J . Phys. Chem. 1989, 93, 3107-31 12
3107
Computer Modeling of a Premixed Laminar Formaldehyde Flame Jim 0.OIsson,* Ingrid B. M. Olsson, Department of Physical Chemistry, Chalmers University of Technology, S-412 96 Gothenburg, Sweden
and Mitchell D. Smooke Department of Mechanical Engineering, Yale University, New Haven, Connecticut 06520 (Received: May 11, 1988; In Final Form: October 3, 1988)
A premixed laminar flame 17.9% CHzO and 82.1% 02,burning at 22.5 Torr and measured by Oldenhove et al. with a sampling cone/MBMS technique, was analyzed numerically by using two kinetic mechanisms. The experimental temperature profile at the sampling point, with the cone present, was used in the computations of species profiles. The first mechanism was based on the compiled work of Westbrook and Dryer (WD), but the rate constant of the reaction HCO M was increased by about a factor of 10, according to Warnatz in his compilation (1983). In the second Warnatz-type mechanism (WT) the rate constants for the 18 most important reactions were taken from compilations by Warnatz. The experimental maximum of HCO is significantly smaller than the computational maxima by a factor of 6 (WD) and a factor of 3 (WT). For H 0 2 the experimental maximum is higher than the computational maxima by a factor of 18 (WD) and a factor of 2 (WT). If the rate constant of the reaction HCO + 0, was increased, the HCO maxima decreased and the HO, maxima increased for both of the mechanisms. Use of the WT value for this rate constant, an increase about a factor of 10, in the WD mechanism gave similar results for these species as with the WT mechanism. IR the WT mechanism the default value 3.0 X lo1,, independent of temperature, increased by a factor of 3 yielded coincidence of the experimental and the computational HCO maxima but slowed down the computational profiles compared to the experiments. The measured temperatures at the sampling point of the sampling cone, and the known cold wall boundary conditions, allowed a unique simulation (WD mechanism) of the effect of the cone on the species profiles. For 22 sampling locations temperature and species profiles were computed, but only the concentrations at the actual sampling location were used in assembling a new series of species profiles. For CH20, 02,HCO, and H 0 2 the main influence of the cone is a broadening of the profiles increasing with increasing distance from the burner. The maximum of H202is decreased by a factor of 3. The species formed late in the flame, except H 2 0 , are clearly delayed and their maxima are reduced significantly.
+
Introduction Formaldehyde reactions are important in all hydrocarbon flames.I Consequently, an evaluation of kinetic mechanisms in such flames is of general interest. Unfortunately, only one accurate experimental study of these flames has been done. Oldenhove et aLz conducted a detailed experimental study of four different premixed laminar formaldehyde flames with equivalence ratios ranging from 0.15 to 1.00. They used a flat flame burner at 22.5-40.0 Torr and modulated molecular beam sampling, followed by mass-spectrometric analysis. Concentration profiles for species such as O,,CH20, HCO, HO,, CO, and CO, were presented. Recently, Vandooren et aL3 analyzed the most accurate (Van Tiggelen, personal communication) of these flames, C H 2 0 / 0 2 (17.9%,82.1%) burning at 22.5 Torr, using a classical kinetic approach. The importance of formaldehyde oxidation motivates detailed computer modeling of the flames investigated by Oldenhove et aL2 Furthermore, in our recent study4 of analogous methanol flamesS studied by the same research group using the same equipment, the experimental C H 2 0 H maxima were a factor 10-100 lower than the computational maxima. Possible explanations were either a rate constant for the reaction C H 2 0 H + 0, 10-100 times higher than in the mechanisms evaluated or a reduction of the CHzOH maxima induced by the sampling cone. By modeling the formaldehyde/oxygen flame, (17.9%,82.1%) burning at a pressure of 22.5 Torr, one could check the second possible explanation. For this flame the temperature profile at ( I ) Westbrook, C, K.; Dryer, F. L. Prog. Energy Combust. Sci. 1984, 10, 1. (2) Oldenhove de Guertechin, L.; Vandooren, J.; Van Tiggelen, P. J. J . Chim. Phys. 1983,80, 583. (3) Vandooren, J.; Oldenhove de Guertechin, L.; Van Tiggelen, P. J. Combust. Flame 1986, 64, 127. (4) Olsson, J. 0.;Olsson, I. B. M.; Andersson, L. L. J . Phys. Chem. 1987, 91, 4160. ( 5 ) Vandooren, J.; Balakhin, V. P.; Van Tiggelen, P. J. Arch. Combust. 1981, 1, 229.
0022-3654/89/2093-3 107$01.50/0
the center of the sampling volume of the cone, 0.3 mm below the cone tip2*6,7 was measured (see Figure l ) , allowing a simulation of the flame/cone interaction. The present study has the following aims: (1) to compute species profiles for the formaldehyde/oxygen flame (17.9%,82.1%) at 22.5 Torr measured by Oldenhove et aL2 using mechanisms based on compilations by Westbrook-Dryer-Schugh8 and Warnatz: (2) to determine the most important reactions in the kinetic mechanism by sensitivity analysis, and to evaluate parts of the formaldehyde mechanisms sensitive to the experiments, and (3) to model the influence of the cone tip and the cold wall on which the cone is mounted on this flame.
Numerical Method In this study of burner-stabilized premixed laminar flat flames we make the normal assumptions (idealizations). The premixed ideal gas flows with a constant mass flow through a wide porous burner, inducing a laminar flow in the gas. The steady flame burns at a constant pressure with no temperature or concentration gradients parallel to the burner; Le., the flame is one-dimensional.1° Computations of Species Profiles and Sensitivity Analysis. Two different flame codes have been used: (1) The timedependent code developed by Andersson and Ol~son"-'~for analyzing experimental flames with a known temperature profile and (2) the (6) Revet, J. M.; Puechberty, D.; Cottereau, M. J. Combust. Flame 1978, 33, 5. ( 7 ) Cattolica, R.J.; Yoon, S.; Knuth, E.L.Combust. Sci. Technol. 1982, 28, 225. (8) Westbrook, C. K.; Dryer, F. L.; Schug, K. P. Symp. (In?.) Combust. [Proc.],19fh 1982 1982, 153. ( 9 ) Warnatz, J. In Combustion Chemistry; Gardiner, W. C. Jr., Ed. Springer: New York, 1984. (10) Dixon-Lewis, G. In Combustion Chemistry; Gardiner, W. C. Jr., Ed. Springer: New York, 1984. (11) Olsson, J. 0.;Andersson, L. L. J . Comput. Phys. 1985, 59, 369. (12) Andersson, L. L.;Olsson,J. 0 .Combust. Sci. Technol. 1986,46,95. Karlsson, L. S.; Andersson, L. L. J. Phys. Chem. 1986, (13) Olsson, J. 0.; 90, 1458.
0 1989 American Chemical Societv
3108 The Journal of Physical Chemistry, Vol. 93, No. 8, 1989
Olsson et al.
TABLE I”
Westbrook et al.
+ +
reaction
+ +
1. H 0 2 = 0 OH 2. H2 O = H OH 3. H20 0 = OH OH 4. H2O H = H2 OH 5. H202 OH = H2O H02 6. H20 M’ = H OH M‘ 7 . H 0 2 M’ = HO2 M’ 8. H02 0 = OH 0 2 9. H02 H = OH OH 10. H02 H = H2 0 2 11. HO2+ OH = H2O + 0 2
+
+ + + + + + + + + + + + + + + 12.H202 + 0 2 = H02 + H02 13. H202 + M’ OH + OH + M’ 14.H202+ H = HO2+ H2 15. 0 + H + M’ = O H + M’ 16. 0 2 + M’ = 0 + 0 + M’ 17. H2 + M’ = H + H + M’ 18. CO + OH = C02 + H 19. CO + H02 = CO2 + OH 20. CO + 0 + M’ = C02 + M’ 21. co2+ 0 = co + o2 22. HCO + OH = CO + H20 23. HCO + M’ = H + CO + M’ 24. HCO + H = CO + H2 25. HCO + 0 = CO + OH 26. HCO + H02 = CH2O + 0 2 27. HCO + 0 2 = CO + H02 28. CHZO + M = HCO + H + M 29. CH2O + OH = HCO + H2O 30. CH2O + H = HCO + H2 31. CH2O + 0 = HCO + OH 32. CH20 + H02 = HCO + H202 + +
log A 14.27 10.26 13.53 13.98 13.00 16.34 15.22 13.70 14.40 13.40 13.70 13.60 17.08 12.23 16.00 15.71 14.34 7.1 1 14.18 15.77 12.44 14.00 14.16 14.30 14.00 14.00 12.60 16.52 12.88 14.52 13.70 12.00
b 0.00 10.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 I .30 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Warnatz type log A
b
17.08 7.18 10.18 8.66
-0.91 2.00 1.14 1.60
16.51 7.55 17.24 18.55
105.00 -1 .oo 1.oo 1.90 0.70 1.oo 42.64 45.50 3.75 0.00 1 15.00 96.00 -0.77 23.65 4.10 43.83
16.39 18.30
0.0 -0.80
114.24 0.00
14.18
0.00
1 .oo
13.30
0.00
0.00
6.64
1.so
-0.74
0.00
13.70 14.85 14.30 13.48 14.00 12.48
0.00 0.00 0.00 0.00 0.00 0.00
0.00 16.81 0.00 0.00 3.01 0.00
13.48 13.40 13.54
0.00 0.00 0.00
1.19 3.99 3.51
E, 16.79 8.90 18.35 20.30
E*
1.80
19.00
0.00 0.00 3.00 7.00 8 1.OO 0.17 10.50 4.60
8.00
“The first mechanism used in this study is a subunit of the mechanism compiled by Westbrook et aL8 and their numbering is used. The units are in em3,mole, s, kcal. The most important change is that we use a 10 times higher value for the rate constant of reaction HCO + M’ (23) taken from the review by warn at^.^ Furthermore, the rate constants for the third-body reactions are changed. The description of Dove and Warnatz’* for third-body efficiencies in H2/02reactions and in HCO + M’ was chosen. For the second Warnatz-type mechanism the values for the most important reactions were taken from Dove and Warnatz” if given or alternatively from the review by Warnatz9 for reactions 6 and 25. combined time-dependent/Newton iteration method developed by Smooke and ~ o - w o r k e r s . ~The ~ , ~second ~ code has the ability to calculate temperature profiles as well as adiabatic flame speeds. The first code was used in most of the computations, including sensitivity analysis, and it provided starting values for the second code. The temperature profile at the center of the sampling volume was used. This gives a first-order approximation of the effect of the sampling cone on the measured profiles. The second code was used to investigate, in more detail, the effect of the sampling quartz cone and the associated cold wall (see below). In the flame analysis the species profiles are computed as functions of the distance from the burner. Multicomponent diffusion models were employed and the Lennard-Jones parameters used in the computation of the diffusion coefficients were taken from Kee et In both codes, we used the measured mass flow as an input parameter. The flame codes have been extensively calibrated against each other. I n the sensitivity analysis” we start with a reference solution at steady state and then proceed to perturb each rate constant in turn keeping the equilibrium constant fixed. For each perturbation the system relaxes to a new steady state with a relaxation time depending on the degree of the perturbation and the importance of the perturbed parameter. The computer program continues the computations until approximately the same time gradient
M. D. J . Comput. Phys. 1982, 48, 72. (15) Smooke, M. D.; Miller, J. A,; Kee, R. J. Combust. Sci. Techmi. 1983,
(14) Smooke,
34, 79. (16) Kee, R. J.; Warnatz, J.; Miller, J. A. Sandia National Laboratories Report, SAND80-8003, 1983. ( 1 7 ) Olsson, J. 0.;Anderson, L. L. Combusi. Flame 1987, 67, 99.
is reached in the perturbed state as in the initial reference state. The changes in the concentration profiles provide a sensitivity measore for the reaction rate constant perturbed. For each species i we compute sensitivities for a reaction rate 0‘)
where the mass fraction &,ne,,, symbolizes the new steady state and &,ref symbolizes the reference state. The sum over all species gives a total sensitivity measure for the system for reaction rate
0’).
Simulation of the Flame/Cone Interaction. In this study, the code of Smooke et al. was used in a new way to simulate the influences of the sampling cone, on the experimental profiles. The influence of the water-cooled burner, the sampling cone, and the associated cold wall on the experimental species profiles was modeled by restricting the computed temperature in two ways. First, the temperature at the center of the sampling volume (see Figure l ) , taken from the experiments, was fixed at its position z cm above the burner surface. Second, the temperature of the cold wall on which the cone is mounted 3.0 cm + z cm above the burner surface, was set to 300 K. For 22 sampling positions z of the cone tip the species profiles of the flame were computed from the burner surface up to a distance of 1.O cm from the burner. The species concentrations for these calculations at the 22 sampling locations were then assembled to provide a new set of species profiles. Chemical Kinetics Mechanisms Two different kinetic mechanisms were used. The first mechanism is a modified subset of the mechanism developed by Westbrook et a1.* The mechanism (see Table I) consists of 12 species and 32 elementary reversible reactions. The species can be divided into three groups: (a) H-0 species H, 0, H2, H 2 0 ,
The Journal of Physical Chemistry, Vol. 93, No. 8, 1989 3109
Premixed Laminar Formaldehyde Flame
I 2000.
NO CONE
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0.5 1 .o D I S T A N C E (CM) Figure 2. The flame C H 2 0 / 0 2(17.9%,82.1%) at 22.5 Torr. Illustration
0.0
of comparisons between computational and experimental (Oldenhove et a1.: long dashes) profiles of (a, top) C H 2 0 and C02, (b, bottom) O2and H20, using the temperature profile at the cone tip directly (see Figure 1). The first mechanism (solid line) is from Westbrook et al. (WD),8 but the rate constant of reaction HCO M and the third body (M') efficiencies in H 2 / 0 2 reactions and in HCO M' are according to Warn a t ~ The . ~ second mechanism (short dashes) is a Warnatz-type mechanism (WT).
+
.I
+
01.0 x0.8
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A
_n
:\PCO*IOO
1
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Figure 3. Comparisons of HCO (a, top) and H 0 2 (b, bottom) profiles computed with the WD mechanism (solid lines) and WT mechanism (short dashes) and measured experimentally by Oldenhove et aL2 (long dashes). Same conditions as in figure 1.
(18) Dove, J. E.; Warnatz, J. Ber. Bunsen-Ges. Phys. Chem. 1983,87, 1040.
maxima are a factor 1.1 less (WT) and 1.8 higher (WD) than the experimental maxima. The experimental maximum of C O is slightly lower than the computational maxima for both the mechanisms. A sensitivity analysis was performed by varying the input conditions and by increasing and decreasing reaction rates up to
Olsson et al.
3110 The Journal of Physical Chemistry, Vol. 93, No. 8, 1989
I
HCO+M 1001
v;
1 1 HC: 0 + H
I
HC0+02
100,
v;
80
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10 15 20 25 30 35 R E A C T I O N NUMBER
1
HC0+02 Z v;
5
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15 20 2 5 30 35 REACTION NUMBER 10
,,,,,/,
Z
'
I,,
40
~
I
I
2ol 0
0
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15 20 25 30 35 REACTION NUMBER 10
Figure 4. Reaction rate sensitivity of HCO (a, top) and H 0 2 (b, bottom) (see text) with the WD mechanism; the numbering is from Westbrook et aL8 The rate constants were multiplied by 3 for each reaction in turn
and the system was allowed to relax to a new steady state. The results are given as a percent of the sensitivity due to the most important reaction and are for the same conditions as in Figures 1 and 2. a factor 3. (This is the maximum uncertainty for all the reaction 4 and 5 show sensitivity rates in the used m e ~ h a n i s m . ~Figures ) spectra for the species H C O and H 0 2 for the WD and W T mechanisms, respectively. The H C O and H 0 2 profiles were studied in detail when the rate constants for the reactions dominating their sensitivity spectra, CO O H (18), HCO O H (22), HCO + M (23), HCO + 0 (25), and HCO + O2 (27) (see Table I), were changed by a factor of 3. The H 0 2 and H C O maxima changed less than 5% as a result of changes in the rate constants for reactions 18, 22, 23, and 25. However, an increase in the rate constant of reaction 23 moved the profiles down stream (toward the burner) as expected from previous studies.I7 For the reaction H C O + O2 (27), approximate relationship was found between increases of its rate constant and decreases and increases of the HCO and the HOz maxima, respectively. It was concluded that the rate constant (27) used in the WD mechanism 4.0 X 10I2exp(-3523/T) was the main reason for the discrepancies between the computed maxima for H C O and HOz using this mechanism and the corresponding experimental maxima (see above). A separate computation using the same rate constant for reaction (27) as in the W T mechanism was also performed. (Note that the important rate constant for the reaction HCO + M (23) in the WD mechanism in this study was modified by taking the same value as in the W T mechanism originating from the Dove-Warnatz mechanism.) The new computed profiles were moved upstream (slowed down) about 0.5 mm and the agreement with experiments concerning H C O and H 0 2 was improved considerably. The new computed H C O and H 0 2 maxima were a factor of 2.1 higher and a factor of 2.7 smaller than the corresponding experimental maxima, respectively. This changes the computed ratio of H 0 2 / H C 0 maxima from 0.49 to 9.6 to compare with the corresponding experimental ratio 53.3. In the original determination by Westbrook et the rate constant of reaction 27 was determined at 1100 K together with the rate constant of reaction 23. This work indicates that the rate constants of the reactions 23 and 27 determined in that work were too low about a factor of IO but the ratio of them was more reasonable.
+
+
(19) Westbrook. C. K.; Creighton. J.; Lund, C.; Dryer, F. L. J . Phys. Chem. 1977, 81, 2542.
0
,
5
-_A
10 15 2 0 25 3 0 3 5 REACTION NUMBER
Figure 5. Reaction rate sensitivity of HCO (a, top) and H 0 2 (b, bottom) (see text) with the WT mechanism; the numbering is from Westbrook et aL8 The rate constants were multiplied by 3 for each reaction in turn and the system was allowed to relax to a new steady state. The results
are given as a precent of the sensitivity due to the most important reaction and are for the same conditions as in Figures 1 and 2. In the WT mechanism the default value (27) 3.0 X 10l2exp(-O.O/T) increased by a factor of 3 gave coincidence of compu-
tational and experimental H C O maxima, but slowed down the computational profiles compared to the experimental ones. These experimental maxima are only slightly affected by the cone according to the simulation below. Interestingly, the rate constant 9.0 X 10l2 is a factor of two less at 1100 K than the value from expression 2.7 X lOI3 exp(dOO/ r ) determined by Vandooren et al.3 Recent studies have yielded some room temperature data for the reaction HCO O2 CO H02;Temps and WagneP have reported k = (3.1 f 0.6) X lo1*cm3/(mol-s) at 298 K. Langford and Moore2' determined k = (2.8 f 0.3) X 10l2cm3/(mol-s) at 298 K. Both these values agree well with the values reported previously by other scientists at 300 K in Warnatz's re vie^.^ Effect of the Sampling Cone on the Species Profiles. We modeled the flame using the experimental mass flow for different positions of the sampling cone. The temperature and species profiles were computed with fixed temperatures at the center of the sampling -iolume, 0.3 mm below the cone tip, and the cold boundary (see above). The computations presented here were performed with the modified WD mechanism. Figure 6 shows the computed species profiles for C H 2 0 and H C O as a function of the height above the burner for different locations of the tip of the sampling cone. Figures 7 and 8 illustrate computed species profiles, produced by combining the calculated values for the 22 sampling locations. The influence of the sampling cone on the species profiles, in this study and according to our simulation, depends on the species studied. First of all, we found no effect at the sampling point of the cold wall 3 cm away. The changes in the profiles began about 1.5 cm from the cold wall. Consequently, the effects of the cone given below are attributed to the local effect induced by the cone at the sampling point. The formaldehyde profile has a small tail starting at a level of 10% of the cold flow mole fraction. Oxygen is more affected due to the excess of oxygen in the flame and the
+
-
+
(20) Temps, F.; Wagner, H. Gg. Ber. Bunsen-Ges. Phys. Chem. 1984,88, 410. (21) Langford, Andrew 0.;Moore, C. Bradley J . Chem. Phys. 1984,80, 4211.
Premixed Laminar Formaldehyde Flame
The Journal of Physical Chemistry, Vol. 93, No. 8, 1989 3111
El.0 "2.01
0.0
01.01
0.5 D I S T A N C E (CM)
I
1.o
0.0
0.5 D I S T A N C E (CM)
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HCO*100
"0.81
L0.6 6
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=,%n
I
y0.51
60.2. .
0.0
0.5 D I S T A N C E (CM)
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Figure 6. The flame CH20/02(17.9%,82.1%)at 22.5 Torr. Illustration of computed profiles of CH20(a, top) and HCO (b, bottom) (solid lines) at different distances from the burner,of the tip of the sampling cone (see text). The mechanism (WD) is from Westbrook et ai.,* but the rate constant of reaction HCO + M and the third body (M') efficiencies and HCO + M are according to warn at^.^
0.0
0.5 D I S T A N C E (CM)
I
0.0
1.o
I
0.5 D I S T A N C E (CM)
1 .o
Figure 7. The flame CH20/02(17.9%,82.1%)at 22.5 Torr. Illustration of computed (solid lines) and experimental (broken lines, Oldenhove et aL2) profiles of CH2O and C02 (a , top), O2 and H20 (b, bottom), simulating the influence of the sampling cone. The profiles are analogous to those in Figure 6.
later consumption. The species H 2 0 2 was strongly affected by the introduction of a cone (or other effects on the temperature). Its maximum decreased a factor of 3. The maxima of the intermediate species H 0 2 and H C O are only slightly affected, but their decreases were significantly affected. The computational maximum of H2 is increased by a factor of 2 due to the introduction of the cone in the simulations. The computations (WD) produce a maximum 2.8 times higher than
0.0
0.5 D I S T A N C E (CM)
1.o
Figure 8. Illustration of computed (solid lines) and experimental (broken lines, Oldenhove et aL2) profiles of HCO (a, top), H 0 2 and 0 (b, bottom), simulating the influence of the sampling cone. The profiles are analogous to those in Figure 6 .
the experimental value. The water profile is slightly decreased and the maximum is somewhat lower compared to the corresponding profile without a sampling cone. The computational profile of C O has no distinct maximum and the level is 3 times lower than the experiments. For COzthe increase is delayed and the plateau is decreased by a factor of 3 compared to the corresponding profile without a sampling cone. Similar marked effects are found for the species 0, H, and OH. These decreases of the maxima of these species have not been documented experimentally as far as we know. How representative is the present cone simulation for other flames? This is an important question as MBMS studies have been shown to be the most important technique, hitherto, for analyzing flames. Therefore, experimental studies using optical spectrometric techniques to investigate the effect of a sampling cone on the sampled concentration profiles are important. In the past there have been only a few such studies. First, Revet et aL6 used optical absorption spectrometry to measure the O H radical in a lean propane/oxygen flame at 25 Torr. In 1981 the same research group, Stepowski et a1.,22used laser-absorption spectrometry and laser-induced fluorescence to measure the OH radical for the same conditions. They found a significant delay for O H profiles from sampling cone/MBMS measurements in relation to optical measurements. Cattolica et a].' measured OH in a stoichiometric methane-air flame at atmospheric pressure. The O H profile from sampling cone/MBMS measurements was strongly delayed compared to the profiles from laser-induced fluorescence and laser absorption, respectively. Both Revet et aL6 and Cattolica et a].' found that optical measurements and the sampling/cone MOMS measurements gave the same maxima for OH, contrary to our simulation. They used partial equilibrium assumptions in the burned gas region about OH and other species in a similar or nearly similar flame to calculate the OH level from the sampling cone/MBMS measurement. However, a flame with a sampling cone inserted is hardly in partial equilibrium with regard to OH and related species. Smith and Chandler23 used laser-induced fluorescence to study the effect of a sampling cone, but without a mass spectrometer, on the C N profiles in a nearly (22) Stepowski, D.; Puechberty, D.; Cottereau, M. J. Symp. (Int.) Combust., [Proc.], 18th 1980 1981, 1567. ( 2 3 ) Smith, 0. I.; Chandler, David W. Combust. Flume 1986, 63, 19.
3112
J . Phys. Chem. 1989, 93, 3 1 12-3 1 17
stoichiometric hydrogen-oxygen-argon flame doped with H C N at 25 Torr. The C N maximum moved away from the burner following the cone. The C N concentration was also reduced up to 48% near the position corresponding to a sampled maximum but with a decreasing reduction at higher distances from the burner. The total effect on a sampling cone/MBMS measurement will be a decreased C N maximum, moved away from the burner, and with a profile showing a tail toward higher temperatures. In our simulation of the effect of a sampling cone on the HCO profile profile), analogous to the C N profile above in many (or the H20z respects, we found similar effects.
For CH20, O,, HCO, and HO, the influence of the cone results in a broadening of the profiles increasing with increasing distance from the burner. However, the maximum of H 2 0 2is decreased a factor of 3. The species formed late in the flame, except H 2 0 , are clearly delayed and their maxima are reduced significantly.
Acknowledgment. This work was supported financially by the Swedish National Research Council and the Swedish National Board for Technological Development. Registry No. CH20, 50-00-0; H202, 7722-84-1; H 2 0 , 7732-18-5; HCO, 2597-44-6; H02, 3170-83-0; 0 2 , 7782-44-7.
Formation and Decay of Exclplexes between 9-Cyanophenanthrene and Mono- and Diaminoakanes Siegfried Schneider,* Peter Geiselhart, Giinter See], Institut fur Physikalische und Theoretische Chemie, Technische Universitat Munchen, Lichtenbergstrasse 4, 08056 Garching, West Germany
Frederick D. Lewis,* Ruth E. Dykstra, and Marshall J. Nepras Department of Chemistry, Northwestern University, Evanston, Illinois 60208 (Received: July 1, 1988; In Final Form: September 12, 1988)
The interaction of singlet 9-cyanophenanthrene with mono- and diaminoalkanes in hexane solution yields weakly fluorescent exciplexes. Whereas the fluorescence quenching constant for 9-cyanophenanthrene is nearly the same for all mono- and diamines (1.5 X 1Olo M-' s-l), the lifetime of the exciplexes appears to be highly dependent upon the diamine chain length. The pronounced shortening of the exciplex lifetimes from about 15 ns to 300 ps for diamines with less than five methylene groups is interpreted in terms of the formation of a triplex between the 9-cyanophenanthrene anion radical and a a-bonded amine dimer cation radical.
Introduction The formation of diamine monocation radicals from conformationally restricted diamines is well documented.' However, little information is available concerning the formation of diamine cation radicals from conformationally mobile acyclic diamines or upon the reaction of a monoamino cation radical with a neutral amine. We recently proposed the formation of diamine cation radicals upon quenching of stilbene-amine exciplexes by ground-state amines in order to account for the pronounced stereoelectronic effect observed for the rate constant for exciplex quenching (Scheme I).* We also proposed the formation of monocyclic diamine monocation radicals upon quenching of singlet stilbene by N,N,N',N'-tetramethyl-a,w-diaminoalkanes capable of forming a cyclic diamine monocation radical without the introduction of significant ring strain. However, the kinetics of this process could not be investigated due to the absence of exciplex fluorescence. The quenching of singlet arenes by a p d i a m i n e s has been extensively investigated by Davidson and c o - w ~ r k e r s .The ~ exciplex fluorescence observed from singlet naphthalene with diaminoethanes and propanes is weak and red-shifted compared to that obtained with monoamines. These changes were attributed (1) Alder, R. W.; Arrowsmith, R. J.; Casson, A.; Sessions, R. B.; Heilbronner, E.; Kovac, B.; Huber, H.; Taagepera, M. J. Am. Chem. SOC.1981, 103, 6137. (2) (a) Hub, W.; Schneider, S.;Dorr, F.; Oxman, J. D.; Lewis, F. D. J. Phys. Chem. 1983.87, 4351. (b) Hub,W.; Schneider, S.; Dorr, F.; Oxman, J. D.; Lewis, F. D. J. Am. Chem. Soc. 1984, 106, 701. (3) (a) Beecroft, R. A.; Davidson, R. S.; Whelan, T. D. J. Chem. SOC., Chem. Commun. 1978,991. (b) Beecroft, R. A.; Davidson, R. S.; Goodwin, D.; Pratt, J. E. Pure Appl. Chem. 1982, 54, 1605. (c) Davidson, R. S.; Wheelan, T. D. J. Chem. SOC.,Perkin Trans. 2 1983, 241.
0022-3654/89/2093-3 1 12$01.50/0
SCHEME I 'A*
+
R3N
'(A.-R3N'*)*
-
'(A*-R3N+'-"R3)* A
+
2R3N
to the formation of a fluorescent triplex in which both amino groups interact with the singlet arene. Red-shifted fluorescence attributed to arene-diamine triplexes has also been reported for several covalently linked arene-diamine trichromophoric molecules with two or three methylenes linking the two amine nitrogem.& In order to obtain further information about the nitrogennitrogen interaction in singlet arene-diamine complexes, we have undertaken an investigation of the kinetics of exciplex formation and decay upon quenching of several singlet arenes by trialkylamines and diamines of variable chain length. The previously investigated stilbene2 and naphthalene3 systems proved to be less than ideal for this purpose due to the short lifetime of singlet stilbene and the highly reversible nature of naphthalene-amine exciplex f ~ r m a t i o n . ~We ~ * selected 9-cyanophenanthrene (CP) for this investigation on the basis of highly exergonic electrontransfer quenching by trialkylamines, the formation of fluorescent exciplexes with both mono- and diamines, and the low quantum yields for chemical reactions in nonpolar solvent (
