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E. LISSI, R. SIMONAITIS, AND J. HEICKLEN
The Bromine Atom Catalyzed Oxidation of Carbon Monoxide by Eduardo Lissi, R. Simonaitis, and Julian Heicklen* Department of Chemistry and Ionosphere Research Laboratory, The Pennsylvania State University, University Park, Pennsylaania 16802 (Received October 4, 1071) Publication costs borne completely by The Journal of Physical Chemistry
Brz was photolyzed at 3660 A and 25' in the presence of O2 and CO. C02 was produced according to the / 2 k = 4.8 X 10-7 Torr-l min-*I2for M = 02. This law is conrate law @ { G O 2 }= k [ C O ] [ 0 2 ] / ( 1 , [ M ] ) ' with
sistent with the simple mechanism
+ hv 2Br rate = I, Br + + CO --+ BrO + COZ 2Br + M +Brz + M Br2
0 2
(a) (b)
The constant k = 2k,/kb'/2 if BrO always produces additional C02,is k,/kb'/2 if BrO never produces additional GOn, and is between ka/kb'/2 and 2k,/kb'/2 otherwise. Since kb = 5.8 X l o 9 i M - 2 sec-l, then k , = 44-88 ;M-2 sec-1. The full mechanism is outlined and the reaction steps are discussed.
Introduction We have examined the bromine-photosensitized oxidation of CO as part of a continuing program on CO oxidation. Apparently the only previous study of this system was made over 40 years ago by Livingston,l who found no evidence for any reaction. Some information does exist on the oxidation of CO sensitized by the lower halogens. With fluorine, even the molecular halogen will initiate the oxidation a t room temperature.a In fact, the oxidation proceeds beyond COz, and the product of the reaction is (FCOz)2.3,4If the milder oxidizing agent FzO is used instead of the Fz-02 mixture, then C02 is p r ~ d u c e d . ~The results of the F20 study were consistent with a mechanism in which CO, in addition to reacting with FzO, reacted with FO via
FO
+ CO-FCO
+0
More recently the reaction has been studied in a shock tube at 800-1400°K,6 and the rate constant for the above reaction was measured to be 7.5 X lo7M-l sec-l independent of temperature. However, the products F, and not of the reaction were assumed to be COZ FCO 0. With Clz it is necessary to photosensitize the oxidation in order for it to proceed. The products are CClzO and COz.788 The ratio [CClzO]/[COZ] was proportional to [C12]/[02] which indicated ClCO as the intermediate
+
+
+ Clz cc1,o + c1 ClCO + +COZ + c10
ClCO
----f
0 2
Support for t.his mechanism came from studies of the photooxidation of CClzO, both in the absenceQ and T h e Journal of Physical Chemistry, Vol. 76, No. 10, 107P
presencelo of CO, where COz was also produced. COz was also found in the flash photolysis of Clz-OZ-CO mixtures.ll
Experimental Section A grease-free high-vacuum system employing Teflon stopcocks with Viton "0" rings was used. All runs were at room temperature. Pressures other than bromine were measured with an alphatron gauge. Br2mine was measured by its optical absorption at 3660 A. Reactions were carried out in a cylindrical quartz cell 5 cm in diameter by 10 cm long, The radiation was from a Hanovia Type 30620 medium-pressure U-shaped lamp and passed through a Corning 7-60 filter before entering th,e reaction vessel. The effective wavelength was 3660 A, and the transmitted intensity was monitored with an RCA 935 phototube. Baker Analyzed reagent bromine was degassed carefully at - 1 O O O ; the COZ impurity was reduced to [OZ], so that the expression reduces to
1419
large values of ( [02]/ [ C O ] )(Ia/[MI)'/', and reaction 10 should be dominant a t sufficiently small values of the same parameter. The data are sufficiently scattered in Figure 1, so that a factor of two difference is difficult to discern. However, three of the four data points for values of ([Oz1/[CO])(I,/[RII])'~'< 1.7 X min- 'Iz lie considerably above the line in Figure 1. If this observation is meaningful, then kg(k,Kz,-z ksK3,-3)/klo2(kg~3,-3)1""-' 600 mini/'.
+
I p and give k6K3,-a = 5.8 X lo9 M - 2 sec-', and again our results would require klz/ks >> 1, which is unlikely. This leaves reactions 9 and 10 as removal steps for BrO. Reaction 9 should be dominant at sufficiently
Acknowledgment. This work was supported by the Atmospheric Sciences Section of the National Science Foundation under Grant No. GA-12385, for which we are grateful.
Photoinduced Ionic Dissociation of Tetracyanobenzene-a-Methylstyrene Complex by Masahiro Irie,* Setsuko Tomimoto, and Koichiro Hayashi Faculty of Engineering, Hokkaido University, Sapporo, J a p a n
(Received J u l y 8, 1971)
Publication costs borne completely by The Journal of Physical Chemistry
The formation and behavior of ions formed from excited charge-transfer states of tetracyanobeneene-a-methylstyrene complex were studied by means of optical absorption, emission, electron spin resonance, and photoconductivity measurements in 1,2-dichloroethane-cyclohexane and n-amyl alcohol solvents a t room temperature and below. The results obtained are: (1) the charge-transfer absorption band has a maximum a t 27.5 x l o a cm-l; (2) the excitation of both charge-transfer band and acceptor band results in the singlet excited electron donor-acceptor complex, giving fluorescence a t 18.9 X l o 3 cm-l a t room temperature; (3) the triplet excited complex is also evidenced by the phosphorescence a t 19.6 X lo*em-l a t 77'K; (4) the excited complex dissociates into ions mostly from its triplet state; ( 5 ) the ions thus formed are in equilibrium between ion pairs and free ions; but (6) the equilibrium constant is so small that most ions are in pairs and recombine with the counterpart ions, by a first-order reaction.
Introduction Excited states of electron donor acceptor (EDA) complexes have recently attracted the interest of molecular photochemists. The lowest singlet excited state of tetracyanobenzene-aromatic hydrocarbon complexes was studied by means of fluorescence measurements' and laser photolysis,2 and the electronic structure in the state was found to be quite polar. The triplet excited state was also studied in detail from emission spectra3 and electron paramagnetic resonance ~pectra.~ As expected, the EDA complexes in polar excited states may readily dissociate into ions in polar solvents and result in photochemical reactions. Potashnik, et u Z . , ~ observed the formation of ions from the triplet excited state of pyromellitic dianhydride-mesitylene complex by laser photolysis at low temperature. The ionic dissociation of the lowest singlet excited state of
tetracyanobenzene-toluene complex was also suggested by Masuhara, et aZn6 However, it has not yet been shown unambiguously from which excited state the ionie dissociation of EDA complex occurs, singlet or tripet. The present authors studied the photoinduced polymerization of a-methylstyrene in the presence of tetracyanobenzene and found that the polymerization pro(1) N. Mataga and Y. Murata, J . Amer. Chem. Soc., 91, 3144 (1969). (2) R. Potashnik and M . Ottolenghi, Chem. P h y s . Lett., 6 , 525 (1970). (3) S. Iwata, J. Tanaka, and S. Nagakura, J . Chem. Phys., 47,2203 (1967). (4) H. Hayashi, 5. Iwata, and 5. Nagakura, ibid., 50, 993 (1969). (5) R. Potashnik, C. R. Goldschmidt, and M . Ottolenghi, J . Phya. Chem., 73, 3170 (1969). (6) H. Masuhara, M. Shimada, and N. Mataga, Bull. Chem. Soc. Jap., 43, 3316 (1969).
The Journal of Physical Chemistry, Vol. 76, N o . 10, 1978
