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A diode laser study of the product branching ratio of the CH+N2O reaction
28 November 1997
Chemical Physics Letters 280 Ž1997. 145–150
A diode laser study of the product branching ratio of the CH q N2 O reaction Nicole Hovda, John F. Hershberger Department of Chemistry, North Dakota State UniÕersity, Fargo, ND 58105, USA Received 20 June 1997
Abstract The reactions of CH and CD radicals with N2 O were studied at room temperature by an excimer photolysisrinfrared absorption technique, using multiphoton photolysis of CHBr3 or CDBr3 at 248 nm. Two product channels were detected: HCN q NO in 72 " 4% yield, and CO q H q N2 in 28 " 4% yield. No detectable change in the branching ratios upon deuteration was observed. q 1997 Published by Elsevier Science B.V.
1. Introduction Reactions of hydrocarbon fragments are of great interest in the kinetic modeling of flames. The methylidyne radical ŽCH., for example, is believed to play a crucial role in the prompt-NO formation mechanism w1x as well as the NO-reburning process in which NO x-containing exhaust gases produced in fossil fuel combustion are recycled into a hydrocarbon-rich zone w2,3x. Previous kinetic investigations of CH radical reactions have concentrated primarily on total rate constant measurements, with comparatively few product studies. The rate constant of the CH q NO reaction has been measured by numerous groups w4–9x. A previous study of the deuterated CD q NO reaction in our laboratory indicated that at least three product channels of this reaction are active at room temperature; the DCN q O channel accounts for the largest fraction of 48% of the total rate w10x. Available kinetic data on other CH reactions with nitrogen oxides includes CH q NO 2 w5x, and CH q N2 O w4,5,11–13x.
In this study we report measurements of the product branching ratios of the CH q N2 O reaction. Several previous studies of the room temperature rate constant show fairly good agreement, with a range of Ž6.9–8.5. = 10y1 1 cm3 moleculey1 sy1 reported w4,5,7,8x. In the first study, Wagal et al. suggested a large number of energetically accessible channels w5x. Possible products include: CH q N2 O ™ NO q HCN, D H s y450.7 kJrmol,
Ž 1a .
CH q N2 O ™ CO q N2 q H, D H s y568.7 kJrmol,
Ž 1b .
CH q N2 O ™ N2 q HCO,
D H s y632.7 kJrmol, Ž 1c .
CH q N2 O ™ N q HCNO,
D H s y29.2 kJrmol, Ž 1d .
0009-2614r97r$17.00 q 1997 Published by Elsevier Science B.V. All rights reserved. PII S 0 0 0 9 - 2 6 1 4 Ž 9 7 . 0 1 0 7 9 - 8
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N. HoÕda, J.F. Hershbergerr Chemical Physics Letters 280 (1997) 145–150
CH q N2 O ™ N q HNCO, D H s y305.2 kJrmol,
2. Experimental
Ž 1e .
CH q N2 O ™ NH q NCO, D H s y168.2 kJrmol,
Ž 1f .
CH q N2 O ™ CN q HNO, D H s y141.5 kJrmol.
Ž 1g .
Two groups have extended measurements to elevated temperatures. Zabarnick et al. obtained k 1 s Ž1.59 " 0.17. = 10y10 exp wŽ500 " 45.rT x cm 3 moleculey1 sy1 over the temperature range 300–670 K w12x. Becker et al. reported k 1 s Ž3.07 " 0.07. = 10y1 1 exp wŽ257 " 9.6.rT x cm3 moleculey1 sy1 over the range 300–1300 K w4x. These studies are in good agreement below 500 K, but deviate somewhat at higher temperatures. Both groups reported the absence of a pressure dependence on the total rate constant. Zabarnick et al. suggested that channels Ž1a. and Ž1c. are most mechanistically reasonable, as they do not require excessive rearrangement, but that HCO formed in channel Ž1c. may have sufficient energy to dissociate to the products in channel Ž1b. w12x. To our knowledge, no direct measurements of the products of this reaction have been reported. We use CHBr3 or CDBr3 as radical precursors via multiphoton absorption of near-UV light: CHBr3 q hn Ž 248 nm . ™ CH q Br q Br2 , CDBr3 q hn Ž 248 nm . ™ CD q Br q Br2 . Deuterated precursor molecules were used in some of the experiments because one of the major products, HCN, lies outside the range of our laser diodes. CHBr3 has been used as a CH radical precursor in several previous studies w6,10,12,14–16x and is known to efficiently generate CH radicals. Excited state CHŽa 4 S . radicals may also be produced in this system, but they are relatively unreactive w17,18x, with a reported rate constant of - 1.0 = 10y1 3 cm3 moleculey1 sy1 with N2 O w18x. Our previous study of the CH q NO reaction demonstrated the effectiveness of using Xe buffer gas to relax electronic excited states of CH w10x. Although no direct measurements of this relaxation process have been reported, xenon is known to efficiently relax NHŽ 1D . to ground state NHŽ 3 S . w19,20x.
The experimental methods have been described in previous publications w10,21,22x. Briefly, ultraviolet light from an excimer laser ŽLambda Physik Compex 200. operating on the KrF line at 248 nm was weakly focused by a 1 m focal length CaF2 lens into the center of a 1.46 m pyrex reaction cell. Infrared probe light from a lead-salt diode laser ŽLaser Photonics. was collimated and made collinear with the UV beam by means of a dichroic beamsplitter. Both beams were copropagated through the cell, after which the UV beam was removed by a second beamsplitter. The infrared light then passed through a 1r4 m monochromator and was focused onto an InSb detector Ž1 ms response time.. Transient absorption signals were collected on a LeCroy 9310A digital oscilloscope and stored on a personal computer. Typical reaction conditions were 0.1 Torr CHBr3 , 0–3 Torr N2 O, 1.0 Torr Xe, and 1.0 Torr SF6 or CF4 buffer gas. CHBr3 and CDBr3 ŽAldrich. were purified by repeated freeze–pump–thaw cycles to remove dissolved air. Other reagents ŽMatheson. were purified by vacuum distillation, primarily to remove traces of CO and NO from CF4 and N2 O, respectively. The HITRAN database w23x was used as an aid in the identification of spectral lines for CO and NO. Other published spectral data were used to search for CN w24,25x, HCO w26x, HNCO w27x, and HCNO w28x. For DCN, a 0.25 cmy1 resolution FTIR scan was used to identify with sufficient accuracy the line positions of the Ž00 0 0. ™ Ž00 0 1. band. Calibration of infrared absorption coefficients of DCN was performed by measuring the fractional absorption of the diode laser radiation as a function of DCN pressure.
3. Results and discussion Typical time-resolved absorption signals are shown in Fig. 1. To obtain these signals, transient signals were obtained with the diode laser tuned on resonance with a desired spectral line as well as detuned ; 0.02 cmy1 off resonance. The off reso-
N. HoÕda, J.F. Hershbergerr Chemical Physics Letters 280 (1997) 145–150
nance signals are due to thermal deflection of the infrared beam upon transient heating of the reagent gas by the photolysis laser. Because of the focused photolysis beam required, these artifacts are quite large in these experiments and are highly sensitive to beam alignment. By fine adjustment of the infrared beam alignment the long-time component of the off-resonant signal was adjustable to acceptable levels, although a large short-time component Ž; 30 ms. remained. The absorption signals shown were obtained by subtraction of the off-resonant signal from the on-resonant signal. Absorption signals were converted to absolute number densities using equations described in earlier publications w10,21,22x. We assume that the probed molecules are relaxed to a room temperature Boltzmann distribution of rotational and vibrational states on the timescale of these experiments. Previous work has shown that this is the case for most molecules provided SF6 buffer gas is included in the reaction mixture. The one exception is CO, which is not
147
vibrationally relaxed by SF6 . CF4 buffer gas, which is more efficient at relaxing vibrationally excited CO, was therefore used whenever CO products were probed. Total pressures were low enough so that pressure broadening effects were negligible. Furthermore, we probed only low rotational levels near the maximum in the rotational Boltzmann distribution, so effects from transient heating of the reaction mixture are minimized. Several control experiments were performed in order to verify that the observed signals originated from the title reaction. When the bromoform precursor was omitted from the reaction mixture, no NO or CO signals were observed. This indicates that multiphoton dissociation of species other than the intended radical precursor Žfor example, NO formation from N2 O photolysis. does not significantly affect these experiments. Fig. 2 shows the effect of addition of Xe buffer gas on the yield of CO. Within experimental uncertainties, no change in product yields were observed
Fig. 1. Transient absorption signals from the CD q N2 O reaction. Lines probed: CO Ž0 ™ 1., PŽ5. at 2123.699 cmy1 ; DCN Ž00 0 0 ™ 00 0 1., PŽ14. at 2594.631 cmy1 ; NO Ž0 ™ 1. RŽ5.5. at 1896.984 cmy1 . PCD BR 3 s 0.1 Torr, P N 2 O s 2.0 Torr, PXe s 1.0 Torr, PCF4 s 1.0 Torr ŽCO transient only., PSF 6 s 1.0 Torr ŽNO and DCN transients..
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N. HoÕda, J.F. Hershbergerr Chemical Physics Letters 280 (1997) 145–150
Fig. 2. Product yields of the CHqN2 O reaction as a function of Xe pressure. Reaction conditions: PCH Br 3 s 0.2 Torr, PN 2 O s 2.0 Torr, PCF 4 s1.0 Torr.
upon addition of Xe. The result was somewhat surprising, because in previous work on the CD q NO reaction, product yields were found to increase substantially with increasing wXex, strongly suggesting that xenon was playing a role in the relaxation of electronically excited states of CD w10x. One possible explanation for the different behavior observed here is that CHŽa 4 S . may be efficiently relaxed in our reaction mixture even in the absence of Xe. In any case, most of the product yield experiments were performed with 1.0 Torr of xenon. The measured product yields of CO and NO from the CH q N2 O reaction are shown as a function of
Fig. 3. Product yields of the CHqN2 O reaction as a function of N2 O pressure. Reaction conditions: PCHBr 3 s 0.1 Torr, PXe s1.0 Torr, PSF 6 s1.0 Torr ŽNO products only., PCF4 s1.0 Torr ŽCO products only..
Fig. 4. Product yields of the CDqN2 O reaction as a function of N2 O pressure. Reaction conditions: PCDBr 3 s 0.1 Torr, PXe s1.0 Torr, PSF 6 s1.0 Torr ŽNO and DCN products only., PCF4 s1.0 Torr ŽCO products only..
reagent N2 O pressure in Fig. 3. The yields of CO, NO, and DCN from the CD q N2 O reaction are shown in Fig. 4. In general, product yields were observed to increase with increasing wN2 Ox, and begin to level off around ; 2 Torr of N2 O. This effect is probably caused by the competition of reaction Ž1. with other CH removal routes at low wN2 Ox. These routes may include self reaction, reaction with Br, Br2 , or CHBr3 , and diffusion out of the probed region. Note that due to the requirement of a moderately focused photolysis laser beam in these experiments, the initial wCHx 0 density is nonuniform, quite large, and not well characterized in the focal region. It therefore is not surprising that quite high N2 O pressures are required to suppress radical–radical chemistry and ensure that most of the CH radicals are removed by the title reaction. The ratios of product yields measured does not depend significantly on wN2 Ox, provided that P N 2 O ) 0.5 Torr, indicating that CH removal by processes other than reaction Ž1. are sufficiently suppressed at these high N2 O pressures so as to not affect the measured branching ratios. The interpretation of the measured product yields as primary branching ratios is quite straitforward for this reaction system because few radicals other than CH react readily with N2 O. For example, OH q N2 O w29x and CN q N2 O w30x are too slow to measure at room temperature, with k < 1.0 = 10y1 5 cm3 mole-
N. HoÕda, J.F. Hershbergerr Chemical Physics Letters 280 (1997) 145–150
culey1 sy1 . In the case of NCO q N2 O and HCO q N2 O, no data exist, but NCO and HCO are generally less reactive than CN, and it is highly unlikely that they react with N2 O. Thus secondary reactions of these possible products with the N2 O reagent are not significant in this experiment. The observed agreement between NO and DCN yields in the CD q N2 O reaction provides further confidence that the observed products are produced solely by the title reaction. Based on the average of five experimental runs, we obtain the a ratio of wDCNxrwCOx s 2.56 " 0.4 and wNOxrwCOx s 2.77 " 0.46 for the CD q N2 O reaction, where the uncertainty represents one standard deviation. For CH q N2 O, we obtain wNOxrwCOx s 2.53 " 0.24. If we assume that only channels Ž1a. and Ž1b. contribute, and that no secondary sources of the observed products are significant, the following branching ratios are obtained: f 1a s 0.28 " 0.04 and f 1b s 0.72 " 0.04. Within the experimental uncertainties, the branching ratios for CH q N2 O and CD q N2 O are identical. Attempts were made to detect several other products, including CN, HCO, HNCO, and HCNO. No measurable transients for any of these species were obtainable; however, trace amounts of HNCO and HCNO were found to appear in the static gas infrared spectra after prolonged photolysis of the reaction mixture Ž; 50 laser shots.. Since we cannot detect these species on a transient signal timescale Ž; 100 ms., we cannot exclude the possibility that any HNCO or HCNO is formed by slow secondary or heterogeneous reactions. Infrared absorption coefficients are unavailable for HNCO and HCNO, preventing an upper limit determination of f 1d or f 1e , but it is clear that these channels are very minor at best. From the lack of CN or HCO signals, we estimate the following upper limits: f 1g - 0.10, f 1c - 0.12. Unfortunately, sensitive detection of shortlived species like CN and HCO is difficult in these experiments due to the off resonant signals described above that are particularly large at early detection times. Thus we cannot exclude the possibility of small contributions by product channels other than Ž1a. and Ž1b.. No attempt was made to detect products from channel Ž1f.. No previous experimental data or ab initio calculations on the product channels of this reaction have
149
been reported. Based on our results, however, the following mechanism is suggested. CH attack of N2 O at the terminal nitrogen atom forms a HCNNO complex; subsequent N–N bond fission yields HCN q NO products. CH attack at the oxygen atom forms a NNOCH complex which can dissociate to N2 q HCO products. The HCO then has enough energy to fragment into CO q H. Alternatively, this complex could dissociate through a cyclic four- or five-center transition state to form HN2 q CO, followed by rapid fragmentation of the HN2 radical, which is well known to be an unstable species w31x. Other possible but less likely mechanisms include attack at the central nitrogen to form a NNŽCH.O complex followed by dissociation via three-center transition states to form the observed product channels. High level ab initio calculations are clearly needed to gain insight regarding these pathways.
4. Conclusions The CH q N2 O reaction was studied at room temperature by infrared diode laser spectroscopic detection of products. The major product channel found was NO q HCN, but a significant CO forming channel was also observed. No HrD isotope effect on the branching ratios was observed.
Acknowledgements This work was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences of the Department of Energy Ž DE-FG0396ER14645.. Acknowledgement is also made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this work.
References w1x J.A. Miller, C.T. Bowman, Prog. Energy Combut. Sci. 15 Ž1989. 287. w2x W.S. Lanier, J.A. Mulholland, J.T. Beard, Symp. ŽInt.. Combust. wProc.x 21 Ž1988. 1171. w3x S.L. Chen, J.M. McCarthy, W.D. Clark, M.P. Heap, W.R. Seeker, D.W. Pershing, Symp. ŽInt.. Combust. wProc.x 21 Ž1988. 1159.
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w4x K.H. Becker, B. Engelhardt, H. Geiger, R. Kurtenbach, P. Wiesen, Chem. Phys. Lett. 210 Ž1993. 135. w5x S.S. Wagal, T. Carrington, S.V. Filseth, C.M. Sadowski, Chem. Phys. 69 Ž1982. 61. w6x D.A. Lichtin, M.R. Berman, M.C. Lin, Chem. Phys. Lett. 108 Ž1984. 18. w7x A.J. Dean, R.K. Hanson, C.T. Bowman, J. Phys. Chem. 95 Ž1991. 3180. w8x S. Okada, K. Yamasaki, H. Matsui, K. Saito, K. Okada, Bull. Chem. Soc. Jpn 66 Ž1993. 1004. w9x J.E. Butler, J.W. Fleming, L.P. Goss, M.C. Lin, Chem. Phys. 56 Ž1981. 355. w10x R. Lambrecht, J.F. Hershberger, J. Phys. Chem. 98 Ž1994. 8406. w11x K.H. Becker, B. Engelhardt, P. Wiesen, K.D. Bayes, Chem. Phys. Lett. 154 Ž1989. 342. w12x S. Zabarnick, J.W. Fleming, M.C. Lin, Int. J. Chem. Kinet. 21 Ž1989. 765. w13x S.M. Anderson, A. Freedman, C.E. Kolb, J. Phys. Chem. 91 Ž1987. 6272. w14x A. McIlroy, F.P. Tully, J. Chem. Phys. 99 Ž1993. 3597. w15x C.A. Taatjes, J. Chem. Phys. 106 Ž1997. 1786. w16x C.A. Taatjes, J. Phys. Chem. 100 Ž1996. 17840. w17x Z. Hou, K.D. Bayes, J. Phys. Chem. 96 Ž1992. 5685.
w18x Z. Hou, K.D. Bayes, J. Phys. Chem. 97 Ž1993. 1896. w19x W. Hack, A. Wilms, J. Phys. Chem. 93 Ž1989. 3540. w20x H.H. Nelson, J.R. McDonald, M.H. Alexander, J. Phys. Chem. 94 Ž1990. 3291. w21x W.F. Cooper, J. Park, J.F. Hershberger, J. Phys. Chem. 97 Ž1993. 3283. w22x R.W. Quandt, J.F. Hershberger, J. Phys. Chem. 100 Ž1996. 9407. w23x L.S. Rothman et al, J. Quant. Spectrosc. Radiat. Transfer 48 Ž1992. 469. w24x D. Cerny, R. Bacis, G. Guelachvili, F. Roux, J. Mol. Spectrosc. 73 Ž1978. 154. w25x P.B. Davies, P.A. Hamilton, J. Chem. Phys. 76 Ž1982. 2127. w26x A.R. McKellar, J.B. Burkholder, J.J. Orlando, C.J. Howard, J. Mol. Spectrosc. 130 Ž1988. 445. w27x D.A. Steiner, S.R. Polo, T.K. McCubbin Jr., K.A. Wishah, J. Mol. Spectrosc. 98 Ž1983. 453. w28x E.L. Ferretti, K.N. Rao, J. Mol. Spectrosc. 51 Ž1974. 97. w29x W. Tsang, J.T. Herron, J. Phys. Chem. Ref. Data 20 Ž1991. 609. w30x N.S. Wang, D.L. Yang, M.C. Lin, Int. J. Chem. Kinet. 23 Ž1991. 151. w31x S.P. Walch, R.J. Duchovic, C.M. Rohlfing, J. Chem. Phys. 90 Ž1989. 3230.
Chemical Physics Letters 280 Ž1997. 145–150
A diode laser study of the product branching ratio of the CH q N2 O reaction Nicole Hovda, John F. Hershberger Department of Chemistry, North Dakota State UniÕersity, Fargo, ND 58105, USA Received 20 June 1997
Abstract The reactions of CH and CD radicals with N2 O were studied at room temperature by an excimer photolysisrinfrared absorption technique, using multiphoton photolysis of CHBr3 or CDBr3 at 248 nm. Two product channels were detected: HCN q NO in 72 " 4% yield, and CO q H q N2 in 28 " 4% yield. No detectable change in the branching ratios upon deuteration was observed. q 1997 Published by Elsevier Science B.V.
1. Introduction Reactions of hydrocarbon fragments are of great interest in the kinetic modeling of flames. The methylidyne radical ŽCH., for example, is believed to play a crucial role in the prompt-NO formation mechanism w1x as well as the NO-reburning process in which NO x-containing exhaust gases produced in fossil fuel combustion are recycled into a hydrocarbon-rich zone w2,3x. Previous kinetic investigations of CH radical reactions have concentrated primarily on total rate constant measurements, with comparatively few product studies. The rate constant of the CH q NO reaction has been measured by numerous groups w4–9x. A previous study of the deuterated CD q NO reaction in our laboratory indicated that at least three product channels of this reaction are active at room temperature; the DCN q O channel accounts for the largest fraction of 48% of the total rate w10x. Available kinetic data on other CH reactions with nitrogen oxides includes CH q NO 2 w5x, and CH q N2 O w4,5,11–13x.
In this study we report measurements of the product branching ratios of the CH q N2 O reaction. Several previous studies of the room temperature rate constant show fairly good agreement, with a range of Ž6.9–8.5. = 10y1 1 cm3 moleculey1 sy1 reported w4,5,7,8x. In the first study, Wagal et al. suggested a large number of energetically accessible channels w5x. Possible products include: CH q N2 O ™ NO q HCN, D H s y450.7 kJrmol,
Ž 1a .
CH q N2 O ™ CO q N2 q H, D H s y568.7 kJrmol,
Ž 1b .
CH q N2 O ™ N2 q HCO,
D H s y632.7 kJrmol, Ž 1c .
CH q N2 O ™ N q HCNO,
D H s y29.2 kJrmol, Ž 1d .
0009-2614r97r$17.00 q 1997 Published by Elsevier Science B.V. All rights reserved. PII S 0 0 0 9 - 2 6 1 4 Ž 9 7 . 0 1 0 7 9 - 8
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N. HoÕda, J.F. Hershbergerr Chemical Physics Letters 280 (1997) 145–150
CH q N2 O ™ N q HNCO, D H s y305.2 kJrmol,
2. Experimental
Ž 1e .
CH q N2 O ™ NH q NCO, D H s y168.2 kJrmol,
Ž 1f .
CH q N2 O ™ CN q HNO, D H s y141.5 kJrmol.
Ž 1g .
Two groups have extended measurements to elevated temperatures. Zabarnick et al. obtained k 1 s Ž1.59 " 0.17. = 10y10 exp wŽ500 " 45.rT x cm 3 moleculey1 sy1 over the temperature range 300–670 K w12x. Becker et al. reported k 1 s Ž3.07 " 0.07. = 10y1 1 exp wŽ257 " 9.6.rT x cm3 moleculey1 sy1 over the range 300–1300 K w4x. These studies are in good agreement below 500 K, but deviate somewhat at higher temperatures. Both groups reported the absence of a pressure dependence on the total rate constant. Zabarnick et al. suggested that channels Ž1a. and Ž1c. are most mechanistically reasonable, as they do not require excessive rearrangement, but that HCO formed in channel Ž1c. may have sufficient energy to dissociate to the products in channel Ž1b. w12x. To our knowledge, no direct measurements of the products of this reaction have been reported. We use CHBr3 or CDBr3 as radical precursors via multiphoton absorption of near-UV light: CHBr3 q hn Ž 248 nm . ™ CH q Br q Br2 , CDBr3 q hn Ž 248 nm . ™ CD q Br q Br2 . Deuterated precursor molecules were used in some of the experiments because one of the major products, HCN, lies outside the range of our laser diodes. CHBr3 has been used as a CH radical precursor in several previous studies w6,10,12,14–16x and is known to efficiently generate CH radicals. Excited state CHŽa 4 S . radicals may also be produced in this system, but they are relatively unreactive w17,18x, with a reported rate constant of - 1.0 = 10y1 3 cm3 moleculey1 sy1 with N2 O w18x. Our previous study of the CH q NO reaction demonstrated the effectiveness of using Xe buffer gas to relax electronic excited states of CH w10x. Although no direct measurements of this relaxation process have been reported, xenon is known to efficiently relax NHŽ 1D . to ground state NHŽ 3 S . w19,20x.
The experimental methods have been described in previous publications w10,21,22x. Briefly, ultraviolet light from an excimer laser ŽLambda Physik Compex 200. operating on the KrF line at 248 nm was weakly focused by a 1 m focal length CaF2 lens into the center of a 1.46 m pyrex reaction cell. Infrared probe light from a lead-salt diode laser ŽLaser Photonics. was collimated and made collinear with the UV beam by means of a dichroic beamsplitter. Both beams were copropagated through the cell, after which the UV beam was removed by a second beamsplitter. The infrared light then passed through a 1r4 m monochromator and was focused onto an InSb detector Ž1 ms response time.. Transient absorption signals were collected on a LeCroy 9310A digital oscilloscope and stored on a personal computer. Typical reaction conditions were 0.1 Torr CHBr3 , 0–3 Torr N2 O, 1.0 Torr Xe, and 1.0 Torr SF6 or CF4 buffer gas. CHBr3 and CDBr3 ŽAldrich. were purified by repeated freeze–pump–thaw cycles to remove dissolved air. Other reagents ŽMatheson. were purified by vacuum distillation, primarily to remove traces of CO and NO from CF4 and N2 O, respectively. The HITRAN database w23x was used as an aid in the identification of spectral lines for CO and NO. Other published spectral data were used to search for CN w24,25x, HCO w26x, HNCO w27x, and HCNO w28x. For DCN, a 0.25 cmy1 resolution FTIR scan was used to identify with sufficient accuracy the line positions of the Ž00 0 0. ™ Ž00 0 1. band. Calibration of infrared absorption coefficients of DCN was performed by measuring the fractional absorption of the diode laser radiation as a function of DCN pressure.
3. Results and discussion Typical time-resolved absorption signals are shown in Fig. 1. To obtain these signals, transient signals were obtained with the diode laser tuned on resonance with a desired spectral line as well as detuned ; 0.02 cmy1 off resonance. The off reso-
N. HoÕda, J.F. Hershbergerr Chemical Physics Letters 280 (1997) 145–150
nance signals are due to thermal deflection of the infrared beam upon transient heating of the reagent gas by the photolysis laser. Because of the focused photolysis beam required, these artifacts are quite large in these experiments and are highly sensitive to beam alignment. By fine adjustment of the infrared beam alignment the long-time component of the off-resonant signal was adjustable to acceptable levels, although a large short-time component Ž; 30 ms. remained. The absorption signals shown were obtained by subtraction of the off-resonant signal from the on-resonant signal. Absorption signals were converted to absolute number densities using equations described in earlier publications w10,21,22x. We assume that the probed molecules are relaxed to a room temperature Boltzmann distribution of rotational and vibrational states on the timescale of these experiments. Previous work has shown that this is the case for most molecules provided SF6 buffer gas is included in the reaction mixture. The one exception is CO, which is not
147
vibrationally relaxed by SF6 . CF4 buffer gas, which is more efficient at relaxing vibrationally excited CO, was therefore used whenever CO products were probed. Total pressures were low enough so that pressure broadening effects were negligible. Furthermore, we probed only low rotational levels near the maximum in the rotational Boltzmann distribution, so effects from transient heating of the reaction mixture are minimized. Several control experiments were performed in order to verify that the observed signals originated from the title reaction. When the bromoform precursor was omitted from the reaction mixture, no NO or CO signals were observed. This indicates that multiphoton dissociation of species other than the intended radical precursor Žfor example, NO formation from N2 O photolysis. does not significantly affect these experiments. Fig. 2 shows the effect of addition of Xe buffer gas on the yield of CO. Within experimental uncertainties, no change in product yields were observed
Fig. 1. Transient absorption signals from the CD q N2 O reaction. Lines probed: CO Ž0 ™ 1., PŽ5. at 2123.699 cmy1 ; DCN Ž00 0 0 ™ 00 0 1., PŽ14. at 2594.631 cmy1 ; NO Ž0 ™ 1. RŽ5.5. at 1896.984 cmy1 . PCD BR 3 s 0.1 Torr, P N 2 O s 2.0 Torr, PXe s 1.0 Torr, PCF4 s 1.0 Torr ŽCO transient only., PSF 6 s 1.0 Torr ŽNO and DCN transients..
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N. HoÕda, J.F. Hershbergerr Chemical Physics Letters 280 (1997) 145–150
Fig. 2. Product yields of the CHqN2 O reaction as a function of Xe pressure. Reaction conditions: PCH Br 3 s 0.2 Torr, PN 2 O s 2.0 Torr, PCF 4 s1.0 Torr.
upon addition of Xe. The result was somewhat surprising, because in previous work on the CD q NO reaction, product yields were found to increase substantially with increasing wXex, strongly suggesting that xenon was playing a role in the relaxation of electronically excited states of CD w10x. One possible explanation for the different behavior observed here is that CHŽa 4 S . may be efficiently relaxed in our reaction mixture even in the absence of Xe. In any case, most of the product yield experiments were performed with 1.0 Torr of xenon. The measured product yields of CO and NO from the CH q N2 O reaction are shown as a function of
Fig. 3. Product yields of the CHqN2 O reaction as a function of N2 O pressure. Reaction conditions: PCHBr 3 s 0.1 Torr, PXe s1.0 Torr, PSF 6 s1.0 Torr ŽNO products only., PCF4 s1.0 Torr ŽCO products only..
Fig. 4. Product yields of the CDqN2 O reaction as a function of N2 O pressure. Reaction conditions: PCDBr 3 s 0.1 Torr, PXe s1.0 Torr, PSF 6 s1.0 Torr ŽNO and DCN products only., PCF4 s1.0 Torr ŽCO products only..
reagent N2 O pressure in Fig. 3. The yields of CO, NO, and DCN from the CD q N2 O reaction are shown in Fig. 4. In general, product yields were observed to increase with increasing wN2 Ox, and begin to level off around ; 2 Torr of N2 O. This effect is probably caused by the competition of reaction Ž1. with other CH removal routes at low wN2 Ox. These routes may include self reaction, reaction with Br, Br2 , or CHBr3 , and diffusion out of the probed region. Note that due to the requirement of a moderately focused photolysis laser beam in these experiments, the initial wCHx 0 density is nonuniform, quite large, and not well characterized in the focal region. It therefore is not surprising that quite high N2 O pressures are required to suppress radical–radical chemistry and ensure that most of the CH radicals are removed by the title reaction. The ratios of product yields measured does not depend significantly on wN2 Ox, provided that P N 2 O ) 0.5 Torr, indicating that CH removal by processes other than reaction Ž1. are sufficiently suppressed at these high N2 O pressures so as to not affect the measured branching ratios. The interpretation of the measured product yields as primary branching ratios is quite straitforward for this reaction system because few radicals other than CH react readily with N2 O. For example, OH q N2 O w29x and CN q N2 O w30x are too slow to measure at room temperature, with k < 1.0 = 10y1 5 cm3 mole-
N. HoÕda, J.F. Hershbergerr Chemical Physics Letters 280 (1997) 145–150
culey1 sy1 . In the case of NCO q N2 O and HCO q N2 O, no data exist, but NCO and HCO are generally less reactive than CN, and it is highly unlikely that they react with N2 O. Thus secondary reactions of these possible products with the N2 O reagent are not significant in this experiment. The observed agreement between NO and DCN yields in the CD q N2 O reaction provides further confidence that the observed products are produced solely by the title reaction. Based on the average of five experimental runs, we obtain the a ratio of wDCNxrwCOx s 2.56 " 0.4 and wNOxrwCOx s 2.77 " 0.46 for the CD q N2 O reaction, where the uncertainty represents one standard deviation. For CH q N2 O, we obtain wNOxrwCOx s 2.53 " 0.24. If we assume that only channels Ž1a. and Ž1b. contribute, and that no secondary sources of the observed products are significant, the following branching ratios are obtained: f 1a s 0.28 " 0.04 and f 1b s 0.72 " 0.04. Within the experimental uncertainties, the branching ratios for CH q N2 O and CD q N2 O are identical. Attempts were made to detect several other products, including CN, HCO, HNCO, and HCNO. No measurable transients for any of these species were obtainable; however, trace amounts of HNCO and HCNO were found to appear in the static gas infrared spectra after prolonged photolysis of the reaction mixture Ž; 50 laser shots.. Since we cannot detect these species on a transient signal timescale Ž; 100 ms., we cannot exclude the possibility that any HNCO or HCNO is formed by slow secondary or heterogeneous reactions. Infrared absorption coefficients are unavailable for HNCO and HCNO, preventing an upper limit determination of f 1d or f 1e , but it is clear that these channels are very minor at best. From the lack of CN or HCO signals, we estimate the following upper limits: f 1g - 0.10, f 1c - 0.12. Unfortunately, sensitive detection of shortlived species like CN and HCO is difficult in these experiments due to the off resonant signals described above that are particularly large at early detection times. Thus we cannot exclude the possibility of small contributions by product channels other than Ž1a. and Ž1b.. No attempt was made to detect products from channel Ž1f.. No previous experimental data or ab initio calculations on the product channels of this reaction have
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been reported. Based on our results, however, the following mechanism is suggested. CH attack of N2 O at the terminal nitrogen atom forms a HCNNO complex; subsequent N–N bond fission yields HCN q NO products. CH attack at the oxygen atom forms a NNOCH complex which can dissociate to N2 q HCO products. The HCO then has enough energy to fragment into CO q H. Alternatively, this complex could dissociate through a cyclic four- or five-center transition state to form HN2 q CO, followed by rapid fragmentation of the HN2 radical, which is well known to be an unstable species w31x. Other possible but less likely mechanisms include attack at the central nitrogen to form a NNŽCH.O complex followed by dissociation via three-center transition states to form the observed product channels. High level ab initio calculations are clearly needed to gain insight regarding these pathways.
4. Conclusions The CH q N2 O reaction was studied at room temperature by infrared diode laser spectroscopic detection of products. The major product channel found was NO q HCN, but a significant CO forming channel was also observed. No HrD isotope effect on the branching ratios was observed.
Acknowledgements This work was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences of the Department of Energy Ž DE-FG0396ER14645.. Acknowledgement is also made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this work.
References w1x J.A. Miller, C.T. Bowman, Prog. Energy Combut. Sci. 15 Ž1989. 287. w2x W.S. Lanier, J.A. Mulholland, J.T. Beard, Symp. ŽInt.. Combust. wProc.x 21 Ž1988. 1171. w3x S.L. Chen, J.M. McCarthy, W.D. Clark, M.P. Heap, W.R. Seeker, D.W. Pershing, Symp. ŽInt.. Combust. wProc.x 21 Ž1988. 1159.
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w4x K.H. Becker, B. Engelhardt, H. Geiger, R. Kurtenbach, P. Wiesen, Chem. Phys. Lett. 210 Ž1993. 135. w5x S.S. Wagal, T. Carrington, S.V. Filseth, C.M. Sadowski, Chem. Phys. 69 Ž1982. 61. w6x D.A. Lichtin, M.R. Berman, M.C. Lin, Chem. Phys. Lett. 108 Ž1984. 18. w7x A.J. Dean, R.K. Hanson, C.T. Bowman, J. Phys. Chem. 95 Ž1991. 3180. w8x S. Okada, K. Yamasaki, H. Matsui, K. Saito, K. Okada, Bull. Chem. Soc. Jpn 66 Ž1993. 1004. w9x J.E. Butler, J.W. Fleming, L.P. Goss, M.C. Lin, Chem. Phys. 56 Ž1981. 355. w10x R. Lambrecht, J.F. Hershberger, J. Phys. Chem. 98 Ž1994. 8406. w11x K.H. Becker, B. Engelhardt, P. Wiesen, K.D. Bayes, Chem. Phys. Lett. 154 Ž1989. 342. w12x S. Zabarnick, J.W. Fleming, M.C. Lin, Int. J. Chem. Kinet. 21 Ž1989. 765. w13x S.M. Anderson, A. Freedman, C.E. Kolb, J. Phys. Chem. 91 Ž1987. 6272. w14x A. McIlroy, F.P. Tully, J. Chem. Phys. 99 Ž1993. 3597. w15x C.A. Taatjes, J. Chem. Phys. 106 Ž1997. 1786. w16x C.A. Taatjes, J. Phys. Chem. 100 Ž1996. 17840. w17x Z. Hou, K.D. Bayes, J. Phys. Chem. 96 Ž1992. 5685.
w18x Z. Hou, K.D. Bayes, J. Phys. Chem. 97 Ž1993. 1896. w19x W. Hack, A. Wilms, J. Phys. Chem. 93 Ž1989. 3540. w20x H.H. Nelson, J.R. McDonald, M.H. Alexander, J. Phys. Chem. 94 Ž1990. 3291. w21x W.F. Cooper, J. Park, J.F. Hershberger, J. Phys. Chem. 97 Ž1993. 3283. w22x R.W. Quandt, J.F. Hershberger, J. Phys. Chem. 100 Ž1996. 9407. w23x L.S. Rothman et al, J. Quant. Spectrosc. Radiat. Transfer 48 Ž1992. 469. w24x D. Cerny, R. Bacis, G. Guelachvili, F. Roux, J. Mol. Spectrosc. 73 Ž1978. 154. w25x P.B. Davies, P.A. Hamilton, J. Chem. Phys. 76 Ž1982. 2127. w26x A.R. McKellar, J.B. Burkholder, J.J. Orlando, C.J. Howard, J. Mol. Spectrosc. 130 Ž1988. 445. w27x D.A. Steiner, S.R. Polo, T.K. McCubbin Jr., K.A. Wishah, J. Mol. Spectrosc. 98 Ž1983. 453. w28x E.L. Ferretti, K.N. Rao, J. Mol. Spectrosc. 51 Ž1974. 97. w29x W. Tsang, J.T. Herron, J. Phys. Chem. Ref. Data 20 Ž1991. 609. w30x N.S. Wang, D.L. Yang, M.C. Lin, Int. J. Chem. Kinet. 23 Ž1991. 151. w31x S.P. Walch, R.J. Duchovic, C.M. Rohlfing, J. Chem. Phys. 90 Ž1989. 3230.