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Kinetics of the CN+CH2CO and NCO+CH2CO reactions
Chemical Physics 234 Ž1998. 231–237
Kinetics of the CN q CH 2 CO and NCO q CH 2 CO reactions Michael A. Edwards, John F. Hershberger
)
Department of Chemistry, North Dakota State UniÕersity, Fargo, ND 58105, USA Received 12 March 1998
Abstract The CN q CH 2 CO and NCO q CH 2 CO reactions were studied over the temperature ranges 296–567 K and 296–556 K, respectively, using time-resolved infrared diode laser absorption and visible laser-induced fluorescence spectroscopy. The total rate constant data were fit to the following expressions: k 1ŽCN q CH 2 CO. s Ž2.37 " 0.5. = 10y11 expw552.6 " 97rT x and k 2 ŽNCO q CH 2 CO. s Ž1.71 " 0.27. = 10y12 expw713.2 " 56rT x cm3 moleculey1 sy1. Detection and quantification of CO product yields suggests that an addition–elimination mechanism producing CH 2 CN q CO or CH 2 NCO q CO dominates these reactions, and that hydrogen abstraction to produce HCNq HCCO or HNCOq HCCO is a minor or negligible product channel. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Kinetics; Ketene; Radical reactions
1. Introduction The kinetics of reactions of ketene, CH 2 CO, are of interest because this compound is a commonly used photolytic precursor of CH 2 radicals in a variety of kinetics, spectroscopy, and energy transfer studies. Knowledge of the kinetics of radical–molecule reactions involving CH 2 CO is currently limited to a few reactions. The O q CH 2 CO, H q CH 2 O, and OH q CH 2 O reactions have been fairly well studied, although agreement between different studies is not very good, with reported rate constants at 298 K of Ž3.0–8.8. = 10y1 3 w1–4x, Ž6.2–24.5. = 10y1 4 w5–7x, and Ž1.2–3.4. = 10y1 1 w8–10x cm3 moleculey1 sy1 , respectively. Positive activation en-
)
Corresponding author.
ergies have been observed for O q CH 2 CO w1x and H q CH 2 O w6,7x, but a negative activation for the OH q CH 2 CO was observed w9x. In addition, the reactions F q CH 2 CO and Cl q CH 2 CO have also been studied at 298 K, with reported rate constants of 8.3 = 10y1 1 and 6.8 = 10y1 1 cm3 moleculey1 sy1 , respectively w11x. In a recent study, Grussdorf et al. w12x measured absolute yields of primary products in the reactions of F, Cl, and OH with ketene in a discharge flow apparatus. They found that these reactions proceed primarily via an addition–elimination mechanism, leading to CH 2 F, CH 2 Cl, or CH 2 OH as well as CO products. Hydrogen abstraction to form the ketenyl radical ŽHCCO. was observed to be a minor or negligible reaction pathway in these systems. CH 2 F products from the F q CH 2 CO reaction have also been detected by laser-induced fluorescence spectroscopy w13x.
0301-0104r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 1 0 4 Ž 9 8 . 0 0 1 7 4 - 8
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M.A. Edwards, J.F. Hershbergerr Chemical Physics 234 (1998) 231–237
In this work we report kinetic investigations of the following two reactions: CN q CH 2 CO ™ CH 2 CN q CO CN q CH 2 CO ™ HCN q HCCO NCO q CH 2 CO ™ CH 2 NCO q CO NCO q CH 2 CO ™ HNCOq HCCO
Ž 1a . Ž 1b . Ž 2a . Ž 2b .
No previous literature data is available for either of these reactions, or any other reactions of nitrogencontaining radicals with ketene. One motivation of the current study is to investigate whether these reactions proceed by similar mechanisms to reactions of halogen atoms with ketene, or whether hydrogen abstraction is a significant reaction channel. If the latter mechanism dominates, these reactions could represent clean sources of the important HCCO radical for subsequent kinetic studies. CN molecules were produced in these experiments by 248 nm excimer laser photolysis of ICN precursor molecules: ICN q hn Ž 248 nm . ™ I q CN Ž 3. In the NCO experiments, NCO molecules were formed by the reaction of CN with molecular oxygen: CN q O 2 ™ NCO q O Ž 4. which has a rate constant of Ž1.8–3.0. = 10y1 1 cm3 moleculey1 sy1 at 298 K w14–16x. Two detection techniques were used in this study. Time-resolved infrared diode laser absorption spectroscopy was used to probe CN and NCO radicals in order to measure total rate constants. This method works well for CN, but somewhat less well for NCO due to rather small infrared absorption coefficients of this radical. As a result, NCO measurements by infrared absorption were limited to experiments at room temperature. Laser-induced fluorescence was therefore used to detect NCO in some of the experiments, especially at elevated temperatures. In addition to total rate constant measurements, the yield of CO product molecules in each of these reactions was measured using infrared absorption spectroscopy. 2. Experimental The time-resolved infrared diode laser apparatus is similar to that described in previous publications
w17–19x. Briefly, 248 nm light from an excimer laser ŽLambda Physik Compex 200. was made collinear with tunable infrared radiation from a lead-salt diode laser ŽLaser Photonics. and co-propagated through a 119.4-cm Pyrex reaction cell. The transmitted infrared light was then passed into a 1r4-meter monochromator for laser mode selection and focused onto a 1-mm InSb infrared detector ŽCincinnati Electronics, ; 1 ms response time.. Transient absorption signals were collected and signal averaged on a Hitachi 6065 digital oscilloscope and transmitted to a personal computer for analysis. Transient signals were also collected with the infrared laser detuned ; 0.02 cmy1 off of the absorption lines. These off resonant signals are due to thermal deflection of the probe beam by the transient heating induced by the photolysis laser. To correct for this effect, the off resonant signals were subtracted from the on-resonant signals. Photolysis pulse energy was measured using a Molectron joulemeter. The following absorption lines were used: CN Ž n s 0, RŽ10. line at 2081.689 cmy1 .; NCO Ž000 PŽ16.5. line at approximately 1906.94 cmy1 .; and CO Ž n s 0, PŽ16. line at 2077.650 cmy1 .. The HITRAN database w20x was used to locate CO lines, while other published spectral data were used to locate CN w21,22x and NCO w23x lines. In the laser-induced fluorescence ŽLIF. experiments, NCO was detected by at approximately 413.3 nm using a 02 0 0 ŽA 2 Sq . § 00 1 0 ŽX 2 P . transition w24x. The probe light was produced by frequency doubling 826 nm light from a dye laser ŽContinuum ND-6000. pumped by the 2nd harmonic of an Nd:YAG laser ŽLumonics JK750.. This probe light was made collinear with the 248 nm excimer light and copropagated down a 119.4-cm reaction cell. Fluorescence through a side window was filtered to block scattered excimer light and detected by a photomultiplier tube ŽR508.. The unamplified PMT output was recorded by a boxcar integrator ŽStanford Instruments Model 250. with a 120 ns delay and a 300 ns gate width and averaged on a personal computer. The computer varied the delay between excimer and probe pulses in order to produce an wNCOx vs. time profile. ICN ŽAldrich. was purified by vacuum sublimation to remove dissolved air. SF6 and CF4 ŽMatheson. were purified by vacuum distillation at 77 K. O 2
M.A. Edwards, J.F. Hershbergerr Chemical Physics 234 (1998) 231–237
ŽMatheson. was used without further purification. CH 2 CO was synthesized by the pyrolysis of acetic anhydride at 970 K and purified by vacuum distillation. A ; 20% impurity of CO 2 as measured by infrared absorption spectroscopy was present in the CH 2 CO samples and could not be easily removed by distillation. The measured CH 2 CO pressures in the kinetics experiments were therefore corrected for the presence of this impurity Žneither CN nor NCO react with CO 2 at moderate temperatures.. Typical reaction conditions were 0.2 Torr ICN, 0.0 Torr O 2 ŽCN experiments., 1.0–2.0 Torr O 2 ŽNCO experiments., 0.0–0.25 Torr CH 2 CO, and 1.0 Torr SF6 or CF4 buffer gas. Typical radical densities were ; 2 = 10 13 moleculesrcm3 for the infrared experiments and - 1 = 10 13 moleculesrcm3 for the laser-induced fluorescence experiments. These experiments were therefore pseudo-first order with wCH 2 COx 4 wCNx,wNCOx. 3. Results and discussion 3.1. Total rate constants Fig. 1 shows a typical transient signal for CN and NCO molecules obtained by transient infrared ab-
Fig. 1. Transient infrared absorption signal for CN and NCO molecules. Trace A: NCO signal without CH 2 CO reactant. Trace B: NCO signal with 0.1 Torr CH 2 CO reactant. Trace C: CN signal without CH 2 CO reactant. Trace D: CN signal with 0.1 Torr CH 2 CO reactant. Other conditions: P ICN s 0.3 Torr, PO 2 s 2.0 Torr ŽNCO signals only., PSF 6 s1.0 Torr.
233
Fig. 2. Pseudofirst order decay rates for CN Žtriangles. and NCO Žcircles. as a function of CH 2 CO pressure. Reaction conditions: P ICN s 0.3 Torr, PO 2 s 0.0 Torr ŽCN data., 2.0 Torr ŽNCO data., PSF 6 s1.0 Torr. Data shown was obtained at 296 K.
sorption spectroscopy. The rapid rise in the CN signal is attributed to formation of CN by direct photolysis of ICN. Although this precursor produces CN with significant amounts of rotational and vibrational excitation, the SF6 buffer gas used is expected to quickly relax these molecules to a Boltzmann distribution. The decay in the transient signals is attributed to reaction of CN molecules as well as diffusion out of the probed volume. When oxygen is included in the reaction mixture, NCO transient signals are obtained, as shown in the upper two traces of Fig. 1. A short induction period of ; 3–5 ms was observed for the NCO signals. This is probably due to a combination of several factors, including the time required for the CN q O 2 reaction to take place, and the vibrational relaxation time of the nascent NCO. The decay portions of the transient signals were fit to exponential decay functions to obtain first order decay rates kX . A pseudofirst order analysis shows that kX s k wCH 2 COx q k D , where k is the desired bimolecular rate constant, and k D represents the rate of other decay processes for the detected radical, including self-reaction, reaction with other radicals, and diffusion out of the probed volume. The dependence of kX on ketene pressure is shown in Fig. 2 for both the CN q CH 2 CO and NCO q CH 2 CO
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M.A. Edwards, J.F. Hershbergerr Chemical Physics 234 (1998) 231–237
reactions. From the slopes of these plots, we obtain the bimolecular rate constants. Fig. 3 shows the NCO signal vs. time obtained using laser-induced fluorescence detection. The infrared and LIF detection techniques produced identical results within the quoted experimental errors for the NCO q CH 2 CO rate constant at 296 K. At elevated temperatures, infrared absorption spectroscopy of NCO gave insufficient signal-to-noise ratios for reliable measurements. NCO rate constants above room temperature were therefore measured exclusively by LIF. Fig. 4 shows Arrhenius plots for the CN q CH 2 CO and NCO q CH 2 CO reactions. Over the limited temperature range of these measurements, no significant deviation from Arrhenius behavior was observed. The rate constants Žin cm3 moleculey1 sy1 , 1 s error bars. are represented by:
Fig. 4. Arrhenius plot for the CNqCH 2 CO Žsquares. and NCOq CH 2 CO Žcircles. reactions.
k CN qCH 2 CO s Ž 2.37 " 0.5 . = 10y1 1 = exp w 552.6 " 97rT x k CN qCH 2 CO Ž 298 K . s Ž 1.5 " 0.2 . = 10y1 0 k NCOqCH 2 CO s Ž 1.71 " 0.27 . = 10y1 2 = exp w 713.2 " 56rT x k NCOqCH 2 CO Ž 298 K . s Ž 1.9 " 0.2 . = 10y1 1 Several secondary reactions could potentially affect these results. For example, in the NCO experi-
Fig. 3. NCO laser-induced fluorescence signal vs. time. Circles: 0.0 Torr CH 2 CO. Crosses: 0.025 Torr CH 2 CO. Other reaction conditions: P ICN s 0.1 Torr, PO 2 s1.0 Torr, PSF6 s1.0 Torr.
ment, several alternative removal mechanisms for NCO in addition to the NCO q CH 2 CO reaction are present. The reaction of NCO with O 2 is extremely slow, with k - 5 = 10y1 5 cm3 moleculey1 sy1 w25x, and is therefore not expected to be significant. Reaction of NCO with ICN precursor molecules is probably partially responsible for the nonzero intercept in Fig. 2, but does not affect the slope and therefore the determined rate constant because wICNx is essentially constant. Radical–radical reactions such as O q NCO ™ CO q NO and NCO q NCO ™ products could affect the results, however, if the radical density is too high. To examine this issue, kinetic modeling simulations were carried out using standard software w26x. We find that at a starting radical concentration of wCNx 0 s 2 = 10 13 molecule cmy3 Žappropriate for the infrared detection experiments., these reactions increase the predicted decay rate of NCO by only ; 5%, which is less than the quoted uncertainties. For the LIF experiments, lower radical densities were used, and radical–radical reactions are negligible. A negative activation energy is observed for both reactions Ž1. and Ž2.. These data suggest that hydrogen abstraction by CN or NCO are most likely not the dominant mechanism in these systems, as such processes usually involve significant potential energy barriers leading to positive activation energies. For
M.A. Edwards, J.F. Hershbergerr Chemical Physics 234 (1998) 231–237
example, hydrogen abstraction reactions of CN and NCO with saturated hydrocarbons invariably yield positive activation energies w27–30x. Instead, our kinetic data suggest that these reactions proceed by addition–elimination mechanism, as has been observed for reactions of CN and NCO with unsaturated hydrocarbons w31–33x. The reaction of CO with ketene is roughly half as fast as reactions of CN with alkenes of similar size. For example, CN q ethylene and CN q propylene have gas-kinetic rate constants at 298 K of 2.61 = 10y1 0 and 2.99 = 10y1 0 cm3 moleculey1 sy1 , respectively w34,35x. For NCO, the opposite trend is apparent. Quite slow rates for NCO q C 2 H 4 and NCO q C 2 H 2 of 5.0 = 10y1 2 Žhigh pressure limit. and 1.1 = 10y1 3 cm3 moleculey1 sy1 , respectively, have been reported w36,37x. 3.2. Product yields In order to better characterize the reaction mechanisms, the yield of carbon monoxide products produced by reactions Ž1a. and Ž2a. was measured at 298 K using infrared absorption spectroscopy. A typical transient absorption signal for CO produced by photolysis of an ICNrCH 2 COrO 2rCF4 mixture is shown in Fig. 5. To check for the possibility that some of this CO was formed by direct dissociation
Fig. 5. Transient signal for CO formation in the NCOqCH 2 CO reaction. Reaction conditions: P ICN s 0.1 Torr, PCH 2 CO s 0.1 Torr, PO 2 s 2.0 Torr, PCF4 s1.0 Torr.
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Fig. 6. CO product yield per initial CN radical as a function of CH 2 CO pressure. Reaction conditions: P ICN s 0.1 Torr, PO 2 s 0.0 Torr ŽCN reaction, crosses., 2.0 Torr ŽNCO reaction, circles., PCF 4 s1.0 Torr, PCH 2 CO s variable.
of ketene, transient signals upon photolysis of a CH 2 COrCF4 mixture were also collected, but were found to be negligible. This is unsurprising, as CH 2 CO has a very low absorption coefficient at 248 nm w38x. The peak amplitudes in these signals were converted to absolute number densities using tabulated linestrengths w20x and equations described in previous publications w19,39x. CF4 buffer gas was used in these experiments, as previous work has shown CF4 to be an efficient relaxer of CO vibrational excitation w19,39,40x. In addition, the initial radical density wCNx 0 was calculated using the measured photolysis laser pulse energy, and an absorption coefficient of ICN of 0.009 Torry1 cmy1 at 248 nm w38,39x. A quantum yield of unity was assumed. Fig. 6 shows the yield of CO Žin terms of wCOxrwCNx 0 . as a function of ketene pressure. When O 2 was omitted from the reaction mixture, the observed wCOxrwCNx0 yield approaches a value near unity in the limit of high CH 2 CO pressure. We attribute this observation to production of CO by the reaction channel Ž1a.. The data clearly indicate that CO formation is the dominant pathway in the CN q CH 2 CO reaction. We estimate an upper limit for the contribution of channel Ž1b. of less than 15%.
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M.A. Edwards, J.F. Hershbergerr Chemical Physics 234 (1998) 231–237
When O 2 is included in the reaction mixture, however, there is a competition between reactions Ž1. and Ž4. for CN radicals. Based on the rate constants of these reactions, we expect that at, for example, 0.4 Torr CH 2 CO and 2.0 Torr O 2 , approximately half of the CN radicals will be converted into NCO radicals by reaction Ž4.. NCO can then react with ketene via reaction Ž2. Žnote that NCO does not react with O 2 .. If the NCO q CH 2 CO reaction had a lower branching ratio into CO products than the CN q CH 2 CO reaction, one would therefore expect the observed CO yield to be lower compared to the yield obtained in the absence of molecular oxygen. In fact, we observe higher CO yields when O 2 is included in the reaction mixture, as shown in Fig. 6. This suggests the presence of additional routes for CO formation when O 2 is included. One likely candidate is O q CH 2 CO ™ CH 2 O q CO
Ž 5.
where the oxygen atom was formed in reaction Ž4.. Although Mack and Thrush w3x, in an early study using end product analysis, suggested that HCO q HCO were the primary products of reaction Ž5., it is likely that this reaction produces CO as an additional channel, or via secondary reactions of HCO radicals. If reaction Ž5. produces exclusively CO, either directly or via subsequent secondary chemistry, the sequence of reaction Ž4. followed by Ž2a. and Ž5. could potentially produce two CO molecules for every NCO radical formed. Since about half of the CN radicals are converted to NCO at the point of Fig. 6 corresponding to 0.4 Torr of CH 2 CO, one then predicts a wCOxrwCNx 0 ratio of approximately 1.5, in excellent agreement with our data. The product yield data is therefore consistent with the hypothesis that the NCO q CH 2 CO reaction produces CO in approximately unity yield. We estimate an upper limit of 20% for the contribution of channel Ž2b.. A final point is that the CO elimination pathways in reaction channels Ž1a. and Ž2a. are not necessarily the only routes to CO formation in these reactions. An alternative path would be CN addition across the carbon–carbon double bond followed by hydrogen atom elimination to form H q CHŽCN.CO, which could conceivably dissociate to HCCNq CO. A similar mechanism can be written for NCO. Since our experiment is only sensitive to the CO yield, we
cannot at present distinguish between this path and direct CO elimination to form CH 2 CN q CO, channel Ž1a.. Comparison with previous studies of halogen atom reactions with ketene w12,13x suggest that the CO elimination pathway as written in Ž1a. and Ž2a. is the more likely.
4. Conclusions Infrared absorption and laser-induced fluorescence techniques were used to study the CN q CH 2 CO and NCO q CH 2 CO reactions. Both reactions are fast, with a small negative activation energy. These observations combined with the observed yields of CO product molecules support the conclusion that these reaction proceed via an addition–elimination mechanism, probably forming CH 2 CN q CO or CH 2 NCO q CO, respectively.
Acknowledgements This work was supported by the Division of Chemical sciences, Office of Basic Energy Sciences of the Department of Energy ŽDE-FE03-96ER14645.. Partial support by the NSF EPSCoR program Žgrant aOSR-9452892. is also acknowledged.
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w26x W. Braun, J.T. Herron, K. Kahaner, Int. J. Chem. Kinet. 20 Ž1988. 51. w27x R.J. Balla, K.H. Casleton, J.S. Adams, L. Pasternack, J. Phys. Chem. 95 Ž1991. 8694. w28x D.L. Yang, T. Yu, N.S. Wang, M.C. Lin, Chem. Phys. 160 Ž1992. 307. w29x B. Atakan, J. Wolfrum, Chem. Phys. Lett. 186 Ž1991. 547. w30x A. Schuck, H.-R. Volpp, J. Wolfrum, Combust. Flame 99 Ž1994. 491. w31x D.L. Yang, T. Yu, N.S. Wang, M.C. Lin, Chem. Phys. 160 Ž1992. 317. w32x K.H. Becker, R. Kurtenbach, F. Schmidt, P. Wiesen, Chem. Phys. Lett. 235 Ž1995. 230. w33x K.H. Becker, R. Kurtenbach, P. Wiesen, J. Phys. Chem. 99 Ž1995. 5986. w34x D.A. Lichtin, M.C. Lin, Chem. Phys. 104 Ž1986. 325. w35x M.T. Butterfield, T. Yu, M.C. Lin, Chem. Phys. 169 Ž1993. 129. w36x K.H. Becker, R. Kurtenbach, P. Wiesen, J. Phys. Chem. 99 Ž1995. 5986. w37x K.H. Becker, R. Kurtenbach, F. Schmidt, P. Wiesen, Chem. Phys. Lett. 235 Ž1995. 230. w38x Okabe, Photochemistry of Small Molecules, Wiley-Interscience, 1978. w39x W.F. Cooper, J. Park, J.F. Hershberger, J. Phys. Chem. 97 Ž1993. 3283. w40x D.C. Richman, R.C. Millikan, J. Chem. Phys. 63 Ž1975. 2242.
Kinetics of the CN q CH 2 CO and NCO q CH 2 CO reactions Michael A. Edwards, John F. Hershberger
)
Department of Chemistry, North Dakota State UniÕersity, Fargo, ND 58105, USA Received 12 March 1998
Abstract The CN q CH 2 CO and NCO q CH 2 CO reactions were studied over the temperature ranges 296–567 K and 296–556 K, respectively, using time-resolved infrared diode laser absorption and visible laser-induced fluorescence spectroscopy. The total rate constant data were fit to the following expressions: k 1ŽCN q CH 2 CO. s Ž2.37 " 0.5. = 10y11 expw552.6 " 97rT x and k 2 ŽNCO q CH 2 CO. s Ž1.71 " 0.27. = 10y12 expw713.2 " 56rT x cm3 moleculey1 sy1. Detection and quantification of CO product yields suggests that an addition–elimination mechanism producing CH 2 CN q CO or CH 2 NCO q CO dominates these reactions, and that hydrogen abstraction to produce HCNq HCCO or HNCOq HCCO is a minor or negligible product channel. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Kinetics; Ketene; Radical reactions
1. Introduction The kinetics of reactions of ketene, CH 2 CO, are of interest because this compound is a commonly used photolytic precursor of CH 2 radicals in a variety of kinetics, spectroscopy, and energy transfer studies. Knowledge of the kinetics of radical–molecule reactions involving CH 2 CO is currently limited to a few reactions. The O q CH 2 CO, H q CH 2 O, and OH q CH 2 O reactions have been fairly well studied, although agreement between different studies is not very good, with reported rate constants at 298 K of Ž3.0–8.8. = 10y1 3 w1–4x, Ž6.2–24.5. = 10y1 4 w5–7x, and Ž1.2–3.4. = 10y1 1 w8–10x cm3 moleculey1 sy1 , respectively. Positive activation en-
)
Corresponding author.
ergies have been observed for O q CH 2 CO w1x and H q CH 2 O w6,7x, but a negative activation for the OH q CH 2 CO was observed w9x. In addition, the reactions F q CH 2 CO and Cl q CH 2 CO have also been studied at 298 K, with reported rate constants of 8.3 = 10y1 1 and 6.8 = 10y1 1 cm3 moleculey1 sy1 , respectively w11x. In a recent study, Grussdorf et al. w12x measured absolute yields of primary products in the reactions of F, Cl, and OH with ketene in a discharge flow apparatus. They found that these reactions proceed primarily via an addition–elimination mechanism, leading to CH 2 F, CH 2 Cl, or CH 2 OH as well as CO products. Hydrogen abstraction to form the ketenyl radical ŽHCCO. was observed to be a minor or negligible reaction pathway in these systems. CH 2 F products from the F q CH 2 CO reaction have also been detected by laser-induced fluorescence spectroscopy w13x.
0301-0104r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 1 0 4 Ž 9 8 . 0 0 1 7 4 - 8
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M.A. Edwards, J.F. Hershbergerr Chemical Physics 234 (1998) 231–237
In this work we report kinetic investigations of the following two reactions: CN q CH 2 CO ™ CH 2 CN q CO CN q CH 2 CO ™ HCN q HCCO NCO q CH 2 CO ™ CH 2 NCO q CO NCO q CH 2 CO ™ HNCOq HCCO
Ž 1a . Ž 1b . Ž 2a . Ž 2b .
No previous literature data is available for either of these reactions, or any other reactions of nitrogencontaining radicals with ketene. One motivation of the current study is to investigate whether these reactions proceed by similar mechanisms to reactions of halogen atoms with ketene, or whether hydrogen abstraction is a significant reaction channel. If the latter mechanism dominates, these reactions could represent clean sources of the important HCCO radical for subsequent kinetic studies. CN molecules were produced in these experiments by 248 nm excimer laser photolysis of ICN precursor molecules: ICN q hn Ž 248 nm . ™ I q CN Ž 3. In the NCO experiments, NCO molecules were formed by the reaction of CN with molecular oxygen: CN q O 2 ™ NCO q O Ž 4. which has a rate constant of Ž1.8–3.0. = 10y1 1 cm3 moleculey1 sy1 at 298 K w14–16x. Two detection techniques were used in this study. Time-resolved infrared diode laser absorption spectroscopy was used to probe CN and NCO radicals in order to measure total rate constants. This method works well for CN, but somewhat less well for NCO due to rather small infrared absorption coefficients of this radical. As a result, NCO measurements by infrared absorption were limited to experiments at room temperature. Laser-induced fluorescence was therefore used to detect NCO in some of the experiments, especially at elevated temperatures. In addition to total rate constant measurements, the yield of CO product molecules in each of these reactions was measured using infrared absorption spectroscopy. 2. Experimental The time-resolved infrared diode laser apparatus is similar to that described in previous publications
w17–19x. Briefly, 248 nm light from an excimer laser ŽLambda Physik Compex 200. was made collinear with tunable infrared radiation from a lead-salt diode laser ŽLaser Photonics. and co-propagated through a 119.4-cm Pyrex reaction cell. The transmitted infrared light was then passed into a 1r4-meter monochromator for laser mode selection and focused onto a 1-mm InSb infrared detector ŽCincinnati Electronics, ; 1 ms response time.. Transient absorption signals were collected and signal averaged on a Hitachi 6065 digital oscilloscope and transmitted to a personal computer for analysis. Transient signals were also collected with the infrared laser detuned ; 0.02 cmy1 off of the absorption lines. These off resonant signals are due to thermal deflection of the probe beam by the transient heating induced by the photolysis laser. To correct for this effect, the off resonant signals were subtracted from the on-resonant signals. Photolysis pulse energy was measured using a Molectron joulemeter. The following absorption lines were used: CN Ž n s 0, RŽ10. line at 2081.689 cmy1 .; NCO Ž000 PŽ16.5. line at approximately 1906.94 cmy1 .; and CO Ž n s 0, PŽ16. line at 2077.650 cmy1 .. The HITRAN database w20x was used to locate CO lines, while other published spectral data were used to locate CN w21,22x and NCO w23x lines. In the laser-induced fluorescence ŽLIF. experiments, NCO was detected by at approximately 413.3 nm using a 02 0 0 ŽA 2 Sq . § 00 1 0 ŽX 2 P . transition w24x. The probe light was produced by frequency doubling 826 nm light from a dye laser ŽContinuum ND-6000. pumped by the 2nd harmonic of an Nd:YAG laser ŽLumonics JK750.. This probe light was made collinear with the 248 nm excimer light and copropagated down a 119.4-cm reaction cell. Fluorescence through a side window was filtered to block scattered excimer light and detected by a photomultiplier tube ŽR508.. The unamplified PMT output was recorded by a boxcar integrator ŽStanford Instruments Model 250. with a 120 ns delay and a 300 ns gate width and averaged on a personal computer. The computer varied the delay between excimer and probe pulses in order to produce an wNCOx vs. time profile. ICN ŽAldrich. was purified by vacuum sublimation to remove dissolved air. SF6 and CF4 ŽMatheson. were purified by vacuum distillation at 77 K. O 2
M.A. Edwards, J.F. Hershbergerr Chemical Physics 234 (1998) 231–237
ŽMatheson. was used without further purification. CH 2 CO was synthesized by the pyrolysis of acetic anhydride at 970 K and purified by vacuum distillation. A ; 20% impurity of CO 2 as measured by infrared absorption spectroscopy was present in the CH 2 CO samples and could not be easily removed by distillation. The measured CH 2 CO pressures in the kinetics experiments were therefore corrected for the presence of this impurity Žneither CN nor NCO react with CO 2 at moderate temperatures.. Typical reaction conditions were 0.2 Torr ICN, 0.0 Torr O 2 ŽCN experiments., 1.0–2.0 Torr O 2 ŽNCO experiments., 0.0–0.25 Torr CH 2 CO, and 1.0 Torr SF6 or CF4 buffer gas. Typical radical densities were ; 2 = 10 13 moleculesrcm3 for the infrared experiments and - 1 = 10 13 moleculesrcm3 for the laser-induced fluorescence experiments. These experiments were therefore pseudo-first order with wCH 2 COx 4 wCNx,wNCOx. 3. Results and discussion 3.1. Total rate constants Fig. 1 shows a typical transient signal for CN and NCO molecules obtained by transient infrared ab-
Fig. 1. Transient infrared absorption signal for CN and NCO molecules. Trace A: NCO signal without CH 2 CO reactant. Trace B: NCO signal with 0.1 Torr CH 2 CO reactant. Trace C: CN signal without CH 2 CO reactant. Trace D: CN signal with 0.1 Torr CH 2 CO reactant. Other conditions: P ICN s 0.3 Torr, PO 2 s 2.0 Torr ŽNCO signals only., PSF 6 s1.0 Torr.
233
Fig. 2. Pseudofirst order decay rates for CN Žtriangles. and NCO Žcircles. as a function of CH 2 CO pressure. Reaction conditions: P ICN s 0.3 Torr, PO 2 s 0.0 Torr ŽCN data., 2.0 Torr ŽNCO data., PSF 6 s1.0 Torr. Data shown was obtained at 296 K.
sorption spectroscopy. The rapid rise in the CN signal is attributed to formation of CN by direct photolysis of ICN. Although this precursor produces CN with significant amounts of rotational and vibrational excitation, the SF6 buffer gas used is expected to quickly relax these molecules to a Boltzmann distribution. The decay in the transient signals is attributed to reaction of CN molecules as well as diffusion out of the probed volume. When oxygen is included in the reaction mixture, NCO transient signals are obtained, as shown in the upper two traces of Fig. 1. A short induction period of ; 3–5 ms was observed for the NCO signals. This is probably due to a combination of several factors, including the time required for the CN q O 2 reaction to take place, and the vibrational relaxation time of the nascent NCO. The decay portions of the transient signals were fit to exponential decay functions to obtain first order decay rates kX . A pseudofirst order analysis shows that kX s k wCH 2 COx q k D , where k is the desired bimolecular rate constant, and k D represents the rate of other decay processes for the detected radical, including self-reaction, reaction with other radicals, and diffusion out of the probed volume. The dependence of kX on ketene pressure is shown in Fig. 2 for both the CN q CH 2 CO and NCO q CH 2 CO
234
M.A. Edwards, J.F. Hershbergerr Chemical Physics 234 (1998) 231–237
reactions. From the slopes of these plots, we obtain the bimolecular rate constants. Fig. 3 shows the NCO signal vs. time obtained using laser-induced fluorescence detection. The infrared and LIF detection techniques produced identical results within the quoted experimental errors for the NCO q CH 2 CO rate constant at 296 K. At elevated temperatures, infrared absorption spectroscopy of NCO gave insufficient signal-to-noise ratios for reliable measurements. NCO rate constants above room temperature were therefore measured exclusively by LIF. Fig. 4 shows Arrhenius plots for the CN q CH 2 CO and NCO q CH 2 CO reactions. Over the limited temperature range of these measurements, no significant deviation from Arrhenius behavior was observed. The rate constants Žin cm3 moleculey1 sy1 , 1 s error bars. are represented by:
Fig. 4. Arrhenius plot for the CNqCH 2 CO Žsquares. and NCOq CH 2 CO Žcircles. reactions.
k CN qCH 2 CO s Ž 2.37 " 0.5 . = 10y1 1 = exp w 552.6 " 97rT x k CN qCH 2 CO Ž 298 K . s Ž 1.5 " 0.2 . = 10y1 0 k NCOqCH 2 CO s Ž 1.71 " 0.27 . = 10y1 2 = exp w 713.2 " 56rT x k NCOqCH 2 CO Ž 298 K . s Ž 1.9 " 0.2 . = 10y1 1 Several secondary reactions could potentially affect these results. For example, in the NCO experi-
Fig. 3. NCO laser-induced fluorescence signal vs. time. Circles: 0.0 Torr CH 2 CO. Crosses: 0.025 Torr CH 2 CO. Other reaction conditions: P ICN s 0.1 Torr, PO 2 s1.0 Torr, PSF6 s1.0 Torr.
ment, several alternative removal mechanisms for NCO in addition to the NCO q CH 2 CO reaction are present. The reaction of NCO with O 2 is extremely slow, with k - 5 = 10y1 5 cm3 moleculey1 sy1 w25x, and is therefore not expected to be significant. Reaction of NCO with ICN precursor molecules is probably partially responsible for the nonzero intercept in Fig. 2, but does not affect the slope and therefore the determined rate constant because wICNx is essentially constant. Radical–radical reactions such as O q NCO ™ CO q NO and NCO q NCO ™ products could affect the results, however, if the radical density is too high. To examine this issue, kinetic modeling simulations were carried out using standard software w26x. We find that at a starting radical concentration of wCNx 0 s 2 = 10 13 molecule cmy3 Žappropriate for the infrared detection experiments., these reactions increase the predicted decay rate of NCO by only ; 5%, which is less than the quoted uncertainties. For the LIF experiments, lower radical densities were used, and radical–radical reactions are negligible. A negative activation energy is observed for both reactions Ž1. and Ž2.. These data suggest that hydrogen abstraction by CN or NCO are most likely not the dominant mechanism in these systems, as such processes usually involve significant potential energy barriers leading to positive activation energies. For
M.A. Edwards, J.F. Hershbergerr Chemical Physics 234 (1998) 231–237
example, hydrogen abstraction reactions of CN and NCO with saturated hydrocarbons invariably yield positive activation energies w27–30x. Instead, our kinetic data suggest that these reactions proceed by addition–elimination mechanism, as has been observed for reactions of CN and NCO with unsaturated hydrocarbons w31–33x. The reaction of CO with ketene is roughly half as fast as reactions of CN with alkenes of similar size. For example, CN q ethylene and CN q propylene have gas-kinetic rate constants at 298 K of 2.61 = 10y1 0 and 2.99 = 10y1 0 cm3 moleculey1 sy1 , respectively w34,35x. For NCO, the opposite trend is apparent. Quite slow rates for NCO q C 2 H 4 and NCO q C 2 H 2 of 5.0 = 10y1 2 Žhigh pressure limit. and 1.1 = 10y1 3 cm3 moleculey1 sy1 , respectively, have been reported w36,37x. 3.2. Product yields In order to better characterize the reaction mechanisms, the yield of carbon monoxide products produced by reactions Ž1a. and Ž2a. was measured at 298 K using infrared absorption spectroscopy. A typical transient absorption signal for CO produced by photolysis of an ICNrCH 2 COrO 2rCF4 mixture is shown in Fig. 5. To check for the possibility that some of this CO was formed by direct dissociation
Fig. 5. Transient signal for CO formation in the NCOqCH 2 CO reaction. Reaction conditions: P ICN s 0.1 Torr, PCH 2 CO s 0.1 Torr, PO 2 s 2.0 Torr, PCF4 s1.0 Torr.
235
Fig. 6. CO product yield per initial CN radical as a function of CH 2 CO pressure. Reaction conditions: P ICN s 0.1 Torr, PO 2 s 0.0 Torr ŽCN reaction, crosses., 2.0 Torr ŽNCO reaction, circles., PCF 4 s1.0 Torr, PCH 2 CO s variable.
of ketene, transient signals upon photolysis of a CH 2 COrCF4 mixture were also collected, but were found to be negligible. This is unsurprising, as CH 2 CO has a very low absorption coefficient at 248 nm w38x. The peak amplitudes in these signals were converted to absolute number densities using tabulated linestrengths w20x and equations described in previous publications w19,39x. CF4 buffer gas was used in these experiments, as previous work has shown CF4 to be an efficient relaxer of CO vibrational excitation w19,39,40x. In addition, the initial radical density wCNx 0 was calculated using the measured photolysis laser pulse energy, and an absorption coefficient of ICN of 0.009 Torry1 cmy1 at 248 nm w38,39x. A quantum yield of unity was assumed. Fig. 6 shows the yield of CO Žin terms of wCOxrwCNx 0 . as a function of ketene pressure. When O 2 was omitted from the reaction mixture, the observed wCOxrwCNx0 yield approaches a value near unity in the limit of high CH 2 CO pressure. We attribute this observation to production of CO by the reaction channel Ž1a.. The data clearly indicate that CO formation is the dominant pathway in the CN q CH 2 CO reaction. We estimate an upper limit for the contribution of channel Ž1b. of less than 15%.
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M.A. Edwards, J.F. Hershbergerr Chemical Physics 234 (1998) 231–237
When O 2 is included in the reaction mixture, however, there is a competition between reactions Ž1. and Ž4. for CN radicals. Based on the rate constants of these reactions, we expect that at, for example, 0.4 Torr CH 2 CO and 2.0 Torr O 2 , approximately half of the CN radicals will be converted into NCO radicals by reaction Ž4.. NCO can then react with ketene via reaction Ž2. Žnote that NCO does not react with O 2 .. If the NCO q CH 2 CO reaction had a lower branching ratio into CO products than the CN q CH 2 CO reaction, one would therefore expect the observed CO yield to be lower compared to the yield obtained in the absence of molecular oxygen. In fact, we observe higher CO yields when O 2 is included in the reaction mixture, as shown in Fig. 6. This suggests the presence of additional routes for CO formation when O 2 is included. One likely candidate is O q CH 2 CO ™ CH 2 O q CO
Ž 5.
where the oxygen atom was formed in reaction Ž4.. Although Mack and Thrush w3x, in an early study using end product analysis, suggested that HCO q HCO were the primary products of reaction Ž5., it is likely that this reaction produces CO as an additional channel, or via secondary reactions of HCO radicals. If reaction Ž5. produces exclusively CO, either directly or via subsequent secondary chemistry, the sequence of reaction Ž4. followed by Ž2a. and Ž5. could potentially produce two CO molecules for every NCO radical formed. Since about half of the CN radicals are converted to NCO at the point of Fig. 6 corresponding to 0.4 Torr of CH 2 CO, one then predicts a wCOxrwCNx 0 ratio of approximately 1.5, in excellent agreement with our data. The product yield data is therefore consistent with the hypothesis that the NCO q CH 2 CO reaction produces CO in approximately unity yield. We estimate an upper limit of 20% for the contribution of channel Ž2b.. A final point is that the CO elimination pathways in reaction channels Ž1a. and Ž2a. are not necessarily the only routes to CO formation in these reactions. An alternative path would be CN addition across the carbon–carbon double bond followed by hydrogen atom elimination to form H q CHŽCN.CO, which could conceivably dissociate to HCCNq CO. A similar mechanism can be written for NCO. Since our experiment is only sensitive to the CO yield, we
cannot at present distinguish between this path and direct CO elimination to form CH 2 CN q CO, channel Ž1a.. Comparison with previous studies of halogen atom reactions with ketene w12,13x suggest that the CO elimination pathway as written in Ž1a. and Ž2a. is the more likely.
4. Conclusions Infrared absorption and laser-induced fluorescence techniques were used to study the CN q CH 2 CO and NCO q CH 2 CO reactions. Both reactions are fast, with a small negative activation energy. These observations combined with the observed yields of CO product molecules support the conclusion that these reaction proceed via an addition–elimination mechanism, probably forming CH 2 CN q CO or CH 2 NCO q CO, respectively.
Acknowledgements This work was supported by the Division of Chemical sciences, Office of Basic Energy Sciences of the Department of Energy ŽDE-FE03-96ER14645.. Partial support by the NSF EPSCoR program Žgrant aOSR-9452892. is also acknowledged.
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