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Reaction rates of the reactions of OH(X2Π ) with NH(a1Δ ) and HN3(X̃)
11 June 1999
Chemical Physics Letters 306 Ž1999. 111–116
Reaction rates of the reactions of OH žX 2 P / with NH ža 1D / and ˜/ HN3 žX W. Hack ) , R. Jordan
1
Max-Planck-Institut fur Bunsenstraße 10, D-37073 Gottingen, Germany ¨ Stromungsforschung, ¨ ¨ Received 11 March 1999; in final form 19 April 1999
Abstract The rate constants of the radical–radical reaction OHŽX 2 P . q NHŽa 1D . ™ products and the radical–molecule reaction OHŽX 2 P . q HN3 ™ products were determined in a quasistatic laser photolysis cell at room temperature and a total pressure of 20 mbar. The radicals OHŽX 2 P . and NHŽa 1D . were generated by exciplex laser photolysis of H 2 O 2 and HN3 , respectively and detected by laser-induced fluorescence. The fluorescence time profiles of these radicals were monitored at variable delay times between photolysis and detection pulses and thus time resolution was obtained. The measurements yielded the rate constants k 1 s Ž7.0 " 2. = 10 14 cm3 moly1 sy1 and k 2 s Ž7.7 " 1. = 10 11 cm3 moly1 sy1. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction There are few direct experimental data for the H 2 –N–O system. The dihydronitrosyl radical H 2 NOŽ 2 B 1 . was observed via its microwave spectrum w1x. The reaction between hydrogen atoms and nitroxyl ŽHNO.: H q HNO ™ H 2 q NO is known to be the fast step in the nitric oxide catalysed recombination of H atoms. The reverse reaction H 2 q NO ™ HNO q H proceeds with a large activation energy of EA s 230 kJ moly1 w2–4x. The atom–radical reaction in the H 2 –N–O system O q NH 2 ™ products was found to be fast at room temperature with a rate coefficient k s 5.3 = 10 13 cm3 moly1 sy1 w5x. It has ) Corresponding author. Fax: q49 551 5176 665; e-mail: [email protected] 1 Present addres: Max-Planck-Institut fur ¨ biophysikalische Chemie, Am Faßberg 11, D-37077 Gottingen. ¨
two reaction pathways O q NH 2 ™ HNO q H Ž k s 4.6 = 10 13 cm3 moly1 sy1 . and O q NH 2 ™ NHŽX. q OH Ž k s 7 = 10 12 cm3 moly1 sy1 . w5x. The atom–molecule reaction in the H 2 –N–O system N q H 2 O ™ product has, to the best of our knowledge, not yet been studied directly. A new approach to the H 2 –N–O system is the radical–radical reaction NH Ž a 1D . q OH Ž X 2 P . ™ products
Ž 1.
The reaction will Ždue to the high excitation energy of NHŽa.. not be important in thermal systems but it might be of interest in photochemical reaction systems. Moreover, it is of fundamental interest as one of the few singlet–doublet reactions of NHŽa 1D .. The reaction of electronically excited imino radicals ŽNHŽa 1D .. with ground state molecules in singlet states like H 2 O or hydrocarbons Že.g. CH 4 , C 2 H 6 , C 3 H 8 . proceeds via complex formation of an intermediate in the singlet electronic ground state.
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 4 3 7 - 6
112
W. Hack, R. Jordanr Chemical Physics Letters 306 (1999) 111–116
The dynamics of these electronic chemically activated molecules was studied for a large number of systems w6–8x. The reaction of the singlet imino radical with a doublet species has been studied only with NOŽX 2 P .. In these reactions, a doublet intermediate is formed which gives rise to crossing over to the triplet surface, i.e. quenching. The aim of this study was to measure the rate of the radical–radical Reaction Ž1. – a doublet species with NH in the first electronically excited singlet state. In this system also, the reaction of the OH radicals with the HN precursor molecule HN3
˜ . ™ products OH Ž X2 P . q HN3 Ž X
Ž 2.
has to be considered quantitatively.
2. Experimental The experiments were performed in a quasistatic reaction cell Ži.e. the duration between two photolysis pulses is large enough to yield a complete exchange of the gas volume; on the time scale between pump and probe pulse the flow is negligible. with a photolysisrLIF system. The experimental arrangement is described in detail elsewhere w7,9x. Briefly, it consists of a photolysis and a detection laser system. The latter consists of a dye laser ŽFL3002, Lambda Physik. that was optically pumped by an exiplex laser ŽLPX 205, Lambda Physik.. The pump laser pulses had energies of 200–400 mJ and a duration of t s 14 ns ŽFMHW. Ž l s 308 nm, XeCl.. The probe beam has a cross-section of about 9 mm2 and pulse energy densities of 20 ( ErmJ cmy2 ( 220. Frequency doubled light was obtained by a KPD crystal ŽFL30, Lambda Physik. and led to energy densities between 2 and 3 mJrcm2 . In all experiments, the pulse energy was high enough to saturate the transition to the electronically excited states. For the photolysis, an exiplex laser ŽLPX 205, Lambda Physik, l s 248 nm, KrF. was used. The photolysis beam had a cross-section of about 1.4 cm2 with pulse energy densities of 0.7 ( ErmJ cmy2 ( 23. A variable delay between the pulse of the photolysis and the probe laser in the range 0 ( t R rms ( 400 yielded the time resolution. The OHŽX 2 P . radicals were produced by the photolysis of H 2 O 2 . This photolysis process has
been investigated in Refs. w10–12x and leads exclusively to OH-radicals in the state OHŽX 2 P, Õ s 0.. The reaction of OH with H 2 O 2 , OH q H 2 O 2 ™ products ,
Ž 3.
has been taken into consideration Žsee below.. The absorption coefficient is ´ 248 nm ŽH 2 O 2 . s 9.3 = 10y2 0 cm2 moleculey1 w13–16x, and the photolysis quantum yield F 248 nm ŽH 2 O 2 . s 2.09 " 0.36 w17,18x. The OH-concentration was observed by the Q 1Ž1.-line of the OHŽA 2 Sq, Õ s 0 § X 2 P, Õ s 0. transition at l s 307.85 nm. The emitted fluorescence was observed perpendicular to the laser beams. After passing a filter, to reduce scattered light, the photons were detected by a photomultiplier tube ŽR955, Hammatsu.. The NHŽa 1D . radicals were produced in the HN3-photolysis which leads mainly to NHŽa 1D . in the vibronic states Õ s 0 and Õ s 1 with high rotational energies w19x. Under the experimental conditions applied Ž20 mbar He., the rotational energies are relaxed by collisions with the inert gas He to room temperature in a time of 1 ms after the photolysis w20,21x. The transition NHŽc 1 P, ÕX s 0,§ a 1D, ÕY s 0. was used to follow the NHŽa 1D . concentration time profile over the wavelength range 325 ( lrnm( 328. All measurements were done with He as the carrier gas at room temperature and a pressure of 20 mbar. Gases were supplied with the highest commercially available purities ŽHe ) 99.9999%, Praxair.. HN3 was produced by melting a mixture of stearic acid ŽC 17 H 35 COOH, Merck. and sodium azide ŽNaN3 , Merck.. The gas was dried with CaCl 2 and stored in a bulb Ž p s 1 bar. diluted in Ar Ž) 99.9999%, Praxair.. The maximum molefraction of HN3 was X HN 3 ( 0.17. Commercially available H 2 O 2 Ž85%, Solvay Interox. was enriched to 99% by trap-to-trap distillation at T s 78 K. The H 2 O 2 concentration in the liquid was measured via KMnO4 titration. The H 2 O 2 concentration in the gas phase on the way to the reaction cell was determined by freezing the gas stream in a N2 Žl.-trap and titrating the amount of H 2 O 2 frozen in a given time.
W. Hack, R. Jordanr Chemical Physics Letters 306 (1999) 111–116
3. Results Reaction Ž1. was investigated by observing NHŽa 1D . concentration time profiles under pseudofirst-order conditions with a large excess of OHŽX 2 P . over NHŽa. in the range 2 = 10 2 ( wOHŽX 2 P .x ts0rwNHŽa 1D .x ts0 ( 2 = 10 3. The OH radicals were produced by the photolysis of H 2 O 2 . The OH concentration was determined from the H 2 O 2 photolysis via:
w OHx s
F P EL P l P ´ P w H 2 O 2 x P ln 10 hPcPA
Ž I.
where: the quantum yield for OH was accepted to be F s 2.0 w18x rather than the lower value given in Ref. w17x Žsee Section 4.; EL s 30 mJ, l s 248 nm, ´ Ž248 nm . s 9.3 = 10 y 20 cm 2 molecule y 1 w16,22,23x wH 2 O 2 x s hydrogen peroxide concentration as given in Table 1, A is the cross-section of the photolysis laser A s 1.4 cm2 . It has to be considered that the concentration of H 2 O 2 in the reaction cell was about a factor of a hundred higher than the OHŽX 2 P . concentration. The NHŽa. depletion in the reaction system is given by: d NH Ž a . dt
s yk 1 NH Ž a . w OH x y k 4 NH Ž a . = w H 2 O 2 x y k 5 NH Ž a . w NH 3 x .
The rate constant for the reaction NH Ž a 1D . q H 2 O 2 ™ products
Ž 4.
was measured under the experimental conditions and was found to be k 4 s 7.1 = 10 13 cm3 moly1 sy1 , in
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good agreement with the literature value k 4 s 7.5 = 10 13 cm3 moly1 sy1 w24,25x. NHŽa. is depleted by HN3 in the reaction NH Ž a . q HN3 ™ products
Ž 5. 13
3
with a rate coefficient k 5 s 6.4 = 10 cm moly1 sy1 w8x. The OH radicals are depleted by other reactions: Reaction Ž3., OH Ž X 2 P . q H 2 O 2 ™ products with k 3 s 1.2 = 10 12 cm3 moly1 sy1 w26x, and Reaction Ž2. with the rate constant given in this Letter. Reaction Ž2. can be neglected due to the small HN3 concentrations applied in the experiments to determine k 1. For wHN3 x s 2 = 10y1 1 mol cmy3 , Reaction Ž2. contributes only a small amount to the consumption of the initial OH concentration i.e. wOHxŽ t s 20 ms.rwOHx o s 0.9997. The OH consumption via Reaction Ž3. has to be taken into consideration. This can be done by taking the initial slope Ži.e. for reaction times smaller than 5 ms. in the ln wNHŽa.x versus time plot to obtain the first-order rate constant. However, no significant deviation from linearity was observed in these plots. The variation of wOHxo was performed in two different ways: Ži. the photolysis laser fluence was varied at constant H 2 O 2 concentration, and Žii. the H 2 O 2 concentration was varied at constant photolysis laser fluence. Ži. To distinguish the NHŽa 1D . depletion by Reaction Ž1. from the depletion by Reaction Ž4. with H 2 O 2 , the photolysis laser power was varied and thus the OHŽX 2 P . concentration as given in Fig. 1.
Table 1 Experimental conditions for the reaction NHŽa 1D, Õ s 0. q OHŽX 2 P . X
T ŽK.
p Žmbar.
wHN3 x Ž10y11 mol cmy3 .
wH 2 O 2 x Ž10y9 mol cmy3 .
wOHŽX 2 P .x Ž10y11 mol cmy3 .
k1 Ž10 4 sy1 .
295 295 295 295 295 295 295 295
20 20 20 20 20 20 20 20
2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2
0.727 1.45 2.09 2.82 3.64 4.36 5.09 5.82
0.8 1.6 2.3 3.1 4.0 4.8 5.6 6.4
0.6 1.2 1.7 2.4 3.0 3.5 4.2 4.8
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W. Hack, R. Jordanr Chemical Physics Letters 306 (1999) 111–116
Fig. 1. First-order rate constant as a function of photolysis laser fluence. v, First-order rate constant for Reaction Ž1..
It was observed that with decreasing laser power, kX1 also decreases. This can be explained by a lower contribution of the fast Reaction Ž1. to the NHŽa 1D . depletion. Žii. A second method to obtain different OH concentrations was applied by varying the H 2 O 2 concentration. The data are given in Table 1. The pseudo-first-order rate constant kX1 for a given H 2 O 2 concentration was obtained by subtracting kX4 at measurement conditions. Reaction Ž1. contributed approximately 10% to the total NHŽa 1D . depletion, the main contribution is NHŽa. q H 2 O 2 .
Fig. 3. Semilogarithmic plot of the concentration profiles of OHŽX 2 P . versus reaction time in the presence of various HN3 concentrations: wHN3 x in =10y9 mol cmy3 ; v, 0.2; B, 2.7; `, 4.1; I, 6.1; ^, 8.9.
By varying the H 2 O 2 concentration at fixed photolysis laser power, the rate constant k 1 was determined from a plot of kX1 versus the OHŽX 2 P . concentration under first-order conditions as shown in Fig. 2. The results look much better than expected from the unfavourable kinetic situation. The slope of the plot in Fig. 2 resulted in the second-order rate constant: k 1 s Ž 7.0 " 2 . = 10 14 cm3 moly1 sy1 . The large error takes into account that the contribution of Reaction Ž1. is smaller than that of Reaction Ž4.. Reaction Ž2. was followed by the OHŽX 2 P . depletion under pseudo-first-order conditions in the presence of a large excess of HN3 over OHTable 2 Experimental conditions for the determination of the rate constant k2 X wOHx 0 wHN3 x T p k2 ŽK. Žmbar. Ž10y12 mol cmy3 . Ž10y9 mol cmy3 . Ž10 3 sy1 .
Fig. 2. First-order rate constant as a function of the OHŽX 2 P . concentration for the reaction of NHŽa 1D, Õ s 0. with OHŽX 2 P ..
294 294 294 294 294 294 294 294
20 20 20 20 20 20 20 20
7 7 7 7 7 7 7 7
0.0 0.2 0.8 2.7 4.1 6.1 7.9 8.9
1.29 1.31 1.40 3.42 4.21 5.57 7.72 7.69
W. Hack, R. Jordanr Chemical Physics Letters 306 (1999) 111–116
Fig. 4. First-order rate constants versus wHN3 x for Reaction 2.
ŽX 2 P . ts0 , i.e. 29 ( wHN3 xrwOHŽX 2 P .x ts0 ( 1270. The OHŽX 2 P . concentration as a function of time is shown at various HN3 concentrations in Fig. 3. The experimental conditions for this reaction are summarized in Table 2. Reaction Ž1. also causes depletion of OH radicals. The initial NHŽa. concentrations are in the range 0.2 ( wNHŽa.xor10y1 2 mol cmy3 ( 9. But the fast reaction of NHŽa. with the HN3 , present in large concentration, reduces the NHŽa. concentration within the first 20 ms to wNHŽa.xŽ t .rwNHŽa.xo ( 1 = 10y5 ; a value which is insignificant for the OH concentration profiles. The pseudo-first-order rate constants as a function of the HN3 concentration is shown in Fig. 4. The intercept is determined by Reaction Ž3. The rate constant calculated from the intercept given in Fig. 3 is k 3 s Ž2.0 " 0.3. = 10 12 cm3 moly1 sy1 . This value can be compared with the value of k 3 s 1.0 = 10 12 cm3 moly1 sy1 given in the literature w22,26x. From the slope in Fig. 4, the rate constant k 2 s Ž7.7 " 0.3. = 10 11 cm3 moly1 sy1 was obtained for Reaction Ž2. at room temperature.
115
though obtained under unfavourable kinetic conditions. The rate constant k 1 was determined by following the NHŽa 1D . depletion in the presence of excess OHŽX 2 P . and H 2 O 2 . The room temperature rate constant is k 1Ž298 K. s 7.0 = 10 14 cm3 moly1 sy1 which is extremely fast compared to other radical–radical reactions. It is by far the fastest reaction of NHŽa. which has been observed. A lower photolysis quantum yield for the H 2 O 2 laser photolysis would lead to an even higher value of k 1. Also taking into account OH consuming reactions would change k 1 in the same direction. From the large value of k 1 it can be stated that it is unlikely that quenching of NHŽa. by OHŽ2 P . is a significant NHŽa. – depletion channel of Reaction Ž1.. Reaction Ž1. has besides the quenching channel NH Ž a 1D . q OH Ž X 2 P . ™ NH Ž X . q OH Ž X . D R H s y150.3 kJ moly1
Ž 1q .
several exothermic reaction pathways:
˜. NH Ž a . q OH ™ NHOH 2AX Ž X D R H s y499.7 kJ moly1
Ž 1a .
NH Ž a . q OH ™ NO Ž X 2 P . q H 2 Ž X . D R H s y474.5 kJ moly1
Ž 1b .
˜. NH Ž a . q OH ™ N Ž 4 S . q H 2 O Ž X D R H s y334.8 kJ moly1
Ž 1c .
˜ 2 B1 . q O Ž 3 P. NH Ž a . q OH ™ NH 2 Ž X D R H s y127.7 kJ moly1
Ž 1d .
Reactions Ž1b. and Ž1c. can be regarded as the decompositions of the initially formed HN–OH radical. For Reaction Ž2., the probable pathway is H atom abstraction via OH q HN3 ™ H 2 O q N3
4. Discussion The rate constant of Reaction Ž1. has not yet been determined directly. It may be of importance in several H–N–O photolytic systems. Therefore it is necessary give this value of the rate constant al-
which is exothermic with D R H s y160 kJ mol -1 . Also the formation of HO–NH via
˜ . ™ HONH Ž X˜ 2AX . q N2 Ž X . OH Ž X 2 P . q HN3 Ž X D R H s y266.9 kJ moly1
W. Hack, R. Jordanr Chemical Physics Letters 306 (1999) 111–116
116
and the formation of NOŽX 2 P . via
˜ . ™ NO Ž X P . q H 2 q N2 OH Ž X P . q HN3 Ž X 2
2
D R H s y241.7 kJ moly1 are possible from the thermodynamic point of view. ˜ . is endothermic: The formation of NH 2 ŽX
˜ . ™ NH 2 Ž X˜ . q N2 Ž X . q O Ž 3 P . OH Ž X . q HN3 Ž X D R H s q105.1 kJ moly1 By comparison of the rate constant with those other H atom abstraction reactions, e.g. for CH 4 , NH 3 and HCN, it can be assumed that the H atom abstraction is the main reaction pathway. Acknowledgements The continued interest of Prof. H.Gg. Wagner is gratefully acknowledged. Thanks are due to the Deutsche Forschungsgemeinschaft ŽSonderforschungsbereich 357 ‘Molekulare Mechanismen unimolekularer Prozesse’. and the Fonds der Chemischen Industrie for financial support. References w1x H. Mikami, S. Saito, S. Yamamoto, J. Chem. Phys. 94 Ž1991. 3415. w2x D.L. Baulch, D.D. Drysdale, D.G. Horne, A.C. Lloyd, Evaluated Kinetic Data for High Temperature Reactions, Butterworth, 1973. w3x D.J. Nesbitt, Report University of Colorado, Boulder, CO, AFOSR-TR-90-0514. w4x L.T. Molina, S.D. Schinke, M.J. Molina, Geophys. Res. Lett. 4 Ž1977. 580. w5x P. Dransfeld, W. Hack, H. Kurzke, F. Temps, H.Gg. Wagner, 20th Symposium ŽInt.. on Combustion, 1984, pp. 655.
w6x W. Hack, K. Rathmann, Ber. Bunsenges. Phys. Chem. 94 Ž1990. 1304. w7x W. Hack, K. Rathmann, Z. Phys. Chem. 176 Ž1992. 151. w8x W. Hack, in: Z.B. Alfassi ŽEd.., NH Radical Reactions in N-centered Radicals, John Wiley, 1998, p. 413. w9x W. Hack, A. Wilms, Z. Phys. Chem. ŽNF. 161 Ž1989. 107. w10x S. Klee, K.-H. Gericke, F.J. Comes, J. Chem. Phys. 85 Ž1986. 40. w11x M.P. Docker, A. Hodgson, J.P. Simons, Chem. Phys. Lett. 128 Ž1986. 264. w12x G. Ondrey, N. van Veen, R. Bersohn, J. Chem. Phys. 78 Ž1983. 3732. w13x L.T. Molina, M.J. Molina, J. Photochem. 15 Ž1981. 97. w14x C.L. Lin, N.K. Rohatgi, W.B. De More, Geophys. Res. Lett. 5 Ž1978. 113. w15x L.T. Molina, S.D. Schinke, M.J. Molina, Geophys. Res. Lett. 4 Ž1977. 580. w16x G.L. Vaghjiani, A.R. Ravishankara, J. Geophys. Res. 94 Ž1989. 3487. w17x A. Schiffman, D.D. Nelson, D.J. Nesbitt, J. Chem. Phys. 98 Ž1993. 6935. w18x G.L. Vaghjiani, A.R. Ravishankara, J. Chem. Phys. 92 Ž1990. 996. w19x T. Mill, Dissertation, MPI fur Gottin¨ Stromungsforschung, ¨ ¨ gen, Report 14, 1990. w20x F. Rohrer, F. Stuhl, J. Chem. Phys. 88 Ž1988. 4788. w21x R. Freitag, F. Rohrer, F. Stuhl, J. Phys. Chem. 93 Ž1989. 3170. w22x W.B. DeMore, M.J. Molina, S.P. Sander, D.M. Golden, R.F. Hampson, M.J. Kurylo, C.J. Howard, A.R. Ravishankara, J. Phys. Lett. 87 Ž1987. 41. w23x W.B. DeMore, M.J. Molina, S.P. Sander, D.M. Golden, R.F. Hampson, M.J. Kurylo, C.J. Howard, A.R. Ravishankara, Chemical Kinetics and Photochemical Data for use in Stratospheric Modeling Evaluation No. 8, JPL Publ. No. 87-41, Jet Propulsion Laboratory, Pasadena, CA, 1987. w24x R. Jordan, Dipl. thesis, Max-Planck-Institut fur ¨ Stromungs¨ forschung, Report 19, University Gottingen, 1996. ¨ w25x W. Hack, R. Jordan, Ber. Bunsenges. Phys. Chem. 101 Ž1997. 545. w26x G.L. Vaghjiani, A.R. Ravishankara, J. Phys. Chem. 93 Ž1989. 7833.
Chemical Physics Letters 306 Ž1999. 111–116
Reaction rates of the reactions of OH žX 2 P / with NH ža 1D / and ˜/ HN3 žX W. Hack ) , R. Jordan
1
Max-Planck-Institut fur Bunsenstraße 10, D-37073 Gottingen, Germany ¨ Stromungsforschung, ¨ ¨ Received 11 March 1999; in final form 19 April 1999
Abstract The rate constants of the radical–radical reaction OHŽX 2 P . q NHŽa 1D . ™ products and the radical–molecule reaction OHŽX 2 P . q HN3 ™ products were determined in a quasistatic laser photolysis cell at room temperature and a total pressure of 20 mbar. The radicals OHŽX 2 P . and NHŽa 1D . were generated by exciplex laser photolysis of H 2 O 2 and HN3 , respectively and detected by laser-induced fluorescence. The fluorescence time profiles of these radicals were monitored at variable delay times between photolysis and detection pulses and thus time resolution was obtained. The measurements yielded the rate constants k 1 s Ž7.0 " 2. = 10 14 cm3 moly1 sy1 and k 2 s Ž7.7 " 1. = 10 11 cm3 moly1 sy1. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction There are few direct experimental data for the H 2 –N–O system. The dihydronitrosyl radical H 2 NOŽ 2 B 1 . was observed via its microwave spectrum w1x. The reaction between hydrogen atoms and nitroxyl ŽHNO.: H q HNO ™ H 2 q NO is known to be the fast step in the nitric oxide catalysed recombination of H atoms. The reverse reaction H 2 q NO ™ HNO q H proceeds with a large activation energy of EA s 230 kJ moly1 w2–4x. The atom–radical reaction in the H 2 –N–O system O q NH 2 ™ products was found to be fast at room temperature with a rate coefficient k s 5.3 = 10 13 cm3 moly1 sy1 w5x. It has ) Corresponding author. Fax: q49 551 5176 665; e-mail: [email protected] 1 Present addres: Max-Planck-Institut fur ¨ biophysikalische Chemie, Am Faßberg 11, D-37077 Gottingen. ¨
two reaction pathways O q NH 2 ™ HNO q H Ž k s 4.6 = 10 13 cm3 moly1 sy1 . and O q NH 2 ™ NHŽX. q OH Ž k s 7 = 10 12 cm3 moly1 sy1 . w5x. The atom–molecule reaction in the H 2 –N–O system N q H 2 O ™ product has, to the best of our knowledge, not yet been studied directly. A new approach to the H 2 –N–O system is the radical–radical reaction NH Ž a 1D . q OH Ž X 2 P . ™ products
Ž 1.
The reaction will Ždue to the high excitation energy of NHŽa.. not be important in thermal systems but it might be of interest in photochemical reaction systems. Moreover, it is of fundamental interest as one of the few singlet–doublet reactions of NHŽa 1D .. The reaction of electronically excited imino radicals ŽNHŽa 1D .. with ground state molecules in singlet states like H 2 O or hydrocarbons Že.g. CH 4 , C 2 H 6 , C 3 H 8 . proceeds via complex formation of an intermediate in the singlet electronic ground state.
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 4 3 7 - 6
112
W. Hack, R. Jordanr Chemical Physics Letters 306 (1999) 111–116
The dynamics of these electronic chemically activated molecules was studied for a large number of systems w6–8x. The reaction of the singlet imino radical with a doublet species has been studied only with NOŽX 2 P .. In these reactions, a doublet intermediate is formed which gives rise to crossing over to the triplet surface, i.e. quenching. The aim of this study was to measure the rate of the radical–radical Reaction Ž1. – a doublet species with NH in the first electronically excited singlet state. In this system also, the reaction of the OH radicals with the HN precursor molecule HN3
˜ . ™ products OH Ž X2 P . q HN3 Ž X
Ž 2.
has to be considered quantitatively.
2. Experimental The experiments were performed in a quasistatic reaction cell Ži.e. the duration between two photolysis pulses is large enough to yield a complete exchange of the gas volume; on the time scale between pump and probe pulse the flow is negligible. with a photolysisrLIF system. The experimental arrangement is described in detail elsewhere w7,9x. Briefly, it consists of a photolysis and a detection laser system. The latter consists of a dye laser ŽFL3002, Lambda Physik. that was optically pumped by an exiplex laser ŽLPX 205, Lambda Physik.. The pump laser pulses had energies of 200–400 mJ and a duration of t s 14 ns ŽFMHW. Ž l s 308 nm, XeCl.. The probe beam has a cross-section of about 9 mm2 and pulse energy densities of 20 ( ErmJ cmy2 ( 220. Frequency doubled light was obtained by a KPD crystal ŽFL30, Lambda Physik. and led to energy densities between 2 and 3 mJrcm2 . In all experiments, the pulse energy was high enough to saturate the transition to the electronically excited states. For the photolysis, an exiplex laser ŽLPX 205, Lambda Physik, l s 248 nm, KrF. was used. The photolysis beam had a cross-section of about 1.4 cm2 with pulse energy densities of 0.7 ( ErmJ cmy2 ( 23. A variable delay between the pulse of the photolysis and the probe laser in the range 0 ( t R rms ( 400 yielded the time resolution. The OHŽX 2 P . radicals were produced by the photolysis of H 2 O 2 . This photolysis process has
been investigated in Refs. w10–12x and leads exclusively to OH-radicals in the state OHŽX 2 P, Õ s 0.. The reaction of OH with H 2 O 2 , OH q H 2 O 2 ™ products ,
Ž 3.
has been taken into consideration Žsee below.. The absorption coefficient is ´ 248 nm ŽH 2 O 2 . s 9.3 = 10y2 0 cm2 moleculey1 w13–16x, and the photolysis quantum yield F 248 nm ŽH 2 O 2 . s 2.09 " 0.36 w17,18x. The OH-concentration was observed by the Q 1Ž1.-line of the OHŽA 2 Sq, Õ s 0 § X 2 P, Õ s 0. transition at l s 307.85 nm. The emitted fluorescence was observed perpendicular to the laser beams. After passing a filter, to reduce scattered light, the photons were detected by a photomultiplier tube ŽR955, Hammatsu.. The NHŽa 1D . radicals were produced in the HN3-photolysis which leads mainly to NHŽa 1D . in the vibronic states Õ s 0 and Õ s 1 with high rotational energies w19x. Under the experimental conditions applied Ž20 mbar He., the rotational energies are relaxed by collisions with the inert gas He to room temperature in a time of 1 ms after the photolysis w20,21x. The transition NHŽc 1 P, ÕX s 0,§ a 1D, ÕY s 0. was used to follow the NHŽa 1D . concentration time profile over the wavelength range 325 ( lrnm( 328. All measurements were done with He as the carrier gas at room temperature and a pressure of 20 mbar. Gases were supplied with the highest commercially available purities ŽHe ) 99.9999%, Praxair.. HN3 was produced by melting a mixture of stearic acid ŽC 17 H 35 COOH, Merck. and sodium azide ŽNaN3 , Merck.. The gas was dried with CaCl 2 and stored in a bulb Ž p s 1 bar. diluted in Ar Ž) 99.9999%, Praxair.. The maximum molefraction of HN3 was X HN 3 ( 0.17. Commercially available H 2 O 2 Ž85%, Solvay Interox. was enriched to 99% by trap-to-trap distillation at T s 78 K. The H 2 O 2 concentration in the liquid was measured via KMnO4 titration. The H 2 O 2 concentration in the gas phase on the way to the reaction cell was determined by freezing the gas stream in a N2 Žl.-trap and titrating the amount of H 2 O 2 frozen in a given time.
W. Hack, R. Jordanr Chemical Physics Letters 306 (1999) 111–116
3. Results Reaction Ž1. was investigated by observing NHŽa 1D . concentration time profiles under pseudofirst-order conditions with a large excess of OHŽX 2 P . over NHŽa. in the range 2 = 10 2 ( wOHŽX 2 P .x ts0rwNHŽa 1D .x ts0 ( 2 = 10 3. The OH radicals were produced by the photolysis of H 2 O 2 . The OH concentration was determined from the H 2 O 2 photolysis via:
w OHx s
F P EL P l P ´ P w H 2 O 2 x P ln 10 hPcPA
Ž I.
where: the quantum yield for OH was accepted to be F s 2.0 w18x rather than the lower value given in Ref. w17x Žsee Section 4.; EL s 30 mJ, l s 248 nm, ´ Ž248 nm . s 9.3 = 10 y 20 cm 2 molecule y 1 w16,22,23x wH 2 O 2 x s hydrogen peroxide concentration as given in Table 1, A is the cross-section of the photolysis laser A s 1.4 cm2 . It has to be considered that the concentration of H 2 O 2 in the reaction cell was about a factor of a hundred higher than the OHŽX 2 P . concentration. The NHŽa. depletion in the reaction system is given by: d NH Ž a . dt
s yk 1 NH Ž a . w OH x y k 4 NH Ž a . = w H 2 O 2 x y k 5 NH Ž a . w NH 3 x .
The rate constant for the reaction NH Ž a 1D . q H 2 O 2 ™ products
Ž 4.
was measured under the experimental conditions and was found to be k 4 s 7.1 = 10 13 cm3 moly1 sy1 , in
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good agreement with the literature value k 4 s 7.5 = 10 13 cm3 moly1 sy1 w24,25x. NHŽa. is depleted by HN3 in the reaction NH Ž a . q HN3 ™ products
Ž 5. 13
3
with a rate coefficient k 5 s 6.4 = 10 cm moly1 sy1 w8x. The OH radicals are depleted by other reactions: Reaction Ž3., OH Ž X 2 P . q H 2 O 2 ™ products with k 3 s 1.2 = 10 12 cm3 moly1 sy1 w26x, and Reaction Ž2. with the rate constant given in this Letter. Reaction Ž2. can be neglected due to the small HN3 concentrations applied in the experiments to determine k 1. For wHN3 x s 2 = 10y1 1 mol cmy3 , Reaction Ž2. contributes only a small amount to the consumption of the initial OH concentration i.e. wOHxŽ t s 20 ms.rwOHx o s 0.9997. The OH consumption via Reaction Ž3. has to be taken into consideration. This can be done by taking the initial slope Ži.e. for reaction times smaller than 5 ms. in the ln wNHŽa.x versus time plot to obtain the first-order rate constant. However, no significant deviation from linearity was observed in these plots. The variation of wOHxo was performed in two different ways: Ži. the photolysis laser fluence was varied at constant H 2 O 2 concentration, and Žii. the H 2 O 2 concentration was varied at constant photolysis laser fluence. Ži. To distinguish the NHŽa 1D . depletion by Reaction Ž1. from the depletion by Reaction Ž4. with H 2 O 2 , the photolysis laser power was varied and thus the OHŽX 2 P . concentration as given in Fig. 1.
Table 1 Experimental conditions for the reaction NHŽa 1D, Õ s 0. q OHŽX 2 P . X
T ŽK.
p Žmbar.
wHN3 x Ž10y11 mol cmy3 .
wH 2 O 2 x Ž10y9 mol cmy3 .
wOHŽX 2 P .x Ž10y11 mol cmy3 .
k1 Ž10 4 sy1 .
295 295 295 295 295 295 295 295
20 20 20 20 20 20 20 20
2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2
0.727 1.45 2.09 2.82 3.64 4.36 5.09 5.82
0.8 1.6 2.3 3.1 4.0 4.8 5.6 6.4
0.6 1.2 1.7 2.4 3.0 3.5 4.2 4.8
114
W. Hack, R. Jordanr Chemical Physics Letters 306 (1999) 111–116
Fig. 1. First-order rate constant as a function of photolysis laser fluence. v, First-order rate constant for Reaction Ž1..
It was observed that with decreasing laser power, kX1 also decreases. This can be explained by a lower contribution of the fast Reaction Ž1. to the NHŽa 1D . depletion. Žii. A second method to obtain different OH concentrations was applied by varying the H 2 O 2 concentration. The data are given in Table 1. The pseudo-first-order rate constant kX1 for a given H 2 O 2 concentration was obtained by subtracting kX4 at measurement conditions. Reaction Ž1. contributed approximately 10% to the total NHŽa 1D . depletion, the main contribution is NHŽa. q H 2 O 2 .
Fig. 3. Semilogarithmic plot of the concentration profiles of OHŽX 2 P . versus reaction time in the presence of various HN3 concentrations: wHN3 x in =10y9 mol cmy3 ; v, 0.2; B, 2.7; `, 4.1; I, 6.1; ^, 8.9.
By varying the H 2 O 2 concentration at fixed photolysis laser power, the rate constant k 1 was determined from a plot of kX1 versus the OHŽX 2 P . concentration under first-order conditions as shown in Fig. 2. The results look much better than expected from the unfavourable kinetic situation. The slope of the plot in Fig. 2 resulted in the second-order rate constant: k 1 s Ž 7.0 " 2 . = 10 14 cm3 moly1 sy1 . The large error takes into account that the contribution of Reaction Ž1. is smaller than that of Reaction Ž4.. Reaction Ž2. was followed by the OHŽX 2 P . depletion under pseudo-first-order conditions in the presence of a large excess of HN3 over OHTable 2 Experimental conditions for the determination of the rate constant k2 X wOHx 0 wHN3 x T p k2 ŽK. Žmbar. Ž10y12 mol cmy3 . Ž10y9 mol cmy3 . Ž10 3 sy1 .
Fig. 2. First-order rate constant as a function of the OHŽX 2 P . concentration for the reaction of NHŽa 1D, Õ s 0. with OHŽX 2 P ..
294 294 294 294 294 294 294 294
20 20 20 20 20 20 20 20
7 7 7 7 7 7 7 7
0.0 0.2 0.8 2.7 4.1 6.1 7.9 8.9
1.29 1.31 1.40 3.42 4.21 5.57 7.72 7.69
W. Hack, R. Jordanr Chemical Physics Letters 306 (1999) 111–116
Fig. 4. First-order rate constants versus wHN3 x for Reaction 2.
ŽX 2 P . ts0 , i.e. 29 ( wHN3 xrwOHŽX 2 P .x ts0 ( 1270. The OHŽX 2 P . concentration as a function of time is shown at various HN3 concentrations in Fig. 3. The experimental conditions for this reaction are summarized in Table 2. Reaction Ž1. also causes depletion of OH radicals. The initial NHŽa. concentrations are in the range 0.2 ( wNHŽa.xor10y1 2 mol cmy3 ( 9. But the fast reaction of NHŽa. with the HN3 , present in large concentration, reduces the NHŽa. concentration within the first 20 ms to wNHŽa.xŽ t .rwNHŽa.xo ( 1 = 10y5 ; a value which is insignificant for the OH concentration profiles. The pseudo-first-order rate constants as a function of the HN3 concentration is shown in Fig. 4. The intercept is determined by Reaction Ž3. The rate constant calculated from the intercept given in Fig. 3 is k 3 s Ž2.0 " 0.3. = 10 12 cm3 moly1 sy1 . This value can be compared with the value of k 3 s 1.0 = 10 12 cm3 moly1 sy1 given in the literature w22,26x. From the slope in Fig. 4, the rate constant k 2 s Ž7.7 " 0.3. = 10 11 cm3 moly1 sy1 was obtained for Reaction Ž2. at room temperature.
115
though obtained under unfavourable kinetic conditions. The rate constant k 1 was determined by following the NHŽa 1D . depletion in the presence of excess OHŽX 2 P . and H 2 O 2 . The room temperature rate constant is k 1Ž298 K. s 7.0 = 10 14 cm3 moly1 sy1 which is extremely fast compared to other radical–radical reactions. It is by far the fastest reaction of NHŽa. which has been observed. A lower photolysis quantum yield for the H 2 O 2 laser photolysis would lead to an even higher value of k 1. Also taking into account OH consuming reactions would change k 1 in the same direction. From the large value of k 1 it can be stated that it is unlikely that quenching of NHŽa. by OHŽ2 P . is a significant NHŽa. – depletion channel of Reaction Ž1.. Reaction Ž1. has besides the quenching channel NH Ž a 1D . q OH Ž X 2 P . ™ NH Ž X . q OH Ž X . D R H s y150.3 kJ moly1
Ž 1q .
several exothermic reaction pathways:
˜. NH Ž a . q OH ™ NHOH 2AX Ž X D R H s y499.7 kJ moly1
Ž 1a .
NH Ž a . q OH ™ NO Ž X 2 P . q H 2 Ž X . D R H s y474.5 kJ moly1
Ž 1b .
˜. NH Ž a . q OH ™ N Ž 4 S . q H 2 O Ž X D R H s y334.8 kJ moly1
Ž 1c .
˜ 2 B1 . q O Ž 3 P. NH Ž a . q OH ™ NH 2 Ž X D R H s y127.7 kJ moly1
Ž 1d .
Reactions Ž1b. and Ž1c. can be regarded as the decompositions of the initially formed HN–OH radical. For Reaction Ž2., the probable pathway is H atom abstraction via OH q HN3 ™ H 2 O q N3
4. Discussion The rate constant of Reaction Ž1. has not yet been determined directly. It may be of importance in several H–N–O photolytic systems. Therefore it is necessary give this value of the rate constant al-
which is exothermic with D R H s y160 kJ mol -1 . Also the formation of HO–NH via
˜ . ™ HONH Ž X˜ 2AX . q N2 Ž X . OH Ž X 2 P . q HN3 Ž X D R H s y266.9 kJ moly1
W. Hack, R. Jordanr Chemical Physics Letters 306 (1999) 111–116
116
and the formation of NOŽX 2 P . via
˜ . ™ NO Ž X P . q H 2 q N2 OH Ž X P . q HN3 Ž X 2
2
D R H s y241.7 kJ moly1 are possible from the thermodynamic point of view. ˜ . is endothermic: The formation of NH 2 ŽX
˜ . ™ NH 2 Ž X˜ . q N2 Ž X . q O Ž 3 P . OH Ž X . q HN3 Ž X D R H s q105.1 kJ moly1 By comparison of the rate constant with those other H atom abstraction reactions, e.g. for CH 4 , NH 3 and HCN, it can be assumed that the H atom abstraction is the main reaction pathway. Acknowledgements The continued interest of Prof. H.Gg. Wagner is gratefully acknowledged. Thanks are due to the Deutsche Forschungsgemeinschaft ŽSonderforschungsbereich 357 ‘Molekulare Mechanismen unimolekularer Prozesse’. and the Fonds der Chemischen Industrie for financial support. References w1x H. Mikami, S. Saito, S. Yamamoto, J. Chem. Phys. 94 Ž1991. 3415. w2x D.L. Baulch, D.D. Drysdale, D.G. Horne, A.C. Lloyd, Evaluated Kinetic Data for High Temperature Reactions, Butterworth, 1973. w3x D.J. Nesbitt, Report University of Colorado, Boulder, CO, AFOSR-TR-90-0514. w4x L.T. Molina, S.D. Schinke, M.J. Molina, Geophys. Res. Lett. 4 Ž1977. 580. w5x P. Dransfeld, W. Hack, H. Kurzke, F. Temps, H.Gg. Wagner, 20th Symposium ŽInt.. on Combustion, 1984, pp. 655.
w6x W. Hack, K. Rathmann, Ber. Bunsenges. Phys. Chem. 94 Ž1990. 1304. w7x W. Hack, K. Rathmann, Z. Phys. Chem. 176 Ž1992. 151. w8x W. Hack, in: Z.B. Alfassi ŽEd.., NH Radical Reactions in N-centered Radicals, John Wiley, 1998, p. 413. w9x W. Hack, A. Wilms, Z. Phys. Chem. ŽNF. 161 Ž1989. 107. w10x S. Klee, K.-H. Gericke, F.J. Comes, J. Chem. Phys. 85 Ž1986. 40. w11x M.P. Docker, A. Hodgson, J.P. Simons, Chem. Phys. Lett. 128 Ž1986. 264. w12x G. Ondrey, N. van Veen, R. Bersohn, J. Chem. Phys. 78 Ž1983. 3732. w13x L.T. Molina, M.J. Molina, J. Photochem. 15 Ž1981. 97. w14x C.L. Lin, N.K. Rohatgi, W.B. De More, Geophys. Res. Lett. 5 Ž1978. 113. w15x L.T. Molina, S.D. Schinke, M.J. Molina, Geophys. Res. Lett. 4 Ž1977. 580. w16x G.L. Vaghjiani, A.R. Ravishankara, J. Geophys. Res. 94 Ž1989. 3487. w17x A. Schiffman, D.D. Nelson, D.J. Nesbitt, J. Chem. Phys. 98 Ž1993. 6935. w18x G.L. Vaghjiani, A.R. Ravishankara, J. Chem. Phys. 92 Ž1990. 996. w19x T. Mill, Dissertation, MPI fur Gottin¨ Stromungsforschung, ¨ ¨ gen, Report 14, 1990. w20x F. Rohrer, F. Stuhl, J. Chem. Phys. 88 Ž1988. 4788. w21x R. Freitag, F. Rohrer, F. Stuhl, J. Phys. Chem. 93 Ž1989. 3170. w22x W.B. DeMore, M.J. Molina, S.P. Sander, D.M. Golden, R.F. Hampson, M.J. Kurylo, C.J. Howard, A.R. Ravishankara, J. Phys. Lett. 87 Ž1987. 41. w23x W.B. DeMore, M.J. Molina, S.P. Sander, D.M. Golden, R.F. Hampson, M.J. Kurylo, C.J. Howard, A.R. Ravishankara, Chemical Kinetics and Photochemical Data for use in Stratospheric Modeling Evaluation No. 8, JPL Publ. No. 87-41, Jet Propulsion Laboratory, Pasadena, CA, 1987. w24x R. Jordan, Dipl. thesis, Max-Planck-Institut fur ¨ Stromungs¨ forschung, Report 19, University Gottingen, 1996. ¨ w25x W. Hack, R. Jordan, Ber. Bunsenges. Phys. Chem. 101 Ž1997. 545. w26x G.L. Vaghjiani, A.R. Ravishankara, J. Phys. Chem. 93 Ž1989. 7833.