Ab Initio Chemical Kinetic Study on Cl + ClO and Related Reverse

Oct 5, 2010 - The reaction of ClO with Cl and its related reverse processes have been ... to give ClO + Cl. The rate constants of both forward and rev...
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J. Phys. Chem. A 2010, 114, 11477–11482

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Ab Initio Chemical Kinetic Study on Cl + ClO and Related Reverse Processes Z. F. Xu and M. C. Lin* Department of Chemistry, Emory UniVersity, Atlanta, Georgia 30322, United States ReceiVed: April 1, 2010; ReVised Manuscript ReceiVed: September 17, 2010

The reaction of ClO with Cl and its related reverse processes have been studied theoretically by ab initio quantum chemical and statistical mechanical calculations. The geometric parameters of the reactants, products, and transition states are optimized by both UMPW1PW91 and unrestricted coupled-cluster single and double excitation (UCCSD) methods with the 6-311+G(3df) basis set. The potential energy surface has been further refined (with triple excitations, T) at the UCCSD(T)/6-311+G(3df) level of theory. The results show that Cl2 and O (3P) can be produced by chlorine atom abstraction via a tight transition state, while ClOCl (1A1) and ClClO (1A′) can be formed by barrierless association processes with exothermicities of 31.8 and 16.0 kcal/ mol, respectively. In principle the O (1D) atom can be generated with a large endothermicity of 56.9 kcal/ mol; on the other hand, its barrierless reaction with Cl2 can readily form ClClO (1A′), which fragments rapidly to give ClO + Cl. The rate constants of both forward and reverse processes have been predicted at 150-2000 K by the microcanonical variational transition state theory (VTST)/Rice-Ramsperger-Kassel-Marcus (RRKM) theory. The predicted rate constants are in good agreement with available experimental data within reported errors. 1. Introduction In our recent studies1,2 on the reactions of hypochlorous acid (HOCl) with O, HOn (n ) 0-2), and ClOm (m ) 1-4), we have reported the kinetics and mechanisms of these processes because of their relevance to the chemistry of the ammonium perchlorate (AP) combustion process3,4 as well as to the stratospheric ozone destruction chemistry.5 Chlorine monoxide (ClO) is an important reactive radical involved in AP combustion; it can be generated from the ClO3 decomposition reaction (ClO3 f ClO + O2) and from bimolecular reactions such as ClO2 + O f ClO + O2. The ClO radical can readily react with other active species, such as chlorine atom studied in the present work. To our knowledge, there are a few studies on ClO + Cl, the known products of Cl2 reactions with both O (3P) and O (1D) atoms, for which several experimental investigations have been made.6-13 In 1981, Baulch et al.12 summarized the earlier experimental data for the reaction

Cl2 + O (3P) f ClO + Cl

(-1)

and recommended an Arrhenius expression, k-1 ) 4.17 × 10-12exp(-1368/T) cm3 · molecule-1 · s-1 over the temperature range 174-602 K, with an error limit of (50%. Using this rate constant expression, together with the equilibrium constant from the JANAF Table, they derived the rate constant expression k1 ) (1.74 × 10-12)exp(-4590/T) cm3 · molecule-1 · s-1 for the forward reaction:

ClO + Cl f Cl2 + O (3P)

(1)

In 1985, a new experiment was done by Wine et al.13 for reaction (-1) in the temperature range 245-371 K with the reported rate constant k-1 ) [(7.4 ( 2.4) × 10-12]exp[(-1650 * Corresponding author: e-mail [email protected].

( 100)/T)] cm3 · molecule-1 · s-1, which is slightly lower than the values recommended by Baulch et al.12 by 1.8 and 1.2 times at 254 and 371 K, respectively. Wine et al.13 also determined the rate constant for the singlet state O atom reaction:

Cl2 + O (1D) f ClO + Cl

(-4)

giving k-4 ) (2.81 ( 0.42) × 10-12 cm3 · molecule-1 · s-1 at 298 K. In the present paper, these experimental data will be compared with our theoretically predicted results. 2. Computational Methods The structures of the species ClO, Cl2, ClClO, and ClOCl and transition states have been optimized by the spin-unrestricted MPW1PW91 density functional theory,14,15 which was found to be effective for Cl-containing systems.2 Then the spinunrestricted coupled-cluster single and double excitation (UCCSD) theory16-18 was employed to further improve their geometric parameters. The 6-311+G(3df) basis set19,20 was used with these two methods. The moments of inertia and frequencies of all the species and stationary points are calculated with the corresponding optimization method. For more accurate evaluation of energies, higher-level single-point energy calculations of all the species and transition states have been carried out by the UCCSD with triple excitations [UCCSD(T)]21 using the 6-311+G(3df) basis set based on the optimized geometries. All electronic structure calculations are performed by the Gaussian 03 program.22 Rate constants for all reactions have been computed with VARIFLEX program23 based on the microcanonical transitionstate theory (TST)/RRKM (Rice-Ramsperger-Kassel-Marcus) theory24-26 in the temperature range 150-2000 K and in the pressure range 1 × 10-4 Torr-1 × 103 atm. The component rate constants are evaluated at the E/J-resolved level and the pressure dependence is treated by one-dimensional master equation calculations using the Boltzmann probability of the

10.1021/jp102947w  2010 American Chemical Society Published on Web 10/05/2010

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Figure 1. Potential energy profiles for the reactions of Cl with ClO in units of kilocalories per mole. Top line, UCCSD(T)/6-311+G(3df)// UCCSD/6-311+G(3df); second line, UCCSD(T)/6-311+G(3df)// UMPW1PW91/6-311+G(3df).

complex for the J-distribution. For a barrierless association process, the variational transition-state theory (VTST)27,28 is employed with the Morse potential, which represents the minimum energy path along with the reaction coordinate, and the potentials corresponding to the conserved and transitional degrees of freedom orthogonal to the reaction coordinate. 3. Results and Discussion The potential energy surface (PES) of the ClO + Cl system shown in Figure 1 has been computed at the UCCSD(T)/6311+G(3df) level of theory. The optimized geometric parameters at both UMPW1PW91 and UCCSD methods are illustrated in Figure 2, and their harmonic frequencies and moments of inertia are listed in Table 1. In addition, the calculated heats of reaction are placed in Table 2 with the experimental enthalpies, which are calculated by the heats of formation of the reactants and products taken from the literature.29,30 There is a good consistency in the calculated reaction heats at both UCCSD(T)// MPW1PW91 and UCCSD(T)//UCCSD levels. In comparison with experimental values, the deviations between theory and experiment are about -0.5 kcal/mol for the ClO + Cl f ClOCl and ClO + Cl f Cl2 + O (3P) reactions. But for ClO + Cl f Cl2 + O (1D), the calculated value is about 5.1 kcal/mol (or about 10%) greater than the experimental one because the energy of the excited state O (1D) is overestimated by the single reference wave function. 3.1. Potential Energy Surfaces. As seen in Figure 1, there are two reaction channels considered on the triplet electronic state energy surface from the ClO + Cl reactants to the Cl2 + O (3P) products. The first one takes place via the ClClO (3A′′) complex and transition state TS1 (3A′′). ClClO (3A′′) is an unstable complex lying 1.4 kcal/mol below the reactants at the UCCSD(T)//UCCSD level and 0.1 kcal/mol above the reactants at the UCCSD(T)//MPW1PW91 level. The loose bond length of the Cl-ClO bond is predicted to be 2.359 and 3.227 Å at the UMPW1PW91 and UCCSD levels, respectively, with a notable large difference (0.87 Å). This implies that the ClClO (3A′′) complex would be much looser at the accurate UCCSD level than at the UMPW1PW91 level. The transition state, TS1 (3A′′), is associated with the abstraction process by the Cl atom from ClO to form the Cl2 and O (3P) products. It is 9.5 kcal/ mol higher than the reactants at the UCCSD(T)//UMPW1PW91 level, but it is 11.2 kcal/mol at the UCCSD(T)//UCCSD level.

Figure 2. Optimized geometric parameters (bond lengths in angstroms and angles in degrees) of reactants, products, and transition states. Top line, UCCSD/6-311+G(3df); second line, UMPW1PW91/6311+G(3df).

The unrestricted spin orbital calculation indicated that 〈s2〉 of TS1 (3A′′) is 2.019 at the former calculation level and 2.138 at the latter calculation level. However, the CAS(8,10)/6311+G(3df,2p) calculation for TS1 (3A′′) shows the coefficient of the ground-state configuration is 0.9668, which indicates that the ground-state configuration is dominant and the mixing of the higher excited-state configurations is very small. So the spin contamination at the UCCSD level could be negligible. The geometric parameters of TS1 (3A′′) by both methods are almost the same. At the UMPW1PW91 and UCCSD levels, the Cl-O breaking bonds are 2.199 and 1.908 Å, respectively, while the Cl-Cl forming bonds are 2.009 and 2.082 Å, respectively. This channel is an endothermic process with the predicted reaction heat of 5.7 kcal/mol, which is in close agreement with the experimental value (6.2 kcal/mol). The second reaction channel on the triplet PES is slightly different from the first one. Cl atom first attacks O of ClO to form the ClOCl (3A′′) complex. After the shortening of the Cl-Cl bond, the reaction system proceeds to the Cl2 + O (3P) products via the TS2 (3A′′) transition state. Like the ClClO (3A′′) complex, ClOCl (3A′′) is a loose structure also, in which the approaching bond length (ClO-Cl) is 2.348 and 2.696 Å at the UMPW1PW91 and UCCSD levels, respectively. The structure of TS2 has an acute triangle, with the Cl-Cl-O angle being 74.4° and 86.2° at the UMPW1PW91 and UCCSD levels, respectively. The Cl-Cl forming bond and the Cl-O breaking bond are 2.176 and 1.881 Å, respectively, at the UCCSD level, which are close to the ones evaluated by UMPW1PW91. From Figure 1, one can see that the energy of TS2 is predicted to be

Ab Initio Chemical Kinetic Study on Cl + ClO

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TABLE 1: Frequencies (ω) and Moments of Inertia (I) of All Species UMPW1PW91/6-311+G(3df)

UCCSD/6-311+G(3df) -1

species

Ia, Ib, Ic (au)

ω (cm )

Ia, Ib, Ic (au)

ω (cm-1)

ClO Cl2 ClOCl (1A1) ClOCl (3A′′) ClClO (1A′) ClClO (3A′′) TS1 (3A′′) TS2 (3A′′) TS3 (1A′)

95, 95, 0 245, 245, 0 40, 490, 531 45, 699, 745 55, 427, 483 17, 645, 662 16, 689, 706 144, 314, 459 85, 468, 553

898 575 305, 688, 723 112, 205, 896 267, 435, 1036 165, 236, 880 i147, 102, 501 i916, 247, 443 i650, 250, 1002

96, 96, 0 249, 249, 0 41, 489, 530 55, 816, 872 57, 426, 483 74, 889, 963 15, 638, 653 129, 372, 502 88, 497, 586

863 562 309, 673, 762 63, 118, 870 262, 431, 999 29, 51, 863 i493, 195, 413 i1838, 278, 411 i656, 205, 910

TABLE 2: Comparison of Theoretical Reaction Heats with Experimental Data ∆rH°0, kcal/mol

dominated by the ClClO (1A′) f ClO + Cl channel because of the high barrier for isomerization and the high endothermicity for the Cl2 + O (1D) products.

UCCSD(T)// UCCSD(T)// UCCSD UMPW1PW91 experiment

reaction Cl + ClO f ClOCl (1A1)

-31.8

-31.9

Cl + ClO f ClClO (1A′) Cl + ClO f Cl2 + O (3P)

-16.0 5.7

-16.0 5.7

6.2a

Cl + ClO f Cl2 + O ( D)

56.9

56.7

51.6a

1

-31.4,a -33.9b

a Heats of formation from Chase29 for ∆fH°0(Cl) ) 28.6 kcal/mol, ∆fH°0(ClO) ) 24.2 kcal/mol, ∆fH°0[O (3P)] ) 59.0 kcal/mol, ∆fH°0[O (1D)] ) 104.4 kcal/mol, ∆fH°0(ClOCl) ) 21.4 kcal/ mol. b Heat of formation from Thorn30 for ∆fH°0(ClOCl) ) 18.9 ( 0.8 kcal/mol.

34.1 and 36.5 kcal/mol at the UMPW1PW91 and UCCSD levels, respectively. Compared with the TS1 channel, this second one is not important for the Cl2 + O (3P) production because TS2 is higher than TS1 by as much as 25 kcal/mol. The third and fourth reaction channels take place on the singlet electronic state potential surface. The third one involves the formation of the singlet electronic state ClClO (chlorosyl chloride, 1A′) from the reactants; the ClClO (1A′) intermediate can isomerize to ClOCl (dichlorine monoxide, 1A1) or decompose to Cl2 and O (1D). The formation of ClClO (1A′) and ClOCl (1A1) is predicted to be exothermic by 16.0 and 31.9 kcal/mol, respectively. The formation of ClClO (1A′) is a barrierless association process, as shown in Figure 3A. The minimum energy path can be described by the Morse function, V(RCl-ClO) ) 17.2{1 - exp[-2.9629(R - 2.0879)]}2 kcal/mol. The isomerization between ClClO (1A′) and ClOCl (1A1) can take place via the transition state TS3, as shown in Figure 2, which is a three-membered ring transiton state with the terminal Cl migrating to the O atom. The Cl-Cl breaking and Cl-O forming bonds at TS3 are predicted to be 2.640 and 2.508 Å at the MPW1PW91 level and 2.709 and 2.610 Å at the CCSD level, respectively. The energies of TS3 relative to that of ClO + Cl are 10.8 and 8.6 kcal/mol at the UCCSD(T)//UMPW1PW91 and UCCSD(T)//UCCSD levels, respectively. Another subchannel is the decomposition of ClClO (1A′) to the Cl2 + O (1D) products with a loose transition-state process, which may be represented by the Morse function V(RClCl-O) ) 74.2{1 exp[-2.2352(R - 1.4948)]}2 kcal/mol, as seen in Figure 3C. The energy of the Cl2 + O (1D) products is predicted to be 56.9 kcal/mol higher than that of the reactants, which is 5.1 kcal/mol or about 10% greater than the experimental value (51.6 kcal/mol) attributable to the single reference wave function of the UCCSD(T) calculation, which overestimates the energy of the excited state O (1D), as alluded to in the preceding paragraph. The decomposition of ClClO (1A′) is therefore

Figure 3. Minimum energy paths of (A) Cl + ClO f ClClO (1A′), (B) Cl + ClO f ClOCl (1A1), and (C) Cl2 + O (1D) f ClClO (1A′) and their Morse function fitting curves.

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In the fourth reaction channel, ClO and Cl first associate to the singlet state ClOCl (1A1) barrierlessly with 31.9 and 31.8 kcal/mol exothermicities at the UCCSD(T)//UMPW1PW91 and UCCSD(T)//UCCSD levels, respectively. ClOCl (1A1) is 15.8 kcal/mol more stable than ClClO (1A′) in energy. For the kinetic calculation discussed in the next section, the minimum energy path of this barrierless process can be depicted as V(RCl-OCl) ) 33.1{1 - exp[-3.4725(R - 1.6833)]}2 kcal/mol, as seen in Figure 3B. Following its formation, ClOCl (1A1) can isomerize to ClClO (1A′) via the transition state TS3 as described above. However, this process cannot compete with the decomposition reaction to ClO + Cl because of its higher isomerization barrier [8.6 kcal/mol at the UCCSD(T)//UCCSD level] than that for the dissociation process with loose transition states. 3.2. Rate Constant Prediction. The rate constants of the following reaction channels

ClO + Cl f Cl2 + O (3P)

(1)

ClO + Cl f ClClO (1A′)

(2)

ClO + Cl f ClOCl(1A1)

(3)

ClO + Cl f Cl2 + O (1D)

(4)

have been predicted in the temperature range 150-2000 K and the pressure range 1 × 10-4 Torr-1 × 103 atm by microcanonical VTST and RRKM theory with master equation treatment to study the pressure effect. A standard exponential-down model is used for the energy transfer with 〈∆E〉down ) 350 cm-1. The Lennard-Jones (L-J) parameters of Ar (ε/kB ) 114.0 K and σ ) 3.47 Å) are taken from the literature,31 while those of dichlorine monoxide (ε/kB ) 533 K and σ ) 4.1 Å) are taken from the paper published by Zhu and Lin.32 For a loose transition-state process, the potential for the transitional degrees of freedom orthogonal to the reaction coordinate is described in terms of internal angle with sinusoidal functions.33 The coefficient in the potential expression can be determined by the appropriate force constant matrix [Fij(R)] at the potential minimum, assuming that Fij(R) decays exponentially with the bond distance:

Fij(R) ) Fij(R0) exp[-η(R - R0)] Here, R is the bond distance along with the reaction coordinate; R0 is the bond distance at the equilibrium structure; and η is a decay parameter with R increasing. In this reaction system, there is only one internal angle for each reaction channel. The decay parameters (η) can be obtained as 2.83, 2.35, and 1.52 Å-1 for ClClO (1A′) f ClO + Cl, ClOCl (1A1) f ClO + Cl, and ClClO (1A′) f Cl2 + O (1D), respectively, in the variational range. In addition, the spin-orbit electronic states of the O (3P) atom are considered for their contribution to the electronic partition function. These states are 3P2, 3P1, and 3P0 with electronic energy levels 0.0, 158.2, and 226.9 cm-1, respectively. For Cl (2P), two electronic states, 2P3/2 and 2P1/2, are included with energies of 0.0 and 882.4 cm-1, respectively. As for the singlet-state oxygen atom, its excited state (1S0) can be ignored because it is 17 824.7 cm-1 higher than the O (1D2) state. Figure 4 shows the predicted rate constants of reaction 1. As stated above, the energy of TS1 is overestimated at the

Figure 4. Predicted rate constants of Cl + ClO f Cl2 + O (3P) and comparison with experimental data. (A) Forward reaction k1 and (B) reverse reaction k-1. (a) Ref 6; (b) ref 7; (c) ref 8; (d) ref 9; (e) ref 10; (f) ref 11; (g) ref 12; (h) ref 13.

UCCSD(T)//UCCSD level and is larger than that optimized by UMPW1PW91. The reaction barrier of TS1 calculated by UCCSD(T)//UMPW1PW91 has been adopted for rate constant calculation, and the channel via TS2 was ignored. As expected, this reaction rate constant has a strong positive temperature dependence with no pressure effect because of its positive barrier with no stable intermediate. For the forward reaction, our predicted k1 is in good agreement with the estimation of Baulch et al.12 in the temperature range 174-602 K. For the reverse reaction, our predicted k-1 result agrees well with the recommendation of Baulch et al.12 at 300-600 K but is somewhat lower than their estimate12 at T < 300 K, because two earlier experimental data at 174 and 195 K provided by Clyne and Coxon in 19667 are included in Baulch’s fitting. However, one can see from Figure 4B that three measured results6,7 before 1967 are greater than our predicted values and four other measured data9-11,13 after 1973 are in good agreement with the predicted ones. Additionally we examined the effect of the predicted reaction barrier on k-1 with the assumed deviation of (0.5 kcal/mol. As shown in Figure 4B, the predicted rate constants can effectively cover all of the experimental data with the 3.8 ( 0.5 kcal/mol barrier. In fact, the value 3.5 ( 0.3 kcal/ mol gives an overall best fit; this value is essentially within the accuracy of the method employed. The rate constants for the barrierless association channel (reaction 2) from ClO + Cl to ClClO (1A′) are calculated on the basis of summation of the Morse potential and the conserved and transitional potentials. Figure 5A shows that the rate constant (k2) for formation of ClClO (1A′) has a strong positive pressure

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Figure 5. Predicted rate constants of Cl + ClO f ClClO (1A′). (A) Forward reaction k2; (B) reverse reaction k-2.

dependence in the whole pressure range with a strong negative temperature dependence above 500 K due to the rapid reverse decomposition giving Cl + ClO. The reverse rate constants (k-2) as shown in Figure 5B have a different characteristic, exhibiting a strong positive temperature dependency at T < 1000 K, reflecting the 16 kcal/mol dissociation energy with an expected strong P dependence which also results in the negative T dependence at high temperatures (T > 1000 K). As seen in Figure 6, reaction 3 for the formation of ClOCl (1A1) has an expected similar kinetic feature as reaction 2 because of a similar

Figure 6. Predicted rate constants of Cl + ClO f ClOCl (1A1). (A) Forward reaction k3; (B) reverse reaction k-3.

barrierless association and stability as ClClO (1A′) discussed above. The forward rate constants (k3) are slightly greater than k2 and the reverse rate constants (k-3) are slightly less than k-2 under similar P and T conditions, reflecting the greater dissociation energy of ClOCl (1A1) than that of ClClO (1A′). For reaction 4 producing Cl2 + O (1D) from ClO + Cl via the ClClO (1A′) intermediate, the forward reaction rate constant (k4) is negligible because of the high endothermicity. Conversely, its exothermic reverse process can take place readily; the

TABLE 3: Three-Parameter Arrhenius Formula of Predicted Rate Constants k ) ATB · exp(C/T) A

B

C

T (K)

ClO + Cl f Cl2+O ( P) 3

k1, cm3 · molecule-1 · s-1 k-1, cm3 · molecule-1 · s-1

2.22 × 10-14 3.25 × 10-14

k2∞, cm3 · molecule-1 · s-1 k20, cm6 · molecule-2 · s-1

1.66 4.08 1.29 4.67 1.33 8.85

k-2∞, s-1 k-20, cm3 · molecule-1 · s-1

× × × × × ×

10-10 10-24 10-3 1016 104 1022

× × × × ×

-11

0.997 0.976 ClO + Cl f ClClO (1A′)

-0.128 -3.21 -9.48 -0.723 -4.16 -9.91

-4761 -1803

150-2000 150-2000

-154 -535 -4814 -8467 -8958 -12 818

150-2000 150-700 700-2000 150-2000 150-900 900-2000

-94 -501 -3778 -16 496 -17 297

150-2000 150-700 700-2000 150-2000 150-2000

-45

150-2000

ClO + Cl f ClOCl (1A1) ∞

-1

-1

k3 , cm · molecule · s k30, cm6 · molecule-2 · s-1 3

k-3∞, s-1 k-30, cm3 · molecule-1 · s-1

4.47 1.06 3.56 1.91 8.03

10 10-24 10-11 1017 106

0.073 -2.87 -6.92 -0.753 -4.76 ClO + Cl f Cl2 + O (1D)

-1

k-4, cm · molecule · s 3

-1

1.27 × 10

-10

-0.187

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Xu and Lin have been presented by three-parameter Arrhenius expressions; these results may be employed for practical modeling applications. Acknowledgment. This work was supported by the Office of Naval Research under Grant N00014-02-1-0133. Acknowledgment is also made to the Cherry L. Emerson Center of Emory University for the use of its computational resources. M.C.L. gratefully acknowledges the support from Taiwan’s National Science Council for a distinguished visiting professorship at the Center for Interdisciplinary Molecular Science, National Chiao Tung University, Hsinchu, Taiwan. References and Notes

Figure 7. Predicted rate constants (k-4) of Cl2 + O (1D) f ClO + Cl and comparison with one experimental datum: (a) ref 13.

predicted reverse rate constants (k-4) are shown in Figure 7 in comparison with one experimental datum at 298 K provided by Wine et al.13 in 1985. The comparison exhibits the good agreement of the predicted value with the experimental one. The result illustrates that k-4 is pressure-independent with a small positive temperature dependence because of its loose variational TS with a rapidly increasing vibrational partition function at increasing temperature. The vibrational excited ClClO (1A′) intermediate formed from Cl2 + O (1D) can decompose almost directly to ClO + Cl without encountering collisional deactivation under normal T and P conditions. Although the predicted reaction heat (56.9 kcal/mol) of reaction 4 is overestimated by 5.3 kcal/mol in comparison with the experimental value (51.6 kcal/mol), the predicted rate constants are not expected to be influenced by the 10% deviation because of the absence of back dissociation to Cl2 + O (1D) from ClClO (1A′) in this highly exothermic barrierless process. For practical modeling applications, the above predicted rate constants at high and low pressure limits are listed in Table 3 by three-parameter Arrhenius expressions over the 150-2000 K temperature range. 4. Conclusions The results presented above may be summarized as follows: (1) Four low-energy product channels of the Cl + ClO reaction have been studied by ab initio quantum chemical calculations. The first and second channels producing Cl2 + O (3P) via transition states TS1 and TS2 were predicted with forward potential barriers of 9.5 and 34.1 kcal/mol, respectively, at the UCCSD(T)//UMPW1PW91 level of theory. The third and fourth channels on their singlet electronic state surfaces proceed barrierlessly via loose transition states to form ClClO (1A′) and ClOCl (1A1), respectively, which lie 16.0 and 31.9 kcal/mol, respectively, below the reactants. Also, the reverse reaction, Cl2 + O (1D) f ClO + Cl, can occur with great facility via the ClClO (1A′) intermediate barrierlessly with 56.9 kcal/mol exothermicity at the UCCSD(T)/6-311+G(3df) level. (2) The rate constants for all forward and reverse reaction processes, including the unimolecular dissociations of ClClO (1A′) and ClOCl (1A1), have been predicted for the temperature range 150-2000 K by the microcanonical VTST/RRKM theory. The predicted results agree reasonably with available experimental kinetic data for Cl + ClO f Cl2 + O (3P) and its reverse process and Cl2 + O (1D) f Cl + ClO. These rate constants

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