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The effect of temperature on collision induced intersystem crossing in the reaction of 1CH2 with H2
Proceedings of the
Combustion Institute
Proceedings of the Combustion Institute 30 (2005) 927–933
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The effect of temperature on collision induced intersystem crossing in the reaction of 1CH2 with H2 Mark A. Blitz, Namil Choi, Tama´s Kova´cs, Paul W. Seakins, Michael J. Pilling* Department of Chemistry, University of Leeds, Leeds LS2 9JT, UK
Abstract Laser flash photolysis of ketene at 308 nm, coupled with H atom vacuum ultraviolet laser induced fluorescence, was used to determine the branching ratio for the CH3 + H channel (1a) in the reaction of CH21A1 (1CH2) with H2, over the temperature range 300–500 K. This reaction channel competes with collision induced intersystem crossing (CIISC) to form triplet methylene, CH23B1 (3CH2) (channel 1b). The branching ratio for H formation, k1a/k1, was determined by measuring the relative H atom yield in three time resolved measurements of H: (i) in ketene, H2 mixtures, where H is exclusively formed by reaction 1a, (ii) in ketene, H2, NO mixtures ([NO] [H2]), where H is formed at short times by 1a and at longer times by 3CH2 + NO, following 1b, and (iii) in ketene, He, NO mixtures ([NO] [He]), where H is exclusively formed from 3CH2 + NO, following deactivation of singlet to triplet methylene by He. k1a/k1 was found to increase from 0.85 at 300 K to unity at 500 K, with the yield of CIISC decreasing from 0.15 to zero. This is the first measurement of the temperature dependence of the rate coefficient for CIISC in a reactive system. The rate coefficient for CIISC with an inert gas increases with T. It has been suggested that the fractional yield of CIISC will increase with temperature in reactive systems, thus reducing the rate coefficient for reaction at high temperature, with significant consequences for combustion systems. The present experiments demonstrate that this is not the case for reaction with H2 and implies a different CIISC mechanism for reactive vs inert collision partners. 2004 Published by Elsevier Inc. on behalf of The Combustion Institute. Keywords: Methylene; Hydrogen; Branching ratio
1. Introduction Methylene plays a central role in the combustion of hydrocarbons, and its kinetics have been extensively studied [1]. The ground triplet state CH2X 3B1(=3CH2) and the first singlet state a1A1 (=1CH2) are separated by only 38 kJ mol1
*
Corresponding author. Fax: +44 113 343 6401. E-mail address: [email protected] (M.J. Pilling).
[2]. The triplet state is generally less reactive than the singlet state, especially with closed shell molecules such as H2. While the triplet does react with open shell molecules, such as O2 and NO, the rate coefficients for these reactions are still about an order of magnitude smaller than those for the singlet. The two states are also collisionally coupled, and a proper description of this coupling is essential in combustion models, given the lower reactivity of the triplet. Collision-induced intersystem crossing (CIISC) between the two states [3,4] can be effected by inert gases via a small number of
1540-7489/$ - see front matter 2004 Published by Elsevier Inc. on behalf of The Combustion Institute. doi:10.1016/j.proci.2004.08.073
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Ôrotational gatewayÕ states with mixed singlet triplet character. The function of the collision is to rotationally relax the singlet and triplet states, to ensure an irreversible transition to the energetically lower spin state. Detailed balance ensures that this process also operates in reverse and provides a mechanism for conversion of the less reactive triplet to the more reactive singlet at the elevated temperatures encountered in combustion. Conversion of the singlet to the triplet also occurs in collisions with reactive gases, albeit with comparatively low probability. It has generally been assumed that this process is similar to that occurring with inert gases and has similarly been described as CIISC. Examples of such CIISC fracþ0:10 tional channel yields include 0:100:04 for 1 1 CH2 + H2 [5], 0.1 for CH2 + CH4 [3,6], and 0.22 for 1CH2 + C2H2 [7]. Parallel measurements for 1CH2 + O2, by contrast, have demonstrated unit efficiency for CIISC [8,9]. To date, no measurements on CIISC in reactive systems have been made other than at room temperature. However, there have been measurements of the temperature dependence of 1 CH2+Ar, He, and N2 [4,10,11], for which the CIISC yield is unity. The rate coefficients all increase with temperature; for example, for 1 CH2 + Ar kCIISC increases from 5 to 15 · 1012 cm3 molecule1 s1 between 295 and 860 K [4]. Bley and Temps [3] and Hancock and Heal [4] have analysed this dependence using the mixed state model and ascribed the increase to the temperature dependence of rotational relaxation and to increased population of the mixed states at higher energies at elevated temperatures. Bley and Temps contrasted the positive temperature dependence of CIISC with the negative temperature dependence of the overall rate constant for 1CH2 + CH4 and argued that, if the mixed state model applies to this reaction, then the fractional yield of CIISC will increase with temperature and could become the major channel at combustion temperatures. Beasley and co-workers [12] have discussed the implications of such behaviour for 1CH2 + C2H2, for which the overall rate constant again shows a negative temperature dependence (k = 5.1 · 108 T0.9 cm3 mole1 1 cule s ). It has been suggested that this reaction plays a key role in soot formation through production of C3H3 + H [13]; any significant increase in CIISC with temperature would seriously limit the yield of C3H3 under combustion conditions. There is a clear need for experimental investigations of the temperature dependence of the yield of 3CH2 in reactive systems. In this paper, we report such a measurement for 1CH2 + H2, using LIF measurements of the yield of H. The system is calibrated against the yield of H from 3 CH2 + NO, following formation of the triplet by CIISC.
2. Experimental All experiments were carried out in a six-way cross stainless steel, slow-flow, laser flash photolysis, laser induced fluorescence (LIF) system [14]. Ketene was photolysed at 308 nm using an excimer laser (Lambda Physik, LPX 105), which typically generated 100 mJ/pulse. H atoms were monitored by vacuum ultraviolet (VUV) LIF using the Lyman-a transition (121.6 nm) by frequency tripling 365.8 nm radiation (produced by mixing infrared with the dye fundamental (Continuum/Sirah, 20 mJ/pulse)) in a krypton and argon mixture (Kr:Ar = 1:2.5). 365.8 nm light was focused (f/s = 30 cm) into a glass cell attached directly to the cell containing 800 Torr of Kr/Ar. Lyman-a radiation was coupled into the cell through a MgF2 window. Fluorescence was detected perpendicularly to both laser axes by a solar blind PMT (Electron Tube). The power of the VUV output was monitored by a second solar blind PMT (Thorn EMI). Some of the probe light, upon exiting the reaction cell, was reflected using a quartz flat at 45 through a VUV interference filter (Acton Research) and onto the second PMT. This resulting signal was used to normalise the VUV LIF signal taking into account any temporal power fluctuations of the probe laser and attenuation by absorption. H atom time profiles were obtained by varying the time delay between the photolysis and probe lasers. Fluorescence was collected via a boxcar integrator, digitised, and stored on a PC for subsequent analysis. Typically, each trace was recorded over 0–1500 ls using ca. 1000 points. Sets of three traces were recorded with H monitored (1) in the presence of H2, (2) in the presence of H2 and a small amount of NO (<1014 molecule cm3) and (3) in the presence of He and a small amount of NO. The total pressure and ketene concentration for the three sets of traces was constant. The pressure was varied over the range 5–100 Torr and the temperature over the range 295–500 K. CH2CO was prepared by cracking acetic anhydride at 873 K [15], purified by trap-to-trap distillation at 77 K and identified from its IR and UV spectra. H2 and He (BOC > 99.999%) were directly used from cylinders. NO (99.9%, Air Products) was purified by trap-to-trap distillation until it was a white solid, and then diluted in He and stored in a glass bulb. Pressures were measured using capacitance manometers (MKS). The cell temperature was measured with type K thermocouples; variations between the two thermocouples were never more than ±5 K. The output of the thermocouples was fed into a home-built temperature control box to regulate the temperature of the cell. The gases were flowed through calibrated mass flow controllers, via a mixing manifold and into the cell. Total gas flow rates were
M.A. Blitz et al. / Proceedings of the Combustion Institute 30 (2005) 927–933
of the order of 5 SLM at 100 Torr which was sufficient to ensure the complete replenishment of gas in the reaction zone between photolysis pulses. The total pressure in the cell was controlled with a needle valve between the cell and vacuum pump, and the pressure was measured with a Baratron pressure gauge (MKS 0– 100 Torr).
3. Methodology
CH2 þ H2 !H þ CH3
ðR1aÞ
3
! CH2 þ H2
ðR1bÞ 1
[H2] was sufficiently high that CH2 reacted exclusively with H2 with H atoms formed essentially instantaneously on the timescale of our experiment. Subsequently, the H atoms were lost via diffusion from the monitoring zone, with first order rate constant kd. In the second experiment, a small concentration of NO (4 mTorr) was added to the H2/ketene mix, so only a very small fraction (<0.001) of the 1CH2 was removed via reaction with NO. 3 CH2, formed by CIISC (R1b), reacts with NO 3
CH2 þ NO ! H þ other products
ðR2Þ
Fikri et al. [19] have determined a value of 0.9 for the yield, Yield2H, of H from this reaction at room temperature. While they did not measure the temperature dependence, their calculations indicate that Yield2H may slightly increase with temperature, giving a T dependent yield in the range 0.9–1.0. Finally, in the third experiment, the hydrogen was replaced with helium, 1
CH2 þ He ! 3 CH2 þ He
nentials since (R1a), (R1b) and (R3) have time constants of 2 ls–30 ns compared with the experimental timescales of 0.1–1 ms. Thus, the formation of 3CH2 from 1CH2 via reactions 1 and 3 can be treated as instantaneous. In addition, the time constant for diffusive loss is 2–5 ms, so that kd can be neglected in comparison with k1[H2] and k3[He] in the analysis. With these simplifications, the expressions for [H](t) in the three experiments are: Experiment 1 (E1)
The 308 nm photolysis of ketene leads to the almost exclusive formation of 1CH2, with only a small fractional yield, P, of 3CH2 at this wavelength (P = [3CH2]0/[1CH2]0 = 0.06 ± 0.02 [16–18]). The method we adopt is based on the formation of H in reactions of CH2, and H detection by LIF. A reaction of 3CH2 with known H atom yield is needed to link the LIF signal to the photolysis yield of CH2. For each total pressure, three different experiments were carried out. The first experiment involved only ketene (2–3 mTorr) and hydrogen. 1CH2 was lost with hydrogen via either reaction, forming CH3 + H (R1a), or CIISC (R1b) 1
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ðR3Þ
consequently all the 1CH2 was relaxed to 3CH2 and titrated to H via reaction (2). In all of these experiments, it was necessary to account for the small yield of 3CH2 [16–18] in the photolysis of ketene at 308 nm. The time dependent H atom profiles are tri-exponential, but can be approximated to bi-expo-
½H ¼
k 01a 1 ½ CH2 0 expðk d tÞ: k 01
Experiment 2 (E2) ½H ¼
k 01a 1 ½ CH2 0 expðk d tÞ k 01 k0 k 02 ½1 CH2 0 þ Yield 2H 1b 0 0 k 1 ðk 2 k d Þ k0 expðk d tÞ 0 1 0 expðk 02 tÞ k1 k2 0 1 Yield 2H k 2 P ½ CH2 0 þ k 02 k d expðk d tÞ expðk 02 tÞ :
Experiment 3 (E3) k0 ½1 CH2 0 ½H ¼ Yield 2H 0 2 ðk 2 k d Þ k0 expðk d tÞ 0 3 0 expðk 02 tÞ k3 k2 Yield 2H k 02 P ½1 CH2 0 þ k 02 k d expðk d tÞ expðk 02 tÞ ; where k 01 ¼ k 1 ½H2 , k 03 ¼ k 3 ½He, and [1CH2]0 is the zero time concentration of singlet methylene. In E1, the pre-exponential term gives [H] at the effective zero time of Experiment 1 and is equal to [H] formed from reaction 1a. The first term in E2 has the same interpretation. The second and third terms in E2 relate to H formed from 3CH2 resulting from CIISC with H2 and initial photolysis, respectively. These terms show bi-exponential behaviour, deriving from the production of H via (R2) and its loss via diffusion. The first and second terms in E3 relate to formation and decay of H following production of 3CH2 via reaction 3 and in initial photolysis, respectively. Experiments 1 and 2 are in themselves sufficient to determine the ratio k1a/k1, and the presence (or absence) of CIISC can be seen from the temporal dependence of H traces in Fig. 3; however, conversion of all the methylene first to 3 CH2, with subsequent titration to H, ensures that
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we are accounting for all the methylene with a consequent increase in the robustness of the analysis. 4. Results An example of the three H atom traces is shown in Fig. 1 for 295 K. The data were fitted by fixing k1 [4], k3 [11,20,21], YieldH2 [19], and P [16–18] to the literature values and using [1CH2]0, k1a/k1, k2, and kd, as adjustable parameters. Only relative values of [1CH2]0 were required and were assumed to be constant in a set of three experiments. In the data analysis, the parameters were adjusted using the non-linear least squares fitting routine by Marquardt [22]. Initially, each trace was analysed individually. However, the analysis of the system was extended using the technique of global analysis [23] for the simultaneous fitting of either traces 1 and 2 or all three traces. A global analysis results in increased model sensitivity and more accurate parameter recovery, by exploiting the relationships between individual traces. To analyse three experiments simultaneously, the traces need to be corrected for H atom fluorescence quenching by H2. He does not quench the fluorescence, and [NO] is so small that quenching by NO is negligible. In a previous study [24], we measured the quenching of the H atom fluorescence signal from H2S photolysis as a function of H2 pressure. The data conformed to a Stern– Volmer analysis, and the quenching rate, kQ,H2, was determined to be 1.3 · 109 cm3 molecule1 s at 295 K, using 1.6 · 109 s1 as the fluorescence lifetime [25]. In the present experiments, a comparison of the zero time fluorescence signal from
Fig. 1. Time profiles for Experiments 1, 2, and 3. (s) Experiment 1; (j) Experiment 2; and (n) Experiment 3. The full line represents the best fit to the data using equations E1, E2, and E3 for Experiments 1, 2, and 3, respectively. The traces were recorded at 20 Torr total pressure and at 295 K.
Table 1 H atom quenching rate coefficient, kQ,H2, as a function of T Temperature (K)
109kQ,H2 (cm3 molecule1 s1)
[H2] (Torr)
295 400 500
1.1 ± 0.1 1.1 ± 0.1 1.2 ± 0.2
5–100 5–50 10–100
It is derived from Stern–Volmer analysis, using 1.6 · 109 s for the H atom fluorescence lifetime [25].
Experiment 1 as a function of [H2] can be used to determine kQ,H2. Stern–Volmer analysis yielded values of kQ,H2, (Table 1) which are in good agreement with our previous values and which show that kQ,H2 is almost independent of temperature. These Stern–Volmer values were used to normalise the traces from Experiments 1 and 2 to those from 3. Note that only a relative, rather than absolute, LIF signal is needed for the analysis, since [H] in each time dependent expression is proportional to [1CH2]0. In general, the observation that traces from Experiments 2 and 3 converge at long-times, >1000 ls, implies that all the CH2 in the system has been accounted for by conversion to H, either by reaction of 1CH2 with H2, or by reaction of 3 CH2 with NO, with appropriate correction for the non-unit yield in reaction 1. The observation of convergence at all pressures demonstrates that allowance for the quenching of H fluorescence by H2 has been properly effected. kd corresponds to H loss from the system via diffusion, and was typically a few hundred s1 and decreased as the total pressure was increased. The H atom branching ratio, k1a/k1, at 295 K is shown in Fig. 2 over the pressure range 10–100 Torr. There is no discernible dependence on pressure, and the average value is given in
Fig. 2. The H atom branching ratio for the reaction 1 CH2 + H2 at RT. (s) Two trace analysis; 0.85 ± 0.05, and (j) three trace analysis, 0.85 ± 0.08.
M.A. Blitz et al. / Proceedings of the Combustion Institute 30 (2005) 927–933
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Table 2 H atom branching ratio for the reaction 1CH2 + H2 Temperature (K)
k1a/k1
[H2] (Torr)
Number of sets of experiments
295
0.85 ± 0.08 0.85 ± 0.05a 0.92 ± 0.08 1.01 ± 0.10
10–50 10–100 10–50 5–80
9 11 9 23
400 500
Errors are the standard deviation. a From analysis of Experiments 1 and 2 only.
Table 2. A two trace analysis of Experiments 1 and 2 can also be used to determine k1a/k1 (Fig. 3) and the resulting values are shown in Fig. 2 and in Table 2. This analysis requires no normal-
isation for H fluorescence quenching and shows reduced scatter as the pressure is changed. As noted above, however, the three-trace analysis provides a more exacting and robust determination of the H atom branching ratio of the system, and was employed at the higher temperatures. The resulting values of k1a/k1 are summarised in Table 2. It is clear that the H atom channel yield increases with temperature and is close to unity at 500 K. A comparison of the three experiment analysis at 295 K and at 500 K (Figs. 1 and 4) emphasises this increase. At 295 K, Experiment 2 shows a small but significant increase in [H] at short times, as the 3CH2 formed in reaction 1b is converted into H. The two traces are nearly coincident at 500 K, where, according to the analysis, 3 CH2 derives solely from the initial photolysis. 5. Discussion
Fig. 3. Two experiment analysis at 30 Torr total pressure and 295 K. Visual inspection of the traces for Experiments 1 and 2 shows the small (15%) but significant amount of CIISC. (s) Experiment 1 and (j) Experiment 2.
Fig. 4. Three experiment analysis at 15 Torr total pressure and 500 K. (s) Experiment 1; (j) Experiment 2, and (m) Experiment 3. Note that there is little difference between traces for Experiments 1 and 2 at all times, and between traces for Experiments 1, 2, and 3 at long-times, demonstrating the very small channel yield for reaction 1b and this higher temperature.
The value for k1a/k1 of 0.85 ± 0.08 at 295 K from this study compares favourably with that þ0:04 obtained in our previous investigation of 0:900:10 [5], in which a more indirect titration method, based on OH formation in reaction 2, was employed. The uncertainty range given corresponds to 2r. The effect of the uncertainties in P and in Yield2H was investigated by fitting the data with these two quantities varied over their uncertainty ranges. The effect on the total uncertainty in k1a/ k1 is not significant. The only other direct measure of the reactive branching ratio in 1CH2 + H2 is that by Braun et al. [26], who reported a lower limit of P0.8. Our measurements indicate that the CIISC yield in (R1a) and (R1b) decreases with increasing temperature, in contrast to the suggestions of Bley and Temps [3]. As discussed in Introduction, based on the observed increase in the rate coefficient for CIISC with inert gases and the mixed state model for CIISC, coupled with the observed decrease in the overall rate coefficient for 1 CH2 + CH4, they suggested that the yield of CIISC for this reaction should increase significantly with increasing temperature. Ashfold et al. [20] demonstrated that the rate coefficient for CIISC with inert gases can be related to the strength of the van der Waals interaction between CH2 and M, the inert gas, via a Parmenter–Seaver
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plot [27]. Using He as a good comparator for H2 and taking the recommended rate coefficient for 1CH2+He, (k = 1.1 · 1011 exp(380/T) cm3 molecule1 s1 which is based on the only temperature dependent measurements by Wagener [11]), together with a 295 K value for the CIISC yield in (R1a) and (R1b) of 0.15 and k1 = 1.0 · 1010 cm3 molecule1 s1 gives a value for the fractional CIISC yield at 500 K of 0.25, significantly greater than the value determined here. The implication is that the mixed state model does not apply to reactive systems. Further evidence derives from the room temperature Parmenter– Seaver plot, presented by Ashfold et al. [20], in which ln rM is plotted vs (e)1/2, where r is the cross-section for CIISC and e is the 1CH2–M van der Waals well depth. The cross-sections for the rare gases show an excellent correlation, while the cross-section for (R1b), obtained with k1b/ k1 = 0.15 and the recommended value for k1, lies a factor of 4 above the Parmenter–Seaver line derived from the rare gas correlation. Thus, the magnitude of k1b at room temperature suggests that a mechanism other than the mixed state CIISC may apply to the electronic quenching of 1 CH2 by H2. Reaction 1a proceeds by insertion of 1CH2 into the H–H bond, to form energised CH4, which dissociates to form CH3 + H. The lifetime of CH4 is so short that collisional stabilisation is not significant at ambient pressures. 3CH2 reacts only slowly with H2 and the reaction has an energy barrier. As a consequence, curve crossing between the singlet and triplet surfaces is feasible, providing a new mechanism for CIISC. As the collision complex moves through the crossing region, a wide range of vibronic states on the singlet and triplet manifolds will be brought into near resonance, providing an enhanced means of intersystem crossing. If the crossing takes place at large separations, then the Laundau–Zener (L–Z) mechanism applies. The L–Z mechanism applies strictly to atom–atom collisions, but Child [28] showed that the same mechanism could be applied to (small) polyatomics, with the L–Z cross-section modulated by Franck–Condon factors. A reduced efficiency at higher temperatures is typical of curve crossing processes because the probability of crossing decreases as the velocity through the crossing point increases. If the crossing occurs at shorter separations however, which seems more likely given the large 1CH2–3CH2 separation, then curve crossing will compete with intramolecular vibrational relaxation and the temperature dependence becomes less transparent. Theoretical work is clearly needed on the nature of the crossing and its impact on CIISC. The implications of our experimental observations, that the reactive yield is not reduced at high temperatures as has been suggested previously, have considerable potential significance for combustion mechanisms, and fur-
ther experiments on other reactions are needed. Work is in progress in our laboratory on 1 CH2 + CH4 and C2H2.
Acknowledgements We are grateful to EPSRC (GR/N00913) and the Joint Infrastructure Fund for equipment grants and for funding for M.A.B. and N.C.
References [1] D.L. Baulch, C.J. Cobos, R.A. Cox, P. Frank, G. Hayman, T. Just, J.A. Kerr, T. Murrels, M.J. Pilling, J. Troe, R.W. Walker, J. Warnatz, J. Phys. Chem. Ref. Data 24 (1995) 1609–1630. [2] P. Jensen, P.R. Bunker, J. Chem. Phys. 89 (1988) 1327–1332. [3] U. Bley, F. Temps, J. Chem. Phys. 98 (1993) 1058–1072. [4] G. Hancock, M.R. Heal, J. Phys. Chem. 96 (1992) 10316–10322. [5] M.A. Blitz, M.J. Pilling, P.W. Seakins, Phys. Chem. Chem. Phys. 3 (2001) 2241–2244. [6] T. Bohland, F. Temps, H.G. Wagner, Ber. BunsenGes. Phys. Chem. Chem. Phys. 89 (1985) 1013–1018. [7] W. Hack, M. Koch, H.G. Wagner, A. Wilms, Ber. Bunsenges. Phys. Chem. 92 (1988) 674–678. [8] M.A. Blitz, K.W. McKee, M.J. Pilling, P.W. Seakins, Chem. Phys. Lett. 372 (2003) 295–299. [9] G. Hancock, V. Haverd, Chem. Phys. Lett. 372 (2003) 288–294. [10] F. Hayes, W.D. Lawrance, W.S. Staker, K.D. King, J. Phys. Chem. 100 (1996) 11314–11318. [11] R. Wagener, Z. Naturforsch. 45 (1990) 649–656. [12] M.A. Blitz, M.S. Beasley, M.J. Pilling, S.H. Robertson, Phys. Chem. Chem. Phys. 2 (2000) 805–812. [13] J.A. Miller, C.F. Melius, Combust. Flame 91 (1992) 21–39. [14] M.A. Blitz, D.G. Johnson, M. Pesa, M.J. Pilling, S.H. Robertson, P.W. Seakins, J. Chem. Soc. Faraday Trans. 93 (1997) 1473–1479. [15] G.J. Fisher, A.F. McLean, A.W. Schizer, J. Org. Chem. 18 (1955) 1055. [16] C.G. Morgan, M. Drabbels, A.M. Wodtke, J. Chem. Phys. 104 (1996) 7460–7474. [17] E.A. Wade, H. Clauberg, S.K. Kim, A. Mellinger, C.B. Moore, J. Chem. Phys. Chem. A 101 (1997) 732–739. [18] M. Drabbels, C.G. Morgan, D.S. McGuire, A.M. Wodtke, J. Chem. Phys. 102 (1995) 611–614. [19] M. Fikri, S. Meyer, J. Roggenbuck, F. Temps, Faraday Discuss. 119 (2001) 223–242. [20] M.N.R. Ashfold, M.A. Fullstone, G. Hancock, G.W. Ketley, Chem. Phys. 55 (1981) 245–257. [21] A.O. Langford, H. Petek, C.B. Moore, J. Chem. Phys. 78 (1983) 6650–6659. [22] D.W. Marquardt, J. Soc. Ind. Appl. Math. 11 (1963) 1431. [23] J.R. Knutson, J.M. Beechem, L. Brand, Chem. Phys. Lett. 102 (1983) 501–507. [24] N. Choi, M.A. Blitz, K.W. McKee, M.J. Pilling, P.W. Seakins, Chem. Phys. Lett. 384 (2004) 68–72.
M.A. Blitz et al. / Proceedings of the Combustion Institute 30 (2005) 927–933 [25] H. Okabe, Photochemistry of Small Molecules, Wiley, New York, 1978. [26] W. Braun, A.M. Bass, M. Pilling, J. Chem. Phys. 52 (1970) 5131–5143.
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Comment Horst Hippler, University of Karlsruhe, Germany. Could you please comment on the origin of H-atoms in the 3CH2 + O2 reaction. Are H-atoms primary or secondary reaction products? Reply. H atoms can be formed in the following exothermic channels. (The numbers after each reaction correspond to DrH298/kJ mol1) 3
CH2 þ O2 !CO2 þ 2H
348
ðaÞ
!HCðOÞO þ H
323
ðbÞ
!CO þ OH þ H
246
ðcÞ
Blitz et al. [1] found an overall H atom yield of 0.8 ± 0.2 at room temperature, which is compatible with the IR chemiluminescence results of Hancock and Haverd [2] and the IR diode laser measurements of Alvarez and Moore [3]. The shock tube measurements of Dom-
browsky and Wagner [4] give much lower yields (0.1), suggesting that the mechanism and the relative yields may change with temperature. The time profiles of the H atom yield in the experiments of Blitz et al. [1] clearly show that H is produced with a time constant compatible with that for the reaction of 3CH2, showing that the H atoms are formed directly and not via a secondary mechanism.
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Combustion Institute
Proceedings of the Combustion Institute 30 (2005) 927–933
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The effect of temperature on collision induced intersystem crossing in the reaction of 1CH2 with H2 Mark A. Blitz, Namil Choi, Tama´s Kova´cs, Paul W. Seakins, Michael J. Pilling* Department of Chemistry, University of Leeds, Leeds LS2 9JT, UK
Abstract Laser flash photolysis of ketene at 308 nm, coupled with H atom vacuum ultraviolet laser induced fluorescence, was used to determine the branching ratio for the CH3 + H channel (1a) in the reaction of CH21A1 (1CH2) with H2, over the temperature range 300–500 K. This reaction channel competes with collision induced intersystem crossing (CIISC) to form triplet methylene, CH23B1 (3CH2) (channel 1b). The branching ratio for H formation, k1a/k1, was determined by measuring the relative H atom yield in three time resolved measurements of H: (i) in ketene, H2 mixtures, where H is exclusively formed by reaction 1a, (ii) in ketene, H2, NO mixtures ([NO] [H2]), where H is formed at short times by 1a and at longer times by 3CH2 + NO, following 1b, and (iii) in ketene, He, NO mixtures ([NO] [He]), where H is exclusively formed from 3CH2 + NO, following deactivation of singlet to triplet methylene by He. k1a/k1 was found to increase from 0.85 at 300 K to unity at 500 K, with the yield of CIISC decreasing from 0.15 to zero. This is the first measurement of the temperature dependence of the rate coefficient for CIISC in a reactive system. The rate coefficient for CIISC with an inert gas increases with T. It has been suggested that the fractional yield of CIISC will increase with temperature in reactive systems, thus reducing the rate coefficient for reaction at high temperature, with significant consequences for combustion systems. The present experiments demonstrate that this is not the case for reaction with H2 and implies a different CIISC mechanism for reactive vs inert collision partners. 2004 Published by Elsevier Inc. on behalf of The Combustion Institute. Keywords: Methylene; Hydrogen; Branching ratio
1. Introduction Methylene plays a central role in the combustion of hydrocarbons, and its kinetics have been extensively studied [1]. The ground triplet state CH2X 3B1(=3CH2) and the first singlet state a1A1 (=1CH2) are separated by only 38 kJ mol1
*
Corresponding author. Fax: +44 113 343 6401. E-mail address: [email protected] (M.J. Pilling).
[2]. The triplet state is generally less reactive than the singlet state, especially with closed shell molecules such as H2. While the triplet does react with open shell molecules, such as O2 and NO, the rate coefficients for these reactions are still about an order of magnitude smaller than those for the singlet. The two states are also collisionally coupled, and a proper description of this coupling is essential in combustion models, given the lower reactivity of the triplet. Collision-induced intersystem crossing (CIISC) between the two states [3,4] can be effected by inert gases via a small number of
1540-7489/$ - see front matter 2004 Published by Elsevier Inc. on behalf of The Combustion Institute. doi:10.1016/j.proci.2004.08.073
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Ôrotational gatewayÕ states with mixed singlet triplet character. The function of the collision is to rotationally relax the singlet and triplet states, to ensure an irreversible transition to the energetically lower spin state. Detailed balance ensures that this process also operates in reverse and provides a mechanism for conversion of the less reactive triplet to the more reactive singlet at the elevated temperatures encountered in combustion. Conversion of the singlet to the triplet also occurs in collisions with reactive gases, albeit with comparatively low probability. It has generally been assumed that this process is similar to that occurring with inert gases and has similarly been described as CIISC. Examples of such CIISC fracþ0:10 tional channel yields include 0:100:04 for 1 1 CH2 + H2 [5], 0.1 for CH2 + CH4 [3,6], and 0.22 for 1CH2 + C2H2 [7]. Parallel measurements for 1CH2 + O2, by contrast, have demonstrated unit efficiency for CIISC [8,9]. To date, no measurements on CIISC in reactive systems have been made other than at room temperature. However, there have been measurements of the temperature dependence of 1 CH2+Ar, He, and N2 [4,10,11], for which the CIISC yield is unity. The rate coefficients all increase with temperature; for example, for 1 CH2 + Ar kCIISC increases from 5 to 15 · 1012 cm3 molecule1 s1 between 295 and 860 K [4]. Bley and Temps [3] and Hancock and Heal [4] have analysed this dependence using the mixed state model and ascribed the increase to the temperature dependence of rotational relaxation and to increased population of the mixed states at higher energies at elevated temperatures. Bley and Temps contrasted the positive temperature dependence of CIISC with the negative temperature dependence of the overall rate constant for 1CH2 + CH4 and argued that, if the mixed state model applies to this reaction, then the fractional yield of CIISC will increase with temperature and could become the major channel at combustion temperatures. Beasley and co-workers [12] have discussed the implications of such behaviour for 1CH2 + C2H2, for which the overall rate constant again shows a negative temperature dependence (k = 5.1 · 108 T0.9 cm3 mole1 1 cule s ). It has been suggested that this reaction plays a key role in soot formation through production of C3H3 + H [13]; any significant increase in CIISC with temperature would seriously limit the yield of C3H3 under combustion conditions. There is a clear need for experimental investigations of the temperature dependence of the yield of 3CH2 in reactive systems. In this paper, we report such a measurement for 1CH2 + H2, using LIF measurements of the yield of H. The system is calibrated against the yield of H from 3 CH2 + NO, following formation of the triplet by CIISC.
2. Experimental All experiments were carried out in a six-way cross stainless steel, slow-flow, laser flash photolysis, laser induced fluorescence (LIF) system [14]. Ketene was photolysed at 308 nm using an excimer laser (Lambda Physik, LPX 105), which typically generated 100 mJ/pulse. H atoms were monitored by vacuum ultraviolet (VUV) LIF using the Lyman-a transition (121.6 nm) by frequency tripling 365.8 nm radiation (produced by mixing infrared with the dye fundamental (Continuum/Sirah, 20 mJ/pulse)) in a krypton and argon mixture (Kr:Ar = 1:2.5). 365.8 nm light was focused (f/s = 30 cm) into a glass cell attached directly to the cell containing 800 Torr of Kr/Ar. Lyman-a radiation was coupled into the cell through a MgF2 window. Fluorescence was detected perpendicularly to both laser axes by a solar blind PMT (Electron Tube). The power of the VUV output was monitored by a second solar blind PMT (Thorn EMI). Some of the probe light, upon exiting the reaction cell, was reflected using a quartz flat at 45 through a VUV interference filter (Acton Research) and onto the second PMT. This resulting signal was used to normalise the VUV LIF signal taking into account any temporal power fluctuations of the probe laser and attenuation by absorption. H atom time profiles were obtained by varying the time delay between the photolysis and probe lasers. Fluorescence was collected via a boxcar integrator, digitised, and stored on a PC for subsequent analysis. Typically, each trace was recorded over 0–1500 ls using ca. 1000 points. Sets of three traces were recorded with H monitored (1) in the presence of H2, (2) in the presence of H2 and a small amount of NO (<1014 molecule cm3) and (3) in the presence of He and a small amount of NO. The total pressure and ketene concentration for the three sets of traces was constant. The pressure was varied over the range 5–100 Torr and the temperature over the range 295–500 K. CH2CO was prepared by cracking acetic anhydride at 873 K [15], purified by trap-to-trap distillation at 77 K and identified from its IR and UV spectra. H2 and He (BOC > 99.999%) were directly used from cylinders. NO (99.9%, Air Products) was purified by trap-to-trap distillation until it was a white solid, and then diluted in He and stored in a glass bulb. Pressures were measured using capacitance manometers (MKS). The cell temperature was measured with type K thermocouples; variations between the two thermocouples were never more than ±5 K. The output of the thermocouples was fed into a home-built temperature control box to regulate the temperature of the cell. The gases were flowed through calibrated mass flow controllers, via a mixing manifold and into the cell. Total gas flow rates were
M.A. Blitz et al. / Proceedings of the Combustion Institute 30 (2005) 927–933
of the order of 5 SLM at 100 Torr which was sufficient to ensure the complete replenishment of gas in the reaction zone between photolysis pulses. The total pressure in the cell was controlled with a needle valve between the cell and vacuum pump, and the pressure was measured with a Baratron pressure gauge (MKS 0– 100 Torr).
3. Methodology
CH2 þ H2 !H þ CH3
ðR1aÞ
3
! CH2 þ H2
ðR1bÞ 1
[H2] was sufficiently high that CH2 reacted exclusively with H2 with H atoms formed essentially instantaneously on the timescale of our experiment. Subsequently, the H atoms were lost via diffusion from the monitoring zone, with first order rate constant kd. In the second experiment, a small concentration of NO (4 mTorr) was added to the H2/ketene mix, so only a very small fraction (<0.001) of the 1CH2 was removed via reaction with NO. 3 CH2, formed by CIISC (R1b), reacts with NO 3
CH2 þ NO ! H þ other products
ðR2Þ
Fikri et al. [19] have determined a value of 0.9 for the yield, Yield2H, of H from this reaction at room temperature. While they did not measure the temperature dependence, their calculations indicate that Yield2H may slightly increase with temperature, giving a T dependent yield in the range 0.9–1.0. Finally, in the third experiment, the hydrogen was replaced with helium, 1
CH2 þ He ! 3 CH2 þ He
nentials since (R1a), (R1b) and (R3) have time constants of 2 ls–30 ns compared with the experimental timescales of 0.1–1 ms. Thus, the formation of 3CH2 from 1CH2 via reactions 1 and 3 can be treated as instantaneous. In addition, the time constant for diffusive loss is 2–5 ms, so that kd can be neglected in comparison with k1[H2] and k3[He] in the analysis. With these simplifications, the expressions for [H](t) in the three experiments are: Experiment 1 (E1)
The 308 nm photolysis of ketene leads to the almost exclusive formation of 1CH2, with only a small fractional yield, P, of 3CH2 at this wavelength (P = [3CH2]0/[1CH2]0 = 0.06 ± 0.02 [16–18]). The method we adopt is based on the formation of H in reactions of CH2, and H detection by LIF. A reaction of 3CH2 with known H atom yield is needed to link the LIF signal to the photolysis yield of CH2. For each total pressure, three different experiments were carried out. The first experiment involved only ketene (2–3 mTorr) and hydrogen. 1CH2 was lost with hydrogen via either reaction, forming CH3 + H (R1a), or CIISC (R1b) 1
929
ðR3Þ
consequently all the 1CH2 was relaxed to 3CH2 and titrated to H via reaction (2). In all of these experiments, it was necessary to account for the small yield of 3CH2 [16–18] in the photolysis of ketene at 308 nm. The time dependent H atom profiles are tri-exponential, but can be approximated to bi-expo-
½H ¼
k 01a 1 ½ CH2 0 expðk d tÞ: k 01
Experiment 2 (E2) ½H ¼
k 01a 1 ½ CH2 0 expðk d tÞ k 01 k0 k 02 ½1 CH2 0 þ Yield 2H 1b 0 0 k 1 ðk 2 k d Þ k0 expðk d tÞ 0 1 0 expðk 02 tÞ k1 k2 0 1 Yield 2H k 2 P ½ CH2 0 þ k 02 k d expðk d tÞ expðk 02 tÞ :
Experiment 3 (E3) k0 ½1 CH2 0 ½H ¼ Yield 2H 0 2 ðk 2 k d Þ k0 expðk d tÞ 0 3 0 expðk 02 tÞ k3 k2 Yield 2H k 02 P ½1 CH2 0 þ k 02 k d expðk d tÞ expðk 02 tÞ ; where k 01 ¼ k 1 ½H2 , k 03 ¼ k 3 ½He, and [1CH2]0 is the zero time concentration of singlet methylene. In E1, the pre-exponential term gives [H] at the effective zero time of Experiment 1 and is equal to [H] formed from reaction 1a. The first term in E2 has the same interpretation. The second and third terms in E2 relate to H formed from 3CH2 resulting from CIISC with H2 and initial photolysis, respectively. These terms show bi-exponential behaviour, deriving from the production of H via (R2) and its loss via diffusion. The first and second terms in E3 relate to formation and decay of H following production of 3CH2 via reaction 3 and in initial photolysis, respectively. Experiments 1 and 2 are in themselves sufficient to determine the ratio k1a/k1, and the presence (or absence) of CIISC can be seen from the temporal dependence of H traces in Fig. 3; however, conversion of all the methylene first to 3 CH2, with subsequent titration to H, ensures that
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we are accounting for all the methylene with a consequent increase in the robustness of the analysis. 4. Results An example of the three H atom traces is shown in Fig. 1 for 295 K. The data were fitted by fixing k1 [4], k3 [11,20,21], YieldH2 [19], and P [16–18] to the literature values and using [1CH2]0, k1a/k1, k2, and kd, as adjustable parameters. Only relative values of [1CH2]0 were required and were assumed to be constant in a set of three experiments. In the data analysis, the parameters were adjusted using the non-linear least squares fitting routine by Marquardt [22]. Initially, each trace was analysed individually. However, the analysis of the system was extended using the technique of global analysis [23] for the simultaneous fitting of either traces 1 and 2 or all three traces. A global analysis results in increased model sensitivity and more accurate parameter recovery, by exploiting the relationships between individual traces. To analyse three experiments simultaneously, the traces need to be corrected for H atom fluorescence quenching by H2. He does not quench the fluorescence, and [NO] is so small that quenching by NO is negligible. In a previous study [24], we measured the quenching of the H atom fluorescence signal from H2S photolysis as a function of H2 pressure. The data conformed to a Stern– Volmer analysis, and the quenching rate, kQ,H2, was determined to be 1.3 · 109 cm3 molecule1 s at 295 K, using 1.6 · 109 s1 as the fluorescence lifetime [25]. In the present experiments, a comparison of the zero time fluorescence signal from
Fig. 1. Time profiles for Experiments 1, 2, and 3. (s) Experiment 1; (j) Experiment 2; and (n) Experiment 3. The full line represents the best fit to the data using equations E1, E2, and E3 for Experiments 1, 2, and 3, respectively. The traces were recorded at 20 Torr total pressure and at 295 K.
Table 1 H atom quenching rate coefficient, kQ,H2, as a function of T Temperature (K)
109kQ,H2 (cm3 molecule1 s1)
[H2] (Torr)
295 400 500
1.1 ± 0.1 1.1 ± 0.1 1.2 ± 0.2
5–100 5–50 10–100
It is derived from Stern–Volmer analysis, using 1.6 · 109 s for the H atom fluorescence lifetime [25].
Experiment 1 as a function of [H2] can be used to determine kQ,H2. Stern–Volmer analysis yielded values of kQ,H2, (Table 1) which are in good agreement with our previous values and which show that kQ,H2 is almost independent of temperature. These Stern–Volmer values were used to normalise the traces from Experiments 1 and 2 to those from 3. Note that only a relative, rather than absolute, LIF signal is needed for the analysis, since [H] in each time dependent expression is proportional to [1CH2]0. In general, the observation that traces from Experiments 2 and 3 converge at long-times, >1000 ls, implies that all the CH2 in the system has been accounted for by conversion to H, either by reaction of 1CH2 with H2, or by reaction of 3 CH2 with NO, with appropriate correction for the non-unit yield in reaction 1. The observation of convergence at all pressures demonstrates that allowance for the quenching of H fluorescence by H2 has been properly effected. kd corresponds to H loss from the system via diffusion, and was typically a few hundred s1 and decreased as the total pressure was increased. The H atom branching ratio, k1a/k1, at 295 K is shown in Fig. 2 over the pressure range 10–100 Torr. There is no discernible dependence on pressure, and the average value is given in
Fig. 2. The H atom branching ratio for the reaction 1 CH2 + H2 at RT. (s) Two trace analysis; 0.85 ± 0.05, and (j) three trace analysis, 0.85 ± 0.08.
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Table 2 H atom branching ratio for the reaction 1CH2 + H2 Temperature (K)
k1a/k1
[H2] (Torr)
Number of sets of experiments
295
0.85 ± 0.08 0.85 ± 0.05a 0.92 ± 0.08 1.01 ± 0.10
10–50 10–100 10–50 5–80
9 11 9 23
400 500
Errors are the standard deviation. a From analysis of Experiments 1 and 2 only.
Table 2. A two trace analysis of Experiments 1 and 2 can also be used to determine k1a/k1 (Fig. 3) and the resulting values are shown in Fig. 2 and in Table 2. This analysis requires no normal-
isation for H fluorescence quenching and shows reduced scatter as the pressure is changed. As noted above, however, the three-trace analysis provides a more exacting and robust determination of the H atom branching ratio of the system, and was employed at the higher temperatures. The resulting values of k1a/k1 are summarised in Table 2. It is clear that the H atom channel yield increases with temperature and is close to unity at 500 K. A comparison of the three experiment analysis at 295 K and at 500 K (Figs. 1 and 4) emphasises this increase. At 295 K, Experiment 2 shows a small but significant increase in [H] at short times, as the 3CH2 formed in reaction 1b is converted into H. The two traces are nearly coincident at 500 K, where, according to the analysis, 3 CH2 derives solely from the initial photolysis. 5. Discussion
Fig. 3. Two experiment analysis at 30 Torr total pressure and 295 K. Visual inspection of the traces for Experiments 1 and 2 shows the small (15%) but significant amount of CIISC. (s) Experiment 1 and (j) Experiment 2.
Fig. 4. Three experiment analysis at 15 Torr total pressure and 500 K. (s) Experiment 1; (j) Experiment 2, and (m) Experiment 3. Note that there is little difference between traces for Experiments 1 and 2 at all times, and between traces for Experiments 1, 2, and 3 at long-times, demonstrating the very small channel yield for reaction 1b and this higher temperature.
The value for k1a/k1 of 0.85 ± 0.08 at 295 K from this study compares favourably with that þ0:04 obtained in our previous investigation of 0:900:10 [5], in which a more indirect titration method, based on OH formation in reaction 2, was employed. The uncertainty range given corresponds to 2r. The effect of the uncertainties in P and in Yield2H was investigated by fitting the data with these two quantities varied over their uncertainty ranges. The effect on the total uncertainty in k1a/ k1 is not significant. The only other direct measure of the reactive branching ratio in 1CH2 + H2 is that by Braun et al. [26], who reported a lower limit of P0.8. Our measurements indicate that the CIISC yield in (R1a) and (R1b) decreases with increasing temperature, in contrast to the suggestions of Bley and Temps [3]. As discussed in Introduction, based on the observed increase in the rate coefficient for CIISC with inert gases and the mixed state model for CIISC, coupled with the observed decrease in the overall rate coefficient for 1 CH2 + CH4, they suggested that the yield of CIISC for this reaction should increase significantly with increasing temperature. Ashfold et al. [20] demonstrated that the rate coefficient for CIISC with inert gases can be related to the strength of the van der Waals interaction between CH2 and M, the inert gas, via a Parmenter–Seaver
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plot [27]. Using He as a good comparator for H2 and taking the recommended rate coefficient for 1CH2+He, (k = 1.1 · 1011 exp(380/T) cm3 molecule1 s1 which is based on the only temperature dependent measurements by Wagener [11]), together with a 295 K value for the CIISC yield in (R1a) and (R1b) of 0.15 and k1 = 1.0 · 1010 cm3 molecule1 s1 gives a value for the fractional CIISC yield at 500 K of 0.25, significantly greater than the value determined here. The implication is that the mixed state model does not apply to reactive systems. Further evidence derives from the room temperature Parmenter– Seaver plot, presented by Ashfold et al. [20], in which ln rM is plotted vs (e)1/2, where r is the cross-section for CIISC and e is the 1CH2–M van der Waals well depth. The cross-sections for the rare gases show an excellent correlation, while the cross-section for (R1b), obtained with k1b/ k1 = 0.15 and the recommended value for k1, lies a factor of 4 above the Parmenter–Seaver line derived from the rare gas correlation. Thus, the magnitude of k1b at room temperature suggests that a mechanism other than the mixed state CIISC may apply to the electronic quenching of 1 CH2 by H2. Reaction 1a proceeds by insertion of 1CH2 into the H–H bond, to form energised CH4, which dissociates to form CH3 + H. The lifetime of CH4 is so short that collisional stabilisation is not significant at ambient pressures. 3CH2 reacts only slowly with H2 and the reaction has an energy barrier. As a consequence, curve crossing between the singlet and triplet surfaces is feasible, providing a new mechanism for CIISC. As the collision complex moves through the crossing region, a wide range of vibronic states on the singlet and triplet manifolds will be brought into near resonance, providing an enhanced means of intersystem crossing. If the crossing takes place at large separations, then the Laundau–Zener (L–Z) mechanism applies. The L–Z mechanism applies strictly to atom–atom collisions, but Child [28] showed that the same mechanism could be applied to (small) polyatomics, with the L–Z cross-section modulated by Franck–Condon factors. A reduced efficiency at higher temperatures is typical of curve crossing processes because the probability of crossing decreases as the velocity through the crossing point increases. If the crossing occurs at shorter separations however, which seems more likely given the large 1CH2–3CH2 separation, then curve crossing will compete with intramolecular vibrational relaxation and the temperature dependence becomes less transparent. Theoretical work is clearly needed on the nature of the crossing and its impact on CIISC. The implications of our experimental observations, that the reactive yield is not reduced at high temperatures as has been suggested previously, have considerable potential significance for combustion mechanisms, and fur-
ther experiments on other reactions are needed. Work is in progress in our laboratory on 1 CH2 + CH4 and C2H2.
Acknowledgements We are grateful to EPSRC (GR/N00913) and the Joint Infrastructure Fund for equipment grants and for funding for M.A.B. and N.C.
References [1] D.L. Baulch, C.J. Cobos, R.A. Cox, P. Frank, G. Hayman, T. Just, J.A. Kerr, T. Murrels, M.J. Pilling, J. Troe, R.W. Walker, J. Warnatz, J. Phys. Chem. Ref. Data 24 (1995) 1609–1630. [2] P. Jensen, P.R. Bunker, J. Chem. Phys. 89 (1988) 1327–1332. [3] U. Bley, F. Temps, J. Chem. Phys. 98 (1993) 1058–1072. [4] G. Hancock, M.R. Heal, J. Phys. Chem. 96 (1992) 10316–10322. [5] M.A. Blitz, M.J. Pilling, P.W. Seakins, Phys. Chem. Chem. Phys. 3 (2001) 2241–2244. [6] T. Bohland, F. Temps, H.G. Wagner, Ber. BunsenGes. Phys. Chem. Chem. Phys. 89 (1985) 1013–1018. [7] W. Hack, M. Koch, H.G. Wagner, A. Wilms, Ber. Bunsenges. Phys. Chem. 92 (1988) 674–678. [8] M.A. Blitz, K.W. McKee, M.J. Pilling, P.W. Seakins, Chem. Phys. Lett. 372 (2003) 295–299. [9] G. Hancock, V. Haverd, Chem. Phys. Lett. 372 (2003) 288–294. [10] F. Hayes, W.D. Lawrance, W.S. Staker, K.D. King, J. Phys. Chem. 100 (1996) 11314–11318. [11] R. Wagener, Z. Naturforsch. 45 (1990) 649–656. [12] M.A. Blitz, M.S. Beasley, M.J. Pilling, S.H. Robertson, Phys. Chem. Chem. Phys. 2 (2000) 805–812. [13] J.A. Miller, C.F. Melius, Combust. Flame 91 (1992) 21–39. [14] M.A. Blitz, D.G. Johnson, M. Pesa, M.J. Pilling, S.H. Robertson, P.W. Seakins, J. Chem. Soc. Faraday Trans. 93 (1997) 1473–1479. [15] G.J. Fisher, A.F. McLean, A.W. Schizer, J. Org. Chem. 18 (1955) 1055. [16] C.G. Morgan, M. Drabbels, A.M. Wodtke, J. Chem. Phys. 104 (1996) 7460–7474. [17] E.A. Wade, H. Clauberg, S.K. Kim, A. Mellinger, C.B. Moore, J. Chem. Phys. Chem. A 101 (1997) 732–739. [18] M. Drabbels, C.G. Morgan, D.S. McGuire, A.M. Wodtke, J. Chem. Phys. 102 (1995) 611–614. [19] M. Fikri, S. Meyer, J. Roggenbuck, F. Temps, Faraday Discuss. 119 (2001) 223–242. [20] M.N.R. Ashfold, M.A. Fullstone, G. Hancock, G.W. Ketley, Chem. Phys. 55 (1981) 245–257. [21] A.O. Langford, H. Petek, C.B. Moore, J. Chem. Phys. 78 (1983) 6650–6659. [22] D.W. Marquardt, J. Soc. Ind. Appl. Math. 11 (1963) 1431. [23] J.R. Knutson, J.M. Beechem, L. Brand, Chem. Phys. Lett. 102 (1983) 501–507. [24] N. Choi, M.A. Blitz, K.W. McKee, M.J. Pilling, P.W. Seakins, Chem. Phys. Lett. 384 (2004) 68–72.
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Comment Horst Hippler, University of Karlsruhe, Germany. Could you please comment on the origin of H-atoms in the 3CH2 + O2 reaction. Are H-atoms primary or secondary reaction products? Reply. H atoms can be formed in the following exothermic channels. (The numbers after each reaction correspond to DrH298/kJ mol1) 3
CH2 þ O2 !CO2 þ 2H
348
ðaÞ
!HCðOÞO þ H
323
ðbÞ
!CO þ OH þ H
246
ðcÞ
Blitz et al. [1] found an overall H atom yield of 0.8 ± 0.2 at room temperature, which is compatible with the IR chemiluminescence results of Hancock and Haverd [2] and the IR diode laser measurements of Alvarez and Moore [3]. The shock tube measurements of Dom-
browsky and Wagner [4] give much lower yields (0.1), suggesting that the mechanism and the relative yields may change with temperature. The time profiles of the H atom yield in the experiments of Blitz et al. [1] clearly show that H is produced with a time constant compatible with that for the reaction of 3CH2, showing that the H atoms are formed directly and not via a secondary mechanism.
References [1] M.A. Blitz, K.W. McKee, M.J. Pilling, P.W. Seakins, Chem. Phys. Lett. 372 (2003) 295. [2] G. Hancock, V. Haverd, Chem. Phys. Lett. 372 (2003) 288. [3] R.A. Alvarez,C.B. Moore, J. Phys. Chem. 98 (1994) 174. [4] Ch. Dombrowsky, H.Gg. Wagner, Ber. Bunsenges. Phys. Chem. 96 (1992) 1048.