suppmat396. 500KB Jun 05 2011 09:30:55 PM

Ab Initio Study on the Structures of Fluorinated Formates
and Hydrogen Abstraction Reaction with OH Radical
ASIT K. CHANDRA,1,* TADAFUMI UCHIMARU,2 MASAAKI SUGIE,2 AKIRA SEKIYA2
1

Research Institute of Innovative Technology for the Earth (RITE), AIST Tsukuba Central 5,
Tsukuba 305-8565, Japan
2
National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba
Central 5, Tsukuba 305-8565, Japan
Received 7 June 2002; Accepted 6 August 2002

Abstract: The conformational potential energy surfaces for mono- and difluoromethyl formate have been determined
by using a modified G2(MP2) level of calculations. The structures and vibrational frequencies for the conformers of
mono- and difluoromethyl formate have been reported. The hydrogen abstraction reaction channels between these two
formates and OH radicals have been studied at the same level of theory. Using the standard transition state theory and
taking into account the effect of tunneling across the reaction barrier, we have estimated the rate constant for hydrogen
abstraction by OH radical. The effect of successive fluorine substitution for methyl hydrogen on the conformational
stability and on the hydrogen abstraction rate has been analyzed.
© 2003 Wiley Periodicals, Inc.

J Comput Chem 24: 396 – 407, 2003

Key words: fluorinated formate; ab initio; hydrogen abstraction; OH radical; rate constant

Introduction
The structure of methyl formate (MF) has been studied extensively
both experimentally1,2 and theoretically,3–5 but the structures of
halomethyl formates are not as well known. Hotokka and Dahlqvist6
studied the conformational potential energy surfaces of chloromethyl
formate and fluoromethyl formate (FMF), CH2FOCHO, by using ab
initio calculations at the Hartree-Fock level. However, their study was
restricted only to the rotation of the halomethyl group about the OOC
bond, and other bond lengths and angles were not optimized. Later,
Lopata and Kuczkowski7 identified the syn conformer of FMF for the
first time in their laboratory as a decomposition product from vinyl
fluoride ozonide and determined its structural parameters. They also
predicted the possibility of existence of another conformer of FMF. In
fact, it is well known8 that alkyl formates can exist in two conformations, syn and anti:

If R is hydrocarbon, the syn conformer is largely preferred to the

anti and the barrier to interconversion is around 10 kcal/mol.8
However, when R ⫽ CF3, the two conformers become closer in
energy and also the barrier to interconversion decreases. Thus,
both the conformers of CF3OCHO exist in normal conditions. It is,
therefore, interesting to study how the stability of the two conformers changes with the successive fluorine substitution while
going from CH3OCHO (MF) to trifluoromethyl formate (TFMF),
CF3OCHO through FMF and difluoromethyl formate (DFMF),
CHF2OCHO. In the case of FMF and DFMF, the rotation of the
OCH2F and OCHF2 groups, respectively, along the OOC bond
may generate more conformers than the two shown above. In the
present work, we have calculated for the first time the structures
and vibrational frequencies for all the conformers of FMF and
DFMF by using ab initio MP2(full)/6-311G(d,p) method. Moreover, the energies of all the species have been calculated by using
high level G2(MP2) method. To the best of our knowledge, there
is no experimental or theoretical report on the structure and con*Current address: Chemistry Group, Birla Institute of Technology and
Science (BITS), Pilani 333 031, Rajasthan, India.
Correspondence to: T. Uchimaru; e-mail: t-uchimaru@aist.go.jp
This article includes Supplementary Material available from the authors
upon request or via the Internet at ftp://ftp.wiley.com/public/journals/jcc/
suppmat/24/396 or http://www.interscience.wiley.com/jpages/0192-8651/

suppmat/v24.396.html

© 2003 Wiley Periodicals, Inc.

Fluorinated Formates and Hydrogen Abstraction Reaction

formational stability of DFMF. Our study provides accurate information about the structure, relative stability, and vibrational frequencies of the conformers of FMF and DFMF and those of the
rotational barrier separating the two conformers.
Moreover, we have also studied the hydrogen abstraction reactions of FMF and DFMF with OH radicals. Recent works9 –12
have shown formates as major products resulting from the oxidation of halogenated ether compounds in the atmosphere. The
atmospheric impact of the resulting formates should, therefore, be
included while considering the global warming potential of haloethers. Because hydrogen abstraction reaction with OH radicals
is likely to be one of the major degradation channels for halogenated formates in the atmosphere,11,12 it is important to understand
the reaction of formates with OH radicals. Knowledge of the rate
constants is essential to evaluate the atmospheric lifetime of a
compound. The kinetic data on hydrogen abstraction from formates are very limited. Very recently, we investigated the hydrogen abstraction reaction: CF3OCHO ⫹ OH 3 CF3OCO ⫹ H2O
using ab initio G2(MP2) theory.13 In the present study, we also
investigate for the first time the kinetics and mechanism of the
following hydrogen abstraction reactions of FMF and DFMF with
OH radicals:

CH2 FOCHO ⫹ OH 3 CH2 FOCO ⫹ H2 O

(R1a)

CH2 FOCHO ⫹ OH 3 CHFOCHO ⫹ H2 O

(R1b)

CHF2 OCHO ⫹ OH 3 CHF2 OCO ⫹ H2 O

(R2a)

CHF2 OCHO ⫹ OH 3 CF2 OCHO ⫹ H2 O

(R2b)

The result provides an indication of kinetics of these reactions and
how the reactivity for hydrogen abstraction changes with the
change in fluorine substitution.

Computational Details
Full geometry optimizations for all species were carried out at the
MP2(full)/6-311G(d,p) level. Frequencies of all the stationary
points were then calculated at the same level of theory. This
computational level was chosen as a reasonable compromise between speed and accuracy of calculations, which was based on our
experience in the earlier studies.14,15 After optimizing the geometries, single point calculations of the energies were carried out at
the QCISD(T)/6-311G(d,p) and MP2/6-311 ⫹ G(3df,2p) levels of
theory in order to determine accurately the relative stability of the
various conformers of FMF and DFMF, and also to calculate the
barrier height and thermochemistry of the reactions (R1) and (R2)
at the G2(MP2)16 level of theory. We should point out that the
G2(MP2) calculations in the present study were made with
MP2(full)/6-311G(d,p) geometries and frequencies rather than the
prescribed MP2(full)/6-31G(d) geometries and HF/6-31G(d) frequencies. In our previous work we employed this modified
G2(MP2) procedure on the reaction between CF3OCHO and OH
radical.13 This procedure produced very good results for hydrogen
abstraction from CF3OCHO. We also calculated the hydrogen
abstraction rate constant for the reaction between CH3OCHO and

397

OH by applying the same procedure, and the calculated value of
1.8 ⫻ 10⫺13 cm3 molecule⫺1 s⫺1 is found to be in good agreement
with the experimental value of (1.73 ⫾ 0.21) ⫻ 10⫺13 cm3
molecule⫺1 s⫺1 at 298 K.17 Thus, we employed the same modification of the G2(MP2) procedure in the present study. The
calculated harmonic vibrational frequencies were scaled by a factor of 0.949618 and the zero point energies (ZPEs) and vibrational
contribution to enthalpy were estimated from the scaled frequencies. Spin contamination is often a serious problem in treating
open-shell systems with single configuration wave function. However, in the present case, spin contamination was not high for
either the radicals or transition states (TSs) for hydrogen abstraction; the 具S 2 典 value never exceeded 0.76 for the radicals and 0.78
for TSs. All calculations were performed with the GAUSSIAN98
suite of programs.19

Results and Discussion
Structures and Vibrational Frequencies

FMF and DFMF have two stable conformations based on the
values of the C3OO1OC2OO4 torsional angle (see Figs. 1 and
2): syn when the C3OO1OC2OO4 dihedral angle is around zero
and anti when the C3OO1OC2OO4 dihedral angle is around
180°. The same is true for the radicals generated after a hydrogen

atom abstraction either from the carbonyl site or from the fluorinated methyl group of FMF and DFMF. However, under the syn
and anti arrangement of C3OO1OC2OO4, more than one conformer may exist depending upon the C2OO1OC3OF6 torsional
angle for FMF and the C2OO1OC3OH6 torsional angle for
DFMF. Studying the rotation of the carbonyl group along the
O1OC2 bond and the rotation of the fluoromethyl/difluoromethyl
group along the O1OC3 bond, we have identified two conformers
for FMF and four conformers for DFMF.
Table 1 presents the optimized geometrical parameters (see Fig.
1 for the definition) for both the syn and anti conformers of FMF
and the radicals generated after hydrogen atom abstraction from
both the carbonyl and fluoromethyl groups. Experimental geometrical parameters are available only for the syn conformer of FMF.7
Our calculated values are in excellent agreement with the experimental results. Our calculated value for the optimum C2OO1OC3
angle (114.9°) of syn-FMF is much lower than the value (122°)
obtained from low level ab initio computations.6 Experimental
geometry is not available for the anti-FMF, probably because at
room temperature almost 99% of FMF will be at syn conformation
due to its lower energy (discussed later). The carbonyl oxygen
atom (O4) deviates slightly from the C3OO1OC2 plane of FMF.
Such deviation from planarity was also noticed from the experimental results.7 However, the energy change from the planar to the
equilibrium slightly nonplanar configuration is very small (⬍0.05

kcal/mol), indicating the flatness of the potential energy surface in
this region. The C2OO1OC3OF6 dihedral angle (⬇80°) indicates that the O1OC3OF6 plane is nearly perpendicular to the
C2OO1OC3 plane. This conformation is in clear contrast to those
observed for cis- and trans-fluoroacetic acid,20 cis- and transfluoroacetyl fluoride,20 and cis- and trans-1,3-difluoroacetone,21
where the COF bond was coplanar with an adjacent carbonyl

398

Chandra et al.

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Vol. 24, No. 3

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Journal of Computational Chemistry

Figure 1. Schematic diagrams for the structures of both the syn and anti conformers of CH2FOCHO
(FMF) and the corresponding radicals generated for the hydrogen abstraction from the formyl group and

the fluoromethyl group.

group. The interaction that leads to this contrast can be explained
in terms of anomeric effect resulting from the interaction between
n oxygen and ␴*CF orbitals of the CH2FOO system (see Fig. 3). This
interaction is most favorable when the O1OC3OF6 plane becomes nearly perpendicular to the C2OO1OC3 plane. Except for
the O1OC2OO4 and O1OC2OH5 angles, the bond lengths and
angles in the syn and anti-FMF are not much different. The
O1OC2OO4 angle is more than 3° larger for the syn-FMF,
whereas the O1OC2OH5 angle is smaller by nearly 4° in comparison to that in the anti-FMF. In the TS structure related to the
interconversion of syn and anti conformers of FMF, the
C3OO1OC2OO4 torsional angle becomes 99°. The dipole moments for the syn and anti-FMF are calculated to be 2.18 and 3.31
D, respectively, at the MP2(full)/6-311G(d,p) level. Our calculated
dipole moment value for syn-FMF (2.18 D) is close to the experimental value of 2.24 ⫾ 0.02 D.7 The C2OO1 bond becomes
shorter and the C3OO1 bond becomes longer while going from
FMF molecule to the corresponding CH2FOCO radical. The opposite is true for the CHFOCHO radical. The O1OC2OO4 angle
in the CH2FOCO radical is more than 4° larger than that in the
corresponding parent molecule.
The optimized geometrical parameters for DFMF and the radicals generated due to hydrogen abstraction from it are given in

Table 2 (see also Fig. 2). The two conformers of DFMF where
C3OO1OC2OO4 torsional angle is around 0°, but
C2OO1OC3OH6 torsional angles are 39.1° and 180.0°, are
named syn-1 and syn-2, respectively. Similarly, the two conformers of DFMF under anti conformation (C3OO1OC2OO4 ⬃
180°) are designated as anti-1 (C2OO1OC3OH6 180.0°) and
anti-2 (C2OO1OC3OH6 41.6°). The same type of nomenclature
is used for designating the conformers of CHF2OCO radical. It is
interesting to note that in the most stable syn-1 conformer of
DFMF the O1OC3OF8 plane makes an angle of 82° with the
C2OO1OC3 plane, which gives extra stabilization to the system
due to anomeric effect, as discussed before for FMF. The
C2OO1OC3 angle in syn-2-CHF2OCHO is more than 3° larger
than that in syn-1-CHF2OCHO. The most significant differences
between the structures of the syn and anti conformers of DFMF
can be observed for the O1OC2OO4 and O1OC2OH5 angles
(see Fig. 2). The values of O1OC2OO4 angles in syn-1 and syn-2
conformers are nearly 5° higher than the values of the same angles
in anti-1 and anti-2 conformers. The opposite trend can be observed for the O1OC2OH5 angle. As mentioned earlier, the
position of the H6 atom in syn-1 and anti-2 is different from the
position of the same atom in syn-2 and anti-1 conformers of

DFMF. In the former two conformers, the C2OO1OC3OH6

Fluorinated Formates and Hydrogen Abstraction Reaction

Figure 2. Schematic diagrams for the structures of the four conformers of CHF2OCHO (DFMF) and the
corresponding radicals generated for the hydrogen abstraction from the formyl group and the OCHF2
group.

399

Chandra et al.

400

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Vol. 24, No. 3

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Journal of Computational Chemistry

Table 1. Optimized Geometrical Parameters for the syn and anti Conformers of CH2FOCHO (FMF)
Molecules, CH2FOCO Radical, and CHFOCHO Radical at the MP2(full)/6-311G(d,p) Level.

CH2FOCO

CH2FOCHO (FMF)
Geometrical
parameters

syn

C2OO1
C3OO1
C2OO4
C2OH5
C3OF6/F5
C3OH7
C3OH8/H6
C2OO1OC3
O1OC2OO4
O1OC2OH5
O1OC3OF6/F5
O1OC3OH7
O1OC3OH8/H6
C3OO1OC2OO4
C2OO1OC3OF6/F5
C2OO1OC3OH7

1.360
1.407
1.199
1.096
1.364
1.087
1.089
114.9
125.8
108.2
109.8
105.6
110.6
1.6
80.5
⫺162.0

Expt.a
1.355
1.404
1.194
1.100
1.369
1.082
1.066
115.8
125.8
108.5
109.6
106.8
111.1
1.5
83.9
⫺164.0

anti
1.366
1.395
1.193
1.103
1.369
1.087
1.093
114.7
122.4
112.4
110.1
106.4
111.4
175.3
77.9
⫺164.1

CHFOCHO

syn

anti

syn

1.339
1.423
1.186

1.351
1.410
1.181

1.358
1.090
1.087
114.6
130.2

1.361
1.090
1.087
113.9
126.9

1.363
1.374
1.199
1.095
1.327
1.085

109.7
109.9
105.2
1.8
77.1
⫺42.4

109.7
110.5
105.6
175.0
81.9
⫺37.9

anti
1.381
1.361
1.191
1.099
1.342
1.083

116.0
125.6
107.9
109.5
116.9

115.4
121.5
112.3
113.7
112.7

1.2
⫺164.0
⫺32.1

174.0
64.9
⫺163.0

a

Ref. 7.
Bond lengths and angles are given in Angstrom and degrees, respectively.
See Figure 1.

dihedral angle is around 40°, whereas the same angle amounts to
180° for the latter two conformers. In fact, syn-2 and anti-1
conformers of DFMF molecule have C s symmetry. It is interesting
to compare the key geometrical parameters of syn and anti conformers of FMF and DFMF with those of MF and TFMF. The
C2OO1 bond length increases and the C3OO1 bond length decreases with the increase in fluorine substitution while going from
MF to TFMF through FMF and DFMF. Successive fluorine substitutions at the methyl position enhance the electron withdrawing
ability of this group, and as a result an electron cloud is likely to
shift from C2OO1 bonding region to the C3OO1 bonding region.
This makes the C2OO1 bond weaker and C3OO1 bond stronger
with the successive fluorine substitution at the methyl position.
The dipole moments for the syn-1, syn-2, anti-1, and anti-2 conformers of DFMF are calculated to be 1.48, 2.65, 2.13, and 3.04 D,
respectively, at the MP2(full)/6-311G(d,p) level. The

Figure 3. Schematic representation of the orbital interaction between
the oxygen lone pair (n oxygen) and the antibonding orbital of COF
bond (␴*CF).

C3OO1OC2OO4 torsional angle amounts to 90° for the TS
structure of syn to anti interconversion in DFMF. Hydrogen abstraction from the carbonyl group of DFMF yields CHF2OCO
radicals. Like the parent molecule, CHF2OCO radical has four
stable conformers. The optimized geometrical parameters for these
conformers are given in Table 2. The syn-2 and anti-1 conformers
of CHF2OCO radical remain in C s symmetry as its parent molecule. Table 2 shows that the main structural changes in CHF2OCO
radicals include a shortening of the C2OO1 bond and lengthening
of the C3OO1 bond. The opposite trend is observed for
CF2OCHO radicals, which results from the methyl hydrogen abstraction of DFMF. In the case of CF2OCHO radical, the C2OO1
bond increases and C3OO1 bond decreases in comparison to those
in parent DFMF molecule.
The vibrational frequencies for all the conformers of FMF and
DFMF, and the radicals generated from them after hydrogen
abstraction are given in Table 3. The calculated results cannot be
compared with any other data, because no such data, experimental
or theoretical, are available for these systems. The stretching
frequency for the COH bond (3132 cm⫺1) in the formyl group of
syn-FMF is much higher than that for the same bond in anti-FMF
(3042 cm⫺1), indicating that this COH bond is much weaker in
anti-FMF. The difference in the vibrational frequencies of the
COH bond in the formyl group of syn-1 (3148 cm⫺1), syn-2 (3133
cm⫺1), and anti-1 (3129 cm⫺1) conformers of DFMF is rather
small. The carbonyl COO stretching frequency for syn-FMF
(1828 cm⫺1) is found to be lower by 34 cm⫺1 than that for
anti-FMF (1862 cm⫺1). The C2OO4 bond length in syn-FMF is
somewhat longer (0.006 Å) than the same bond in anti-FMF,
which indicates that the C2OO4 bond in the former is weaker than

Fluorinated Formates and Hydrogen Abstraction Reaction

401

Table 2. Optimized Geometrical Parameters for the Different Conformers of CHF2OCHO (DFMF)

Molecules, CHF2OCO Radical, and CF2OCHO Radical at the MP2(full)/6-311G(d,p) Level.
CHF2OCO

CHF2OCHO (DFMF)
Geometrical
parameters

syn-1

C2OO1
C3OO1
C2OO4
C2OH5
C3OH6/H5
C3OF7
C3OF8/F6
C2OO1OC3
O1OC2OO4
O1OC2OH5
O1OC3OH6/H5
O1OC3OF7
O1OC3OF8/F6
C3OO1OC2OO4
C2OO1OC3OH6/H5
H6OO1OC3OF7
H6OO1OC3OF8
C2OO1OC3OF6

1.364
1.393
1.198
1.095
1.087
1.336
1.342
115.1
125.6
108.0
112.4
106.2
109.4
0.0
39.1
121.1
⫺121.8

syn-2
1.377
1.388
1.193
1.096
1.087
1.342
1.342
118.2
126.5
106.8
106.6
111.1
111.1
0.0
180.0
119.5
⫺119.5

CF2OCHO

anti-2

anti-2

syn-1

syn-2

anti-1

anti-2

syn

anti

1.383
1.374
1.191
1.096
1.085
1.351
1.351
115.8
120.8
113.2
108.1
110.9
110.9
180.0
180.0
120.6
⫺120.6

1.371
1.381
1.192
1.101
1.091
1.332
1.348
115.3
121.9
112.9
113.0
107.0
109.7
175.4
41.6
121.0
⫺121.2

1.342
1.410
1.186

1.353
1.404
1.182

1.367
1.389
1.178

1.354
1.397
1.180

1.377
1.370
1.193
1.094

1.386
1.358
1.189
1.098

1.088
1.337
1.329
114.6
129.9

1.086
1.338
1.338
117.2
130.7

1.086
1.341
1.341
114.4
125.4

1.088
1.340
1.332
113.8
126.6

1.317
1.324
115.5
125.5
107.3

1.313
1.332
114.9
121.1
112.4

111.5
109.2
106.0
⫺1.0
42.0

106.3
110.5
110.5
0.0
180.0

106.7
110.7
110.7
180.0
180.0

112.0
109.0
106.5
176.4
28.7

109.3
113.3
1.2
⫺164.0
⫺32.1

110.3
113.4
174.0
64.9
⫺163.0

72.7

71.1

Bond lengths and angles are given in Angstrom and degrees, respectively.
See Figure 2.

in the latter. This is also reflected in the force constant values for
the C2OO4 bond of syn-FMF (17.68 mDyne/Å) and anti-FMF
(19.76 mDyne/Å). The distance between O4 and H8 atoms (see
Fig. 1) in syn-FMF amounts to 2.40 Å, which indicates the presence of weak hydrogen bonding interaction between these two
atoms. This hydrogen bonding interaction weakens the C2OO4
bond in syn-FMF. Interestingly, the carbonyl COO stretching
frequencies for the syn- (1828 cm⫺1) and anti-FMF (1862 cm⫺1)
are almost the same as those calculated for the syn and anti
conformers of DFMF. The COH stretching frequencies for the
COH bond in the fluoromethyl group for the syn- (3157 cm⫺1)
and anti-FMF (3117 cm⫺1) are higher than those observed for the
syn- (3228 and 3225 cm⫺1) and anti-DFMF (3245 and 3169
cm⫺1).
Energetics

As mentioned before, FMF has two conformers, syn and anti. The
syn conformer is found to be lower in energy than the corresponding anti counterpart. The energy differences between these two
conformers of FMF amount to 3.0 kcal/mol, at the G2(MP2) level
of theory. DFMF has four stable conformers, and the G2(MP2)
calculated energy differences between the most stable syn-1 and
the other three conformers are found to be 2.4, 2.4, and 4.0
kcal/mol for syn-2, anti-1, and anti-2, respectively. These energy
differences are much lower than those observed for MF, where at
the same G2(MP2) level the syn conformer is 4.7 kcal/mol lower
in energy than the anti-CH3OCHO. Thus the energy difference
between the two conformers comes down with the increase in
fluorine substitution for the methyl hydrogen. In fact, the energy

difference between the two conformers of TFMF is only 1.1
kcal/mol.13 The barrier to interconversion (syn 3 anti) is also
found to reduce with increasing fluorine substitution at the methyl
position. The G2(MP2) calculated barrier heights for syn 3 anti
interconversion amount to 12.8, 9.8, and 7.1 kcal/mol, respectively, for MF, FMF, and TFMF. However, in the case of DFMF,
the syn-1 3 anti-2 interconversion barrier (11.0 kcal/mol) is found
to be much higher than the syn-2 3 anti-1 barrier (6.4 kcal/mol).
The syn-1 3 syn-2 and anti-1 3 anti-2 interconversion barriers
for DFMF are found to be 7.1 and 3.2 kcal/mol, respectively, at the
G2(MP2) level. In the case of TFMF, the strong ␲-accepting
influence of the CF3 group is known to be responsible for the
decrease in energy difference between the two conformers and the
barrier separating the two.8,22 The energy differences between the
conformers of FMF and DFMF indicate that the presence of the
anti conformer will be negligible in any sample of FMF or DFMF
at room temperature (298 K). However, the population of anti
conformer will be significant at higher temperature and can be
experimentally detected.
In contrast to FMF and DFMF molecules, the anti conformers
of CH2FOCO and CHF2OCO radicals are found to be 1.5 to 2.7
kcal/mol lower in energy than the corresponding the syn conformers. The reversal in the stability order of the two conformers of
radicals in comparison to the stability order of the two conformers
of FMF and DFMF molecules introduces a significant difference in
the bond strength between the COH bonds at the carbonyl groups
of the syn and anti conformers of FMF and DFMF. The energy
difference between the anti-1 and anti-2 conformers of CHF2OCO
is found to be rather small (0.1 kcal/mol), whereas the energy

Chandra et al.

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Vol. 24, No. 3

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Table 3. Unscaled Harmonic Vibrational Frequencies (␯ in cm⫺1)
at the MP2(full)/6-311G(d,p) Level.

␯ in cm⫺1

System
syn-FMF
(CH2FOCHO)
anti-FMF
(CH2FOCHO)
synCH2FOCO
antiCH2FOCO
synCHFOCHO
antiCHFOCHO
syn-1-DFMF
(CHF2OCHO)
syn-2-DFMF
(CHF2OCHO)
anti-1-DFMF
(CHF2OCHO)
anti-2-DFMF
(CHF2OCHO)
syn-1CHF2OCO
syn-2CHF2OCO
anti-1CHF2OCO
anti-2CHF2OCO
synCF2OCHO
antiCF2OCHO

101, 245, 336, 553, 773, 1009, 1055, 1121, 1177, 1217, 1345, 1424,
1499, 1544, 1828, 3132, 3157, 3242
107, 151, 357, 503, 692, 1042, 1049, 1116, 1156, 1231, 1341, 1447,
1498, 1552, 1862, 3042, 3117, 3229
94, 272, 332, 545, 767, 990, 1120, 1156, 1198, 1332, 1489, 1546,
1883, 3152, 3246
102, 225, 326, 492, 674, 1029, 1123, 1144, 1216, 1342, 1499, 1547,
1908, 3150, 3241
76, 250, 300, 470, 794, 990, 1043, 1080, 1211, 1246, 1388, 1434,
1817, 3147, 3229
116, 143, 322, 521, 695, 963, 1041, 1072, 1182, 1224, 1397, 1436,
1860, 3094, 3260
69, 218, 254, 459, 522, 662, 799, 1053, 1079, 1162, 1198, 1222,
1427, 1436, 1449, 1827, 3148, 3228
120, 204, 209, 487, 535, 606, 839, 1032, 1046, 1154, 1186, 1190,
1424, 1440, 1454, 1845, 3133, 3225
104, 161, 234, 482, 521, 598, 806, 1000, 1039, 1147, 1183, 1199,
1422, 1443, 1467, 1861, 3129, 3245
42, 124, 251, 431, 531, 616, 746, 1037, 1132, 1153, 1180, 1231,
1430, 1449, 1471, 1863, 3061, 3169
64, 211, 302, 456, 518, 664, 782, 1056, 1137, 1188, 1234, 1430,
1446, 1886, 3213
100, 195, 268, 488, 535, 610, 840, 1016, 1109, 1177, 1194, 1427,
1434, 1902, 3235
111, 211, 217, 476, 517, 590, 806, 983, 1156, 1184, 1187, 1434,
1448, 1929, 3240
40, 209, 258, 429, 518, 619, 720, 1127, 1136, 1178, 1220, 1431,
1444, 1913, 3212
77, 191, 238, 453, 517, 670, 801, 1036, 1046, 1163, 1235, 1315,
1429, 1840, 3155
73, 122, 232, 432, 533, 624, 740, 1036, 1087, 1118, 1258, 1323,
1439, 1866, 3103

difference between syn-1 and syn-2 conformers amounts to 1.1
kcal/mol. Interestingly, the syn conformer is found to be more
stable than the anti conformer for CHFOCHO and CF2OCHO
radicals, generated due to hydrogen abstraction from the fluoromethyl group of FMF and DFMF. The interconversion (syn to anti)
barrier for CHFOCHO and CF2OCHO radicals is calculated to be
8.7 and 7.6 kcal/mol, respectively.
Reaction with OH Radical

There are two potential hydrogen abstraction sites in FMF and
DFMF molecules, namely the formyl hydrogen and the hydrogen
atom of the fluoromethyl group. Moreover, in principle, hydrogen
abstraction from both syn and anti conformers of FMF and DFMF
should be considered. The free energy difference at 298 K between
the syn and anti conformers of FMF is calculated to be 2.8
kcal/mol at the G2(MP2) level. It suggests that at 298 K only about
1% of FMF will exist as anti conformer. In the case of DFMF,
syn-1 is the lowest energy conformer, and the free energy differences between syn-2, anti-1, and anti-2 conformers and syn-1
amount to 2.7, 2.4, and 3.5 kcal/mol, respectively. Thus at 298 K

about 1% of the DFMF molecule is likely to exist in syn-2
conformation, whereas about 1.5% of the molecule exists as anti-1
conformer. However, at higher temperature, the presence of anti
conformer of FMF and syn-2 and anti-1 conformers of DFMF
cannot be neglected in any reaction sample of FMF or DFMF. For
example, at 450 K, the percentage of anti conformer of FMF
should be around 4%. The presence of anti-2 conformer of DFMF
can be neglected even at higher temperature. Thus, hydrogen
abstraction from both the syn and anti conformers of FMF and
syn-1, syn-2, and anti-1 conformers of DFMF should be taken into
account while calculating the total hydrogen abstraction reaction
rate of FMF and DFMF at a temperature significantly higher than
the room temperature (298 K).
Six TSs (three TSs for each conformer of FMF) are identified for hydrogen abstraction reactions of OH radicals with
syn-FMF and anti-FMF. Similarly, DFMF has two different
hydrogen abstraction sites, and accordingly six TSs (two TSs
for each of the syn-1, syn-2, and anti-1 conformers) are found
for reaction with OH radicals. Figures 4 and 5 illustrate the
structures and key geometrical parameters of these TSs for the

Fluorinated Formates and Hydrogen Abstraction Reaction

403

Figure 4. Optimized structures for the transition states of the hydrogen abstraction reaction of OH radical
with syn and anti conformers of CH2FOCHO (FMF). Bond lengths and angles are in Angstrom and
degrees, respectively.

hydrogen abstraction from FMF and DFMF. The S-FMF-TS1
and A-FMF-TS1 (see Fig. 4) are the TSs in which the hydroxyl
radical attacks, respectively, the formyl hydrogen of syn- and
anti-FMF to form CH2FOCO radical and H2O [reaction (R1a)].
The other four TSs are associated with the hydrogen abstraction
from the OCH2F group of syn-FMF (S-FMF-TS2 and S-FMFTS3) and anti-FMF (A-FMF-TS2 and A-FMF-TS3), resulting
in the CHFOCHO radical and H2O [reaction (R1b)]. The S1DFMF-TS1, S2-DFMF-TS1, and A1-DFMF-TS1 (see Fig. 5)
are associated with the reaction (R2a) and correspond, respectively, to the hydrogen abstraction from the formyl group of
syn-1, syn-2, and anti-1 DFMF. The breaking COH bonds are
elongated in the TS by nearly 10%; on the other hand, the
forming OOH bonds are longer by almost 33% than the normal
OOH bond length in water molecule. These structural features
indicate the formation of early TS, which is, of course, expected
from Hammond’s postulate23 due to the exothermic nature of

the reaction (discussed later). The other structural parameters
do not change much while going from isolated molecules to TS.
The distance between H(10) and O(1) atoms (see Figs. 4 and 5)
in S-FMF-TS1 (2.67 Å), A-FMF-TS3 (2.61 Å), S1-DFMF-TS1
(2.68 Å), S2-DFMF-TS1 (2.67 Å), and A1-DFMF-TS2 (2.66 Å)
is slightly shorter than the sum of van der Waals radii of these
two atoms (2.72 Å).24 Thus weak hydrogen bonding interactions might be present in these TSs and reduce the barrier height
for hydrogen abstraction. Meanwhile, strong hydrogen bonding
interaction exists between H(10) and O(4) atoms (see Figs. 4
and 5) in S-FMF-TS2 and S1-DFMF-TS2, because the distance
between these two atoms is much shorter than the sum of van
der Waals radii of these two atoms. The hydrogen bonding
interaction is also found to exist between H(10) and F(6) atoms
of S-FMF-TS3 and A-FMF-TS2. The vibrational frequencies
for all the TSs are given as supplementary information in Table
S1. Each TS has one imaginary frequency, the normal mode of

404

Chandra et al.

•

Vol. 24, No. 3

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Journal of Computational Chemistry

Figure 5. Optimized structures for the transition states of the hydrogen abstraction reaction of OH radical
with syn-1, syn-2, and anti-1 conformers of CHF2OCHO (DFMF). Bond lengths and angles are in
Angstrom and degrees, respectively.

which corresponds to the coupling of the breaking COH bond
and the forming OOH bond stretching vibrational modes.
The barrier heights for hydrogen abstraction reactions are given
in Table 4. Owing to the lower COH bond strength, hydrogen
abstraction from the formyl group of FMF and DFMF is expected
to have lower barrier heights than the barrier heights for hydrogen
abstraction from the fluoromethyl group. However, the reaction

channel (R1b) through S-FMF-TS2 has a somewhat lower barrier
(2.6 kcal/mol) than the barrier (2.9 kcal/mol) for hydrogen abstraction from the carbonyl group (S-FMF-TS1) of syn-FMF, presumably due to the presence of strong hydrogen bonding interaction in
S-FMF-TS2. The S-FMF-TS3 and A-FMF-TS3 are significantly
higher in energy in comparison to other hydrogen abstraction
reaction channels in FMF and, therefore, they are unlikely to make

Fluorinated Formates and Hydrogen Abstraction Reaction

405

Table 4. Heats of Reaction and Barrier Heights for the Hydrogen

k TST ⫽ ⌫

Abstraction Reaction of syn and anti Conformers of CH2FOCHO
(FMF), CHF2OCHO (DFMF), and CF3OCHO (TFMF) with
OH Radicals as Calculated at the G2(MP2) Level.

⌬E #0

System
Reactions for FMF
CH2FOC*HO ⫹ OH (S-FMF-TS1)
C*H2FOCHO ⫹ OH (S-FMF-TS2)
C*H2FOCHO ⫹ OH (S-FMF-TS3)
CH2FOC*HO ⫹ OH (A-FMF-TS1)
C*H2FOCHO ⫹ OH (A-FMF-TS2)
C*H2FOCHO ⫹ OH (A-FMF-TS3)
Reactions for DFMF
CHF2OC*HO ⫹ OH (S1-DFMF-TS1)
C*HF2OCHO ⫹ OH (S1-DFMF-TS2)
CHF2OC*HO ⫹ OH (S2-DFMF-TS1)
C*HF2OCHO ⫹ OH (S2-DFMF-TS2)
CHF2OC*HO ⫹ OH (A1-DFMF-TS1)
C*HF2OCHO ⫹ OH (A1-DFMF-TS2)
Reactions for TFMF
CF3OCHO ⫹ OH (syn)
CF3OCHO ⫹ OH (anti)

⌬H r
(298 K)

2.88
2.59
4.88
0.53
2.84
4.91

⫺18.3
⫺16.4

3.33
4.25
2.87
5.70
2.59
6.25

⫺17.7
⫺12.5
⫺19.0
⫺15.0
⫺21.7
⫺13.3

4.11
3.00

⫺17.7
⫺21.3

⫺22.8
⫺17.5

Asterisk (*) indicates the hydrogen abstraction site (see Figs. 4 and 5).
Heats of reaction [⌬H r (298 K)] and barrier heights (⌬E #0 ) are in kcal/mol.

any contribution to the total reaction rate. Similarly, the hydrogen
abstraction reactions (R2b) from the OCHF2 group of syn-1,
syn-2, and anti-1 conformers of DFMF have much larger barrier
height and should not make any contribution to the total reaction
rate. It should be mentioned here that activation entropies (⌬S # )
are always more negative for the hydrogen abstractions (R2b) from
the OCHF2 group than the hydrogen abstractions (R2a) from the
formyl group. Thus, even the entropy factor is not favorable for the
hydrogen abstraction from the OCHF2 group of DFMF. The
barrier height for hydrogen abstraction is seen to increase with
successive substitution of methyl hydrogen by fluorine. For example, the barrier heights for hydrogen abstraction from the formyl
group of syn-CH3OCHO, syn-CH2FOCHO, syn-1-CHF2OCHO,
and syn-CF3OCHO amount to 2.4, 2.9, 3.3, and 4.1 kcal/mol,
respectively. The heats of reaction calculated from the G2(MP2)
results are given in Table 4. Hydrogen abstraction from the carbonyl group is found to be much more exothermic than that from
the fluoromethyl group of FMF and DFMF. This is because the
energies for CHFCHO and CF2OCHO radicals are higher than the
energies of CH2FOCO and CHF2OCO, respectively. Among the
conformers of FMF and DFMF, heats of reaction are always higher
for the anti conformer.

Rate Constants

Using simple standard transition-state theory (TST),25 the rate
constant for hydrogen abstraction reaction was calculated from the
expression

k BT Q TS ⫺⌬E#/RT
e 0
h Q AQ B

(1)

where ⌬E #0 is the barrier height calculated from the energy difference (including the ZPEs) between the TS and the two reactants,
T is the temperature, and Q’s are the respective partition functions.
The ⌫, k B , and h in eq. (1) stand for the transmission coefficient
for tunneling, the Boltzmann constant, and Planck’s constant,
respectively. The tunneling factor, ⌫, was calculated by using
Eckart’s method for unsymmetrical one-dimensional potential barriers.26 In the Eckart’s method, the tunneling factor is estimated as
an integrated sum of the energy dependent transmission probability, ␬(E). The barrier heights given in Table 4 and the imaginary
frequencies (scaled by 0.9496) for the TSs mentioned in Table S1
were used for tunneling correction. As described in the section of
“Computational Details,” we observed during our studies on the
hydrogen abstraction reactions of CF3OCHO and CH3OCHO that
the rate constants calculated from the TST equation in conjunction
with the tunneling correction by Wigner’s method were somewhat
lower than the corresponding experimental values. On the other
hand, the rate constants calculated from the TST equation but with
tunneling correction by Eckart’s method were in very good and
satisfactory agreement with the experimental values.13 Thus we
used the same procedure (i.e., TST with Eckart’s method of
tunneling correction) for the present study. The partition functions
were evaluated with the rigid-rotator and quantum harmonic approximation27 using the scaled frequencies at the (U)MP2(full)/6311G(d,p) level. The electronic partition function of the OH radical was evaluated by taking into account the splitting of 139.7
cm⫺1 in the 2⌸ ground state.28
It is evident from the barrier heights that both syn- and antiFMF have two competitive hydrogen abstraction channels; one
channel is originated from the formyl site (S-FMF-TS1 and AFMF-TS1) and the other channel (S-FMF-TS2 and A-FMF-TS2) is
from the fluoromethyl group, which both contribute to the total
reaction rate. As discussed before, the contribution from the anti
conformer of FMF and DFMF will slowly rise with the increase in
temperature and its contribution should also be accounted for in
the total reaction rate. The total hydrogen abstraction rate constants
(k total) for FMF and DFMF were, therefore, calculated from the
weighted sum of the individual rate constants of each channel, as
expressed in eqs. (2) and (3):
k total共FMF兲 ⫽ wsyn k共syn兲 ⫹ wanti k共anti兲

(2)

Table 5. Total Rate Constants (in cm3 molecule⫺1 s⫺1) for Hydrogen

Abstraction Reactions between Formates and OH Radicals at 298 K.
Molecule
CH3OCHO (MF)
CH2FOCHO (FMF)
CHF2OCHO (DFMF)
CF3OCHO (TFMF)
a

Ref. 17.
Ref. 12.

b

Calculated

Experimental

1.8 ⫻ 10⫺13
5.9 ⫻ 10⫺14
3.6 ⫻ 10⫺14
2.0 ⫻ 10⫺14

(1.73 ⫾ 0.21) ⫻ 10⫺13 a

(1.68 ⫾ 0.20) ⫻ 10⫺14 b

406

Chandra et al.

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Vol. 24, No. 3

k total共DFMF兲 ⫽ wsyn-1 k共syn-1兲 ⫹ wsyn-2 k共syn-2兲 ⫹ wanti-1 k共anti-1兲

•

Journal of Computational Chemistry

(3)

where wsyn and wanti are the weight factors for the syn and anti
conformers calculated from the free energy difference between
them. The k(syn) and k(anti) are the individual hydrogen abstraction rate constants for syn and anti conformers, respectively, at
a particular temperature. In the case of DFMF, the contribution
from the anti-2 conformer is not considered in eq. (3), because
the population of anti-2 is negligibly small within the temperature region studied here. In the case of FMF, k(syn) and k(anti)
include contributions from both the hydrogen abstraction reaction channels (R1a) and (R1b). For DFMF, the hydrogen abstraction takes place mainly from the formyl site [reaction
(R2a)], because contribution for hydrogen abstraction from the
OCHF2 site is much lower. For example, the k(R2a) is found to
be almost 18 times higher than k(R2b) at 298 K for the syn-1DFMF, while the k(R2a)/k(R2b) for the anti-1-DFMF amounts to
98. Thus k values for each conformer in eq. (3) primarily
correspond to the rate constant for the hydrogen abstraction
from the formyl group.
The calculated rate constant values for FMF and DFMF at 298
K are reported in Table 5. The values for CH3OCHO and
CF3OCHO are also given in Table 5 for comparison. It should be
noted here that even at room temperature both syn and anti
conformers of CF3OCHO contribute to the total reaction rate,
whereas contribution from the anti conformers is negligible for the
other three formates at lower temperature. The branching ratio of
the two reactions (R1a) and (R1b) of FMF can be calculated from
the ratio of the two rate constant values [k (R1a)/k (R1b)] at any
temperature. For example, the k (R1a) and k (R1b) amount to 4.8 ⫻

Figure 6. Arrhenius plot for the total and individual rate constants (in
cm3 molecule⫺1 s⫺1) of the hydrogen abstraction reaction of OH
radicals with syn and anti conformers of CH2FOCHO (FMF). The total
rate constant is estimated from the weighted sum [eq. (2)] of the rate
constants for the syn and anti-CH2FOCHO.

Figure 7. Arrhenius plot for the total and individual rate constants (in
cm3 molecule⫺1 s⫺1) of the hydrogen abstraction reaction of OH
radicals with syn-1, syn-2, and anti-1 conformers of CHF2OCHO
(DFMF). The total rate constant is estimated from the weighted sum
[eq. (3)] of the rate constants for the three conformers of CHF2OCHO
(DFMF).

10⫺14 and 1.1 ⫻ 10⫺14 cm3 molecule⫺1 s⫺1 at 298 K and this
produces a branching ratio of 4.4. Therefore, OH radical initiated
hydrogen abstraction reaction predominantly (81%) takes place at
the formyl hydrogen of FMF. The [k (R2a)/k (R2b)] value for DFMF
at 298 K is 18.8 and thus almost 95% of the reaction is from the
carbonyl site of DFMF. The rate constant value is found to
decrease with the increase in fluorine substitution at the methyl
position. Figures 6 and 7 display Arrhenius plots for the hydrogen
abstraction rate constants of FMF and DFMF in the temperature
region of 250 to 450 K. Hydrogen abstraction rate from the
carbonyl site of syn-FMF is always found to be larger than that
from the fluoromethyl group. This is because of the lower preexponential factor for the latter. The entropy value for S-FMF-TS2
(83.5 cal/mol-K) is much lower than that for S-FMF-TS1 (88.4
cal/mol-K), due to the presence of strong hydrogen bonding interaction in S-FMF-TS2. Thus the entropy of activation ⌬S # is much
lower for S-FMF-TS2 (⫺31.2 cal/mol-K) than that for S-FMF-TS1
(⫺26.3 cal/mol-K). As a result, the pre-exponential factor (which
depends upon ⌬S # exponentially) is much smaller for the reaction
channel through S-FMF-TS2. In the case of DFMF, hydrogen
abstraction from the carbonyl group of syn-2 and anti-1 conformers is faster than that for the syn-1 conformer. However, due to
higher population, syn-1 always remains the main contributor
toward the total hydrogen abstraction rate constant. Arrhenius
expressions for the rate constants of FMF and DFMF within the
temperature range of 250 – 450 K are found to be 8.3 ⫻
10⫺13exp(⫺773/T) and 8.0 ⫻ 10⫺13exp(⫺908/T) cm3 molecule⫺1 s⫺1, respectively.

Fluorinated Formates and Hydrogen Abstraction Reaction

Summary
The structures and vibrational frequencies for both the syn and anti
conformers of FMF and DFMF have been reported for the first
time. The syn conformer is found to be lower in energy. However,
the energy difference between the syn and anti conformers and the
barrier to interconversion between them are found to decrease with
the increase in fluorine substitution at the methyl position of
formates. The energy differences between the most stable syn and
anti conformers of FMF and DFMF are calculated to be 3.0 and
2.4 kcal/mol at the G2(MP2) level. Thus the experimental detection of anti-FMF and -DFMF may be possible at an elevated
temperature. The kinetics and mechanism of the hydrogen abstraction reactions of OH radical with FMF and DFMF have been
studied. The reaction rate is always found to be higher for the
hydrogen abstraction from the carbonyl site of the anti conformer,
primarily due to the lower COH bond strength. However, owing
to much larger population, the syn conformer of FMF and DFMF
is always found to be the main contributor to the total rate constant.
Arrhenius expressions for the temperature dependent rate constants of FMF and DFMF are estimated to be 8.3 ⫻
10⫺13exp(⫺773/T) and 8.0 ⫻ 10⫺13exp(⫺908/T) cm3 molecule⫺1 s⫺1, respectively.

9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.

Acknowledgments
A.K.C. thanks RITE, Japan, for providing him with a senior
researcher position.
20.
21.

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