Theoretical Study and Rate Constants Calculation forthe Reactions X 1 CF3CH2OCF3 (X 5 F, Cl, Br)

Hui Zhang,*[a] Yang Liu,[a] Jing-Yao Liu,[b] and Ze-Sheng Li*[c,d]

The multiple-channel reactions X þ CF3CH2OCF3 (X ¼ F, Cl, Br) are

theoretically investigated. The minimum energy paths (MEP) are

calculated at the MP2/6-31þG(d,p) level, and energetic information

is further refined by the MC-QCISD (single-point) method. The rate

constants for major reaction channels are calculated by canonical

variational transition state theory (CVT) with small-curvature

tunneling (SCT) correction over the temperature range 200–2000 K.

The theoretical three-parameter expressions for the three channels

k1a(T) ¼ 1.24 � 10�15T1.24exp(�304.81/T), k2a(T) ¼ 7.27 �10�15T0.37exp(�630.69/T), and k3a(T) ¼ 2.84 � 10�19T2.51

exp(�2725.17/T) cm3 molecule�1 s�1 are given. Our calculations

indicate that hydrogen abstraction channel is only feasible channel

due to the smaller barrier height among five channels considered.VC 2011Wiley Periodicals, Inc.

DOI: 10.1002/jcc.22897

Introduction

Chemicals containing chlorofluorocarbons (CFCs) and hydro-

chlorofluorocarbons (HCFCs), mainly used as refrigerant and

cleaning fluids in the industrial society for years, are able to

destroy stratospheric ozone and are greenhouse gases becom-

ing ever more important relative to other greenhouse gases

(e.g., CO2, CH4). They are being phased out according to the

Montreal Protocol. Hydrofluoroethers (HFEs) have been pro-

posed for alternative compounds for chlorofluorocarbons

(CFCs), because they contain no chlorine and bromine atoms,

and do not contribute to ozone depletion; however, they are

potential greenhouse gases and their atmospheric degradation

products may not be benign to the environment. Therefore, it

is important to investigate the reaction mechanism of

halogen with HFEs. The reactivity of OH/Cl with HFEs has been

theoretical and experimental studied in the past literatures,[1–8]

and few rate constants data are available for the Cl atom

reactions of CF3CH2OCH3,[1] CH3CH2OCF3,

[1] CF3CH2OCHF2,[2–4]

CF3CH2OCClF2,[4] and CF3CHFOCF3,

[5] the corresponding rate

values at 298 K are (1.8 6 0.9) � 10�11, (2.2 6 0.8) � 10�12,

(3.1 6 0.1) � 10�14, (3.2 6 0.2) � 10�15, and (3.1 6 2.5) �10�14 cm3 molecule�1 s�1, respectively. The theoretical investi-

gation of the reaction halogen with CF3CH2OCF3 is desirable to

give a further understanding of the mechanism of this multiple

channel reaction and to evaluate the rate constant. To the best

of our knowledge, no previous theoretical work has been per-

formed on this reaction. The result aids our understanding of

CF3CH2OCF3 with halogen’s atmospheric chemistry.

For reactions X þ CF3CH2OCF3 (X ¼ F, Cl, Br), the hydrogen

abstracted atom can be from CH2 group, and fluorine atom can

be abstracted from CF3 group by halogen, as a result, five reac-

tion channels are feasible for each reaction, denoted as R1a

(R2a, R3a), R1b-in (R2b-in, R3b-in), R1b-out (R2b-out, R3b-out),

R1c-in (R2c-in, R3c-in), and R1c-out (R2c-out, R3c-out), respec-

tively. The calculations indicate that two reaction channels exist

in the reaction channels R1b (R2b, R3b) and R1c (R2c, R3c),

namely ‘‘in-plane fluorin abstraction’’ (channels R1b-in, R2b-in or

R3b-in) and ‘‘out-of-plane fluorin abstraction’’ (channels R1b-out,

R2b-out or R3b-out), two channels of in-plane and out-of-plane

lead to the same products, as follows:

FþCF3CH2OCF3 !CF3CHOCF3þHF ðR1aÞ!CF2CH2OCF3þF2 ðR1b-in and R1b-outÞ!CF3CH2OCF2þF2 ðR1c-in and R1c-outÞ

ClþCF3CH2OCF3 !CF3CHOCF3þHCl ðR2aÞ!CF2CH2OCF3þCIF ðR2b-in and R2b-outÞ!CF3CH2OCF2þCIF ðR2c-in and R2c-outÞ

BrþCF3CH2OCF3 !CF3CHOCF3þHBr ðR3aÞ!CF2CH2OCF3þBrF ðR3b-in and R3b-outÞ!CF3CH2OCF2þBrF ðR3c-in and R3c-outÞ

[a] H. Zhang, Y. Liu

College of Chemical and Environmental Engineering, Harbin University of

Science and Technology, Harbin 150080, People’s Republic of China

E-mail: hust_zhanghui11@hotmail.com

[b] J.-Y. Liu

Institute of Theoretical Chemistry, State Key Laboratory of Theoretical and

Computational Chemistry, Jilin University, Changchun 130023, People’s

Republic of China

[c] Z.-S. Li

Academy of Fundamental and Interdisciplinary Sciences, Department of

Chemistry, Harbin Institute of Technology, Harbin 150080, People’s Republic

of China

[d] Z.-S. Li

School of Sciences, Beijing Institute of Technology, Beijng100081, People’s

Republic of China

E-mail: zeshengli@jlu.edu.cn

Contract/grant sponsor: National Natural Science Foundation of China;

Contract/grant numbers: 20333050, 20303007, 20973049; Contract/

grant sponsor: Foundation for the Department of Education of

Heilongjiang Province; Contract/grant numbers: 1152G010, 11551077;

Contract/grant sponsor: SF (Academe of Harbin of China); Contract/

grant number: 2011RFJGS026; Contract/grant sponsors: New Century

Excellent Talents in University (NCET), Doctor Foundation (Ministry of

Education), Key subject of Science and Technology (Ministry of

Education of China).

VC 2011 Wiley Periodicals, Inc.

Journal of Computational Chemistry 2012, 33, 685–690 685

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In this paper, dual-level direct dynamics method[9–13] is

employed to study the kinetics of the major reaction channels

X þ CF3CH2OCF3 ! CF3CHOCF3 þ HX (X¼ F, Cl, Br). The potential

energy surface information, including geometries, energies,

gradients, force constants of all the stationary points (reactants,

complexes, products, and transition states) and some extra points

along the minimum energy path (MEP), is obtained directly from

electronic structure calculations. Single-point energies are calcu-

lated by the MC-QCISD method.[14] Subsequently, by means

of the POLYRATE 9.7 program,[15] the rate constants of these

reaction channels are calculated by the variational transition state

theory (VTST)[16,17] proposed by Truhlar and coworkers. Our

results may be helpful for further experimental investigations.

Computational Method

In this work, the equilibrium geometries and frequencies of all

the stationary points (reactants, complexes, products, and tran-

sition states) are optimized at the restricted or unrestricted

second-order Møller-Plesset perturbation (MP2)[18–20] level with

the 6-31þG(d,p) basis set. The MEP is obtained by intrinsic

reaction coordinate (IRC) theory with a gradient step-size of

0.05 (amu)1/2 bohr. Then, the first and second energy deriva-

tives are obtained to calculate the curvature of the reaction

path and the generalized vibrational frequencies along the

reaction path. To obtain more accurate energies and barrier

heights, the energies are refined by the MC-QCISD method

(multicoefficient correlation method based on quadratic con-

figuration interaction with single and double excitations) pro-

posed by Fast and Truhlar[11] based on the MP2/6-31þG(d,p)

geometries. All the electronic structure calculations are

performed by the GAUSSIAN03 program package.[21]

VTST[16,17] is employed to calculate the rate constants by

the POLYRATE 9.7 program.[15] The theoretical rate constants

for the three reaction channels over the temperature range

200–2000 K are calculated by canonical variational transition

state theory (CVT)[22] incorporating small-curvature tunneling

(SCT)[23,24] contributions proposed by Truhlar and coworkers.[22]

For the title reaction, most of the vibrational modes are treated

as quantum-mechanical separable harmonic oscillators except

for the lowest modes. The hindered-rotor approximation of

Truhlar and Chuang[25,26] is used for calculating the partition

function of the five transitional state modes. In the calculation

of the reactant electronic partition function, two electronic

states of Cl atoms, 2P1/2 and 2P3/2, are included, with

an 881 cm�1 splitting due to spin-orbit coupling. The 2P1/2 and2P3/2 electronic state of the fluorine and bromine atoms with

404 and 3685 cm�1 splitting due to the spin–orbit coupling,

respectively. The curvature components are calculated by using

a quadratic fit to obtain the derivative of the gradient with

respect to the reaction coordinate.

Results and Discussions

Stationary points

The optimized geometries of the reactants (CF3CH2OCF3), com-

plexes (CR1aR and CR2aR), products (CF3CHOCF3, CF2CH2OCF3,

CF3CH2OCF2, HF, HCl, HBr, F2, ClF, and BrF), and transition

states (TS1a, TS1b-in, TS1b-out, TS1c-in, TS1c-out, TS2a, TS2b-

in, TS2b-out, TS2c-in, TS2c-out, TS3a, TS3b-in, TS3b-out, TS3c-

in, and TS3c-out) calculated at the MP2/6-31þG(d,p) level are

presented in Figure 1, along with the available experimental

values.[27–29] The theoretical geometric parameters of HF, HCl,

HBr, F2, ClF, and BrF are in good agreement with the corre-

sponding experimental values.[27–29] Furthermore, two com-

plexes (CR1aR and CR2aR) are presented on the reactants sides

of reaction R1a and R2a, which means that reaction R1a and

R2a may proceed via an indirect mechanism. At the MP2/6-

31þG(d,p) level, the H���F and H���Cl bonds distance in CR1aR

and CR2aR are 2.92 and 3.03 A, respectively, while the other

bond lengths are very close to those of the corresponding

reactants. Seen from Figure 1, both of transition states have

the same symmetry C1. In TS3a, TS3b-in, TS3b-out, TS3c-in,

and TS3c-out structures, the breaking bonds CAH and CAF

increase by 41%, 45%, 44%, 40%, and 45% compared to the

equilibrium bond length in CF3CH2OCF3; the forming bonds

HABr and BrAF stretch by 7%, 3%, 3%, 3%, and 3% over the

equilibrium bond lengths in isolated HBr and BrF, respectively.

The elongation of the breaking bond is larger than that of the

forming bond, indicating that TS3a, TS3b-in, TS3b-out, TS3c-in,

and TS3c-out are all product-like, i.e., the five reaction chan-

nels will proceed via ‘‘late’’ transition states, which is consistent

with Hammond’s postulate,[30] applied to for an endothermic

reaction. The same conclusion can be drawn from the transi-

tion state structures of reactions R1 and R2.

Supporting Information Table S1 lists the harmonic vibra-

tional frequencies of the reactants, complexes, products, and

transition states calculated at the MP2/6-31þG(d,p) level as

well as the available experimental values[31–33]. The imaginary

frequency of transition state corresponds to the stretching

modes of coupling between breaking and forming bonds.

Energetics

The reaction enthalpies (DH0298) and potential barrier heights

(DETS) with zero-point energy (ZPE) corrections for the fifteen

reaction channels calculated at the MC-QCISD//MP2/6-

31þG(d,p) level are listed in Table 1. From Table 1, it is shown

that the five individual reaction channels for reaction R3 are all

endothermic reactions, consistent with the discussion above of

Hammond’s postulate.[30]

Table 2 lists the calculated bond dissociation energies

(D298�) of the CAH and CAF bonds in methyl ethyl ether’s hal-

ogenated derivatives at the MC-QCISD//MP2/6-31þG(d,p) level,

along with several experimental data[1] of CAH bond dissocia-

tion energy. The results of the corresponding experimental

rate constants with Cl atom reactions at 298 K are also

compared in Table 2. The CAH bond dissociation energies in

Table 2 shows that the CAH bond is the strongest

(100.82 kcal/mol) for CF3(CAH)HOCClF2. The CAF bond dissoci-

ation energies of CF3CH2O(CAF)F2 and F2(CAF)CH2OCF3,

128.25 and 126.89 kcal/mol, are stronger than CAH bonds,

which means that the H-abstraction reaction channels will be

an absolute preponderant channel, and the F-abstraction

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686 Journal of Computational Chemistry 2012, 33, 685–690 WWW.CHEMISTRYVIEWS.COM

reaction channels will be negligible. The high D298� of the

CAH bond of CF3(CAH)HOCF3 is due to the combined elec-

tron-withdrawing inductive effects of three fluorine atoms and

one oxygen atom adjacent to the hydrogen atom, which effec-

tively compensate for the electron-donating conjugative effect

of the oxygen atom. A comparison among the bond dissocia-

tion energies of these CAH bonds indicates that the electron-

withdrawing inductive effects of the AOCH3 group are prob-

ably more effective than those of the CH3(CAH)HOA group.

The D298� (CAH) of CF3(CAH)HOCH3 compared with

CF3(CAH)HOCHF2 and CF3(CAH)HOCF3, the values becomes

more large as the number increases of F-substituent, the lower

D298� value of the CF3(CAH)HOCH3 bond (95.29 kcal/mol)

compared to those of the CF3(CAH)HOCHF2 (98.76 kcal/mol)

and CF3(CAH)HOCF3 bonds (100.31 kcal/mol) can be explained

by considering the higher negative inductive effects of the

CHF2OA group, compared to the CH3OA group, the inductive

effects of the more distant fluorine atoms are not able to com-

pensate efficiently the conjugative effects of the adjacent oxy-

gen atom. The higher D298� value for the CF3(CAH)HOCClF2

bond may be attributed to the above indication on the rela-

tive strengths of the inductive effects of the CF3CH2OA and

Figure 1. Optimized geometries of the reactants, products, and transition states at the MP2/6-31þG(d,p) level. The values in parentheses are the experi-

mental values (Ref. [27] for HBr, HCl, HF, and F2, Ref. [28] for ClF, Ref. [29] for BrF). Bond lengths are in angstrom and angles are in degree.

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Journal of Computational Chemistry 2012, 33, 685–690 687

CHF2OA groups. The corresponding reaction rate constants

with Cl will be decrease gradually, indicating that the

H-abstraction channel in CF3(CAH)HOCF3 will be slowest

compare with CF3(CAH)HOCH3 and CF3(CAH)HOCHF2. The

D298� (CAH) values of CF3(CAH)HOCH3, CH3(CAH)HOCF3,

CF3(CAH)HOCHF2, CF3CH2O(CAH)H2, and CF3CH2O(CAH)F2with 95.29, 98.03, 98.76, 96.95, and 102.72 kcal/mol shows

good consistency with the previous literature results,[1] 95.22,

96.89, 98.09, 97.13, and 102.39 kcal/mol, respectively. No

comparison between theory and experiment can be made

due to the lack of the experimental D298� (CAH) values in

CF3(CAH)HOCF3, CF3CH2O(CAH)HF, CF3(CAH)HOCClF2, and

D298� (CAF) values. The good agreement between the theoreti-

cal and experimental above-mentioned five results implies that

the MC-QCISD//MP2/6-31þG(d,p) level is a suitable method to

compute the bond dissociation energies and our calculated

D298� (CAH) and D298

� (CAF) value may be expected to pro-

vide reliable reference information for future laboratory investi-

gations. Thus, we use MC-QCISD//MP2/6-31þG(d,p) method to

calculate the potential energy barriers as well as the energies

along the MEP in the following studies.

The schematic potential energy diagrams of the reactions F,

Cl, Br atoms with CF3CH2OCF3 with ZPE corrections obtained

at the MC-QCISD//MP2/6-31þG(d,p) level are plotted in

Figures 2–4. Note that the energy of reactant is set to zero for

Table 1. The reaction enthalpies at 298 K (DH0298), the barrier heights

TSs (DETS) (kcal/mol) with zero-point energy (ZPE) correction for the

reactions of X 1 CF3CH2OCF3 (X 5 F, Cl, Br) at the MC-QCISD//MP2/

6-311G(d,p) level.

MC-QCISD//

MP2

DH0298 F þ CF3CH2OCF3 ! CF3CHOCF3 þ HF (R1a) �38.62

F þ CF3CH2OCF3 ! CF2CH2OCF3 þ F2 (R1b) 91.31

F þ CF3CH2OCF3 ! CF3CH2OCF2 þ F2 (R1c) 92.69

Cl þ CF3CH2OCF3 ! CF3CHOCF3 þ HCl (R2a) �3.30

Cl þ CF3CH2OCF3 ! CF2CH2OCF3 þ ClF (R2b) 66.78

Cl þ CF3CH2OCF3 ! CF3CH2OCF2 þ ClF (R2c) 68.16

Br þ CF3CH2OCF3 ! CF3CHOCF3 þ HBr (R3a) 10.43

Br þ CF3CH2OCF3 ! CF2CH2OCF3 þ BrF (R3b) 66.35

Br þ CF3CH2OCF3 ! CF3CH2OCF2 þ BrF (R3c) 67.74

D(ETS þ ZPE) F þ CF3CH2OCF3 ! CF3CHOCF3 þ HF (R1a) �0.87

F þ CF3CH2OCF3 ! CF2CH2OCF3 þ F2 (R1b-in) 97.61

F þ CF3CH2OCF3 ! CF2CH2OCF3 þ F2 (R1b-out) 98.33

F þ CF3CH2OCF3 ! CF3CH2OCF2 þ F2 (R1c-in) 102.03

F þ CF3CH2OCF3 ! CF3CH2OCF2 þ F2 (R1c-out) 98.71

Cl þ CF3CH2OCF3 ! CF3CHOCF3 þ HCl (R2a) 3.04

Cl þ CF3CH2OCF3 ! CF2CH2OCF3 þ ClF (R2b-in) 74.49

Cl þ CF3CH2OCF3 ! CF2CH2OCF3 þ ClF (R2b-out) 75.40

Cl þ CF3CH2OCF3 ! CF3CH2OCF2 þ ClF (R2c-in) 79.80

Cl þ CF3CH2OCF3 ! CF3CH2OCF2 þ ClF (R2c-out) 75.10

Br þ CF3CH2OCF3 ! CF3CHOCF3 þ HBr (R3a) 11.67

Br þ CF3CH2OCF3 ! CF2CH2OCF3 þ BrF (R3b-in) 71.70

Br þ CF3CH2OCF3 ! CF2CH2OCF3 þ BrF (R3b-out) 72.99

Br þ CF3CH2OCF3 ! CF3CH2OCF2 þ BrF (R3c-in) 77.15

Br þ CF3CH2OCF3 ! CF3CH2OCF2 þ BrF (R3c-out) 72.38

Table 2. Calculated and experimental bond dissociation energies

(kcal/mol) at 298 K for methyl ethyl ether’s halogenated derivatives at the

MC-QCISD//MP2/6-311G(d,p) level, and the corresponding experimental

rate constants with Cl atom at 298 K.

D�298

MC-QCISD//

MP2/6-31þG(d,p) Expt. Rate constants

CF3(CAH)HOCH3 95.29 95.22[a] (1.8 6 0.9) � 10�11[a]

CH3(CAH)HOCF3 98.03 96.89[a] (2.2 6 0.8) � 10�12[a]

CF3(CAH)HOCHF2 98.76 98.09[a] (1.5 6 0.4) � 10�14[a]

CF3(CAH)HOCF3 100.31 7.2 � 10�15[b]

CF3(CAH)HOCClF2 100.82 (3.2 6 0.2) � 10�15[c]

CF3CH2O(CAH)H2 96.95 97.13[a]

CF3CH2O(CAH)HF 96.97

CF3CH2O(CAH)F2 102.72 102.39[a]

CF2(FAC)H2OCF3 126.89

CF3CH2O(CAF)F2 128.25

[a] Ref. [1]. [b] This work. [c] Ref. [4].

Figure 2. Schematic potential energy surface for the reaction F þCF3CH2OCF3. Relative energies are calculated at the MC-QCISD//MP2/6-

31þG(d,p) þ ZPE level [in (kcal/mol)]. The values in parentheses are calcu-

lated at the MP2/6-31þG(d,p) þ ZPE level.

Figure 3. Schematic potential energy surface for the reaction Cl þCF3CH2OCF3. Relative energies are calculated at the MC-QCISD//MP2/

6-31þG(d,p) þ ZPE level [in (kcal/mol)]. The values in parentheses are

calculated at the MP2/6-31þG(d,p) þ ZPE level.

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688 Journal of Computational Chemistry 2012, 33, 685–690 WWW.CHEMISTRYVIEWS.COM

reference. For reaction R1a (and R2a), the attack of F (and Cl)

atoms on the CAH bond of CF3CH2OCF3 would proceed via a

complex CR1aR (and CR2aR) on the reactants side, which is

1.40 (and 1.84) kcal/mol lower than the reactants. The reaction

channel of the abstraction from the in-plane fluorin (R1b-in)

has a lower barrier than the out-plane fluorin (R1b-out) chan-

nel, and R1c-out out-plane fluorin abstraction reaction channel

has a lower barrier than the in-plane fluorin (R1c-in) channel.

At the same time, the reaction channels of forming

CF2CH2OCF3 are less endothermic than forming CF3CH2OCF2by about 0.37 kcal/mol. For the reaction CF3CH2OCF3 with F

atoms, the potential barrier height of reaction channel R1a

(A0.87 kcal/mol) is much lower than the ones of R1b-in (97.61

kcal/mol), R1b-out (98.33 kcal/mol), R1c-out (99.98 kcal/mol),

and R1c-in (102.03 kcal/mol) at the MC-QCISD//MP2/6-

31þG(d,p) level. Thus, to yield CF3CHOCF3 þ HF channel is

more favorable and to yield CF2CH2OCF3 þ F2 and

CF3CH2OCF2 þ F2 channels are negligible. At the same time,

reaction R1a is a exothermic reaction, and R1b and R1c are

endothermic reactions, which indicates that the former reac-

tion channel R1a is more thermodynamically and kinetically

favorable than the later channels, and the rate constants of

reaction R1a will be much faster than that of the reaction

channel R1b and R1c, and the later channels are negligible.

The similar conclusion can be draw from Figures 3 and 4 in

the reaction CF3CHOCF3 with Cl and Br atoms, which indicates

that the reaction channels R2b, R2c, R3b, and R3c is also negli-

gible. Thus we perform the rate constant calculations only for

the H-abstraction reaction channels.

Rate constants

Dual-level direct dynamics calculations[9–13] of the three

H-abstraction reaction channels are carried out at the

MC-QCISD//MP2/6-31þG(d,p) level. The rate constants are eval-

uated by conventional transition state theory (TST), canonical

variational transition state theory (CVT), and the CVT with the

small-curvature tunneling (SCT) contributions in a wide tem-

perature range from 200 to 2000 K. The calculated CVT/SCT

rate constants of the three channels, k1a, k2a, and k3a, are plot-

ted against the reciprocal of temperature in Figure 5, and list

Table S2 as Supporting Information. The theoretical CVT/SCT

rate constant of reaction channel Br þ CF3CH2OCF3 !CF3CHOCF3 þ HBr is 4.87 � 10�17 cm3 molecule�1 s�1, which

is smaller than 7.19 � 10�15 and 5.32 � 10�13 cm3 molecule�1 s�1

of reaction channels Cl þ CF3CH2OCF3 ! CF3CHOCF3 þ HCl

and F þ CF3CH2OCF3 ! CF3CHOCF3 þ HF at 298 K.

Theoretical activation energy (Ea) is estimated based on the

calculated CVT/SCT rate constants, and it is found that the

corresponding Ea value for reaction channel F þ CF3CH2OCF3! CF3CHOCF3 þ HF, 1.42 kcal/mol, is lower than that for

reaction channels Cl þ CF3CH2OCF3 ! CF3CHOCF3 þ HCl

(1.49 kcal/mol) and Br þ CF3CH2OCF3 ! CF3CHOCF3 þ HBr

(7.05 kcal/mol) in 200–600 K. Those are consistent with a

qualitative assessment based on the potential energy diagram

Figures 2–4 above-mentioned of the three reactions.

Figure 5 shows that it can also be found that the values of

k1a is much larger than those of k2a and k3a by about 1–2 and

2–7 orders of magnitude in the temperature range 200–400 K.

This is in line with the potential energy barrier heights and the

reaction enthalpies results calculated above, namely, which is

in accordance with its kinetic superiority.

Due to the limited experimental knowledge on the kinetics

of the title reaction, we hope that our study may provide

useful information for future laboratory investigations. For

convenience of future experimental measurements, the three-

parameter fits of the CVT/SCT rate constants of three reaction

channels in the temperature range from 200 to 2000 K are

performed and the expressions are given as follows (in unit of

cm3 molecule�1 s�1):

k1aðTÞ ¼ 1:24� 10�15T1:24 expð�304:81=TÞk2aðTÞ ¼ 7:27� 10�15T0:37 expð�630:69=TÞk3aðTÞ ¼ 2:84� 10�19T2:51 expð�2725:17=TÞ

Figure 4. Schematic potential energy surface for the reaction Br þCF3CH2OCF3. Relative energies are calculated at the MC-QCISD//MP2/6-

31þG(d,p) þ ZPE level [in (kcal/mol)]. The values in parentheses are calcu-

lated at the MP2/6-31þG(d,p) þ ZPE level.

Figure 5. The CVT/SCT rate constants calculated at the MC-QCISD//MP2/6-

31þG(d,p) level for the reaction channels R1a (k1a), R2a (k2a), and R3a (k3a)

(in cm3 molecule�1 s�1) versus 1000/T between 200 and 2000 K.

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Journal of Computational Chemistry 2012, 33, 685–690 689

Conclusion

In this paper, the mechanism of multichannel reactions X þCF3CH2OCF3 (X ¼ F, Cl, Br) are investigated theoretically at MC-

QCISD//MP2/6-31þG(d,p) level using a dual-level direct

dynamics method. For the title reaction, two type reaction

channels are identified, one for hydrogen abstraction from the

CH2 group, and the others for halogen abstraction from the CF3group. The calculated potential barriers show that only feasible

reaction channel is H-abstraction. The results of the theoretical

investigation show that for the three reactions, with n increas-

ing for the attack atoms F, Cl, Br, the reaction rate constants

decrease in the order of F þ CF3CH2OCF3 > Cl þ CF3CH2OCF3> Br þ CF3CH2OCF3. The three-parameter rate-temperature for-

mulae for the three H-abstraction reaction in the temperature

range from 200 to 2000 K are fitted and reported. It is expected

the present theoretical results may be helpful for estimating the

kinetics of the above-mentioned reactions.

Acknowledgments

The authors thank Prof. Donald G. Truhlar for providing POLYRATE

9.7 program.

Keywords: gas-phase reaction � transition state � rate constants

How to cite this article: H. Zhang, Y. Liu, J.-Y. Liu, Z.-S. Li, J.

Comput. Chem. 2012, 33, 685–690. DOI: 10.1002/jcc.22897

Additional Supporting Information may be found in the

online version of this article.

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Received: 24 May 2011Revised: 17 October 2011Accepted: 3 November 2011Published online on 29 December 2011

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