N2 are in excess relative to the 1CH2 concentration)
Figure 1. Pseudo-first-order rate constants for 1CH2 removal by CH3OH at 450 (0), 355 (4), and 298 K ()) versus reactant pressure. The size of the symbols is indicative of the uncertainties in the rate constants.
Figure 2. Pseudo-first-order rate constants for 1CH2 removal by N2 at 298 (0), 335 (4), and 455 K ()) versus reactant pressure. The size of the symbols is indicative of the uncertainties in the rate constants.
[1CH2]/[1CH2]0 ) exp(-k1stt)
(1)
where k1st, the pseudo-first-order rate constant, is given by
k1st ) k1[R] + k2[CH2CO] + k3[N2]
(2)
Typical experimental curves and the fits to these curves have been shown previously.14,16 Absolute removal rate constants, k1, for 1CH2 were obtained by weighted linear least-squares fitting of eq 2 using decay rates obtained with a N2 pressure of 4.0 Torr and a CH2CO pressure of 0.07 Torr and reactant pressures in the range 0.05-2.0 Torr, with the upper limit imposed by fast removal rates. Typical plots are shown in Figures 1 and 2. The absolute removal rate constants for the reactants of interest have been measured at ambient temperature (298 ( 2 K) by us previously12,15,16 and are summarized in Table 1. As commented previously,12,15,16 good agreement was obtained between our work and that of other research groups.7,8,10,22-28 All errors quoted correspond to 2σ statistical errors from the weighted linear least-squares analysis plus estimated uncertainties in flow rates, total pressure, and temperature. As discussed previously, the relative precision of the rate constants is better than is indicated by the quoted errors.15 The temperature dependences of the 1CH2 bimolecular removal rate constants were investigated typically at six temperatures for each substrate over the range of ca. 298-462 K. The rate constants were fitted to the normal Arrhenius equation given by
k1 ) A exp(-E/RT)
(3)
11316 J. Phys. Chem., Vol. 100, No. 27, 1996
Hayes et al. TABLE 2: Arrhenius Parameters, A (10-10 cm3 molecule-1 s-1) and E (kJ mol-1) for Removal of 1CH2 this worka reactant
A
He Ar
Figure 3. Arrhenius plot of the second-order removal rate constant for 1CH2 by CH3OH (0) over the temperature range 298-450 K.
N2 C2F6 NO HF CH3F H2
0.21 ( 0.06 0.38 ( 0.26
CH4 C2H4 C2H6 C6H6 H2CO CH2CO H2O CH3OH
0.32 ( 0.08 0.88 ( 0.24 0.46 ( 0.16
0.43 ( 0.10
0.88 ( 0.34 1.01 ( 0.38 0.91 ( 0.72 0.89 ( 0.29
previous workb,c E
A
0.10 ( 0.08 0.11 ( 0.08 0.24 ( 0.04 1.8 ( 0.9 0.21 ( 0.11 1.6 ( 2.0 1.82 ( 0.45 0.46 ( 0.76 -0.7 ( 0.7 0.70 ( 0.31 0.86 ( 0.23 -2.2 ( 0.8 0.29 ( 0.19 -2.6 ( 0.8 0.99 ( 0.63 -3.3 ( 1.0 0.66 ( 0.48 2.22 ( 1.75 -3.0 ( 0.9 -2.2 ( 1.1 1.17 ( 0.22 -1.8 ( 2.2 -3.5 ( 1.0
E
ref
3.2 ( 1.7 1.9 ( 1.5 3.7 ( 0.6 2.5 ( 1.2
7 7 10 7
0.3 ( 0.7 -1.7 ( 3.7
10 8
-1.3 ( 1.0 -0.2 ( 0.8 -2.2 ( 1.4 -2.1 ( 1.4 -2.2 ( 1.3 -1.5 ( 1.6
7 10 7 7 7 7
-1.4 ( 0.6
10
a Temperature range: 298-462 K. b Temperature ranges: 210-475 K (ref 7); 295-859 K (Ar, ref 10); 296-645 K (NO, ref 10); 293676 K (H2, ref 10); 300-855 K (CH2CO, ref 10); 295-485 K (ref 8). c In refs 7, 8, and 10 the temperature dependences were reported in the form k1 ) A(T/295 K)n, where A is not the Arrhenius A-factor but corresponds to the removal rate constant at 295 K. The Arrhenius parameters quoted here were obtained by us from least-squares fitting of the original data (removal rate constants at given temperatures) of refs 7, 8, and 10.
Figure 4. Arrhenius plot of the second-order removal rate constant for 1CH2 by N2 (0) over the temperature range 298-455 K.
TABLE 1: Ambient Temperature (298 ( 2 K) Removal Rate Constants, k1 (10-10 cm3 molecule-1 s-1) reactant
k1
ref
N2 C2F6 CH3F CH4 C 2 H6 H 2O C2H4 CH2CO H2CO CH3OH
0.108 ( 0.006 0.212 ( 0.031 0.549 ( 0.025 0.79 ( 0.03 1.83 ( 0.10 1.86 ( 0.38 2.49 ( 0.11 2.50 ( 0.14 3.02 ( 0.15 3.54 ( 0.19
16 12 12 16 16 15 16 16 15 15
Discussion
where A is the frequency factor or Arrhenius A-factor and E is the activation energy. For each of the reactants, a plot of ln k1 versus 1/T demonstrates a linear dependence (typical plots are shown in Figures 3 and 4). There was no significant indication of curvature in the Arrhenius plots over the temperature range of the experiments. Linear least-squares fitting of these data yields A and E for the removal of 1CH2 by each of the reactants. The results are shown in Table 2 along with the results for all previous temperature dependence studies for comparison. Previous publications did not report the results in the form of Arrhenius parameters; the parameters reported here were obtained by us from least-squares fitting of the data given in refs 7, 8, and 10 to the Arrhenius form. Other workers have reported the temperature dependences of 1CH2 removal rate constants in the form
k1 ) A(T/295 K)n
removal rate constant at 295 K. Wagener,7 who was the first to present the results in this form, has pointed out that although this expression gives a slightly better correlation than the Arrhenius expression the difference is not significant.
(4)
Here A is not the Arrhenius A-factor but corresponds to the
The CIISC between the singlet and triplet CH2 states is rapid. The probe laser, tuned to a 1CH2 transition, cannot distinguish CIISC from chemical reaction; both processes deplete the 1CH population. The removal rate constants reported here are 2 thus equal to the sum of the reaction and CIISC rate constants. For hydrocarbons, Wagner’s group8,25,28-31 has shown that chemical reaction is the dominant removal channel at room temperature, accounting for 70-85% of the removal rate constant. However despite the large branching ratio in favor of reactive removal at ambient temperature, the opposing temperature dependences observed for reaction and CIISC mean that this is not going to be the case at elevated temperatures. Hence knowledge of the extent to which the observed temperature dependences of the removal rate constants reflect the dependences of the reaction rate constants or the CIISC rate constants is necessary for a full understanding of methylene reaction systems. Where there is overlap with previous work, very good agreement is found, within experimental error, in all cases. Thus all studies find negative temperature dependences for the removal rate constants for CH4, C2H4, C2H6, and CH2CO and a positive temperature dependence for N2. An examination of all results shown in Table 2 reveals that the 1CH2 removal rate constants for reactive substrates decrease with increasing temperature (except for NO and one study of H2) while the removal rate constants for substrates where only CIISC occurs (He, Ar, N2, C2F6) show an increase with increasing temperature. The ambient temperature CIISC rate constant for the “large molecule” C2F6 is 6.4 times larger than that for He,15 but the temperature dependence appears not to be significantly different
Singlet Methylene Removal Rates from those observed for the smaller species He, Ar, and N2. Within the group of nonreacting species, the A-factors show a slight increase in molecular size from He to Ar to N2 to C2F6 while the activation energy for the polyatomic C2F6 is slightly smaller than the values for the monatomic and diatomic species. A positive temperature dependence for CIISC rate constants is consistent with recent theoretical calculations. The most successful model for CIISC in methylene is the “mixed-state” model of Gelbart and Freed.32 It attributes the rate for this process to relaxation within the triplet manifold from states of mixed singlet-triplet parentage.32,33 Thus the rate constants for CIISC are largely those for rotational relaxation within the triplet and are relatively insensitive to the collision partner. Quantitative analysis of this mixed-state model for 1CH2 to 3CH transfer without the need for adjustable parameters has 2 been carried out by Bley et al.33,34 Hancock and co-workers9,10 have considered the effect of temperature on the mixed-state model estimates of the CIISC rate constants for Ar collider and compared the results with their experimental observations. They pointed out that provided the redistribution of population within the singlet manifold is fast and that vibrational relaxation within the triplet manifold is not rate determining, then the effect of temperature will simply be to alter the relative proportion of singlet molecules in the perturbed levels and to change the rotational relaxation rate constant out of the perturbed triplet levels. The predicted increase of the CIISC rate constant for Ar collider (relative to their most recent experimental value of 5.5 × 10-12 cm3 molecule-1 s-1 at 295 K10) was found to be in excellent agreement with experiment up to a temperature of about 500 K.10 At temperatures >500 K their predicted relative CIISC rate constant was found to be less than experiment with the difference increasing with temperature. Possible reasons for the deviations between experiment and theory at high temperatures have been discussed by Hancock and Heal.10 More detailed calculations of CIISC rate constants for methylene according to the mixed-state model have been carried out by Bley et al.33,34 They have evaluated ambient temperature CIISC rate constants for 12 selected collision partners, including He, Ne, Ar, Kr, Xe, N2, and SF6 as nonreactive colliders and H2, D2, CH4, C2H6, and H2O as examples of reactive colliders. In addition, temperature dependent CIISC rate constants over the range 200-500 K were calculated for Ar, N2, and CH4 colliders.34 Calculations were carried out for both the ortho and para nuclear spin forms of 1CH2. The nuclear spin degeneracies of the ortho and para forms are three and one, respectively,33 and a statistical distribution for nuclear spin in the initial distribution of 1CH2 formed on photolysis of CH2CO at 308 nm has been observed (see evidence referenced by Hancock and Heal10). The calculated ambient temperature (300 K) CIISC rate constants are in very good agreement with experiment for He, Ne, Ar, Kr, N2, and CH4 colliders. For the other gases (Xe, SF6, H2, D2, C2H6, and H2O), the calculated CIISC rate constants are less than the experimental values by up to a factor of 3. Because of certain simplifications, the model calculations of Bley and Temps34 are not as accurate for the molecular colliders as they are for the monatomics, and therefore these calculated CIISC rate constants are considered to be lower limits. For comparison with our experimental Arrhenius parameters, we have combined the ortho and para CIISC rate constants calculated by Bley and Temps34 for a bulk statistical sample of 1CH2 and fitted the resultant overall CIISC rate constants for Ar, N2, and CH4 colliders to the normal Arrhenius equation. The calculated Arrhenius A-factors are 0.17, 0.45, and 0.55 × 10-10 cm3 molecule-1 s-1, and the activation energies are 3.3, 4.0, and 4.7 kJ mol-1 for Ar, N2, and CH4,
J. Phys. Chem., Vol. 100, No. 27, 1996 11317 respectively. There is reasonable agreement with the experimental results listed in Table 2 for Ar and N2 colliders. Of course direct comparison is not possible for CH4 because the experimental values include reactive removal as well as CIISC. For reactive species the results uniformly indicate negative temperature dependences for the removal rate constants. The only reactants for which a non-negative temperature dependence has been observed are NO and H2, as reported by Hancock et al.9,10 The results for H2 reported by Wagener7 show a significant negative temperature dependence, but reasons for the difference between his results and those of Hancock et al.9,10 are not known. The Arrhenius A-factors are, in general, larger than those for removal by CIISC. Within the group of reacting species, there are no significant trends in the Arrhenius parameters although the parameters for the larger species are, in general, greater than those for the smaller species, NO, HF, and H2, as well as the relatively unreactive CH3F. The temperature dependences for the larger species may reflect the temperature dependences of multiple reaction pathways (e.g., 1CH reacts with alkenes predominantly through addition to the 2 double bonds, but it can also react via C-H insertion). We have shown previously15 that at ambient temperature the oxygencontaining species are more reactive than their hydrocarbon analogues, and this appears to be reflected in the temperature dependences as well. The positive temperature dependences observed for CIISC imply that the 1CH2 chemical reaction rate constants have to decrease with increasing temperature even more strongly than the removal rate constants. That is, the temperature dependences of the removal rate constants results from a partial compensation of the positive temperature dependence for CIISC and the negative temperature dependence for chemical reaction. Thus CIISC competes increasingly favorably with reaction as the temperature is raised. This has important consequences for modeling of combustion processes. In the case of NO and H2 the results of Hancock et al.9,10 indicate that there is almost full compensation of the opposing temperature dependences for CIISC and reactive removal. Given that removal due to CIISC is largely independent of the nature of the collision partner, this suggests that the temperature dependences for reactive removal by NO and H2 are weaker than for the other reactive substrates. Hancock and Heal10 indicate that ab initio calculations35 of a zero-activation energy barrier for 1CH2 insertion into H2 is in agreement with their results. The negative temperature dependence for reactive removal of 1CH2 is consistent with a mechanism involving formation of an intermediate complex which then undergoes decomposition or rearrangement. It is known that the reactions of 1CH2 with saturated hydrocarbons occur via insertion of the radical into the C-H bonds to form excited alkane adducts.36,37 For example 1
CH2 + CH4 f [C2H6]* f CH3 + CH3
Direct C-H insertion may also occur with alkenes, but the major reaction pathway is via addition to the double bonds.36,38 For example
CH2 + C2H4 f [c-C3H6]* f CH3CHdCH2
1
Of the other species shown in Table 2, reaction of 1CH2 with alcohols probably involves insertion into the O-H bond which is significantly more reactive than C-H bonds,15 and, as discussed by us previously,15 aldehydes and ketones, including formaldehyde, can react with 1CH2 to form CdO insertion products.39 This may occur via attachment of 1CH2 to the
11318 J. Phys. Chem., Vol. 100, No. 27, 1996 oxygen atom forming the [H2C-O-CH2]* intermediate complex in the case of formaldehyde or the [CH2C-O-CH2]* complex in the case of ketene, followed by rearrangement, or by direct insertion into the CdO double bond. Conclusions Absolute removal rate constants for singlet methylene, 1CH2 (a˜ 1A1) in the temperature range 298-462 K exhibit positive temperature dependences for removal by the nonreacting species, N2 and C2F6, and negative temperature dependences for removal by the reacting species, CH4, C2H4, C2H6, H2O, H2CO, CH2CO, CH3F, and CH3OH. The results for removal by CIISC are in general agreement with recent theoretical calculations. Although the temperature range in this study is below typical combustion temperatures, the results obtained do give a guide to trends in removal rate constants at higher temperatures. The scenario is that the removal rate for CIISC will rise with temperature while that for the reaction will fall. Thus even though reaction dominates the removal of 1CH2 by reactive substrates at ambient temperature, at higher temperatures (e.g., those reached in combustion systems) CIISC may start to compete favorably with chemical reactions provided the Arrhenius plots extrapolate to the higher temperatures without exhibiting curvature. That is, for reactive substrates the 1CH2 removal rate may change from predominantly reactive removal at ambient temperature to predominantly CIISC at higher temperatures. Since the chemical reaction pathways of 1CH2 and 3CH2 are different, this has important implications for the high-temperature chemistry. Acknowledgment. This work was supported by the Australian Research Council, Flinders University, and the University of Adelaide. W.S.S. is grateful for an Australian Postgraduate Research Award. The technical support provided by the Flinders University School of Physical Sciences electronic, mechanical, and glass-blowing workshops is gratefully acknowledged. We thank Dr. Gus Hancock (Oxford University) for the communication of results prior to publication and Professor John Barker (University of Michigan) for helpful discussions and comments on the manuscript. References and Notes (1) Homann, K. H.; Schweinfurth, H. Ber. Bunsen-Ges. Phys. Chem. 1981, 85, 569. Homann, K. H.; Wellmann, Ch. Ber. Bunsen-Ges. Phys. Chem. 1983, 87, 609. (2) Homann, K. H.; Warnatz, J.; Wellmann, Ch. Symp. (Int.) Combust. [Proc.] 16th 1977, 853. (3) Warnatz, J. Symp. (Int.) Combust. [Proc.] 18th 1981, 369. (4) Warnatz, J. Ber. Bunsen-Ges. Phys. Chem. 1983, 87, 1008. (5) Strobel, D. F. J. Atmos. Sci. 1973, 30, 489. Bossard, A.; Toupance, G. Nature 1980, 288, 243. Kuhn, W. R.; Atreya, S. K. Geophys. Res. Lett. 1977, 4, 203. (6) McKellar, A. R. W.; Bunker, P. R.; Sears, T. J.; Evenson, K. M.; Saykally, R. J.; Langhoff, S. R. J. Chem. Phys. 1983, 79, 5251. Jensen, P.; Bunker, P. R. J. Chem. Phys. 1988, 89, 1327. (7) Wagener, R. Z. Naturforsch. 1990, 45a, 649.
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