Theoretical Mechanistic Study on the Ion− Molecule Reaction of CHCl

Feb 10, 2010 - Yan Li, Hui-ling Liu, Yan-bo Sun, Zhuo Li, Xu-ri Huang* and ... in the most feasible pathways all lie below the reactant, the title rea...
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Theoretical Mechanistic Study on the Ion-Molecule Reaction of CHCl- with CS2 Yan Li, Hui-ling Liu, Yan-bo Sun, Zhuo Li, Xu-ri Huang,* and Chia-chung Sun State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin UniVersity, Changchun 130023, People’s Republic of China ReceiVed: September 6, 2009; ReVised Manuscript ReceiVed: December 28, 2009

A detailed theoretical study for the poorly understood ion-molecule reaction of CHCl- with CS2 is explored at the B3LYP/6-311++G(d,p) and CCSD(T)/6-311++G(3df,2p) (single-point) levels. Various possible reaction pathways are considered. On the doublet potential energy surface, five dissociation products are both thermodynamically and kinetically possible. Among these products, P7 (SCHCl- + CS) may be the most favorable product with predominant abundances, whereas P1 (Cl- + SCHCS) and P2 (Cl- + HCCSS) may be the second and third feasible products followed by the almost negligible P3 (Cl- + HSCCS), P4 (CClS+ HCS), and P6 (S-cCCS- + HCl). Because the isomers and transition states involved in the most feasible pathways all lie below the reactant, the title reaction is expected to be fast, which is consistent with the measured large rate constant in recent experiment. The present paper may provide a useful guide for understanding other analogous ion-molecule reactions such as CHF- and CHBr- with CS2, COS, and CO2. 1. Introduction Halocarbenes play a crucial role in atmospheric chemistry due to their role in destroying the ozone layer.1 In addition, halocarbenes can be used for etching of semiconductor materials in plasma chemistry.2 In view of these factors, halocarbenes have long been the subject of extensive studies.3-9 As the simplest monochlorocarbene, CHCl radical can be formed by photolysis of chloroform and other halons.10-14 Up to now, a large number of experimental and theoretical investigations have been reported on the spectroscopic properties15-19 and formation enthalpy20 of CHCl, as well as its reactions.21-26 To our great surprise, in sharp contrast to the rich knowledge of CHCl, the anion CHCl- has received rather little attention. Up to now, only a few experimental studies have been reported on CHCl- reactions with CH3X (X ) F, Cl, Br, I) and ROH (R ) CH3, C2H5, (CH3)2CH, (CH3)2CHCH2), CHnCl4-n (n ) 0-3), CS2, CSO, CO2, O2, CO, and N2O.27-30 Among the CHCl- studies, the reaction with CS2 attracts our great interest. In 2008, Villano et al. studied the CHCl- + CS2 reaction using the flowing afterglow-selected ion flow tube instrument (FA-SIFT).30 The measured single temperaturedependent rate constant at 298 K is k ) (10.5 ( 0.25) × 10-10 cm3 s-1. According to Villano et al.’s experimental observations, the products and distributions are as

CHCl-+ CS2 f Cl-+ C2HS2 f C2S2- + HCl

0.07 0.02

f CHClS- + CS

0.73

f CClS- + HCS

0.18

It should be pointed out that, in Villano et al.’s experiment, only the ionic products are detected; the real structure of the ionic and the neutral products are inferred. Without detailed * To whom correspondence should be addressed.

potential energy surface (PES) investigation, it is difficult to discuss the mechanism of the ion-molecule reaction between CHCl- and CS2. Unfortunately, to the best of our knowledge, no theoretical study has been reported. Thus, in the present paper, we set out to study the title reaction by applying the quantum chemical methods. Our main goal is to provide elaborated isomerization and dissociation pathways thereby to interpret previous experimental observations. 2. Computational Methods The optimized geometries of reactant, products, isomers, and transition states are obtained at the B3LYP/6-311++G(d,p) level. Frequency calculations are performed at the same level of theory to check whether the obtained structure is an isomer (with all real frequencies) or a transition state (with only one imaginary frequency). Note that the frequencies are not scaled in the present work. Connections of the transition state between designated isomers are confirmed by intrinsic reaction coordinate (IRC) calculations at the B3LYP/6-311++G(d,p) level. Furthermore, to get more reliable energetic data, single-point calculations are carried out at the CCSD(T)/6-311++G(3df,2p) level using the B3LYP/6-311++G(d,p) optimized geometries. Unless otherwise specified, the relative energies at CCSD(T)/ 6-311++G (3df,2p)//B3LYP/6-311++G(d,p)+ZPVE level are used throughout. All the calculations are carried out using the Gaussian 98 program package.31 3. Results and Discussion The optimized structures of the reactant and various products are shown in Figure 1, whereas the optimized structures of isomers and transition states are presented in Figures 2 and 3, respectively. The energetic data for isomers and products are listed in Table 1, whereas those for transition states are listed in Table 2. For convenient discussion, the total energy of the reactant R (CHCl- + CS2) is set as zero for reference. The symbol TSm/n is used to denote the transition state connecting isomers m and n. By means of the interrelationship of reactant, products, isomers, and transition states as well as their relative

10.1021/jp908601v  2010 American Chemical Society Published on Web 02/10/2010

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Figure 1. Optimized structures of the reactant and products at the B3LYP/6-311++G(d,p) level. Distances are given in angstroms and angles in degrees.

energies, the schematic PES for the CHCl- + CS2 reaction is plotted in Figure 4. 3.1. Entrance Channels. CHCl- has Cs symmetry in its 2A′ ground state. At the B3LYP/6-311++G(d,p) level, the negative charge and spin density is mainly positioned at the C-atom. Thus, the C-atom can be considered as the most active site of CHCl-. The association with CS2 may have two attack patterns, that is, (i) middle-C attack to form S2CCHCl- 1 (-69.4) (as shown in Figure 4A); (ii) end-S attack to form SCSCHCl- 2

(2a, 2b, 2c) (-19.8, -24.8, -22.0) (as shown in Figure 4B). The values in parentheses are relative energies in kcal/mol with reference to R (CHCl- + CS2) (0.0). No barrier can be found for the formation of isomers S2CCHCl- 1 and SCSCHCl- 2 (2a, 2b, 2c). As shown in Figure 4B, further evolution pathways of 2b and 2c involve high-energy transition states TS2b/8a (5.8), TS2b/P7 (43.8), TS2c/7 (1.9), which indicates that these processes may have little contribution to the final product. They will not be considered further. Thus, in the following passage,

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Figure 2. Optimized structures of the isomers at the B3LYP/6-311++G(d,p) level. Distances are given in angstroms and angles in degrees.

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Figure 3. Optimized structures of the transition states at the B3LYP/6-311++G(d,p) level. Distances are given in angstroms and angles in degrees.

we concentrate on the subsequent evolutions of isomers S2CCHCl- 1 and SCSCHCl- 2a. 3.2. Isomerization and Dissociation. Starting from S2CCHCl- 1 (-69.4), four different reaction channels are identified: (i) concerted S-shift along with Cl-extrusion to generate P1 (Cl- + SCHCS) (-50.8); (ii) S-Cl exchange to form SCHCClS- 3a (-80.9); (iii) Cl-extrusion accompanied by S-shift (from C-atom to S-atom) to generate P2 (Cl- + HCCSS) (-38.4); (iv) H migration (from C-atom to S-atom) to produce HSC(S)CCl- 4a (-35.4). Once the channel ii isomer

SCHCClS- 3a is formed, two processes may then occur. One possibility is conversion of SCHCClS- 3a to its structural isomer SCHCClS- 3b (-83.2), followed by concerted H-shift along with Cl-elimination to yield P3 (Cl- + HSCCS) (-42.7). Another possibility is concerted C-C bond cleavage along with C-S bond formation to generate SCHSCCl- 7 (-27.1). Starting from SCHSCCl- 7, two processes are located. One is dissociation to P4 (CClS- + HCS) (3.2) via the weakly bound complex HCS · · · SCCl- 10 (-9.0). The other is conversion of SCHSCCl7 to HSCSCCl- 8c (12.6). However, the latter process has to

Ion-Molecule Reaction of CHCl- with CS2

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TABLE 1: Total (a.u.), Zero-Point (kcal/mol), and Relative Energies in Parentheses (kcal/mol) as Well as Those Including Zero-Point Energies of the Reactant, Products, and Isomers for the CHCl- + CS2 Reaction species -

R (CHCl + CS2) P1 (Cl- + SCHCS) P2 (Cl- + HCCSS) P3 (Cl- + HSCCS) P4 (CClS- + HCS) P5 (Cl- + HS-cCCS) P6 (S-cCCS- + HCl) P7 (SCHCl- + CS) P8 (HSCCl- + CS) P9 (Cl- + CS + HCS) P10 (3SCClSC- + H) 1 S2CCHCl2a SCSCHCl2b SCSCHCl2c SCSCHCl3a SCHCClS3b SCHCClS4a HSC(S)CCl4b HSC(S)CCl5 SC · · · SCHCl6 HCl · · · S-cCCS7 SCHSCCl8a HSCSCCl8b HSCSCCl8c HSCSCCl8d HSCSCCl9a SCClSCH9b SCClSCH9c SCClSCH9d SCClSCH10 HCS · · · SCCl-

B3LYP

ZPVE

-1333.4104545 -1333.5039918 -1333.5554499 -1333.4868058 -1333.4047579 -1333.4254152 -1333.4509634 -1333.4227181 -1333.3541380 -1333.3980673 -1333.2956438 -1333.5233808 -1333.4371911 -1333.4463875 -1331.4410559 -1333.5443159 -1333.5467334 -1333.4632685 -1333.4646106 -1333.4396584 -1333.4737100 -1333.4500447 -1333.3874534 -1333.3837543 -1333.3763694 -1333.3767940 -1333.4328660 -1333.4352845 -1333.4421190 -1333.4406755 -1333.4215602

10.39385 13.48986 12.22855 11.76523 9.484320 11.33091 9.606690 11.04376 8.436650 9.207510 5.859600 14.70529 12.73285 12.74181 12.75386 14.65344 14.89412 11.99404 11.88184 11.97482 10.48680 12.73164 10.80200 11.04333 10.58801 10.75104 12.33965 12.60178 12.49988 12.60060 10.328810

CCSD + ZPVE

CCSD (0.0) (3.1) (1.8) (1.4) (0.9) (-0.9) (-0.8) (0.6) (-2.0) (-1.2) (-4.5) (4.3) (2.3) (2.3) (2.4) (4.3) (4.5) (1.6) (1.5) (1.6) (0.1) (2.3) (0.4) (0.6) (0.2) (0.4) (1.9) (2.2) (2.1) (2.2) (-0.1)

-1331.7576897 -1331.8435532 -1331.8217693 -1331.8278690 -1331.7510727 -1331.7760780 -1331.8027069 -1331.7754543 -1331.7089027 -1331.7494655 -1331.6548947 -1331.8751275 -1331.7929427 -1331.8008818 -1331.7965057 -1331.8933619 -1331.8974757 -1331.8166446 -1331.8176647 -1331.7901229 -1331.8236282 -1331.8046090 -1331.7498382 -1331.7458035 -1331.7379578 -1331.7393242 -1331.7924809 -1331.7956749 -1331.8004906 -1331.7996052 -1331.7718894

(0.0) (-53.9) (-40.2) (-44.0) (-4.2) (-11.5) (-28.2) (-11.1) (30.6) (5.2) (64.5) (-73.7) (-22.1) (-27.1) (-24.4) (-85.1) (-87.7) (-37.0) (-37.6) (-20.4) (-41.4) (-29.4) (4.9) (7.5) (12.4) (11.5) (-21.8) (-23.8) (-26.9) (-26.3) (-8.9)

0.0 -50.8 -38.4 -42.7 3.2 -10.6 -29.0 -10.5 28.7 4.0 60.0 -69.4 -19.8 -24.8 -22.0 -80.9 -83.2 -35.4 -36.1 -18.8 -41.3 -27.1 5.3 8.1 12.6 11.9 -19.9 -21.6 -24.8 -24.1 -9.0

TABLE 2: Total (a.u.), Zero-Point (kcal/mol), and Relative Energies in Parentheses (kcal/mol) as Well as Those Including Zero-Point Energies of the Transition States for the CHCl- + CS2 Reaction species

B3LYP

ZPVE

TS1/3a TS1/4a TS1/P1 TS1/P2 TS2a/5 TS2a/P1 TS2b/8a TS2b/P8 TS2c/7 TS3a/3b TS3a/7 TS3b/9b TS3b/P3 TS4a/4b TS4b/6 TS7/8c TS7/10 TS8a/8b TS8b/8c TS8b/9a TS8c/8d TS8d/P5 TS9a/9b TS9a/9c TS9b/9d TS9b/P10 TS9c/9d TS9c/P9 TS9d/P9

-1333.4638499 -1333.4501706 -1333.4923989 -1333.4384882 -1333.4322098 -1333.4217298 -1333.3858972 -1333.3277738 -1333.4012689 -1333.4989227 -1333.4270529 -1333.3961007 -1333.4556901 -1333.4445703 -1333.4521662 -1333.3332293 -1333.4134286 -1333.3599203 -1333.3626713 -1333.3263987 -1333.3684064 -1333.3515491 -1333.4183087 -1333.4183606 -1333.4171841 -1333.2990263 -1333.4225268 -1333.4068723 -1333.4079768

12.24053 10.41921 13.16346 12.88913 12.08101 12.54797 9.50574 9.58887 9.77923 13.46320 12.20526 11.43054 10.68068 11.58909 10.82673 8.02640 10.887590 9.69760 10.33995 9.24238 10.44674 10.10850 12.20529 11.37531 11.43035 6.02003 12.15431 10.99001 10.92737

CCSD(T) + ZPVE

CCSD(T) (1.8) (0.0) (2.8) (2.5) (1.7) (2.2) (-0.9) (-0.8) (-0.6) (3.1) (1.8) (1.0) (0.3) (1.2) (0.4) (-2.4) (0.5) (-0.7) (-0.1) (-1.2) (0.1) (-0.3) (1.8) (1.0) (1.0) (-4.4) (1.8) (0.6) (0.5)

-1331.8054537 -1331.7718098 -1331.8387845 -1331.7933032 -1331.7849829 -1331.7781876 -1331.7470984 -1331.6865439 -1331.7536775 -1331.8508488 -1331.7801371 -1331.7524062 -1331.8070278 -1331.7962847 -1331.7988054 -1331.6337489 -1331.7610209 -1331.7214118 -1331.7258989 -1331.6868562 -1331.7278312 -1331.7060345 -1331.7784148 -1331.7767269 -1331.7751707 -1331.6550659 -1331.7812248 -1331.7568338 -1331.7570384

(-30.0) (-8.9) (-50.9) (-22.3) (-17.1) (-12.9) (6.6) (44.6) (2.5) (-58.5) (-14.1) (3.3) (-31.0) (-24.2) (-25.8) (77.8) (-2.1) (22.8) (19.9) (44.4) (18.7) (32.4) (-13.0) (-11.9) (-11.0) (64.4) (-14.8) (0.5) (0.4)

-28.1 -8.8 -48.1 -19.9 -15.4 -10.7 5.8 43.8 1.9 -55.4 -12.3 4.4 -30.7 -23.0 -25.4 75.4 -1.6 22.1 19.9 43.3 18.8 32.1 -11.2 -11.0 -9.9 60.0 -13.0 1.1 0.9

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Figure 4. Potential energy surface (PES) for the CHCl- + CS2 reaction. Erel are the relative energies (kcal/mol).

surmount a significant high-energy transition state TS7/8c (75.4). Thus, this process is kinetically unfeasible and not discussed

further. Channel iv isomer HSC(S)CCl- 4a can easily convert to HSC(S)CCl- 4b (-36.1). Subsequently, HSC(S)CCl- 4b

Ion-Molecule Reaction of CHCl- with CS2 undergoes side-HCl extrusion to form the weakly bound complex HCl · · · S-cCCS- 6 (-41.3). Finally, HCl · · · S-cCCS6 can directly dissociate to P6(S-cCCS- + HCl) (-29.0). As a result, the respective pathways for channels i-iv can be depicted as Path i

R f 1 f P1 (Cl- + SCHCS)

Path ii1

R f 1 f 3a f 3b f P3 (Cl- + HSCCS)

Path ii2

R f 1 f 3a f 7 f 10 f P4 (CClS- + HCS)

Path iii

R f 1 f P2 (Cl- + HCCSS)

Path iv

R f 1 f 4a f 4b f 6 f P6 (S-cCCS- + HCl)

For SCSCHCl- 2a (-19.8), there are two different channels: (v) C-S bond rupture to form the weakly bound complex SC · · · SCHCl- 5 (-18.8) before the final product P6 (SCHCl+ CS) (-10.5); (vi) concerted C-C bond formation, Clelimination, and S-C bond dissociation to generate P1 (Cl- + SCHCS) (-50.8). The pathways correspond to channels v-vi can be written as

Path v

R f 2a f 5 f P7 (SCHCl- + CS)

Path vi

R f 2a f P1 (Cl- + SCHCS)

4. Reaction Mechanism For the ion-molecule reaction of CHCl- with CS2, we have obtained seven important pathways, i.e., paths i-vi. Generally, the energy barrier controls the reaction rate in a pathway. Thus, we use barrier height to discuss the feasibility of paths i-vi. Considering the rather high barriers of 41.3(1 f 3a), 25.5 (3a f 3b), and 52.5 (3b f P3) kcal/mol in path ii1, 41.3(1 f 3a), 68.6 (3a f 7), and 25.5 (7 f 10) kcal/mol in path ii2, and 60.6 (1 f 4a) kcal/mol in path iv, formation of P3 (Cl- + HSCCS), P4 (CClS- + HCS), and P6 (S-cCCS- + HCl) is quite uncompetitive. With respect to the remaining four paths, the least competitive channel should be path iii because the energy barrier of 49.5 (1 f P2) kcal/mol in path iii is much higher than 4.4 (2a f 5) kcal/mol in path v, 21.3 (1 f P1) kcal/mol in path i, and 8.1 (2a f P1) kcal/mol in path vi. Furthermore, the barrier of path v is much lower than those of path i and path vi. Thus, we expect path i and path vi cannot compete with path v. As a consequence, reflected in the final product distributions, P7 (SCHCl- + CS) may be the most favorable product, P1 (Cl- + SCHCS) and P2 (Cl- + HCCSS) are the second and third feasible products, respectively. P3 (Cl- + HSCCS), P4 (CClS- + HCS), and P6 (S-cCCS- + HCl) may be the least possible products. 5. Comparison with Experiments It is useful to compare our calculated results with previous experimental findings. To our best knowledge, only one experimental study has been performed by Villano et al. using the FA-SIFT instrument.30 In their studies, four products Cl+ C2HS2, C2S2- + HCl, CHClS- + CS, and CClS- + HCS with respective branching ratios 0.07, 0.02, 0.73, and 0.18 were observed. The experimental detected CHClS- + CS and C2S2+ HCl correspond to the major and minor product P7 (SCHCl+ CS) and P6 (S-cCCS- + HCl), respectively. Furthermore, the experimental observed Cl- + C2HS2 corresponds to products P1 (Cl- + SCHCS), P2 (Cl- + HCCSS), P3 (Cl- + HSCCS), and P5 (Cl- + HS-cCCS). However, based on our calculations,

J. Phys. Chem. A, Vol. 114, No. 8, 2010 2883 the formation pathway of P5 (Cl- + HS-cCCS) contains highenergy transition state TS7/8c (75.4). Formation of P5 (Cl- + HS-cCCS) is unfeasible. Thus, the experimental detected Cl+ C2HS2 may be the combination of P1 (Cl- + SCHCS), P2 (Cl- + HCCSS), and P3 (Cl- + HSCCS). Moreover, the experimentally detected product CClS- + HCS corresponds to P4 (CClS- + HCS) in our results. As for these aspects, our calculated results are quite consistent with previous experimental findings. However, there exist discrepancies. As shown in Table 1, the relative energies of P1 (Cl- + SCHCS), P2 (Cl- + HCCSS), and P4 (CClS- + HCS) are -50.8, -38.4, and 3.2 kcal/mol, respectively. Furthermore, the energy barriers involved in the formation pathways of P1 (21.3 kcal/mol in path i and 8.1 kcal/mol in path vi), and P2 (49.5 kcal/mol in path iii) are much lower than those involved in the formation pathway of P4 (41.3, 68.6, and 25.5 kcal/mol in path ii2). From both thermodynamic and kinetic considerations, formation of P1 and P2 should be more competitive than that of P4. Thus, we believe that products P1 and P2 may have a larger branching ratio than P4. The abundance of CClS- + HCS obtained by Villano et al. may be overestimated. In view of this discrepancy, future reinvestigation of the title reaction is very desirable. 6. Conclusion The ion-molecule reaction of CHCl- with CS2 is theoretically studied for the first time at the B3LYP/6-311++G(d,p) and CCSD(T)/6-311++G(3df,2p) levels. Our results indicate that the barrierless association of CHCl- with CS2 generates four initial isomers S2CCHCl- 1 and SCSCHCl- 2 (2a, 2b, 2c) which can undergo a variety of isomerzation and dissociation pathways lead to six dissociation products P1 (Cl- + SCHCS), P2 (Cl- + HCCSS), P3 (Cl- + HSCCS), P4 (CClS- + HCS), P6 (S-cCCS- + HCl), and P7 (SCHCl- + CS). Among these, P7 may be the most favorable product, P1 and P2 are the lesser followed products, P3, P4, and P6 may be the least possible products. The present paper may be helpful for understanding the chemistry of halocarbenes. Acknowledgment. This work is supported by the National Natural Science Foundation of China (No. 20773048). References and Notes (1) Wayne, R. P. Chemistry of the Atomspheres; Oxford University Press: Oxford U.K., 1985. (2) Moss, S. J.; Ledwith, A. The Chemistry of the Semiconductor Industry, 1986. (3) Rablen, P. R.; Paiz, A. A.; Thuronyi, B. W.; Jones, M., Jr. J. Org. Chem. 2009, 74, 4252. (4) Tao, C.; Reid, S. A.; Schmidt, T. W.; Kable, S. H. J. Chem. Phys. 2007, 126, 051105. (5) Mukarakate, C.; Tao, C.; Jordan, C. D.; Polik, W. F.; Reid, S. A. J. Phys. Chem. A 2008, 112, 466. (6) Krogh-Jespersen, K.; Yan, S.; Moss, R. A. J. Am. Chem. Soc. 1999, 121, 6269. (7) Hancock, G.; Ketley, G. W.; MacRobert, A. J. J. Phys. Chem. 1984, 88, 2104. (8) Za´rate, A. O.; Martinez, R.; Rayo, M. N. S.; Castano, F.; Hancock, G. J. Chem. Soc., Faraday Trans. 1992, 88, 535. (9) Browsword, R. A.; Hancock, G.; Oum, K. W. J. Phys. Chem. 1996, 100, 4840. (10) Ibuki, T.; Hiraya, A.; Shobatake, K. J. Chem. Phys. 1992, 96, 8793. (11) Felder, P.; Demuth, C. Chem. Phys. Lett. 1993, 208, 21. (12) Scientific Assessment of Ozone Depletion; World Meteorological Organization: Geneva, Switzerland, 1995. (13) Chowdhury, P. K. J. Phys. Chem. 1995, 99, 12084. (14) Montzka, S. A.; Butler, J. H.; Myers, R. C.; Thompson, T. M.; Swanson, T. H.; Clarke, A. D.; Lock, L. T.; Elkins, J. W. Science 1996, 272, 1318. (15) Tao, C.; Mukarakate, C.; Reid, S. A. J. Chem. Phys. 2006, 124, 224314.

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(16) Tao, C.; Mukarakate, C.; Terranova, Z.; Ebben, C.; Judge, R. H.; Reid, S. A. J. Chem. Phys. 2008, 129, 104309. (17) Tao, C.; Mukarakate, C.; Judge, R. H.; Reid, S. A. J. Chem. Phys. 2008, 128, 171101. (18) Tarczay, G.; Miller, T. A.; Czako´, G.; Csa´sza´r, A. G. Phys. Chem. Chem. Phys. 2005, 7, 2881. (19) Shin, S. K.; Dagdigian, P. J. J. Chem. Phys. 2008, 128, 064309. (20) Schwartz, M.; Marshall, P. J. Phys. Chem. A 1999, 103, 7900. (21) Clark, D. T.; Tedder, J. M. Trans. Faraday Soc. 1966, 62, 399. (22) Wagener, R.; Wagner, H. Gg. Z. Phys. Chem. (Munich) 1992, 175, 9. (23) Carstensen, H. H.; Rehbein, C.; Wagner, H. Gg. Ber. Bunsen-Ges. Phys. Chem. 1997, 101, 1429. (24) Baren, R. E.; Erickson, M. A.; Hershberger, J. F. Int. J. Chem. Kinet. 2002, 34, 12. (25) Zhang, J. X.; Liu, J. Y.; Li, Z. S.; Sun, C. C. J. Comput. Chem. 2004, 25, 1184. (26) Liu, J. J.; Ding, Y. H.; Yu, G. T.; Feng, J. K.; Sun, C. C. J. Comput. Chem. 2002, 23, 625. (27) Born, M.; Ingemann, S.; Nibbering, N. M. M. J. Am. Chem. Soc. 1994, 116, 7210.

Li et al. (28) Van Doren, J. M.; Barlow, S. E.; DePuy, C. H.; Bierbaum, V. M. Int. J. Mass Spectrom. 1987, 81, 85. (29) Villano, S. M.; Eyet, N.; Lineberger, W. C.; Bierbaum, V. M. J. Am. Chem. Soc. 2008, 130, 7214. (30) Villano, S. M.; Eyet, N.; Lineberger, W. C.; Bierbaum, V. M. Int. J. Mass Spectrom. 2009, 280, 12. (31) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.6; Gaussian, Inc.: Pittsburgh, PA, 1998.

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