2136
J. Phys. Chem. 1996, 100, 2136-2140
Kinetic Studies on the Pyrolysis of H2S Hiroumi Shiina,† Masaaki Oya,‡ Koichi Yamashita,§ Akira Miyoshi,† and Hiroyuki Matsui*,† Department of Chemical Systems Engineering, The UniVersity of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113 Japan, National Institute for Resources and EnVironment, Onogawa, Tsukuba, Ibaraki, 305 Japan, and Department of Applied Chemistry, The UniVersity of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113 Japan ReceiVed: August 23, 1995; In Final Form: NoVember 7, 1995X
The kinetics and the mechanism of the thermal decomposition of H2S and subsequent reactions have been studied. The rate constant for the initiation reaction H2S + M f products (1) was determined by a shock tube-infrared emission spectroscopy at temperatures 2740-3570 K to be k1 ) 10-10.44(0.31 exp[(268.6(18.4)kJ mol-1/RT] cm3 molecule-1 s-1, which is about one-fifth to one-tenth of the recent results reported by Woiki and Roth (J. Phys. Chem. 1994, 98, 12958) and Olschewski et al. (J. Phys. Chem. 1994, 98, 12964). An ab initio (MRCI+Q) calculation suggested that a spin-forbidden product channel (fS(3P) + H2) is energetically favorable compared to a H-S bond fission channel; that is, the singlet-triplet intersystem crossing occurs at an energy lower than the dissociation threshold for HS + H by about 17 kJ mol-1. The present rate constant for reaction 1 could be well reproduced by an unimolecular decomposition theory with the calculated energy for the crossing and with a reasonable collision parameter, βc. The rate constants for important subsequent reactions, S(3P) + H2 f products (3) and S(3P) + H2S f products (4), were also determined by a laser photolysis-shock tube-atomic resonance absorption spectrometry method: k3 ) 10-9.58(0.16 exp[-(82.5(4.0) kJ mol-1/RT] (1050-1660 K) cm3 molecule-1 s-1, and k4 ) 10-9.86(0.17 exp[-(30.9(4.1) kJ mol-1/RT] (1050-1540 K) cm3 molecule-1 s-1. The ARAS measurement of H atoms revealed that the main products for reaction 3 are HS + H at pressures below 2 atm.
reactions following the initial decomposition of H2S are
Introduction The mechanism of the high-temperature oxidation processes of sulfur-containing fuels, in relation to SOx formation in combustion, has not been well established. The thermal decomposition of H2S has been investigated by several groups.1-5 In early investigations,1-3 the primary decomposition step was assumed to be a simple S-H bond fission;
H2S + M f HS + H, M, ∆H°0K ) 375.1 kJ mol-1 (1a)
S + H2S f H + S2H
where the rate constants were derived from the formation rates of S2 detected by UV absorption,1 the decay rates of H2S were detected by UV absorption,2 or the formation rates of H atom were detected by VUV absorption.3 Recently, however, Woiki and Roth4 and Olschewski et al.5 simultaneously reported rate constants for H2S thermal decomposition 3-10 times larger than the earlier measurements2,3 from S atom formation rates monitored by VUV absorption4 or H2S decay rates monitored by UV absorption.5 Combination of these two studies strongly suggests that the primary step is a spinforbidden channel:
S + H2S f H2 + S2
H2S + M f S(3P) + H2 + M, ∆H°0K ) 292.3 kJ mol-1 (1b) Olschewski et al.5 reported that their rate measurements are consistent with a theoretical calculation assuming that the singlet surface crosses the triplet surface at the energy close to that of S(3P) + H2. The reason for the difference between these results and the previous ones is not still clear. Main subsequent * To whom correspondence should be addressed. † Department of Chemical System Engineering, The University of Tokyo. ‡ National Institute for Resources and Environment. § Department of Applied Chemistry, The University of Tokyo. X Abstract published in AdVance ACS Abstracts, January 15, 1996.
0022-3654/96/20100-2136$12.00/0
∆H°0K ) -57.0 kJ mol-1
(2)
S + H2 f H + HS
∆H°0K ) 82.8 kJ mol-1
(3)
S + H2S f HS + HS
∆H°0K ) 25.8 kJ mol-1
(4a)
H + H2S f H2 + HS
S + HS f S2 + H
(4b) ∆H°0K ) -128.8 kJ mol-1 ∆H°0K ) -71.9 kJ mol-1
(4c) (5)
where enthalpies of formation, ∆Hf°,0K, were obtained from JANAF Thermochemical Tables6 except that for HS, which was obtained from Continetti et al.7 ∆Hf° for S2H is not well-known. ∆H°298K for (4b) ) -38.4 kJ mol-1 is used in ref 4. For reaction 2, two studies have been reported at high temperatures above 1000 K,3,8 but the measured rate constants significantly differ from each other. The rate constants in ref 3 were reported, by the same group,4 to be the result of an incorrect assumption of the dominance of the decomposition channel 1a. Woiki and Roth4,9 studied reactions 3 and 4. They reported4 the branching fraction for 4b to be 0.35-0.57 in addition to the rate constants k3 and k4. No other measurement has been reported for these subsequent reactions at high temperatures. Further experimental and theoretical investigations are needed to clarify the entire view of the thermal decomposition mechanism. In the present study, the rate constant for the initial thermal decomposition of H2S was measured via infrared emission spectroscopy. An ab initio (MRCI+Q) calculation was also © 1996 American Chemical Society
Kinetic Studies on the Pyrolysis of H2S
J. Phys. Chem., Vol. 100, No. 6, 1996 2137
performed to examine the mechanism for reaction 1. The rate constants for important subsequent reactions, S + H2 f HS + H (3) and S + H2S f products (4), were directly measured by monitoring the pseudo-first-order decrease of S atoms in the photolysis of COS/H2/Ar or COS/H2S/Ar mixtures behind reflected shock waves. Experimental Section Two shock tube studies were conducted in the present study. In both, special caution was used to discriminate the effects due to the wall contamination and the resident impurities. During the experiments, blank tests with pure Ar (instead of sample mixtures) were repeated, confirming the absence of background signals due to contaminations or impurities. A conventional stainless-steel shock tube (5 cm i.d., 4 m long) was used to measure the overall rate constant for reaction 1. Sample gas mixtures of 200, 500, 1000, or 2000 ppm H2S diluted in Ar were heated in incident shock waves between 2740 and 3570 K. Initial concentrations of H2S were 2.4 × 1014 to 2.4 × 1015 molecules cm-3. The variation of the concentrations of H2S was monitored by infrared emission at 2.65 µm using an IR band-pass filter (peak of transmission 2.65 µm; 0.42 µm full width at half-maximum) or a quartz filter (cutoff wavelength 3.5 µm) with an In-Sb detector. A diaphragmless stainless-steel shock tube (5 cm i.d., 4 m long) was used for the studies on reactions 3 and 4. The details of the shock tube-laser flash photolysis apparatus have been described previously.10,11 At 100 µs after the reflected shock wave passed through the observation station, sample gas mixtures were irradiated by an ArF (193 nm) or KrF (248 nm) excimer laser through a rectangular quartz window (3 cm × 1 cm) located at the end plate of the shock tube. The time-dependent concentrations of the H(2S) atoms and the S(3P) atoms were monitored by using ARAS (atomic resonance absorption spectrometry). The resonant radiation at 121.6 nm for H(2S) or 182.6 nm for S(3P) from a microwaveexcited discharge lamp (1% H2/He or 0.1% SO2/He) was isolated by a 20 cm VUV monochromator and detected by a solar-blind photomultiplier tube (Hamamatsu R972). The ARAS system was located 3 cm upstream from the end plate; windows were MgF2. A calibration curve for the determination of absolute concentrations of S(3P) atoms was obtained by the thermal decomposition of COS diluted in Ar. The calibration procedure for H atoms has been described previously.12 Measurements of the rate constants for reactions 3 and 4 were carried out by the photolysis of H2/COS or H2S/COS mixtures diluted in Ar and detecting S(3P) atoms ([H2] ) 2360-20300 ppm, [H2S] ) 49-102 ppm, [COS] ) 35-61 ppm). Here, COS was used as a precursor for S(3P). S(1D) atoms produced by ArF or KrF excimer laser photolysis of COS were rapidly quenched (within less than 1 µs) to their ground state S(3P) under the present experimental conditions. The initial concentrations of S(3P) atoms were always kept low enough to maintain the pseudo-first-order conditions for the decay of S atoms. Present experiments were limited to temperatures below 1700 K to avoid the influence of the thermal decomposition of COS. The previously reported13 rate constant for the side reaction
S(3P) + COS f products
(6)
was extended to a wider temperature range, 860-1680 K, yielding
k6 ) 10-10.31(0.07 exp[-(31.0(1.8)kJ mol-1/RT] cm3 molecule-1 s-1
Figure 1. Arrhenius plot for the reaction H2S + M f products + M. Present study: (b) 200 ppm H2S/band-pass filter; (O) 200 ppm H2S/ quartz filter; (0) 500 ppm H2S/quartz filter; (]) 1000 ppm H2S/quartz filter; (4) 2000 ppm H2S/quartz filter; (×) 200 ppm H2S/without filter. The error bars represent 2σ of a nonlinear least-squares fit of the exponential decay curve. Solid lines denote previous studies: (a) ref 1; (b) ref 2; (c) ref 3; (d) ref 4; (e) ref 5. The broken line denotes calculated rate constants by unimolecular reaction theory with E0 ) 357.9 kJ mol-1 and βc ) 0.025 at 3200 K (∝T-1). Insert: IR emission time profile at 2.65 µm behind incident shock waves in 200 ppm H2S diluted in Ar. T ) 3300 K; P ) 0.54 atm. The time profile is plotted against particle time.
The contribution of reaction 6 was kept less than 20% of the observed decay rate of S atoms, and a correction was made for each measurement. The indicated error limits for experimentally determined values are at the two standard deviations level throughout the paper. Results Thermal Decomposition of H2S. A. Experimental Results. As shown in the inset of Figure 1, the time profile of the IR emission intensity at 2.65 µm showed an instantaneous increase at the arrival of the incident shock front, followed by an exponential decay due to the decomposition reaction 1 and rapid subsequent reactions 2 and/or 4. Because the subsequent reactions are much faster than the initial step of decomposition, decomposition of one H2S molecule results in quick consumption of another H2S, no matter which dissociation channel, 1a or 1b, dominates. The apparent second-order rate constants kapp for the consumption of H2S were derived from a least-squares fit to the equation
[H2S] ) [H2S]0 exp(-kapp[M]t) A kinetic simulation showed that the rate constant k1 can be simply obtained as kapp/2. An Arrhenius plot of the measured rate constants for reaction 1 is shown in Figure 1 with previous measurements. The measured rate constants were found to be independent of the initial concentration of H2S (200-2000 ppm) or the difference of used filters (IR band-pass filter or quartz filter). A leastsquares fit of the present experimental data gives the following Arrhenius expression for the temperature range 2740-3570 K.
k1 ) 10-10.44(0.31 exp[-(268.6(18.4)kJ mol-1/RT] cm3 molecule-1 s-1 The present study agrees well with the previous study by Bowman and Dodge2 and the extrapolation of the former
2138 J. Phys. Chem., Vol. 100, No. 6, 1996
Shiina et al.
Figure 2. Energy diagram for reaction 1. Indicated values are experimentally known enthalpies (∆H°0K, in kJ mol-1) relative to H2S. Values in parentheses are corresponding calculated values, i.e. the calculated energies corrected for zero-point energies. The energy for the singlet-triplet crossing (marked by an asterisk) was evaluated from the calculated crossing point energy relative to S(3P) + H2 with the experimental ∆H°0K for H2S f S(3P) + H2. Inset: the low-lying potential energy curves for the C2V approach of the S atom to the H2.
experimental result by Roth et al.,3 but is significantly (5-10 times) smaller than the recent two studies.4,5 B. Comparison with Theoretical InVestigation. To examine the disagreement in k1 as well as to investigate the mechanism for reaction 1, ab initio potential energy surface (PES) calculations for the dissociation channels H2S f S(1D) + H2 and f S(3P) + H2 were performed by using the MOLPRO code14 with the valence triple-ζ (VTZ) basis set,15 which includes three d and an f functions on the sulfur and two p and a d functions on the hydrogens. For the complete active space self-consistentfield (CASSCF) calculations, eight electrons and six orbitals were chosen as the active space, and the four states (1A1, 3B1, 3B , and 3A ) were optimized in a state-averaged procedure with 2 2 equal weights. The multireference single and double configuration interaction (MRSDCI) calculations, taking into account all the singly and doubly excited configurations from the reference configurations, were then performed with a higher order correction based on the formula of Feller and Davidson.16 At this theoretical level of calculation, the dissociation energies of H2S to S(3P) + H2 and S(1D) + H2 could be well reproduced as shown in Figure 2. Figure 2 also shows the low-lying potential energy curves for the C2V approach of the sulfur atom to the hydrogens, where the distance r between the two hydrogen atoms is fixed at the equilibrium distance of H2, 0.74 Å. R is the distance between the sulfur atom and the center of the H-H bond. The crossings between the triplet surface (3B1 and 3B2) and the singlet surface (1A1) occur at around R ) 1.9 Å. The contour plots of two-dimensional PESs are given in Figure 3 for (a) 1A1 and (b) 3B2. The PESs of 3B1 and 3A2 are also repulsive, similar to that of 3B2. The broken lines in Figure 3 are the crossing seams between the 1A1 and the triplet states. The lowest energy point of the crossing between 1A1 and 3B2 is found to be located where r ) 0.78 Å and R ) 1.94 Å with an energy of 65.4 kJ mol-1 relative to S(3P) + H2. Here, B1 and B2 denote the species corresponding to A′ and A′′, respectively, when they are distorted to Cs in the molecular plane. The present calculation shows that the singlet-triplet crossing occurs at an energy ∼17 kJ mol-1 below the dissociation
Figure 3. Contour plots of two-dimensional potential energy surfaces: (a) 1A1; (b) 3B2. Broken lines indicate the crossing seams between the 1A1 and the triplet surfaces.
threshold for HS + H, as shown in Figure 2. This suggests that the major dissociation pathway is 1b (fS(3P) + H2), as was pointed out by Woiki and Roth4 and Olschewski et al.5 However, the energy of the intersystem crossing is obviously higher than that suggested by Olschewski et al.,5 who indicated that the crossing occurs at energies close to S(3P) + H2 from an analysis of their rate measurements for reaction 1 with the unimolecular reaction theory.17 Recalculation of their analysis with E0 (the crossing point energy) obtained here leads to
Kinetic Studies on the Pyrolysis of H2S
J. Phys. Chem., Vol. 100, No. 6, 1996 2139
S(3P) + H2 f H + HS
(3)
k3 ) 10-9.58(0.16 exp[-(82.5(4.0)kJ mol-1/RT] cm3 molecule-1 s-1 Since the measured rate constant was not affected by the change of the initial concentration ratio (d[H2]0/[S]0 ) 160028 000), the effect of the side reaction was considered to be negligible. Pressure dependence of the rate constant was not observed from 0.68 to 2.4 atm. This suggests that the contribution of the recombination reaction to H2S (reverse of reaction 1b),
S(3P) + H2 + M f H2S + M
Figure 4. Arrhenius plot for the reaction H2 + S f products (3). The total number densities are (3) 3.57-5.53; (4) 7.37-8.70; (O) 9.3510.5; (0) (10.7-12.5) × 1018 molecules cm-3. The error bars represent 2σ of a nonlinear least-squares fit of the exponential decay curve. Broken lines: (Woiki et al.10) (-‚-) caluclated bimolecular rate for recombination based on the rate constant for decomposition of H2S measured in the present study; (-‚‚-) calculated bimolecular rates for recombination based on the rate constant reported in ref 4. Inset: time profile of the S atom concentration in the laser flash photolysis of H2/ COS/Ar mixtures behind reflected shock waves. [Ar] ) 1.1 × 1019 molecules cm-3; [H2]0 ) 2360 ppm; [COS]0 ) 35 ppm; T ) 1400 K.
anomalous values of βc (1.03 at 1900 K and 0.25 at 2800 K) with their rate constants for reaction 1. On the other hand, more reasonable values for βc or -〈∆E〉 can be derived for the present measurements for k1 (βc ) 0.039-0.020 and -〈∆E〉 ) 1099693 J mol-1 at temperatures 2700-3600 K). A calculated rate constant assuming βc ∝ T-1 (βc ) 0.025 at 3200 K) is also shown in Figure 1 by a broken line. It seems that the present measurements for k1 and those by Bowman and Dodge2 are preferable from the theoretical energy of singlet-triplet crossing. However, the contradiction with the measurements by Woiki and Roth4 or by Olschewski et al.5 should be left as a still unresolved problem since no serious error with experimental procedures can be found in any of the experiments. S(3P) + H2 Reaction. Absorption time profiles of S(3P) atoms were measured over the temperature range 1050-1660 K at total number densities of 3.5 × 1018 to 1.3 × 1019 molecules cm-3 in the 193 or 248 nm photolysis of H2/COS/Ar mixtures. A typical example of the time dependence of absorption intensity at 182.6 nm is shown in the inset of Figure 4. Instantaneous production of S(3P) atoms and subsequent exponential decay are clearly seen. The yields of S atom from COS in the 193 and 248 nm photolysis were found to be nearly independent of temperature and were estimated to be σΦS ) 1.5 × 10-18 cm2 at 193 nm and 3.7 × 10-19 cm2 at 248 nm, respectively, where σ is the absorption cross section and ΦS is the quantum yield of S atoms. The absorption signal subtracted with background absorption caused by COS was converted to the absolute concentration of S atoms using the calibration curve. Initial concentrations of S atom were (0.6-1.3) × 1013 molecules cm-3. The first-order decay rate of S atoms was determined by a least-squares fit. An Arrhenius plot of the S(3P) + H2 reaction is shown in Figure 4. A least-squares fit of the experimental data gives the following Arrhenius expression for the rate constant for reaction 3 in the temperature range 10501660 K.
(-1b)
is negligible in the present experimental conditions because a strong density dependence is expected for this recombination reaction. Absorption time profiles of H atoms were measured to determine the products for reaction 3. Time dependence of absorption intensity at 121.6 nm is shown in Figure 5a. S atoms were also monitored separately at the same experimental condition but without H2 to determine the initial S atom concentration (Figure 5b). It is clearly shown in Figure 5 that one S atom produced in the photolysis is successively converted into two H atoms through the reactions S + H2 f H + HS (3) and HS + H2 f H + H2S (-2) with a large excess of H2. The main products of the S + H2 reaction are confirmed to be HS + H. A numerical simulation assuming that S + H2 exclusively produces HS + H and including other side reactions could well reproduce the observed time profile of H atoms (the solid line in Figure 5a). The numerical simulation also showed that the contribution of side reactions was negligible in the S atom decay measurements under the present experimental conditions. S(3P) + H2S Reaction. The decay profiles of S atom concentration were measured over the temperature range 10501540 K at total number densities of (7.5-9.5) × 1018 molecules cm-3 in the 248 nm photolysis of H2S/COS/Ar mixtures by the same manner as described above. It was found that 193 nm photolysis is not adequate for this measurement because a substantial fraction of H2S is photolyzed at 193 nm to produce H and HS, and S atom concentrations are affected seriously by the side reactions HS + HS f S + H2S (-4a) and HS + H f S + H2 (-3). Such difficulty could be avoided by using KrF laser (248 nm) photolysis of COS, where a very small fraction of H2S is photolyzed. The H atom yield in 248 nm photolysis of H2S increased with elevating temperature but was limited to be only σΦH ∼ 1 × 10-19 cm2 at 1600 K. The production of S atoms due to side reactions in the 248 nm photolysis of H2S was confirmed to be below the detection limit in the present experimental conditions. An Arrhenius plot of the S(3P) + H2S reaction is shown in Figure 6. The initial concentrations of S atoms were (0.81.8) × 1013 molecules cm-3, and the initial concentration ratio of [H2S]0/[S]0 was 27-110. No systematic deviation of the measured rate constants was found by changing the initial concentrations. An Arrhenius expression for the rate constants for reaction 4,
S(3P) + H2S f products
(4)
was evaluated to be (1050-1540 K)
k4 ) 10-9.86(0.17 exp[-(30.9(4.1)kJ mol-1/RT] cm3 molecule-1 s-1
2140 J. Phys. Chem., Vol. 100, No. 6, 1996
Figure 5. Time profiles of the H and S atom concentrations in the laser flash photolysis of H2/COS/Ar or COS/Ar mixtures behind reflected shock waves. Upper trace (a): time profile of the H atoms observed after the photolysis of a COS(71ppm)/H2(4.2%)/Ar mixture. Lower trace (b): time profile of the S atoms observed after the photolysis of a COS(71ppm)/Ar mixture. Experimental conditions: T ) 1300 K, total density ) 8.7 × 1018 molecules cm-3. Solid lines indicate the results of a numerical simulation for the COS/H2/Ar mixture.
Shiina et al. The extrapolation of the rate constants for reaction 1b reported by Woiki and Roth4 and Olschewski et al.5 seems to be too large to account for the observed disappearance rate of S atoms (at a lower temperature range; T < 1200 K). However, since it is a large extrapolation, such a comparison cannot lead to a conclusive discussion for the disagreement of k1 at the present stage. Further measurements for the S + H2 reaction extended to a much higher pressure range will be of great help to resolve the disagreement in k1. Also, poor agreement was found for reaction 4 between the present measurements and those by Woiki and Roth.4 The differences of the precursor molecule for S atoms (COS or CS2) or of the resonance lines (182.5 or 147.4 nm) are not responsible for the disagreement. This disagreement can be clearly explained by the difference of wavelength used for the photolysis (248 or 193 nm). Disappearance rates of S atoms similar to those by Woiki and Roth were observed in this study when 193 nm photolysis was used. The present measurements for k3 and k4 show that the activation energies are very close to the endothermicity, i.e., Ea ) 82.5 and ∆H°0K ) 82.8 kJ mol-1 for reaction 3; Ea ) 30.9 and ∆H°0K ) 25.8 kJ mol-1 for reaction 4. This indicates that no pronounced barriers are present for the reverse reactions -3 and -4. This fact is in clear contrast with analogous reactions, O + H2 and O + H2S, both of which proceed via obvious barriers between reactants and products. It is a speculation but worth examining in the future that reactions 3 and 4 do not proceed via direct abstraction mechanisms, which are supposed to have substantial barriers, but proceed via an insertion channel involving the triplet-singlet crossing S(3P) + H2 f H2S* f HS + H at low pressure ranges, where the reaction intermediate H2S* cannot be quickly stabilized to H2S. For S + H2, the ab initio calculation shows that this crossing occurs at an energy below that for the product channel of HS + H. References and Notes
Figure 6. Arrhenius plot for the reaction H2S + S f products (4). The solid line represents the Arrhenius fit of the present results. The broken line denotes the rate constant reported by Woiki and Roth.4
It is likely that the major channel of this reaction is 4a (f2HS), but the existence of channel 4b (fH + HS2) can be inferred by analogy with the O(3P) + H2S reaction.12 Therefore, a trial was made to measure the branching fraction for channel 4b by monitoring H atoms. The rapid consumption of H atoms by reaction 2 and a generation of a small amount of H atoms from H2S photolysis obscured this measurement for branching fraction. The upper limit of the branching fraction for channel 4b was evaluated to be 0.2 by comparing the observed time profile with numerical simulations. Discussion The measured rate constants for the S + H2 reaction in the present study agree well with those reported by Woiki and Roth9 in an overlapping temperature range, as shown in Figure 4. The bimolecular rates for S + H2 (+M) f H2S (+M) at the typical pressure of this study were evaluated via the equilibrium constant from the present measurements of k1 and those by Woiki and Roth,4 and the results are also shown in Figure 4.
(1) Higashihara, T.; Saito, K.; Yamamura, H. Bull. Chem. Soc. Jpn. 1976, 49, 965. (2) Bowman, C. T.; Dodge, L. G. Symp. (Int.) Combust. [Proc.] 1976, 16, 971. (3) Roth, P.; Lo¨hr, R.; Barner, U. Combust. Flame 1982, 45, 273. (4) Woiki, D.; Roth, P. J. Phys. Chem. 1994, 98, 12958. (5) Olschewski, H. A.; Troe, J.; Wagner, H. Gg. J. Phys. Chem. 1994, 98, 12964. (6) Chase, M. W., Jr.; Davies, C. A.; Downey, J. R., Jr.; Frurip, D. J.; McDonald, R. A.; Syverud, A. N. JANAF Thermochemical Tables Third Edition. J. Phys. Chem. Ref. Data 1985, 14, Suppl. 1. (7) Continetti, R. E.; Balko, B. A.; Lee, Y. T. Chem. Phys. Lett. 1991, 182, 400. (8) Yoshimura, M.; Koshi, M.; Matsui, H. Chem. Phys. Lett. 1992, 189, 199. (9) Woiki, D.; Roth, P. Int. J. Chem. Kinet. 1995, 27, 547. (10) Koshi, M.; Yoshimura, M.; Fukuda, K.; Matsui, H.; Saito, K.; Watanabe, M.; Imamura, A.; Chen, C. J. Chem. Phys. 1990, 93, 8703. (11) Matsui, H.; Koshi, M.; Oya, M.; Tsuchiya, K. Shock WaVes 1994, 3, 287. (12) Tsuchiya, K.; Yokoyama, K.; Matsui, H.; Oya, M.; Dupre, G. J. Phys. Chem. 1994, 98, 8419. (13) Oya, M.; Shiina, H.; Tsuchiya, K.; Matsui, H. Bull. Chem. Soc. Jpn. 1994, 67, 2311. (14) MOLPRO is a package of ab initio programs written by H. J. Werner and P. J. Knowles, with contributions from J. Almlof, R. D. Amos, M. J. O. Deegan, S. T. Elbert, C. Hampel, W. Meyer, K. Peterson, R. Pitzer, A. J. Stone, and P. R. Taylor. (15) Dunning, T. H., Jr. J. Chem. Phys. 1989, 90, 1007. (16) Feller, D.; Davidson, E. R. J. Chem. Phys. 1984, 80, 1006. (17) Troe, J. J. Chem. Phys. 1977, 66, 4758.
JP952472J
