theoretical study of the reactions cf3ch2ochf2 + oh/cl and its product radicals and parent...

Theoretical Study of the Reactions CF3CH2OCHF21OH/Cl

and its Product Radicals and Parent Ether

(CH3CH2OCH3) with OH

LEI YANG,1JING-YAO LIU,

1* LI WANG,1HONG-QING HE,

1,2YING WANG,

1ZE-SHENG LI

1

1State Key Laboratory of Theoretical and Computational Chemistry, Institute of TheoreticalChemistry, Jilin University, Changchun 130023, People’s Republic of China

2State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics,Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China

Received 23 January 2007; Revised 29 May 2007; Accepted 5 July 2007DOI 10.1002/jcc.20813

Published online 17 August 2007 in Wiley InterScience (www.interscience.wiley.com).

Abstract: A dual-level direct dynamic method is employed to study the reaction mechanisms of CF3CH2OCHF2(HFE-245fa2; HFE-245mf) with the OH radicals and Cl atoms. Two hydrogen abstraction channels and two dis-

placement processes are found for each reaction. For further study, the reaction mechanisms of its products

(CF3CH2OCF2 and CF3CHOCHF2) and parent ether CH3CH2OCH3 with OH radical are investigated theoretically.

The geometries and frequencies of all the stationary points and the minimum energy paths (MEPs) are calculated at

the B3LYP/6-311G(d,p) level. The energetic information along the MEPs is further refined at the G3(MP2) level of

theory. For reactions CF3CH2OCHF2 1 OH/Cl, the calculation indicates that the hydrogen abstraction from

��CH2�� group is the dominant reaction channel, and the displacement processes may be negligible because of the

high barriers. The standard enthalpies of formation for the reactant CF3CH2OCHF2, and two products CF3CH2OCF2and CF3CHOCHF2 are evaluated via group-balanced isodesmic reactions. The rate constants of reactions

CF3CH2OCHF2 1 OH/Cl and CH3CH2OCH3 1 OH are estimated by using the variational transition state theory

over a wide range of temperature (200–2000 K). The agreement between the theoretical and experimental rate con-

stants is good in the measured temperature range. From the comparison between the rate constants of the reactions

CF3CH2OCHF2 and CH3CH2OCH3 with OH, it is shown that the fluorine substitution decreases the reactivity of the

C��H bond.

q 2007 Wiley Periodicals, Inc. J Comput Chem 29: 550–561, 2008

Key words: density functional theory; direct dynamics; rate constant; variational transition-state theory

Introduction

Depletion of stratospheric ozone by chemicals containing chlo-

rine and bromine has attracted considerable international atten-

tion in the past decade. The productions which deplete ozone

in the stratosphere are being phased out under the Montreal

Protocol and its subsequent amendments and adjustments.1 A

large number of compounds, such as hydrochlorofluorocarbons,

hydrofluorocarbons, and hydrofluoroethers (HFEs), have been

proposed for alternative compounds for chlorofluorocarbons

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

and do not contribute to ozone depletion. Nevertheless, the

infrared absorbing properties of such fluorinated compounds

may potentially cause the global warming effect. Since the

introduction of ether linkage ��O�� may lead to an even

greater reactivity in the troposphere,2 HFEs are considered to

be the promising substitutes of CFCs. In order to assess its

environmental impact, it is necessary to determine the atmos-

pheric lifetime of HFEs in the troposphere. The degradation of

HFEs in the troposphere is most probably initiated by OH radi-

cals. Also, Cl atoms may be a significant part of the degrada-

tion of HFEs because of its higher reactivity.3 Thus, the reac-

This article contains supplementary material available via the Internet at

http://www.interscience.wiley.com/jpages/0192-8651/suppmat

Correspondence to: J.-Y. Liu; e-mail: [email protected]

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

contract/grant numbers: 20333050, 20073014, 20303007

Contract/grant sponsor: Ministry of Education, China

Contract/grant sponsor: Innovation Foundation, Jilin University

q 2007 Wiley Periodicals, Inc.

tivity of HFEs against OH radicals and Cl atoms is crucial for

the evaluation of the lifetime of the HFEs. In the present paper,

our attention will be focused on the reactions of CF3CH2-

OCHF2 with OH radicals and Cl atoms.

For the reaction CF3CH2OCHF2 with OH (R1) or Cl (R2),

two H-abstraction channels are feasible.

CF3CH2OCHF2 þ OH ! CF3CH2OCF2 þ H2O (R1a)

! CF3CHOCHF2 þ H2O (R1b)

CF3CH2OCHF2 þ Cl ! CF3CH2OCF2 þ HCl (R2a)

! CF3CHOCHF2 þ HCl (R2b)

Three experimental studies were reported for reaction R1 by

Zhang et al.,4 Beach et al.,5 and Oyaro et al.6 in the temperature

range of 292–402 K, and the measured rate constants show good

agreement. The kinetics of reaction R2 has attracted consider-

able attention experimentally. Five experimental studies of the

rate constants5–9 of R2 have been carried out in the temperature

range of 273–398 K. Most of these results show good mutual

agreement except for the values taken from ref. 8, which are

about two to three times larger than the others in the measured

temperature range. To the best of our knowledge, no theoretical

study has been reported.

Here, we employ a dual-level (X//Y) direct dynamics

method10–12 to investigate the reaction mechanisms of the reac-

tions CF3CH2OCHF2 1 OH/Cl ? products. Both H-abstraction

and displacement processes (R1c–1d and R2c–2d) are considered.

CF3CH2OCHF2 þ OH ! CF3CH2OHþ CHF2O (R1c)

! CF3CH2Oþ CHF2OH (R1d)

CF3CH2OCHF2 þ Cl ! CF3CH2Clþ CHF2O (R2c)

! CF3CH2Oþ CHF2Cl (R2d)

The electronic structure information and the minimum energy

paths (MEPs) are obtained at the B3LYP/6-311G(d,p) level,13,14

and the energies information are further refined at the G3(MP2)

level.15

Bond dissociation energies (BDEs) of the breaking C��H

bonds are strongly correlated with the observed reactivity trend

for the hydrogen abstraction reaction, and the ether linkage

(��O��) is important for the reactivity of the ethers; thus, we

present BDE results of the two types of C��H and C��O bonds

of CF3CH2OCHF2.

In addition, with respect to the product radicals CF3CH2

OCF2 and CF3CHOCHF2, their reactivities in the stratosphere

are important to assess their environmental impact. However,

there is no literature to report how they further react with OH

radical and which products may be produced through the reac-

tions if the concentration of OH radical is large. In this work,

the mechanisms of the product radicals with OH are studied at

the G3(MP2)//B3LYP/6-311G(d,p) level.

CF3CH2OCF2 þ OH ! products (R3)

CF3CHOCHF2 þ OH ! products (R4)

For the purpose of comparison, the reactions of the parent

molecule CH3CH2OCH3 with OH radical are also studied. In the

case of ethylmethylether, H-abstractions by hydroxyl radical

occur at three special sites:

CH3CH2OCH3 þ OH ! CH3CH2OCH2 þ H2O (R5a)

! CH3CHOCH3 þ H2O (R5b)

! CH2CH2OCH3 þ H2O (R5c)

The total rate constants of this reaction were reported by Starkey

et al.16 However, there is no information available on the

branching ratio of each channel, since it is difficult to determine

which hydrogen atom is abstracted experimentally. In the pres-

ent study, the rate constants of R5 and branching ratios are

obtained and the comparison with CF3CH2OCHF2 is made.

Calculational Methods

All of the electronic structure calculations are carried out with

the Gaussian 98 program.17 The optimized geometries and har-

monic vibrational frequencies of all the stationary points (reac-

tants, products, transition-states, and complexes) are calculated

by the B3LYP method (Becke’s three-parameter nonlocal-

exchange functional with the nonlocal correlation functional of

Lee, Yang and Parr) and at the MP2 level of theory (restricted

or unrestricted second-order Møller-Plesset perturbation theory)

with the 6-311G(d,p) basis set. At the B3LYP/6-311G(d,p) level,

the MEP is calculated with a gradient step size of 0.02 (amu)1/2

bohr to confirm whether the transition states connect the desig-

nated reactants and products. Also, the energy derivatives,

including gradients and Hessians at geometries along the MEP,

are obtained at the same level. To improve the accuracy of the

energetics, the extra energies along the MEP are carried out at

the G3(MP2) level of theory (Gaussian-3(G3) theory with a

reduced order of perturbation theory)15 using the B3LYP/6-

311G(d,p) optimized geometries.

An accurate knowledge of the enthalpy of formation of the

species is important in determining the thermodynamic proper-

ties and the kinetics of atmospheric processes. However, no the-

oretical or experimental study of standard enthalpy has been

reported for these species, except for CF3CH2OCHF2. The en-

thalpy of CF3CH2OCHF2 was calculated using bond additivity

corrected MP2 method (BAC-MP2/6-31G**) and atom additivity

corrected MP2 method (AAC-MP2/6-31G**).18 Here, three iso-

desmic reactions19 for each species are used to estimate the

enthalpies of formation of the species (CF3CH2OCHF2,

CF3CHOCHF2, and CF3CH2OCF2). It is known that isodesmic

551Reaction Mechanisms of CF3CH2OCHF2 with the OH Radicals and Cl Atom

Journal of Computational Chemistry DOI 10.1002/jcc

reactions, in which the number of each type of bond is con-

served, will cancel the systematic errors in the ab initio calcula-

tions and lead to quite accurate results because of the similarity

of bond type in both reactants and products. The used isodesmic

reactions are given as follows:

a. For CF3CH2OCHF2:

CF3CH2OCHF2 þ H2Oþ CH3F

! CH3OHþ CH3OHþ CF3CH3 (R6)

CF3CH2OCHF2 þ CH3F ! CH3OCH3 þ CF3CF3 (R7)

CF3CH2OCHF2 þ CH4 þ CH4

! CH3OCH3 þ CH2F2 þ CF3CH3 (R8)

b. For CF3CH2OCF2

CF3CH2OCF2 þ CH2Oþ CH3F

! CH3OCH3 þ HCOþ CF3CF3 (R9)

CF3CH2OCF2 þ CH3Fþ CH4

! CH3OCH3 þ CH3 þ CF3CF3 (R10)

CF3CH2OCF2 þ CH4 ! CF3CH2OCHF2 þ CH3 (R11)

c. For CF3CHOCHF2

CF3CHOCHF2 þ CH2Oþ CH3F

! CH3OCH3 þ HCOþ CF3CF3 (R12)

CF3CHOCHF2 þ CH3Fþ CH4

! CH3OCH3 þ CH3 þ CF3CF3 (R13)

CF3CHOCHF2 þ CH4 ! CF3CH2OCHF2 þ CH3 (R14)

By means of the POLYRATE 8.4.1 program,20 the dynamics

calculations are performed by using the variational transition

state theory (VTST)21–23 with the interpolated single-point ener-

gies (ISPE) method.24 The ISPE method is a dual-level direct

dynamics scheme that uses a low-level (LL) MEP and corrects

the energy by interpolating the energy differences at some points

along this LL MEP and single-point energy calculations at a

higher-level (HL). Only single-point energies at the stationary

points as well as a few nonstationary points are required to be

calculated. The rate constants are calculated using canonical

VTST (CVT)25 with the small-curvature tunneling (SCT) correc-

tion proposed by Truhlar and coworkers.26,27 Most of the vibra-

tional modes are treated as quantum-mechanical separable har-

monic oscillators. The hindered-rotor approximation of Chuang

and Truhlar28 was used for calculating the partition functions of

the lower modes associated with the torsion. In the calculation

of the electronic partition functions, the excited state of the OH

radical is included, with a 140 cm–1 splitting; the 2P3/2 and 2P1/2electronic states of Cl atoms are also included, with a 881 cm–1

splitting due to the spin–orbit coupling. The total rate constant of

each reaction is obtained as the sum of the individual rate constant

of each H-abstraction channel.

Results and Discussion

Stationary Points

At the B3LYP/6-311G(d,p) and MP2/6-311G(d,p) levels, some

of the optimized geometric parameters of the stationary points

involved in R1a–1b and R2a–2b are shown in Figure 1 along with

the limited experimental values,29,30 and others involved in R1–

R5 are given in the Supporting Information (Fig. S1). From Fig-

ure 1, it is seen that the optimized parameters of the reactants

(CF3CH2OCHF2, OH, and Cl) and the products (CF3CH2OCF2,

CF3CHOCHF2, H2O, HCl) obtained at the two levels are reason-

ably consistent with each other, and the largest discrepancies are

0.033 A for the calculated bond lengths and 2.38 for the bond

angles. Also, both of them are in reasonable accord with the ex-

perimental ones when the comparisons are possible, with the

maximum deviation within a factor of 1.9%. At the B3LYP/6-

311G(d,p) level, the reactant complexes (CR1a, CR1b, CR2b) and

product complexes (CP1a, CP1b, CP2a, CP2b) are located at the

entrances and exists of the four H-abstraction channels, which

means that the two reactions may proceed via indirect mecha-

nisms. Since the two hydrogen atoms in ��CH2�� group are

equivalent, only one transition state is located for reaction chan-

nels R1a and R2a. As to the two transition states (TS1a and TS1b)

for the CF3CH2OCHF2 1 OH reaction, the length of the break-

ing C��H bond are elongated by 13.6% and 11.0% as compared

to the C��H equilibrium bond length in CF3CH2OCHF2, respec-

tively, while the length of the forming H��O bond are 31.3%

and 37.6% longer than the equilibrium bond length of H2O,

respectively. The elongation of the breaking bond is smaller

than that of the forming bond, indicating that the transition

states are reactant-like and the reactions may proceed via early

transition states. While for the transition states (TS2a and TS2b)

of the CF3CH2OCHF2 1 Cl reaction, the elongation of the

breaking C��H bond (41.0% and 31.1%) is greater than that of

the forming H��Cl bond (10.0% and 13.5%), which indicates

these two transition states are product-like and the reactions may

proceed via late transition state. For the displacement processes

of reaction CF3CH2OCHF2 with OH or Cl, the attacks of OH or

Cl at the two a-C (��CH2�� and ��CHF2 groups) in molecule

are considered. TS1c, TS1d, TS2c, and TS2d are located for reac-

tion channels R1c, R1d, R2c, and R2d, respectively. For the reac-

tion CH3CH2OCH3 1 OH, four H-abstraction channels (R5a,

R5b, R5c, and R5c0) are found. For the three former channels, the

reactant complexes (CR5a, CR5b) and product complexes (CP5a,

CP5b, CP5c) are located at the entrances and exists of the chan-

nels, and so they proceed via indirect mechanisms, while for H-

abstraction from ��CH3, another direct channel (R5c0) is also

found. On the singlet potential energy surfaces (PESs) of reac-

tions CF3CH2OCF2 1 OH (R3) and CF3CHOCHF2 1 OH (R4),

the addition of OH forms R��OH adducts followed by decompo-

sition to final products. The indirect H-abstraction channels are

found on the triplet PESs of each reaction. All the optimized

structures involved in R3–R5 are shown in the Supporting Infor-

552 Yang et al. • Vol. 29, No. 4 • Journal of Computational Chemistry


mation (Fig. S1). All the reactants, products, and complexes are

confirmed with only real frequencies, which indicate a minimum

has been located, and all the transition states are confirmed to

have only one imaginary frequency corresponding to the nega-

tive eigenvalue of the respective Hessian matrix. The harmonic

vibrational frequencies of the stationary points of the reactions

of CF3CH2OCHF2 1 OH/Cl calculated at the B3LYP/6-

311G(d,p) and MP2/6-311G(d,p) levels, along with the available

experimental values, are listed in Supporting Information (Table

S1). The results show that the two methods yield similar calcu-

lated values and that the agreement between the theoretical and

experimental values29,31 is good.

Figure 1. Optimized geometries of reactants, products, complexes, and transition states at the B3LYP/6-

311G(d,p) and MP2/6-311G(d,p) (in parentheses) levels of the reactions R1a–1b and R2a–2b. The numbers in

square brackets are the experimental values.29,30 Bond lengths are in angstroms, and angles are in degrees.



Energetics

The reaction enthalpies (DH0298) are calculated at various levels,

i.e., B3LYP/6-311G(d,p), G3(MP2)//B3LYP/6-311G(d,p), and

G3(MP2)//MP2/6-311G(d,p) levels, and the corresponding values

are listed in Table 1. As is shown in Table 1, the values of

DH0298 at the G3(MP2)//MP2/6-311G(d,p) level are very close to

those obtained at the G3(MP2)//B3LYP/6-311G(d,p) level, withthe maximum error within 0.2 kcal/mol. The products of reac-tion channels R1a, R1b and R2a, R2b are more exothermic thanR1c, R1d and R2c, R2d, i.e., the products of H-abstraction reac-tions are more thermodynamically stable than those of displace-

Figure 1. (continued)



ment processes. For H-abstraction reactions, channels R1b and

R2b are more thermodynamically favorable than channels R1a

and R2a, respectively. Unfortunately, it is difficult to make a

direct comparison with the experiment, because neither the

enthalpies of reactions R1 and R2 nor the heats of formation for

CF3CH2OCHF2, CF3CH2OCF2, and CF3CHOCHF2 have been

studied experimentally. Here, an attempt is made to estimate the

enthalpies of formation for these species via the isodesmic reac-

tions R6–R14. We calculate the reaction enthalpies of R6–R14

and combine them with the known enthalpies of formation of

the reference compounds involved in these reactions. (CF3CF3:

2321.20 kcal/mol32; CH4: 217.89 kcal/mol32; CH3F: 255.999

kcal/mol32; CH2F2: 2107.71 kcal/mol32; CH3: 34.82 kcal/mol32;

CF3CH3: 2178.94 6 0.76 kcal/mol33; CH3OCH3: 243.99 60.12 kcal/mol34; CH3OH: 248.0 kcal/mol35; H2O: 257.799

kcal/mol36; CH2O: 227.701 kcal/mol36; HCO: 10.40 kcal/mol36)

to evaluate the required enthalpies of formation. All of the geo-

metrical parameters of the species in the isodesmic reactions are

calculated at the B3LYP/6-311G(d,p) level, and energies of the

species are refined at the G3(MP2) level. The calculated values

of enthalpies of formation (DH0f;298) as well as the available liter-

ature data18 are listed in Table 2. They are 2315.93 6 0.33,

2260.96 6 0.12, and 2266.72 6 0.19 kcal/mol for

CF3CH2OCHF2, CF3CH2OCF2, and CF3CHOCHF2, respectively,

which are obtained as the unweighted average of these results.

The error limits are calculated by adding the maximum uncer-

tainties of DH0f;298 values of reference species. It is seen that our

calculated DH0f;298 value of CF3CH2OCHF2 is well consistent

with the available literature values.18

Note that the geometries and frequencies calculated at the

MP2/6-311G(d,p) and B3LYP/6-311G(d,p) levels are close, and

the HL DH0298 values show good agreement at the G3(MP2)//

MP2 and G3(MP2)//B3LYP levels. Since the B3LYP calculation

requires significantly less computational cost compared with the

MP2 calculation, we just employ the G3(MP2)//B3LYP/6-

311G(d,p) method to calculate the potential energy barriers as

well as the single-point energies information along the MEP in

the following studies.

The schematic PESs of reactions R1–R5 obtained at the

G3(MP2)//B3LYP/6-311G(d,p) level are plotted in Figures 2a–2e.

From Figure 2a, we can see that for channels R1a and R1b, two

reactant complexes (CR1a and CR1b) are first formed with energies

about 2.53 and 0.77 kcal/mol lower than the reactants. Then start-

ing form the complexes, the reactions proceed via two reactant-

Table 2. Enthalpies of Formation (kcal/mol) of CF3CH2OCHF2 and Fluorinated Ether Radicals

(CF3CH2OCF2 and CF3CHOCHF2) at the G3(MP2)//B3LYP/6-311G(d,p) Level.

Compound Isodesmic scheme DH0f;298

Average

values

Literature

valuesa

CF3CH2OCHF2 CF3CH2OCHF21CH3F 2316.28 2315.93 6 0.33 2319.38

CF3CH2OCHF21CH41CH4 2315.15 2318.66

CF3CH2OCHF21H2O1CH3F 2316.36

CF3CH2OCF2 CF3CH2OCF21CH2O1CH3F 2259.28 2260.96 6 0.12

CF3CH2OCF21CH3F1CH4 2260.77

CF3CH2OCF21CH4 2262.82

CF3CHOCHF2 CF3CHOCHF21CH4 2266.99 2266.72 6 0.19

CF3CHOCHF21CH3F1CH4 2267.34

CF3CHOCHF21CH2O1CH3F 2265.84

aFrom ref. 18.

Table 1. The Enthalpies (DH0298) Calculated at the B3LYP/6-311G(d,p), G3(MP2)//B3LYP/

6-311G(d,p), and G3(MP2)//MP2/6-311G(d,p) Levels.

Levels B3LYP

G3(MP2)//

B3LYP

G3(MP2)//

MP2

CF3CH2OCHF21 OH?CF3CH2OCF21 H2O (R1a) 29.43 210.71 210.73

CF3CH2OCHF21OH?CF3CHOCHF21H2O (R1b) 216.14 217.36 217.23

CF3CH2OCHF21OH?CF3CH2OH1CHF2O (R1c) 25.05 20.19 0.15

CF3CH2OCHF21OH?CF3CH2O1CHF2OH (R1d) 29.52 24.37 24.23

CF3CH2OCHF21Cl?CF3CH2OCF21HCl (R2a) 3.05 3.56 3.59

CF3CH2OCHF21Cl?CF3CHOCHF21HCl(R2b) 23.66 23.09 22.92

CF3CH2OCHF21Cl?CF3CH2Cl1CHF2O (R2c) 7.59 9.63 9.83

CF3CH2OCHF21Cl?CF3CH2O1CHF2Cl (R2d) 19.17 20.17 20.23



like transition states (TS1a and TS1b) to form two complexes (CP1aand CP1b) in the exit routes, which are about 1.1 and 1.9 kcal/mol

more stable than the products. For channels R2a and R2b, two com-

plexes (CP2a and CP2b) with the energies being 2.64 and 0.81

kcal/mol lower than the products are located at the product sides,

while only one complex (CR2b), which is 2.04 kcal/mol lower

than the reactants, is located at the reactant side of channel R2b.

Let us now consider the barrier heights of the reactions R1 and

R2. At the G3(MP2)//B3LYP/6-311G(d,p) level, the barrier heights

of displacement reactions R1c and R1d are about 40 and 50 kcal/

mol much higher than that of reaction channel R1a (and R1b),

respectively. Thus, the hydrogen abstraction reaction channels are

the dominant reaction pathways and the displacement processes

should be negligible. Similar conclusion can be obtained for R2.

Furthermore, for reactions R1a–1b and R2a–2b, the H-abstraction

from ��CH2�� group is more favored, since the barrier heights of

channels R1b and R2b are about 2.2 and 1.3 kcal/mol lower than

those of channels R1a and R2a, respectively. This is in accordance

with the fact that the BDE (D0298) of H-abstraction from ��CH2��

group is smaller than that from ��CHF2 group, as discussed in the

following paragraph. For the purpose of comparison, the CH3CH2

OCH3 1 OH reaction is investigated. It is seen that the barrier

heights of the H-abstraction from ��CH2�� and ��CHF2 group in

CF3CH2OCHF2 by OH radical are 4.97 and 6.55 kcal/mol, respec-

tively, higher than those of the H-abstraction from corresponding

sites in CH3CH2OCH3. This indicates that the fluorine atom substi-

tution for hydrogen atom on carbon atom reduces the reactivity of

C��H toward hydrogen abstraction. Similar conclusion can be

obtained from the reactions OH with CH3OCH3 and

CF3OCH3.37 In addition, for H-abstraction reaction from

��CHF2 group, the barrier heights of channels R1a (6.68 kcal/

mol) and R2a (3.15 kcal/mol) are lower than those of reactions

CF3OCHF2 1 OH and CF3OCHF2 1 Cl, 7.01 and 6.27 kcal/

mol,38 respectively. This energy decrease can be analyzed by

the changes in electron density distribution. Because CF3CH2O�� is less electron-withdrawing than CF3O��, the electron

density on the carbon atom of ��CHF2 is less reduced in

CF3CH2OCHF2 than in CF3OCHF2. Consequently, the lower

barrier heights are found in channels R1a and R2a.

With respect to the subsequent reactions of product radicals

with OH, both singlet and triplet PES of CF3CH2OCF2 1 OH

(R3) and CF3CHOCHF2 1 OH (R4) are considered. On the sin-

glet PESs, because each reactant radical has a single electron,

the attacks of doublet OH radical at CF3CH2OCF2 and

CF3CHOCHF2 are static attracting process with no barrier. The

additions of OH to those two radical species lead to one adduct

M3a for R3 and two adducts M4a and M4b for R4. The binding

energies of M3a, M4a, and M4b are 2111.85, 297.25, and

298.11 kcal/mol, respectively, at the G3(MP2)//B3LYP/6-

311G(d,p). For R3, the transition state (TS3a) of elimination

reaction of H2O from M3a, along with C��O bond rupture, lies

10.82 kcal/mol higher than the reactant. Clearly, formation of

CF3CHO, CH2, and H2O, i.e.,

CF3CH2OCF2 þ OH ! CF3CH2OCðOHÞF2! CF3CHOþ CF2 þ H2O (R3a)

is energetically inaccessible. The hydrogen transfer process of

reaction R3 is located on the triplet PES.

CF3CH2OCF2 þ OH ! CF3CHOCF2 þ H2O (R3b)

The barrier height of the hydrogen transfer process (3TS3b) is

4.85 kcal/mol higher than that of the reactants, but it is about 6

kcal/mol lower than that of addition–elimination process. Thus,

for reaction R3, only the addition reaction is dominant channel,

while the triplet state products 3CF3CHOCF2 and H2O may be

produced via hydrogen transfer process at higher temperatures.

On the other hand, Figure 2d shows that adducts M4a and M4b,

if formed, would rapidly take subsequent elimination and disso-

ciation processes to form final products in two ways:

CF3CHOCHF2 þ OH ! CF3CHðOHÞOCHF2 ðM4aÞ! CF3CHOþ CF2 þ H2O (R4a)

! CF3CHðOHÞOCHF2 ðM4bÞ ! CF3COCHF2 þ H2O (R4b)

While the barrier heights of the hydrogen transfer processes of

R4 on the triplet PES (3TS4c and3TS4d) are much higher (about

30 kcal/mol) than those of R4a and R4b.

CF3CHOCHF2 þ OH ! CF3CHOCF2 þ H2O (R4c)

! CF3COCHF2 þ H2O (R4d)

Thus the formation of products CF3CHO, CF2, CF3COCHF2,

and H2O are energetically accessible for reaction R4. Because

the transition states and isomers involved in the addition–elimi-

nation channel all lie below the reactants, reaction R4 is

expected to be rapid. No experimental data is reported for these

two reactions, and it is desirable to perform laboratory investiga-

tions on these two reactions.

The calculated BDEs (D0298) of C��H bond in molecules

CF3CH2OCHF2 and CH3CH2OCH3, along with several literature

data8,39–41 are listed in Table 3. The D0298 (C��H) values

obtained at the G3(MP2)//B3LYP/6-311G(d,p) level in ��CH2��and ��CHF2 group of CF3CH2OCHF2 are 100.21 and 105.05

kcal/mol, respectively, which show good consistency with the

values in the literature. The change of bond energies is in line

with the change of barrier heights mentioned earlier. Similar

conclusion can be obtained for CH3CH2OCH3. In addition, the

BDEs (D0298) of C��O bond in CF3CH2OCHF2 and

CH3CH2OCH3 are also presented in Table 3. From Table 3, we

can see that the D0298 (C��O) values obtained from reactions

CF3CH2OCHF2 ? CF3CH2O 1 CHF2 and CF3CH2OCHF2 ?CF3CH2 1 OCHF2 are 106.10 and 92.50 kcal/mol, respectively,

at the G3(MP2)//B3LYP level. Moreover, since the D0298 value

of C��O bond in group ��OCH2CF3 is about 12.55 and 7.71

kcal/mol, respectively, lower than that of two types of C��H

bond, the broken of C��O bond may play an important role for

the CF3CH2OCHF2 at the stratospheric level. Similarly, the D0298

values of C��O bonds in CH3CH2OCH3 are much lower than

those of C��H bonds, and as a result, all the product radicals

CH3CH2O�, �CH3, �CH2CH3, and CH3O� may be present in these



Figure 2. Schematic potential energy surface for the reactions CF3CH2OCHF2 1 OH/Cl, CF3CH2

OCF2/CF3CHOCHF2/CH3CH2OCH3 1 OH, Relative energies (in kcal/mol) are calculated at the

G3(MP2)//B3LYP/6-311G(d,p)1ZPE level, the values in parentheses are calculated at the B3LYP//6-

311G(d,p)1ZPE level.

dissociation processes. Furthermore, seen from Table 3, the D0298

values of C��O and C��H bonds in CF3CH2OCHF2 are higher

than the corresponding values in CH3CH2OCH3, indicating that

the reactivities of both C��O and C��H bonds are reduced due

to the fluorine substitution effect.

Dynamic Calculations

The total rate constants of reactions of R1a–1b, R2a–2b, and R5

are obtained from the sum of the individual rate constants asso-

ciated with each reaction channel. The PES information of each

reaction channel, which is obtained at the G3(MP2)//B3LYP/6-

311G(d,p) level, is put into the POLYRATE 8.4.1 program to

produce the VTST rate constants in a wide temperature range

from 200 to 2000 K. The rate constants for the H-abstraction

reaction channels are evaluated by the conventional transition

state theory (TST), CVT, and CVT/SCT correction (CVT/SCT).

The TST, CVT, and CVT/SCT rate constants of channel R1a is

plotted in Figure 3. At the temperatures 200 and 2000 K, the

kCVT:kTST results are 0.69 and 0.62 for channel R1a, respectively.

This indicates that the variational effect is to some extent large

within the whole temperature range. Moreover, the CVT/SCT

rate constants are much larger than the CVT ones at lower tem-

peratures, but the two curves are asymptotic to each other in the

higher temperature range. So the small curvature effect plays an

important role only for the lower temperatures. Similar conclu-

sions can be obtained for reaction channels R1b and R2a–2b (in

Supporting Information Fig. S3).

The individual CVT/SCT rate constants of each reaction

channel (k1a, k1b, k2a, k2b and k5a, k5b, k5c, k5c0), the total rate

constant k1, k2, k5, and the available experimental values5–11 are

plotted in Figure 4, the temperature dependence of the k1a/k1,

k1b/k1, k2a/k2, k2b/k2 and k5a/k5, k5b/k5, k5c/k5. k5c0/k5 branching

ratios are plotted in Figure 5. From these figures, we can find

1. The calculated rate constants are in good agreement with the

available experimental values4–9,16 if the experimental uncer-

tainties are taken into account. In the measured temperature

range, for the CF3CH2OCHF2 1 OH reaction, the calculated

results slightly overestimate the experimental values of Zhang

et al.4 and Oyaro et al.,6 while they are in good agreement

with the values obtained by Beach et al.,5 within a factor

~0.7–1.3. For the CF3CH2OCHF2 1 Cl reaction our values are

Table 3. Bond Dissociation Energies (D0298) (kcal/mol) for the CF3CH2OCHF2 at the

G3(MP2)//B3LYP/6-311G(d,p) Level.

Bond dissociation

G3MP2//

B3LYP/6-311G(d,p) Literature values

C��H bond

CF3CH2OCHF2?�CF2OCH2CF31H 106.86 93.57,a 93.82,a 98.90,a 103.4,b

103.5,b 102.31,c 105.5d

CF3CH2OCHF2?CF3(�)CHOCHF21H 100.21 90.66,a 91.05,a 95.91,a 98.9,b

98.8,b 97.92,c 100.8,d 99.7d

CH3CH2OCH3?�CH2OCH2CH31H 95.94

CH3CH2OCH3?CH3(�)CHOCH31H 94.82

CH3CH2OCH3?�CH2CH2OCH31H 102.12

C��O bond

CF3CH2OCHF2?CF3CH2O�1�CHF2 106.10

CF3CH2OCHF2?�CH2CF31�OCHF2 92.50

CH3CH2OCH3?CH3CH2O�1�CH3 83.11

CH3CH2OCH3?�CH2CH31CH3O� 85.49

aCalculations at the MP2/6-31G(d), MP2/6-311G(d), and MP2/6-311G(d,p) levels, respectively,

from ref. 8.bCalculations at the (RO)B3LYP/6-311G(d,p) and (RO)B3LYP/6-31111G(2d,p) from ref. 39.cEstimation by using the artificial neural (ANN) technique from ref. 40.dCalculations at the B3P86/6-31G(d) and B3P86/6-31111G(3df,2p) levels, respectively, from

ref. 41.

Figure 3. Computed TST, CVT, and CVT/SCT rate constants as a

function of 103/T for the reaction channel CF3CH2OCHF2 1 OH ?CF3CH2OCF2 1 H2O.



slightly higher than those determined by Wallington et al.,7

Beach et al.,5 Oyaro et al.,6 and Hickson et al.,9 while they

show better agreement with the values reported by Kambanis

et al.8 The deviation between our calculated values and the

experimental values8 obtained by Kambanis remains within a

factor 0.75–0.78; For the CH3CH2OCH3 1 OH reaction, the

calculated rate constants are also in good agreement with the

corresponding experimental values16 in the considered tem-

perature ranges within a factor 0.6–1.0. The rate constants of

reaction OH with CF3CH2OCHF2 are about two to three

orders of magnitude lower than those of reaction R5 below

500 K, because the substitution of hydrogen atoms in

CH3CH2OCH3 by fluorine atoms reduces the reactivity of the

C��H bond.

2. The Arrhenius expressions of k1 5 1.91 3 10224 exp(21324/

T) and k2 5 3.04 3 10212 exp (21466/T) cm3/(mol s) are fit-

ted by the CVT/SCT rate constants in the temperature range of

292–402 and 273–398 K, respectively, for the reactions

CF3CH2OCHF2 1 OH and CF3CH2OCHF2 1 Cl. The calcu-

lated activation energy for the CF3CH2OCHF2 1 OH reaction,

2.63 kcal/mol, from this fit is slightly lower than the experi-

mental values of 4.19 kcal/mol (ref. 6), while it is reasonably

close to the result of 3.19 kcal/mol taken from ref. 42. The cal-

culated activation energy for reaction CF3CH2OCHF2 1 Cl,

2.91 kcal/mol, is in good accordance with the experimental

value of 2.89 kcal/mol (ref. 8).

3. Figures 5a–5b show the temperature dependence of k1b/k1 and

k2b/k2 branching ratios are 96, 91, 81, 68% and 97, 90, 73,

68% at 200, 500, 1000, and 2000 K, respectively, indicating

that the H-abstraction from ��CH2�� group will be the pre-

dominant pathway of the reactions R1 and R2 over the whole

temperature range (200–2000 K). Similarly, from Figure 5c, we

Figure 4. Calculated rate constant k1a, k1b for the reaction channels of CF3CH2OCHF2 1 OH, k2a, k2bfor CF3CH2OCHF2 1 Cl, k5a, k5b, k5c, k5d for reaction channels of CH3CH2OCH3 1 OH, and the total

rate constant k1(k1 5 k1a 1 k1b), k2(k2 5 k2a 1 k2b), k5(k5 5 k5a 1 k5b 1 k5c 1 k5d) are obtained at the

G3(MP2)//B3LYP/6-311G(d,p) level along with the experimental values4–9,16 as a function of 103/T.



can see that channel R5b (H-abstraction from ��CH2�� group)

is the dominant reaction route at the lower temperatures. How-

ever, with the increasing temperature, channel R5a (H-abstrac-

tion from ��CH3 group) plays a more important role in the

total reaction, and the contributions of R5a, R5b, R5c, and R5c0

should all be taken into account at higher temperatures.

Because of the lack of the experimental data in other temper-

ature range, and we hope our results may provide a good esti-

mate for future laboratory investigation. Finally, the theoretical

rate constants reactions R1, R2, and R5 over a wide temperature

range of 200–2000 K are fitted by the three-parameter Arrhen-

nius expressions as follows (in units of cm3/(mol s21):

k1 ¼ 9:48310�24 T4:14 exp ð108=TÞ

k2 ¼ 6:96310�20 T2:57 exp ð�566=TÞ

k5 ¼ 3:57310�19 T2:71 exp ð303=TÞ

Conclusions

Theoretical studies on the reactions of CF3CH2OCHF21OH and

CF3CH2OCHF21Cl are performed by the dual-level direct dynam-

ics method. The PES information is obtained at the B3LYP/6-

311G(d,p) level, and the HL energies for the stationary points

and extra points along the MEP are refined at the G3(MP2)

theory. By using group-balanced isodesmic reaction as working

reaction, the calculated values of standard enthalpies of formation at

the G3(MP2)//B3LYP/6-311G(d,p) level are 2315.93 6 0.33,

2260.966 0.12, and2266.726 0.19 kcal/mol for CF3CH2OCHF2,

CF3CH2OCF2, and CF3CHOCHF2, respectively. The reaction mech-

anisms of the two products (CF3CH2OCF2 and CF3CHOCHF2) with

OH radical are investigated theoretically. We found that addition–

elimination processes may be the major product pathway of

CF3CHOCHF2 1 OH, leading to major products CF3CHO, CF3,

CF3COCHF2, and H2O. However, only addition reaction may be im-

portant for CF3CH2OCF2 1 OH reaction, that is, the adduct

CF3CH2OC(OH)F2 may be the major product. The rate constants of

Figure 5. Calculated branching ratio for CF3CH2OCHF2 1 OH, CF3CH2OCHF21Cl and

CH3CH2OCH3 1 OH reactions as a function of 103/T at the G3(MP2)//B3LYP/6-311G(d,p) level.



the H-abstraction reaction channels are calculated by using CVT/

SCT correction. The theoretical results are consistent with the

available experimental values in the measured temperature range.

For the four H-abstraction reaction channels, variational effect is

somewhat important over the whole temperature region, and the

SCT correction has large contribution to the rate constants in the

lower temperature range. Substitution of hydrogen atoms in

CH3CH2OCH3 by fluorine atoms reduces the reactivity of the

C��H bond, and as a result, the rate constants for reaction CF3CH2

OCHF2 1 OH become much smaller than CH3CH2OCH3 1 OH.

To provide good estimation for future laboratory investigation, the

fitted three-parameter expressions in the wide temperature range of

200–2000 K for reactions R1, R2, and R5 are given, k1 5 9.48 310224 T4.14 exp(108/T), k2 5 6.963 10220 T2.57 exp(2566/T), k5 53.573 10219 T2.71 exp (303/T) cm3/(mol s).

Acknowledgment

We thank Professor Donald G. Truhlar for providing the POLY-

RATE 8.4.1 program.

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theoretical study of the reactions cf3ch2ochf2 + oh/cl and its product radicals and parent...

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