J. Phys. Chem. 1989, 93, 4785-4790 leading to chlorine atoms, also plays a role, albeit minor. Addition of benzene to a sample of C102 in trichlorofluoromethane led, upon laser excitation, to a transient with ,A, 490 nm, readily characterized as the r-complex between chlorine atoms and benzene.20 Given the known excitation coefficients of the acomplex and of C102, it is possible to estimate that -8% of the initial (fast) bleaching is due to reaction 4.
-
c102
-
C1'
+0 2
(4)
Our results indicate rapid (within the laser pulse) formation of Cl'. This tends to eliminate the C10 + 0 reaction as a possible source of CI', unless the process was to occur in a geminate fashion before the products of reaction 1 leave the solvent cage. Thus in solution, C10 remains as the most likely intermediate to combine with C102 (reaction 5). C10
+ C102
-
C1203
(5)
This appears to be the first time that formation of C1203in C102 photolysis has been clearly demonstrated although McHale and
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(20) Buhler, R. E. Radiat. Res. Rev. 1972,4, 233. McGimpsey, W. G.; Scaiano, J. C. Can. J . Chem. 1988, 66, 1474.
4785
von Elbe" and Lipscomb et al.12 have previously reported circumstantial evidence supporting its existence. Considering the small solvent-induced red shift (-8 nm) in the UV spectrum of C102 in C13CF2' as due to the different conditions, the results agree well with the gas-phase spectra obtained by Cox and Hayman in the photolysis of C1023and by Molina and Molina in the C1' + C10, systema4 In all cases maxima are observed at 270-280 nm, and therefore, on the basis of both spectral and chemical evidence, it appears reasonable to conclude that they can be attributed to C1203.22 Further work will be reported e l s e ~ h e r e . ~ ~ ? ~ ~ Registry No. CIOz, 10049-04-4; CI2O3,17496-59-2; T M E , 2741606-4. (21) Wahner, A,; Tyndall, G. S.; Ravishankara, A. R. J . Phys. Chem. 1987, 91, 2734. (22) Molina, M. J. Personal communication. (23) At higher TME concentrations a noticeable reduction in the time constant for the secondary C102bleaching was observed. This can be ascribed to the competitive reaction of TME with C10 via addition or H atom abstraction. To be published. (24) Churio et al. (Churio, M. S.; Brusa, M. A.; Perissinoti, L. J.; Ghibaudi, E.; Colussi, A. J. To be published) have found that the initial quantum yield of C102disappearance in this system under steady illumination at 366 nm is @ = 2.09 k 0.20, in agreement with the present results.
Reaction between the Amidogen Radical, NH2, and Molecular Oxygen in Low-Temperature Matrices John N. Crowleyt and John R. Sodeau* School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ, England (Received: October 17, 1988; In Final Form: January 3, 1989)
The reaction between NH2 and O2has been examined in a low-temperature matrix for the first time. Results from a series of experiments employing isotopically substituted ammonia and oxygen indicate that the primary reaction products are,HONO and H atoms. A reaction mechanism is proposed that involves the intermediacy of the aminoperoxy radical, N H 2 0 0 , which is stabilized by the low-temperature/high-pressure environment of the matrix before undergoing intramolecular decomposition or reaction with oxygen.
Introduction The reaction between the amidogen radical, NH2, and oxygen is a potential source of NO, species in the troposphere and has therefore stimulated a considerable amount of research. Despite this, neither the end products nor the rate coefficients or mechanism involved in the overall photooxidation of ammonia is well characterized.' Channels 1-7 have been suggested to follow the primary reaction between NH3 and OH radicals in the atmospheree2 Early
+ H,O + H,
NO,
+ 02
A H f / k J mol-' = - 1 6 7
(2)
NH200+ NH200
H
A H f / k J mol-' = - 5 0
(3)
NH200+ NH20
6H
A H , / k J mol-' = - 4 2
(4)
A H f / k J mol-' = - 2 1
(5)
0
A H f / k J mol-' = + 1 2 5
(6)
+ Hb2
A / f f / k J mol-' = +188
(7)
HNO
+
+
N H ,ob
H,NO NH
---
(1)
HONO
kH2
+
A H , / k J mol-' = - 3 4 7
4 NO
by 213.9-nm light at room temperature led to the proposal of the so-called Gesser m e ~ h a n i s m ~in. ~which channel 1 is most important. A secondary reaction between NH2 and N O was thought .to account for the observation of N2 product in these experiments. Jayant? conducted similar experiments and, by adding CO to the reaction mixture, was able to show that channel 4 which produces H N O and O H radicals was not important because C 0 2 was not observed. Instead a mechanism involving N H 2 0 0 , the addition product of N H 2 and 02,was proposed to account for the formation of N 2 0 and N2: N H 2 O2 N H 2 0 0 (5)
+
experiments in which mixtures of N H 3 and O2were photolyzed To whom correspondence should be sent. 'Present address: Max-Planck-Institut fur Chemie, Air Chemistry Department, Saarstrasse 23, Postfach 3060, D-6500 Mainz, w. Germany.
NH20
+ NH20
2NH20
+ O2
N 2 0 + 2H20
(8) (9)
N2 + 2H20
(10) Flash photolysis of N H 3 / 0 2 mixtures was found to lead to a (1) Chemical Kinetics and Photochemical Data for use in Stratospheric Modelling, JPL Publications, 1987; pp 87-41. ( 2 ) Melius, C . F.; Binkley, J. S. ACS. Symp. Ser., Chem. Combust. 1984, 249, 103. (3) Bacon, H. E.; Duncan, A. 8. F. J . Am. Chem. SOC.1934, 56, 336. (4) Gesser, H. J. Am. Chem. SOC.1953, 77, 2626. (5) Jayanti, R.K. M.; Simomaitis, R.; Heicklen, J. J . Phys. Chem. 1976, 80, 433.
0022-3654/89/2093-4785!§01.50/0 0 1989 American Chemical Society
4786
The Journal of Physical Chemistry, Vol. 93, No. 12, 1989
different set of products, and accordingly channel 2 was favored by Hussain and Norrish.6 In this case, it was suggested that the observed N20product arose from H N O self-reaction: HNO
+ HNO
-+
N20
+ H2O
(11)
Two further flash photolysis concluded that the measured rapid production of H 0 2 results only from reaction between H atoms and 02.This step is followed by H02 reaction with N H 2 and accounts for N H 2 loss in the system rather than its direct removal by 02:
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Hb2
+
