Experimental and theoretical investigation of rate coefficients of the reaction S(P3)+OCS in the temperature range of 298–985K

Abstract

The reaction S(3P)+OCS in Ar was investigated over the pressure range of 50-710 Torr and the temperature range of 298-985 K with the laser photolysis technique. S atoms were generated by photolysis of OCS with light at 248 nm from a KrF excimer laser; their concentration was monitored via resonance fluorescence excited by atomic emission of S produced from microwave-discharged SO2. At pressures less than 250 Torr, our measurements give k(298 K)=(2.7+/-0.5)x10(-15) cm3 molecule-1 s-1, in satisfactory agreement with a previous report by Klemm and Davis [J. Phys. Chem. 78, 1137 (1974)]. New data determined for 407-985 K connect rate coefficients reported previously for T>or=860 and T<or=478 K and show a non-Arrhenius behavior. Combining our results with data reported at high temperatures, we derived an expression k(T)=(6.1+/-0.3)x10(-18) T1.97+/-0.24 exp[-(1560+/-170)/T] cm3 molecule-1 s-1 for 298<or=TK<or=1680. At 298 K and P>or=500 Torr, the reaction rate was enhanced. Theoretical calculations at the G2M(CC2) level, using geometries optimized with the B3LYP6-311+G(3df) method, yield energies of transition states and products relative to those of the reactants. Rate coefficients predicted with multichannel Rice-Ramsperger-Kassel-Marcus (RRKM) calculations agree satisfactorily with experimental observations. According to our calculations, the singlet channel involving formation of SSCO followed by direct dissociation into S2(a 1Deltag)+CO dominates below 2000 K; SSCO is formed via intersystem crossing from the triplet surface. At low temperature and under high pressure the stabilization of OCS2, formed via isomerization of SSCO, becomes important; its formation and further reaction with S atoms partially account for the observed increase in the rate coefficient under such conditions.

Oxidation of Reduced Sulfur Species: Carbonyl Sulfide

“…The thermodynamic properties for selected species in the sulfur subset are shown in Table . The data were drawn from the Goos et al database , except for the properties of OCS 2 .…”

“…The heat of formation for OCS 2 , as well as rotational constants and frequencies to calculate entropies and heat capacities, was derived from the computational results of Lu et al .…”

“…Atomic sulfur, formed from dissociation of OCS at high temperature, reacts with OCS to form CO and S 2 : This reaction is an important consumption step for OCS at higher temperatures, and it has been studied experimentally over a wide range of conditions . We adopt the rate coefficients of Lu et al ; their recommendation provides a good fit of available data over an extended temperature range.…”

Evidence for carbonyl sulfide (OCS) conversion to CO in the lower atmosphere of Venus

“…The key reaction in this case is the in , OCS + S → CO + S 2 . This reaction was considered by Prinn [1975] and Krasnopolsky [2007], and was measured by Lu et al [2006] in the temperature range 298–985 K. This is adequate for our modeling. The crucial requirement is that the number density of S atoms at 30 km must be ∼10 5 cm −3 .…”

“…This scheme is known as photosensitized dissociation (for a discussion of this process see, e.g., Yung et al [1984, section 2]). The rate coefficient for the key reaction (R7) has recently been measured in the temperature range of 298 -985 K by Lu et al [2006] and their expression is given in Table 1. At 500 K, k 7 = 5.41 Â 10 À14 cm 3 s À1 is much smaller than the value 5.30 Â 10 À13 cm 3 s À1 used by Prinn [1975] but is close to the value 6.29 Â 10 À14 cm 3 s À1 estimated by Krasnopolsky [2007].…”