JOURNAL O F T H E A M E R I C A N CHEM1CA.L S O C I E T Y Registered i n U.S. Palcnl Ofice.
@ Copyright,
1972, by !he American Chemical Society
AUGUST9, 1972
VOLUME94, NUMBER 16
Mechanism of Carbon Dioxide Formation in 3 130-A Irradiated Mixtures of Sulfur Dioxide and Carbon Monoxide Fred B. Wampler, Abraham Horowitz, and Jack G. Calvert" Contribution from the Chemistry Department, The Ohio State University, Columbus, Ohio 43210. Received August 25, 1971 Abstract: Emission and product quantum yield measurements reported here provide a new insight into the mechanism .of carbon dioxide formation in SO?-CO mixtures irradiated within the first singlet absorption band (24003200 A). The quantum yields of sulfur dioxide triplet-sensitized phosphorescence in biacetyl were determined in SOz-CO-biacetyl mixtures. The kinetic treatment of the data gives the fraction of the excited sulfur dioxide singlet molecules (IS02) which generate the emitting triplet ( 3S02)on undergoing quenching collisions with CO: 'SO2 M + (SOz-M or products) (1); 'SO2 M + 3SOz M (2); k2/(kl k2) = 0.017 + 0.010, where M = CO. The corresponding rate ratio for M = Nz is 0.033. Other emission experiments with SOz-Nz-biacetyl mixtures show that there is an additional source of SO2triplets in experiments above about 5 Torr of reactants. As a result of these two effects the rate of triplet formation is increased markedlyeon the addition of Nz to an SO2-CO mixture. Quantum yields of COz formation have been determined in 3130-A irradiated mixtures of SO2and CO at different CO and SO2 reactant pressures and with added C G Hand ~ N2 gases. The kinetics of the CGHG-inhibition experiments are consistent with the involvement of the excited fluorescent singlet and the phosphorescent triplet sulfur dioxide molecules in the C02formation reactions. The results confirm the observations of Cehelnik, Spicer, and Heickle? that some state (or states) of SO2 in addition to the emitting singlet and triplet states is (are) involved in the 3130-A irradiated S02-C0 system at high pressures. The tentative mechanism of excess triplet formation in runs at high pressures which is favored here includes an undefined excited state of SO2 or one of its isomers. The at high pressures in a collisionspecies is not a reactant leading to COZformation, but it presumably generates ally induced reaction.
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arbon monoxide is oxidized when its mixtures with sulfur dioxide are irradiated withino either the first allowed absorption ban$ (2400-3200 A) l s 2 or the forbidden band (3400-3970 A)3 of sulfur dioxide. SOs is not photodissociated at wavelengths greater than about 2180 A, so the oxidation of C O must result from a n oxygen atom transfer reaction involving some excited species of SOz. Jackson and Calvert3 have shown that this excited species is the 3B1state of SO,, designated here as %CZ,when the SOz is excited directly to this state by light absorption within the "forbidden" band. However, the situation is not this simple when SOz is excited within the first allowed band. Neither the results of Timmonsl nor those of Cehelnik, Spicer, and Heicklenz appear to be compatible with a COSforming mechanism involving 3SOz-C0 interactions (1) R. B. Timmons, Photochem. Phofobiol., 12, 219 (1970). (2) E. Cehelnik, C. W. Spicer, and J. Heicklen, preliminary results presented at the Ninth Informal Photochemistry Conference, The Ohio State University, Columbus, Ohio, Sept 1970; J. Amer. Chem. Soc., 93, 5371 (1971). (3) G. E. Jackson and J. G. Calvert, ibid., 93, 2593 (1971).
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alone. Thus Timmons found ~ ' c to o ~be practically independent of the pressure of added CO in his limiteg series of observations made in experiments at 3130 A and at a fixed SO2 pressure. If 3SOzwere the precursor to COS formation in this system, one expects CO, formation to be sensitive to the [S02]/[CO]ratio. I n contrast to Timmons' observations, Cehelnik, et a1.,2found that ~ C O increased , markedly with increase in CO pressure, and it was relatively insensitive to SOz pressure. They observed further that nitrogen gas addition to the S02-C0 mixture caused little inhibition of the COz formation. These results seemed inconsistent with the involvement of the known emitting excited states of SOz as the only reactants for COaformation, since both the emitting singlet and triplet states of SO, are quenched by SOs, CO, and Nz gases with efficiencies that are not greatly different.415 Cehelnik, et al., proposed that C 0 2 formation involved the (4) H. D. Mettee, J. Phys. Chem., 73, 1971 (1969). (5) H. W. Sidebottom, C. C. Badcock, G. E. Jackson, .I. G. Calvert. G. W. Reinhardt, and E. I
