10) Bock, H.; Goebel, I.; Zdenek, H.; Liedle, S.; Oberbammer, H. Angew. Chem., Int. Ed. Engl. 1991, 30, 187. (11) (a) Leonard, N. J.; Coll, J. C.; Wang, A. H.-J.; Missavage, R. J.; Paul, I. C . J. Am. Chem. S a . 1971,93,4628. (b) Wan& A. H.-J.; Missavage, R. J.; Byrn, S. R.; Paul, I. C. (17) Frisch, M. J.; Head-Gordon, M.; Trucks, G. W.; Foresman, J. B.; Schlegel, H. B.; Raghavachari, K.; Robb, M. A.; Binkley, J. S.; Gonzalez, C.; Defrees, D. J.; Fox, D. J.; Whiteside, R. A.; Seeger, R.; Melius, C. F.; Baker, J.; Martin, R. L.; Kahn, L. R.; Stewart, J. J. P.; Topiol, S.; Pople, J. A.; Gaussian 90, Rev. I; Gaussian,Reactions of the ethynyl (C2H) radical with C2H2, H2, and D2 were studied over the temperature range 298-438 K by time-resolved mass spectrometry. The rate of the reaction with C2H2 was followed by measuring the appearance rate of C4H2. The rates of the reactions with H2 and D2 were deduced by measuring the dependence of C4H2 production (arising from the reaction of C2H with the C2H2 precursor) on the partial pressure of added H2 or D2. The rate constants for the reaction C2H + C2H2 -C I H~ + H were also measured following reflected shock waves by monitoring H atom resonant absorption at 121.6 nm. In both experiments, the C2H radical was generated by ArF (193-nm) laser photolysis of C2H2. A rate constant of (1.5 f 0.3) X cm3 molecule-' s-l was obtained for reaction 1 without any temperature dependence at T = 298-2177 K. The results for the reactions C2H + H2 -C2H2 + H and C2H + D2 -C2HD + D could be represented by the Arrhenius expressions, k2 = (1.8 f 1.0) X lo-" exp(-(1090 f 299)/T) and k3 = (1.4 f 0.8) X lo-" exp(-(1377 f 301)/T) cm3 molecule-' s-', over the range of T = 298-438 K. The classical barrier height for reactions 2 and 3 was estimated to be 2 kcal/mol on the basis of conventional transition-state theory. The isotope effects on reactions 2 and 3 calculated with Wigner tunneling correction were in good agreement with the present results.

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The mechanism of SiO formation in the laser photolysis of SiH 4 /O 2 /CCl 4 mixtures was investigated using a laser-induced fluorescence method. Measured rates for the SiO production corresponded to the decay rates of SiH 3 radical and depended linearly on the O 2 concentration. The yield of SiO was estimated on the basis of LIF intensity, and it was found that SiO was one of the major products in the SiH 3 + O 2 reaction. The bimolecular rate constant for the SiO production was determined to be (1.14 ( 0.18) × 10 -11 cm 3 molecules -1 s -1 . Ab initio molecular orbital calculations were performed for various pathways of the SiH 3 + O 2 reaction. Geometries were optimized at the MP2(full)/6-31G(d) level of theory, and relative energies and barrier heights were calculated at the G2(MP2) level of theory. Silyl radical and O 2 react to form SiH 3 OO, which irreversibly decomposes to various excited products. A new transition state for the production of cyclic H 2 SiO 2 (siladioxirane) + H from SiH 3 OO adduct was found. Possible decomposition channels of the vibrationally excited products of the SiH 3 + O 2 reaction to produce SiO are discussed.

numbers of studies were performed until very recently on the fundamental oxidation reaction mechanism of hydrogen sulfide and related H 9 S 9 O reaction systems.The oxidation process of H 2 S (and some times the catalytic effect of SO 2 ) have been investigated in several flame studies [1 -5], and the reaction mechanisms were discussed based on the measured product distributions. These studies have provided much useful information on the elementary reactions of H 9 S 9 O systems, however, it is generally very difficult to judge the reliability of kinetic/thermodynamic parameters of individual elementary reactions proposed in these indirect studies. It is also noted that even a conventional shock tube study was not performed for clarifying oxidation mechanism of H 2 S (as far as we know), except an ignition delay mea- INTRODUCTIONThe oxidation of hydrogen sulfide in combustion of fossil fuels and the consecutive atmospheric processes of sulfur oxides are creating serious pollution problems on a global scale.Many practical studies for reducing SO x in combustion systems have been conducted from the engineering point of view. In contrast, only very limited (1), the rate constants evaluated by numerical simulations are summarized as; k 1 ϭ 3.1 ϫ 10 Ϫ11 exp[Ϫ 75 kJ mol Ϫ1 /RT ] cm 3 molecule Ϫ1 s Ϫ1 (T ϭ 1400 -1850 K) with an uncertainty factor of about 2. Direct measurements of the rate constants for S ϩ O2 : SO ϩ O (2), and SO ϩ O 2 : SO 2 ϩ O (3) yield k 2 ϭ (2.5 Ϯ 0.6) ϫ 10 Ϫ11 exp[Ϫ(15.3 Ϯ 2.5) kJ mol Ϫ1 /RT ] cm 3 molecule Ϫ1 s Ϫ1 (T ϭ 980 -1610 K) and, k 3 ϭ (1.7 Ϯ 0.9) ϫ 10 Ϫ12 exp[Ϫ(34 Ϯ 11) kJ mol Ϫ1 /RT ] cm 3 molecule Ϫ1 s Ϫ1 (T ϭ 1130 -1640 K), respectively. By summarizing these data together with the recent experimental results on the H 9 S 9 O reaction systems, a new kinetic model for the H 2 S oxidation process is constructed. It is found that this simple reaction scheme is consistent with the experimental result on the induction time of SO 2 formation obtained by Bradley and Dobson.

The potential energy surfaces of electronically excited states of SO2 have been investigated with ab initio molecular orbital method: Especially, the two-dimensional potential energy surface for the \ ildeC1B2 (21A′) state as a function of both the bond angle and bond distance has been carefully analyzed, including the spin-orbit interaction. Energetically, 23A′ (13A1 in C2v) was found to be the most grobable state which concerns the reaction mechanism for the photodissociation pathway from the \ ildeC state (SO2 (\ ildeC1B2)→SO(3∑−)+O(3P)); however, other pathway may also be possible to go over the avoided crossing on a singlet potential surface.