Computational Studies on Metathetical and Redox Processes of HOCl

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J. Phys. Chem. A 2010, 114, 833–838

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Computational Studies on Metathetical and Redox Processes of HOCl in the Gas Phase: (II) Reactions with ClOx (x ) 1-4) Z. F. Xu and M. C. Lin* Department of Chemistry, Emory UniVersity, Atlanta, Georgia 30322 ReceiVed: September 14, 2009; ReVised Manuscript ReceiVed: October 12, 2009

The reactions of HOCl + ClOx (x ) 1-4) have been studied theoretically by ab initio quantum chemical and statistical mechanical methods. The structures of reactants, intermediates, products, and transition states were optimized at the MPW1PW91/6-311+G(3df,2p) level of theory, and the potential energy surface of each reaction was refined at the G2M and CCSD(T)/6-311+G(3df,2p) levels of theory. The most favorable reaction channels are predicted to be Cl-abstraction in HOCl + ClO with a barrier of 18.5 kcal/mol and H abstraction in HOCl + OClO with a barrier of 23.9 kcal/mol. In the HOCl + ClO3 reaction both processes can occur; the barriers of Cl and H abstraction are 16.4 and 17.1 kcal/mol, respectively. In the HOCl + ClO4 reaction, the H abstraction transition state lies below that of the reactants by 1.4 kcal/mol. The rate constants for all low barrier channels have been calculated in the temperature range 200-3000 K by statistical theory. In addition, the rate constant for the reverse of the HOCl + ClO reaction, Cl2O + OH f HOCl + ClO, has been predicted; the result is in good agreement with the bulk of available experimental data. 1. Introduction Hypochlorous acid (HOCl) is a temporary reservoir of atmospheric chlorine; it plays an important role in stratospheric chemistry.1 In the combustion of ammonium perchlorate (AP), a practically important propellant in use today,2 the decomposition of perchloric acid (HClO4)3 in the gas phase produces initially ClO3 and OH radicals. The reactive chlorine trioxide (ClO3) can be rapidly converted to ClO and HOCl by reduction reactions involving OH and NHx (x ) 1-3) in high temperature media. Consequently, the reactions of HOCl with other reactive radicals can be expected to take place and should play a significant role in the chemistry of the AP propulsion process.4 In our recent publication on HOCl reactions,5 the kinetics and mechanisms of its reactions with H, O, HO, and HO2 were investigated, and the results were compared with available experimental and theoretical data. In the present work, we examine its metathetical and redox reactions with a series of chlorine oxide radicals, ClOx (x ) 1-4):

(a)

HOCl + ClO f products

(b)

HOCl + OClO f products

(c)

HOCl + ClO3 f products

(d)

HOCl + ClO4 f products

To our knowledge, there have been no experimental and theoretical studies available in the literature. In a related study on the reaction of HOCl with Cl, there are several experimental measurements6-9 performed around ambient temperature; the results showed that the chlorine atom abstraction reaction producing Cl2 is predominant. This conclusion had been * Corresponding author. E-mail: [email protected].

confirmed by a theoretical work published in 2003 by Wang et al.,10 who calculated two low barrier reaction channels:

HOCl + Cl f Cl2 + OH f ClO + HCl On the basis of the predicted reaction path energetics and transition state geometries, they computed the rate constants for the two product channels which are in good agreement with the experimental data. In this paper, we shall focus on the four unknown ClOx reactions with HOCl to investigate their mechanisms and the associated kinetics by using accurate quantum chemical and statistical theories as described below. 2. Computational Methods In the present study, the structures of the reactants, products, intermediates, and transition states related to the HOCl + ClOx (x ) 1-4) reactions have been optimized at the MPW1PW91/ 6-311+G(3df,2p) level of theory.11 The moments of inertia and frequencies of all species and stationary points were calculated with the corresponding optimization method. For a more accurate evaluation of energies, higher-level single-point energy calculations of all species and stationary points have been carried out by the CCSD(T)/6-311+G(3df,2p)12-14 method based on the optimized geometries at the MPW1PW91/6-311+G(3df,2p) level. For HOCl reactions with ClO and OClO, we have also employed the CCSD(T)/6-311+G(3df,2p)//BH&HLYP/6311+G(3df,2p) scheme. The results obtained by both methods agree within 0.5 kcal/mol for all major species involved. Also, for comparison, the G2M(CC5) method15 was employed to calculate the single point energies for all stationary points. All of the electronic structure calculations were preformed by the Gaussian03 program.16 Rate constant calculations were carried out with the Variflex program17 based on the microcanonical RRKM (RiceRamsperger-Kassel-Marcus) theory18-20 in the temperature

10.1021/jp908882b  2010 American Chemical Society Published on Web 11/03/2009

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Figure 1. Potential energy profiles of the reaction of HOCl with ClOx (x ) 1-4) in units of kcal/mol. The numbers on the top and second lines are obtained at G2M//MPW1PW91/6-311+G(3df,2p) and CCSD(T)/ 6-311+G(3df,2p)//MPW1PW91/6-311+G(3df,2p) levels of theory, respectively.

range 200-3000 K. Eckart tunneling permeability coefficients21 were used to correct the rate constants for the hydrogen transfer quantum effects at low temperatures. For a barrierless association/decomposition process, the variational transition state theory (VTST)22,23 was employed with the fitted Morse function, V(R) ) De{1-exp[-β(R-Re)]}2, which represents the minimum potential energy path (MEP). In the V(R) equation, De is the binding energy excluding zero-point vibrational energy for an association reaction, R is the reaction coordinate (i.e., the distance between the two bonding atoms), and Re is the equilibrium value of R at the stable intermediate structure. 3. Results and Discussion The potential energy surfaces (PESs) of HOCl reactions with ClOx (x ) 1-4) were predicted at both the CCSD(T)/ 6-311+G(3df, 2p) and the G2M levels of theory based on the geometric parameters optimized by MPW1PW91/6-311+G(3df, 2p) as alluded to above. All PESs are presented in Figure 1, and the structures of reactants, intermediates, and transition states are shown in Figure 2. The relative energies at the G2M and

Xu and Lin CCSD(T) levels of theory are placed on the top line and the second line, respectively, in Figure 1 for comparison. Unless specified otherwise, only the CCSD(T) energies are cited in the following discussion and rate constant calculations. Table 1 summarizes the vibrational frequencies and moments of inertia for the reactants, intermediates, and transition states computed at the MPW1PW91/6-311+G(3df, 2p) level of theory. 3.1. Potential Energy Surfaces. 3.1.1. HOCl + ClO. In this reaction system, three potential reaction channels have been considered; they include Cl abstraction, H abstraction, and OH transfer reactions as shown in Figure 1A. Comparing the relative energies of the stationary points calculated by G2M and CCSD(T), we note that the deviations between those predicted by the two methods are less than 1.5 kcal/mol. In the Cl abstraction reaction, chlorine monoxide radical (ClO) may approach HOCl by trans- and cis-configurations to the complexes a-LM1 and a-LM3, respectively. From the trans complex a-LM1, the reaction can proceed to the Cl2O + OH products via a-TS1 and another trans complex a-LM2. The relative energies of a-LM1, a-TS1, and a-LM2 are predicted to be -2.0, 18.5, and 17.9 kcal/mol, respectively. From the cis complex a-LM3, almost the same reaction channel as the trans pathway may take place along the cis type reaction path, with the energies of a-TS2 and complexes (a-LM3 and a-LM4) to be almost the same as those of the corresponding stationary points in the trans reaction channel. Since the structures and energies of transition states are very close to those of productside complexes, the predicted imaginary frequencies of a-TS1 and a-TS2 are only i56 and i76 cm-1. Also, the Cl2O + OH products are predicted to lie above the reactants by 19.4 kcal/ mol, which is 0.9 kcal/mol greater than that of a-TS1. It implies that the reverse Cl abstraction reaction should occur rather readily as will be discussed later. Similar to the Cl abstraction process, there are two H abstraction reactions which result in ClO exchange, ClO + HOCl′ f ClOH + OCl′. The trans one occurs via the hydrogen bonding complex a-LM5 and the transition state a-TS4 with the relative energies of -3.8 and 19.2 kcal/mol, respectively, while the cis channel takes place via the hydrogen bonding complex a-LM6 and a-TS5 with the relative energies of -4.1and 8.6 kcal/mol, respectively. Although a-LM5 is almost equal to a-LM6 in energy, the cis transition state a-TS5 is predicted to lie below the trans transition state a-TS4 by 10.6 kcal/mol. So, the cis H abstraction channel is predominant. However, we will not address the H abstraction reaction kinetically because it does not yield new products. The OH transfer channel can takes place via the transition state a-TS3 with an imaginary frequency of i738 cm-1 to produce the products Cl + HOOCl. The forward and reverse reaction barriers are predicted to be 31.8 and 11.1 kcal/mol, respectively. Comparing with the Cl and H abstraction reactions, the OH transfer reaction has the highest reaction barrier and will be neglected in our kinetic study. 3.1.2. HOCl + OClO. Four reaction channels are investigated in this system as seen in Figure 1B. The first one produces the ClO + HOClO products via the hydrogen bonding complex b-LM1, transition state b-TS1, and another hydrogen bonding complex b-LM2 on the product side with the relative energies of -4.0, 23.9, and 23.0 kcal/mol, respectively. However, the energies of these three stationary points predicted at the G2M level are lower than those computed at the CCSD(T) level, though the relative energies of the products ClO + HOClO obtained by G2M and CCSD(T), 23.4 and 23.2 kcal/mol, respectively, are very

Reactions with ClOx (x ) 1-4)

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Figure 2. Geometric parameters (bond length in Å and bond angle in degree) of reactants, intermediates, and transition states optimized by MPW1PW91/6-311+G(3df,2p).

close. For the hydrogen bonding complex b-LM1, the energy predicted by the G2M method was found to be lower by as much as 6.2 kcal/mol. Apparently the G2M method overes-

timates the hydrogen bonding energy of b-LM1. Another ClO + HOClO product channel occurs via b-TS2, at which one oxygen atom of OClO is abstracted by HOCl with a very

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TABLE 1: Vibrational Frequencies and Moments of Inertia Calculated at the MPW1PW91/6-311+G(3df, 2p) Level of Theory for Reactants, Intermediates, and Transition States of the Reactions HOCl + ClOx (x ) 1-4) species

IA, IB, IC (au)

frequancies (cm-1)

HOCl ClO ClO2 ClO3 ClO4 a-LM1 a-LM2 a-LM3 a-LM4 a-LM5 a-LM6 a-TS1 a-TS2 a-TS3 a-TS4 a-TS5 b-LM1 b-LM2 b-LM3 b-LM4 b-TS1 b-TS2 b-TS3 b-TS4 c-LM1

2.8, 117.2, 120.1 95.3, 95.3 34.6, 178.3, 213.0 175.5, 175.5, 333.2 291.2, 312.5, 336.9 71.0, 1531.4, 1602.5 78.5, 1235.7, 1314.3 70.2, 1548.7, 1618.9 90.7, 1201.7, 1292.5 80.0, 1738.6, 1818.6 165.1, 1352.5, 1449.0 80.6, 1198.2, 1275.5 89.4, 1175.4, 1262.1 126.7 1184.3, 1297.7 63.5, 1420.3, 1483.9 194.6, 766.6, 961.3 198.0, 1580.1, 1778.1 316.9, 1021.3, 1216.9 97.6, 2181.3, 2277.1 185.9, 1589.0, 1696.2 296.7, 1209.2, 1329.8 156.9, 1747.7, 1904.7 148.6, 1915.4, 1982.6 169.7, 1563.3, 1640.6 427.5, 1646.2, 1739.3

c-LM2

423.9, 1642.2, 1741.1

c-LM3

314.9, 2734.3, 2757.9

c-LM4

346.0, 1719.5, 1745.7

c-TS1

324.0, 1742.0, 1833.6

c-TS2

337.5, 1601.5, 1700.5

e-LM1

435.4, 1524.6, 1622.8

d-LM2

413.1, 2101.0, 2197.8

d-LM3

377.2, 1959.4, 2019.0

d-TS1

421.6, 1552.7, 1651.6

d-TS2

376.0, 1873.6, 1928.6

781, 1276, 3857 898 468, 1023, 1183 490, 490, 586, 978, 1156, 1156 390, 393, 404, 573, 593, 676, 962, 1229, 1327 26, 40, 71, 76, 130, 770, 902, 1271, 3856 51, 79, 103, 117, 298, 353, 677, 722, 3772 11, 40, 72, 75, 128, 769, 901, 1270, 3858 52, 82, 105, 123, 301, 368, 674, 723, 3772 20, 32, 66, 184, 402, 787, 914, 1369, 3692 15, 38, 78, 168, 516, 785, 916, 1345, 3685 i56, 77, 103, 168, 306, 384, 672, 722, 3774 i76, 74, 112, 162, 310, 392, 668, 723, 3774 i738, 73, 159, 334, 375, 438, 772, 1192, 3812 i2092, 46, 103, 154, 603, 873, 905, 1717, 2708 i1504, 57, 110, 221, 303, 618, 893, 901, 1567 15, 40, 45, 110, 139, 422, 483, 784, 1016, 1171, 1351, 3720 63, 85, 114, 140, 216, 374, 706, 744, 892, 1031, 1309, 3504 19, 24, 42, 53, 62, 88, 473, 780, 1021, 1179, 1279, 3852 27, 55, 79, 109, 128, 277, 330, 384, 539, 646, 1057, 3771 i194, 64, 77, 115, 220, 386, 699, 827, 880, 1045, 1412, 2774 i641, 31, 50, 101, 141, 222, 272, 458, 772, 930, 1301, 3698 i640, 62, 88, 144, 247, 334, 376, 472, 729, 1092, 1222, 3811 i99, 47, 84, 147, 170, 284, 376, 399, 520, 633, 1058, 3774 i8, 20, 30, 77, 98, 254, 488, 488, 578, 783, 979, 1152, 1161, 1309, 3804 9, 19, 38, 110, 172, 380, 458, 519, 565, 692, 924, 1097, 1228, 1309, 3462 2, 13, 20, 22, 43, 66, 490, 490, 585, 781, 979, 1156, 1157, 1281, 3847 23, 40, 61, 100, 105, 190, 300, 356, 421, 489, 596, 749, 1124, 1276, 3775 i1554, 24, 43, 89, 142, 466, 473, 506, 662, 869, 982, 1051, 1158, 1201, 1459 i72, 8, 51, 102, 166, 216, 343, 375, 404, 484, 591, 744, 1123, 1280, 3783 57, 89, 157, 163, 184, 423, 435, 561, 579, 606, 614, 746, 821, 1029, 1190, 1292, 1404, 3446 11, 32, 37, 108, 181, 408, 441, 591, 598, 602, 606, 780, 927, 1097, 1302, 1310, 1416, 3431 39, 77, 100, 115, 121, 219, 359, 399, 399, 541, 579, 606, 669, 753, 1093, 1326, 1345, 3764 i609, 43, 86, 152, 209, 431, 441, 579, 613, 634, 684, 916, 937, 991, 1162, 1282, 1473, 1786 i131, 53, 73, 139, 181, 264, 353, 401, 503, 545, 581, 604, 661, 735, 1092, 1325, 1344, 3766

high reaction barrier of 55.9 kcal/mol; thus the reaction producing ClO + HOClO is not expected to be kinetically competitive. The third reaction channel is Cl abstraction giving the ClOClO + OH products. It undergoes via the complex b-LM3, transition state b-TS4, and the product complex b-LM4 with the relative energies of -2.1, 40.2, and 38.9 kcal/mol, respectively. The structure of b-TS4 is very close to that of b-LM4 and its imaginary frequency is predicted to be i99 cm-1 at the MPW1PW91 level. The products ClOClO + OH have almost the same energy as b-TS4. The forth reaction channel invloves OH migration from HOCl to OClO to form the Cl + HOOClO products, which has an endothermicity of 42.0 kcal/mol. The forward and reverse reaction barriers of this channel are 47.0 and 5.0 kcal/mol, respectively. The imaginary frequency of b-TS3 is i640 cm-1. Both channels have much higher reaction barriers than the first process, and they are not expected to be kinetically relevant. 3.1.3. HOCl + ClO3. From the above discussion on the reactions of HOCl with ClO and OClO, one can see that the O

abstraction and OH transfer processes have significantly higher reaction barriers than the H and Cl abstraction channels; accordingly in the following discussion on the reactions of HOCl with ClO3 and ClO4, we only consider the H and Cl abstraction processes in order to save CPU time. In the case of the reaction of HOCl with ClO3 as shown in Figure 1C, the hydrogen abstraction channel occurs via the hydrogen bonding complex c-LM1, transition state c-TS1, and hydrogen bonding complex c-LM2 to yield the ClO + HOClO2 products. At the CCSD(T) level, the relative energies of c-LM1, c-TS1, and c-LM2 are predicted to be -3.2, 17.1, and -10.0 kcal/mol, respectively, with the ClO + HOClO2 products lying below the reactants by 4.4 kcal/mol. The Cl abstraction channel includes the stationary points of c-LM3, c-TS2, and c-LM4 with the relative energies of -1.3, 16.4, and 11.2 kcal/mol, respectively. The OH + ClOClO2 products lie above the reactants by 12.8 kcal/mol, which is slightly less than that of c-TS2. Despite the exothermicity of ClO + HOClO2 and the endothermicity of OH + ClOClO2, both H and Cl abstraction channels may be strongly competitive because c-TS1 is higher than c-TS2 only

Reactions with ClOx (x ) 1-4) by 0.7 kcal/mol. Also, the imaginary frequency of c-TS1 (i1554 cm-1) is much larger than that of c-TS2 (i72 cm-1), suggesting that the H abstraction reaction will have a much greater tunneling effect than the Cl atom abstraction process and thus dominates the metathetical reaction. 3.1.4. HOCl + ClO4. In the first step of this reaction, HOCl and ClO4 can form a complex (d-LM1) by both a hydrogen bond and a loose Cl-O bond with a six-membered ring as shown in Figure 1D. The complex lies below the reactants by 1.1 kcal/mol. From the complex, hydrogen abstraction can take place via d-TS1 and the complex d-LM2 to the ClO + HOClO3 products. The energy of d-TS1 is 1.4 kcal/mol lower than that of the reactants and even below d-LM1 by 0.3 kcal/mol, which is caused by the zero point vibrational energy corrections. Because of the large exothermicity (14.5 kcal/mol), this hydrogen abstraction reaction can occur readily. The Cl abstraction reaction is predicted to occur through d-TS2 and the complex d-LM3 to the OH + ClOClO3 products. The relative energies of d-TS2, d-LM3, and OH + ClOClO3 are 5.7, 3.5, and 5.8 kcal/mol, respectively. By comparing with the exothermic H abstraction channel, the endothermic Cl abstraction process is not competitive kinetically. 3.2. Rate Constants. From the above discussions about potential energy surfaces, the most favorable reaction channels may be summarized as follows: (a)HOCl + ClO f a-LM1 f a-TS1 f a-LM2 f Cl2O + OH f a-LM3 f a-TS2 f a-LM4 f Cl2O + OH

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Figure 3. Predicted rate constants of the HOCl + ClOx (x ) 1-4) reaction.

energy of d-TS1. As for the reaction of HOCl with ClO3, the hydrogen abstraction channel with a greater tunneling correction becomes faster at low temperatures (T < 500 K) than the Clabstraction process despite a lower barrier for the latter. The trend is reversed at T > 500 K with kc(HOClO2 + ClO) < kc(ClOClO2 + OH). For kinetic modeling applications, the predicted rate constants for the four reactions are fitted to modified three-parameter Arrhenius expressions in units of cm3 molecule-1 s-1 in the temperature range 200-3000 K:

(b)HOCl + OClO f b-LM1 f b-TS1 f b-LM2 f HOClO + ClO

ka(Cl2O + OH) ) 2.29 × 10-17 T 1.77 exp(-9386/T)

(c)HOCl + ClO3 f c-LM1 f c-TS1 f c-LM2 f ClO + HOClO2 f c-LM3 f c-TS2 f c-LM4 f OH + ClOClO2

kb(HOClO + ClO) ) 1.08 × 10-21 T 2.48 exp(-11666/T)

(d)HOCl + ClO4 f d-LM1 f d-TS1 f d-LM2 f ClO + HOClO3

kc(HOClO2 + ClO) ) 3.88 × 10-30 T 5.45 exp(-3867/T) kc(ClOClO2 + OH) ) 2.79 × 10-17 T 2.04 exp(-8231/T).

For rate constant calculations, the loose transition state process of the Cl-O bond fission can be presented by Morse function of V(RCl-O) ) De(1 - exp(-1.762(RCl-O-2.731)))2 kcal/mol, while that of the H-O hydrogen bond fission can be described by another Morse function of V(RH-O) ) De(1 exp(-1.449(RH-O-1.981)))2 kcal/mol. The vibrational frequencies and moments of inertia are adopted from those listed in Table 1, in which the small frequencies related to molecular internal rotations were treated as free rotors except a-TS1, for which the two low frequencies were replaced with two hinder rotors. The rotational constant and barrier of one rotor were calculated to be 19.563 and 433 cm-1, respectively, and those of another one were calculated to be 8.725 and 805 cm-1, respectively. However, the hindered rotation assumption for the two frequencies gave essentially the same values of rate constant as those obtained by harmonic oscillator model calculations. The predicted results for these four reactions show that there is no pressure effect in the temperature range from 200 to 3000 K on account of the shallow well depths of the prereaction complexes. The predicted rate constants, displayed in Figure 3, all exhibit positive T-dependence, except kd(HOClO3 + ClO), which has a very small negative temperature dependence at low temperatures with a small increase with temperature above 1000 K, consistent with the PES shown in Figure 1. The rate constant for the HOCl + ClO4 reaction was predicted with inclusion of multiple reflections above d-LM1 because of the comparable entrance VTS and d-TS1. Such multiple reflection correction reduces the rate constant at 200 K by 35% because of the low

kd(HOClO3 + ClO) ) 1.35 × 10-18 T 1.73 exp(1017/T). In addition to the above four reactions, we have also computed the rate constant kar for the reverse reaction

(ar) Cl2O + OH f HOCl + ClO, which has been kinetically studied by four research groups. At 298 K, the rate constant was determined to be (6.5 ( 0.5) × 10-12 and (9.4 ( 1.0) × 10-12 cm3 molecule-l s-l by Leu and Lin24 in 1979 and by Ennist and Birks25 in 1988, respectively, by discharge flow with resonance fluorescence detection. In 1992, Stevens and Anderson26 measured temperature dependent rate constant, (1.7 ( 0.8) × 10-12 exp[(420 ( 170)/T] cm3 molecule-l s-l, over the temperature range 230-400 K with the same experimental method. More recently, Hansen, Friedl, and Sander27 reported the rate constant expression, (5.1 ( 1.5) × 10-12 exp(100 ( 92/T) cm3 molecule-l s-l, in the temperature range 223-307 K also using the same experimental technique. We predicted the reaction rate constant for the reaction by micro-canonical VTST/RRKM master equation analysis with rate flux reflections above a-LM2. The predicted rate constant exhibits a weak negative-temperature dependence at low temperatures with a rapid positive temperature dependence at high temperatures (T > 500 K), as shown in Figure 4. The theoretical curve is in good agreement with the bulk of experimental data.24,26,27 However, those of Stevens and Anderson,26 measured

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Xu and Lin edgment 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 4. Predicted rate constants of the Cl2O + OH f HOCl + ClO reaction and comparison with the experimental data from literature. (a) Reference 24. (b) Reference 25. (c) Reference 26. (d) Reference 27.

at three temperatures 338, 358, and 383 K, are noticeably lower than the predicted values. The theoretical results can be fitted to the three parameter formula in units of cm3 molecule-1 s-1 in the temperature range of 200-3000 K:

kar ) 6.46 × 10-16 T1.39 exp(415/T) 4. Conclusions Four potential energy surfaces of HOCl reactions with ClOx (x ) 1-4) have been studied by CCSD(T)/6-311+G(3df,2p)// MPW1PW91/6-311+G(3df,2p). The low energy transition states predicted for these reactions are mainly for H- or Cl-abstraction processes: a-TS1 (and a-TS2) for HOCl + ClO giving Cl2O + OH, b-TS1 for HOCl + OClO giving HOClO + ClO, c-TS1 (and c-TS2) for HOCl + ClO3 giving HOClO2 + ClO (and ClOClO2 + OH), and d-TS1 for HOCl+ ClO4 giving HOClO3 + ClO, with the energies of 18.5 (and 18.7), 23.9, 17.1 (and 16.4), and -1.4 kcal/mol, respectively, relative to the corresponding reactants. The predicted rate constants of the former three low energy channels all exhibit positive temperaturedependences in the temperature range 200-3000 K. However, the H-abstract rate constant for HOCl + ClO4, kd(HOClO3 + ClO), appears to have a weak negative temperature dependence at low temperatures and positive temperature dependence at temperatures above 500 K. In addition, the predicted rate constants for a reverse process of HOCl + ClO, (ar) Cl2O + OH f HOCl + ClO, is shown to be in good agreement with the experimental data. Acknowledgment. This work was supported by the Office of Naval Research under Grant N00014-02-1-0133. Acknowl-

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