Browsing by Author "Zhu, RS"

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Ab initio studies of ClOx radical reactions: V. Evidence for a new path in the Cl+ClOOCl reaction
(2003-05-15)
Zhu, RS
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Lin, MC
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交大名義發表
;
National Chiao Tung University
The mechanism for the reaction Cl + ClOOCl has been investigated by ab initio molecular orbital and transition-state theory calculations. The result shows that the reaction can produce both Cl-2 + ClOO and Cl2O + ClO. The former product pair can be formed by direct and indirect abstraction paths. The direct abstraction path, which dominates at low temperatures, takes place barrierlessly with the rate constant k(1) = 1.53 x 10(-13)T(1.1) exp(118/T) cm(3) molecule(-1) s(-1) in the temperature range of 200-1000 K. The rate constant of the indirect path, which occurs via the two isomers of the Cl-O(Cl)OCl complex lying 3 kcal/mol below the reactants, k(1)' = 7.14 x 10(-12)T(0.86) exp(-2370/T) cm(3) molecule(-1) s(-1), accounting for less than 0.1% of the Cl-2 yield at 298 K but increasing to about 46% at 1000 K. The new Cl2O + ClO products can be produced by the fragmentation of the two complexes; the rate constant for their formation was found to be pressure-independent and can be expressed by k(2) = 2.19 x 10(-14)T(0.70) exp(-1110/T) cm(3) molecule(-1) s(-1) covering the temperature range of 200-1000 K. Above room temperature, Cl2O formation becomes competitive; for example, its branching ratios, k(2)/k(t), are predicted to be 0.19, 0.55, and 0.62 at 298, 500, and 1000 K, respectively. The predicted total rate constant agrees closely with available experimental results. The heat of formation of ClOOCl has been examined in detail using different reactions and computational methods. The results of these calculations converge to Delta(f)Hdegrees(0)(ClOOCl) = 30 +/- 2 kcal/mol.
Ab initio studies of ClOx reactions. 3. Kinetics and mechanism for the OH plus OClO reaction
(2003-02-20)
Xu, ZF
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Zhu, RS
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Lin, MC
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交大名義發表
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National Chiao Tung University
The mechanism for the OH + OClO reaction on the singlet and triplet surfaces and its rate constants for formation of various products have been investigated by means of ab initio molecular orbital theory and variational RRKM theory calculations. The geometric parameters of stationary points were optimized at the B3LYP level of theory with the 6-311G(d,p) and 6-311+G(3df,2p) basis sets, and the potential energy surfaces were evaluated at the G2M(CC2)//B3LYP/6-311+G(3df,2p) level of theory. Three main product channels, all located on the singlet PES, have been identified: (1) HOO + ClO, (2) HOCl + O-1(2), and (3) HOClO2, the association product. The predicted results show that the rate constants for channels 1 and 2 are pressure-independent up to 1000 atm and that for channel 3 is strongly pressure dependent. Below 1000 K, all rate constants were found to vary negatively with temperature. The individual and total rate constants in the temperature range from 200 to 1000 K at 1 Torr He pressure can be represented by k(1)(T) = 1.22 x 10(-22)T(2.75) exp(1682/T), k(2)(T) = 5.47 x 10(-20)T(2.07) exp(2064/T), k(3)(T) = 1.37 x 10(4)T(-6.61) exp(-536/T) (200-500 K) and 4.99 x 10(54)T(-22.36) exp(-9807/T) (500-1000 K), and k(tot)(T) = 1.78 x 10(-20)T(2.25) exp(2100/T) in units of cm(3) molecule(-1) s(-1). The predicted rate constant, with the HOCl + O-1(2) as the major products in the 300-500 K range, agrees well with available experimental data obtained at 1 Torr He pressure. The high- and low- pressure limits of k(3) can be effectively given by k(3)(infinity)(T) = 3.24 x 10(-11) T-0.28 exp(- 18/T) cm(3) molecule(-1) s(-1) in 200-2500 K and k(3)degrees(T) = 1.28 x 10(-13)T(-6.36) exp(-635/T) for 200-800 K, 7.37 x 10(84)T(-36.02) exp(-22134/T) for 800-1000 K, and 2.91 x 10(-13)T(-8.42) exp(11500/T) for 1000-2500 K in units of cm(6) molecule(-2) s(-1) with N-2 as the third body.
Ab initio studies of ClOx reactions. IV. Kinetics and mechanism for the self-reaction of ClO radicals
(2003-03-01)
Zhu, RS
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Lin, MC
;
交大名義發表
;
National Chiao Tung University
The self-reaction of ClO radicals has been investigated by ab initio molecular orbital and variational transition state theory calculations. Both singlet and triplet potential energy surfaces were predicted by the modified Gaussian-2 method. The reaction was shown to take place mainly over the singlet surface by forming ClOOCl (k(1)) and ClOClO (k(1)'). These association processes were found to be strongly pressure dependent and the predicted total rate constants are in good agreement with the experimental data. The predicted second- and third-order rate constants for formation of ClOOCl and ClOClO can be expressed, respectively, in units of cm(3) molecule(-1) s(-1) and cm(6) molecule(-2) s(-1) by k(1)(infinity)=1.6x10(-9)T(-0.67) exp(-64/T), k(1)('infinity)=6.4x10(-9)T(-0.78) exp(-76/T), and k(1)(0)=8.31x10(-20)T(-4.96) exp(-336/T), k(1)(0),=1.72x10(-14)T(-6.99) exp(-926/T) in the temperature range 180-500 K for N-2 as the third body. The observed T, P-dependent data could be best accounted for with the heat of formation of ClOOCl, Delta(f)H(0)(o)(ClOOCl)=29.4+/-1 kcal/mol. The formation of Cl-2+O-2 (2), Cl+ClOO (3), and Cl+OClO (4) products have been confirmed, with the predicted pressure-independent rate constants: k(2)=1.09x10(-13)T(0.66) exp(-1892/T); k(3)=1.36x10(-13)T(0.77)exp(-2168/T); k(4)=6.26x10(-11)T(0.005) exp(-2896/T), respectively, in units of cm(3) molecule(-1) s(-1), covering the temperature range 200-1500 K. These results are also in reasonable agreement with existing experimental kinetic data. (C) 2003 American Institute of Physics.
Experimental and theoretical investigations of rate coefficients of the reaction S(P-3)+O-2 in the temperature range 298-878 K
(2004-11-01)
Lu, CW
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Wu, YJ
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Lee, YP
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Zhu, RS
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Lin, MC
Rate coefficients of the reaction S+O-2 with Ar under 50 Torr in the temperature range 298-878 K were determined with the laser photolysis technique. S atoms were generated by photolysis of OCS with a KrF excimer laser at 248 nm; their concentration was monitored via resonance fluorescence excited by atomic emission of S produced from microwave-discharged SO2. Our measurements show that k(298 K)=(1.92+/-0.29)x10(-12) cm(3) molecule(-1) s(-1), in satisfactory agreement with previous reports. New data determined for 505-878 K show non-Arrhenius behavior; combining our results with data reported at high temperatures, we derive an expression k(T)=(9.02+/-0.27)x10(-19)T(2.11+/-0.15) exp[(730+/-120)/T] cm(3) molecule(-1) s(-1) for 298less than or equal toTless than or equal to3460 K. Theoretical calculations at the G2M (RCC2) level, using geometries optimized with the B3LYP/6-311+G(3df) method, yield energies of transition states and products relative to those of the reactants. Rate coefficients predicted with multichannel RRKM calculations agree satisfactorily with experimental observations; the reaction channel via SOO((1)A') dominates at T<500 K, whereas channels involving formation of SOO((3)A") followed by isomerization to SO2 before dissociation, and formation of SOO((1)A") followed by direct dissociation, become important at high temperatures, accounting for the observed rapid increase in rate coefficient. (C) 2004 American Institute of Physics.
Thermal decomposition of iso-propanol: First-principles prediction of total and product-branching rate constants
(2002-12-22)
Bui, BH
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Zhu, RS
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Lin, MC
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交大名義發表
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National Chiao Tung University
The unimolecular decomposition of iso-C3H7OH has been studied with a modified GAUSSIAN-2 method. Among the six low-lying product channels identified, the H2O-elimination process (2) via a four-member-ring transition state is dominant below 760 Torr over the temperature range 500-2500 K. At higher pressures and over 1200 K, the cleavage of a C-C bond by reaction (1) producing CH3+CH3C(H)OH is predicted to be dominant. The predicted low- and high-pressure limit rate constants for these two major product channels can be given by k(1)(0)=6.3x10(42) T-16.21 exp(-47 400/T), k(2)(0)=7.2x10(44) T-14.70 exp(-35 700/T) cm(3) molecule(-1) s(-1), k(1)(infinity)=8.0x10(29) T-3.75 exp(-45 800/T), and k(2)(infinity)=2.0x10(6) T-2.12 exp(-30 700/T) s(-1), respectively. Predicted k(1) values compare reasonably with available experimental data; however, k(2) values are lower than the experimentally determined apparent rate constant for C3H6 formation, which may derive in large part from secondary radical reactions. Other minor decomposition products were predicted to have the barriers H-2+CH3C(O)CH3, E-3(0)=82.8 kcal/mol; H2O+(CH3CCH3)-C-1, E-4(0)=77.9 kcal/mol; CH4+CH3C(H)O, E-5(0)=84.3 kcal/mol; and CH4+(CH3COH)-C-1, E-6(0)=81.9 kcal/mol. The triplet-singlet energy gap for CH3CCH3 was predicted to be 5.2 kcal/mol, favoring the singlet state. (C) 2002 American Institute of Physics.