Absolute rate constants for the reactions of Cl atoms with CH3Br, CH2Br2, and CHBr3

Article abstract of DOI:10.1021/jp9719671

The rate constants for the reactions of chlorine atoms with the complete series of the three bromomethanes CH₃Br (1), CH₂Br₂ (2), and CHBr₃ (3) were measured in a very low pressure reactor, employing a microwave discharge for the generation of Cl atoms with mass spectrometric detection of reactants and products. The experiments were performed in the temperature range 273-363 K and at total pressures approximately 1 m Torr. The reactions proceed via hydrogen atom transfer leading to HCl product and the corresponding bromomethyl radicals. Their rate constant expressions are (in cm³ molecule^-1 s^-1): k₁ = (1.66±0.14)×10^-11 exp(-1072±46/T), k₂ = (0.84±0.15)×10^-11 exp(-911±101/T), and k₃ = (0.43±0.11)×10^-11 exp(-809±142/T). The activation energy of the reaction decreases with additional bromine substitution, which is attributed to the gradual weakening of the corresponding C-H bond strength. Ab initio theoretical calculations performed at the MP2/6-31++G(2d,2p) level of theory suggest C-H bond strengths for CH₃Br, CH₂Br₂, and CHBr₃ of 416.58, 407.03, and 396.60 kJ mol^-1, respectively.

Full text of DOI:10.1021/jp9719671

8496
J. Phys. Chem. A 1997, 101, 8496-8502
Absolute Rate Constants for the Reactions of Cl Atoms with CH3Br, CH2Br2, and CHBr3
Kyriakos G. Kambanis, Yannis G. Lazarou, and Panos Papagiannakopoulos*
Department of Chemistry, UniVersity of Crete, Heraklion 714 09, Crete, Greece
X
ReceiVed: June 17, 1997
The rate constants for the reactions of chlorine atoms with the complete series of the three bromomethanes
CH Br (1), CH Br (2), and CHBr (3) were measured in a very low pressure reactor, employing a microwave
3 2 2 3
discharge for the generation of Cl atoms with mass spectrometric detection of reactants and products. The
experiments were performed in the temperature range 273-363 K and at total pressures ∼1 mTorr. The
reactions proceed via hydrogen atom transfer leading to HCl product and the corresponding bromomethyl
3
-1 -1
-11
radicals. Their rate constant expressions are (in cm molecule s ): k
1
) (1.66 ( 0.14) × 10
exp(-
-
11
-11
1
(
072 ( 46/T), k
2
) (0.84 ( 0.15) × 10 exp(-911 ( 101/T), and k ) (0.43 ( 0.11) × 10 exp(-809
3
142/T). The activation energy of the reaction decreases with additional bromine substitution, which is
attributed to the gradual weakening of the corresponding C-H bond strength. Ab initio theoretical calculations
2
performed at the MP2/6-31++G(2d,2p) level of theory suggest C-H bond strengths for CH Br, CH Br ,
3
2
-
1
3
and CHBr of 416.58, 407.03, and 396.60 kJ mol , respectively.
Introduction
Bromine atoms are considered very effective in consuming
TABLE 1: Rate Constants for the Reactions of Cl Atoms
with CH Br, CH Br , and CHBr
3
2
2
3
1
3
11
1,2
10 k298
10 A
E
a
/R
T range
ref
stratospheric ozone; therefore, the concentrations and chemical
transformations of their atmospheric precursors have to be prop-
erly evaluated. Although biogenic processes release bromine
containing organic compounds into the atmosphere, man-made
contributions are suspected to contibute significantly, as several
bromocompounds (CF3Br, CF2Br2, CH3Br) have found a variety
of applications (fire extinguishers, pesticides, fumigants). The
most abundant gas-phase bromocompound is CH3Br, which has
CH
3
Br
5
4
4
4
.53 ( 1.7 3.16 ( 0.63
.16 ( 0.14 1.55 ( 0.18
.45 ( 0.60 1.78 ( 0.25
1205 ( 69
1070 ( 50
1095 ( 60
273-368
222-393.5
231-296
8
9
10
11
this work
[3.2 × 10 T1.26 exp(-670/T)] 197-690
-15
.40
4.83 ( 0.12 1.66 ( 0.14
1072 ( 46
273-363
2 2
CH Br
5.30 ( 1.6 9.53 ( 1.8
4.17 ( 0.08 0.64 ( 0.06
4.42 ( 0.60 0.97 ( 0.15
1547 ( 68
810 ( 50
906 ( 80
911 ( 101
273-368
222-394.5
231-295
273-363
8
9
1
,3
natural as well as anthropogenic sources,
followed by
4
10
this work
CH2Br2 and CHBr3, which are biogenic in origin. The atmos-
pheric lifetime of the hydrogen-containing bromocompounds
CH3Br and CH2Br2 is primarily determined by degradation
processes at tropospheric heights, mostly by their reactions with
OH radicals. In the case of CHBr3, which absorbs at the UV-A
region of solar spectrum, rapid tropospheric photolysis may
constitute its major removal pathway. However, at stratospheric
heights, photolysis becomes the dominant degradation process
of bromocompounds, leading to production of bromine atoms.
Chlorine atoms are also important stratospheric species, and
they participate extensively in ozone destruction cycles. Their
4
.20 ( 0.21 0.84 ( 0.15
CHBr
809 ( 142
3
3
.04 ( 0.24 0.43 ( 0.11
273-363
this work
atoms and mass spectrometric monitoring of reactants and
products. The thermostated cylidrical reactor (Vcell ) 168 cm )
was internally coated by a thin film of Teflon (Du-Pont Teflon
3
1
20) to inhibit the recombination of Cl atoms and radicals on
the reactor walls and had two capillary inlets for the admission
of each reactant. The reaction mixture was escaping through a
cyclic orifice (5 mm diameter) to the first stage of a differentially
pumped chamber. Thus, an effusive molecular beam was
formed, which was collimated by a conical skimmer and entered
the second vacuum chamber. It was further modulated by a
tuning fork chopper with a frequency of 200 Hz and was
analyzed with a quadrupole mass spectrometer (BALZERS
QMG 511, electron impact ion source). A lock-in amplifier
5
tropospheric significance is considered to be low, although it
has been proposed that they attain significant concentrations in
coastal areas, generated from heterogeneous processes on sea-
salt aerosols.6 The gas-phase reactions of chlorine atoms are
often used as a probe of the reactivity of several atmospheric
species and the facile generation of the corresponding free
radicals via hydrogen atom abstraction.
,7
(
NF LI-570) was used to select and amplify the modulated
component of the mass spectrometric signal. A microcomputer
DEC microPDP-11) was used to collect the output of the lock-
The reactions of Cl atoms with CH3Br and CH2Br2 have been
studied by several investigators,8 and the corresponding rate
constants are shown in Table 1. However, the reaction of Cl
with CHBr3 has not been studied in the past. The present study
attempts to extend the kinetic information to CHBr3 and allows
the examination of the bromomethanes reactivity as a function
of bromine substitution in a more systematic way.
-11
(
in amplifier and control the operation of the mass spectrometer.
-1
The flow rates (in molecule s ) of the reactants were
determined from the pressure drop in their buffer volumes as
they were flowing into the reactor through long capillary
resistances. The escape rate constant kesc,M of several species
out of the reactor was measured as a function of their molecular
weight M using their mass spectrometric signal first-order decay
after a fast halt of their flow. Thus, kesc,M was given by the
Experimental Section
The reactions were studied in a very low pressure reactor
1
/2
-1
1
2
expression 1.86 (T/M) (in s ), where T is the reactor
temperature. The total pressure inside the reactor was calculated
to be in the range 0.6-1.5 mTorr, and the partial pressure of
(
VLPR), employing a microwave discharge as a source of Cl
X
Abstract published in AdVance ACS Abstracts, October 15, 1997.
S1089-5639(97)01967-1 CCC: $14.00 © 1997 American Chemical Society

Reactions of Cl Atoms with Bromomethanes
J. Phys. Chem. A, Vol. 101, No. 45, 1997 8497
2 2
Figure 1. Plot of (R - 1)kesc,Cl vs [CH Br ] at T ) 303 K. Error bar reflects the propagated error (2σ). Solid line is the linear least-squares fit to
the data.
TABLE 2: Mass Spectra of CH
3
Br, CH
2
Br
2
, and CHBr
3
at Two Electron Energies of 19 and 70 eV. Intensities Are Reported
CH Br
Relative to the Most Intense Peak Intensity
3
mass
12
13
14
15
79
80
81
82
91
92
93
94
95
96
1
9 eV
99.7
100
3.5
4.4
100
26.7
3.5
3.6
98.3
23.9
70 eV
0.9
2.6
5.3
3.5
0.6
3.5
0.6
2.0
1.1
2 2
CH Br
mass
12
13
14
79
21.1
80
81
82
91
15.4
93
95
172
174
176
1
9 eV
0 eV
0.3
21.3
100
100
95.7
76.2
11.0
4.6
21.4
9.4
9.1
4.1
7
4.5
10.6
2.0
21.0
2.0
CHBr
82
3
mass
12
13
14
3.2
79
80
81
91
100
92
44.0
93
97.5
94
171
173
175
1
9 eV
0 eV
16.7
85.2
8.7
9.4
22.7
90.7
10.9
9.1
43.5
18.6
100
43.8
54.7
21.7
7
17.8
44.8
48.7
helium was 0.5 mTorr. The residence time of a reactant in the
reactor, given by the reciprocal of the sum of the rate constants
of its loss processes (escape out of the reactor and chemical
reactions), was calculated to be less than 0.2 s for Cl atoms,
while for CH3Br, CH2Br2, and CHBr3 it was less than 0.3, 0.4,
and 0.5 s, respectively.
and 97%, respectively, and they were frequently subjected to
degassing procedures. The liquid reactants CH2Br2 and
CHBr3 were kept in darkened bulbs, above molecular sieves
(0.4 nm) that absorb moisture and ethanol, used as a stabilizer
for CHBr3. Their purity was checked by mass spectrometry,
and in all cases no decomposition products were found;
35
+
Chlorine atoms were produced by a microwave discharge in
a mixture of 5% Cl2 in helium flowing through a quartz tube
coated with a phosphoric-boric acid mixture in order to in-
hibit wall recombination of chlorine atoms. The conversion of
Cl2 to Cl atoms and HCl molecules (as a side product inside
the discharge tube) was always complete as verified by the
however, two minor peaks at m/e 49 and 51 (CH2 Cl and
3
7
+
CH2 Cl , respectively) in the mass spectrum of CH2Br2 were
attributed to CH2ClBr impurity, which could not be removed.
The mass spectra of CH3Br, CH2Br2, and CHBr3 at two electron
energies (70 and 19 eV) are displayed in Table 2. The parent
peak at m/e 94 was used to monitor CH3Br, while the most
+
+
+
absence of m/e 70 (Cl2 ) in the mass spectra. The electron
intense peaks at m/e 93 (CH2Br ) and 173 (CHBr2 ) were used
to monitor CH2Br2 and CHBr3, respectively; these peaks are
not likely to have contributions from the mass spectrometric
fragmentation of the corresponding free radical products of their
reactions with Cl atoms. The correlation of the mass spectro-
metric peak intensity IS of a species with its steady-state
energy of the ion source was set to 19 eV in order to sup-
+
press HCl fragmentation to m/e 35 (Cl ) to negligible levels
(
∼0.3%) and thus eliminate the mass spectrometric inter-
ference of HCl in the measurements of Cl atoms. The stated
purities of CH3Br, CH2Br2, and CHBr3 were >99.5%, >99%,

8498 J. Phys. Chem. A, Vol. 101, No. 45, 1997
Kambanis et al.
TABLE 3: Rate Constants for the Reactions of Cl Atoms
Results
3 2 2 3
with CH Br, CH Br , and CHBr , Measured at
The mass spectrometric analysis of the reaction products for
the three bromomethanes reveals an increase of the peak at m/e
36 that is attributed to HCl product. In the case of CH3Br and
CH2Br2, there was no evidence for the production of Br atoms
Temperatures of 273, 303, 333, and 363 K
rate constant
(1013 cm molecule s-1
3
-1
temp (K)
)
no. of points
Reaction Cl + CH
3
Br
(at m/e 79, 80) or BrCl molecules (at m/e 114, 116, 118),
273
303
333
363
3.21 ( 0.30
17
11
15
10
indicating that bromine substitution or abstraction pathways were
not occurring. However, in the case of CHBr3, small peaks at
m/e 114, 116, and 118 were observed, while the intensity of
4.83 ( 0.12
6.75 ( 0.63
8.62 ( 0.35
7
9
35
+
Reaction Cl + CH
2.91 ( 0.20
2
Br
2
the weak peak at m/e 114 ( Br Cl ) was ca. 4% of the
intensity loss of Cl atoms when CHBr3 was added. The
calibration factor for the parent peak of BrCl at m/e 114 was
measured to be 1.29 ( 0.09 times higher than that of Cl atoms
at m/e 35 (vide infra); thus, a yield of ca. 3% is derived for the
bromine abstraction pathway in the reaction of CHBr3 with Cl.
Since the yield of BrCl is small, the kinetic effects of its possible
secondary reaction with Cl atoms were considered negligible.
Moreover, in all three cases the HCl product yield was found
always equal to the Cl atoms consumption, within an accuracy
of 10%. Therefore, the dominant pathway for the three reactions
is the abstraction of a hydrogen atom:
273
303
333
363
28
70
19
48
4.20 ( 0.21
5.59 ( 0.56
6.72 ( 0.34
Reaction Cl + CHBr
2.12 ( 0.25
3
273
303
333
363
15
49
14
16
3.04 ( 0.24
3.90 ( 0.54
4.56 ( 0.27
concentration [S] was determined by calibration plots of the
expression
CH Br + Cl f CH Br + HCl
(1)
(2)
(3)
3
2
I ) R Vcellkesc,S[S]
(I)
S
S
CH Br + Cl f CHBr + HCl
2
2
2
where RS is a characteristic mass spectrometric calibration factor.
The mass spectrometric intensity measurements had a deviation
of (5% (2σ).
CHBr + Cl f CBr + HCl
3
3
All bromomethanes were introduced into the reactor undi-
luted. The flow of chlorine atoms was always kept constant,
by maintaining the Cl2/He mixture in the buffer volume at a
certain pressure, while the flow of bromomethanes to the reactor
was controlled by varying the pressure in their buffer volumes
accordingly.
In addition, there was no mass spectrometric evidence for
radical recombination reactions either with Cl atoms or with
themselves; therefore, the kinetic scheme is free from secondary
reactions complications. The parallel reaction of Cl atoms with
the CH2ClBr impurity in CH2Br2 was ignored since its rate
-
13
3
-1 -1 8
constant was reported to be 4.18 × 10 cm molecule
s
Figure 2. Arrhenius plot for reaction of Cl atoms with CH Br, filled circles; error bars reflect the propagated errors (2σ); solid line is the linear
3
least-squares fit to the data. Dashed line drawn from rate parameters in ref 8; triangles and rhombohedrons represent the results of refs 9 and 10,
respectively. Dotted line drawn from the non-Arrhenius expression in ref 11.

Reactions of Cl Atoms with Bromomethanes
J. Phys. Chem. A, Vol. 101, No. 45, 1997 8499
2 2
Figure 3. Arrhenius plot for reaction of Cl atoms with CH Br ; filled circles; error bars reflect the propagated errors (2σ); solid line is the linear
least-squares fit to the data. Dashed line drawn from rate parameters in ref 8; triangles and rhombohedrons represent the results of refs 9 and 10,
respectively.
Figure 4. Arrhenius plot for reaction of Cl atoms with CHBr . Error bars reflect the propagated errors (2σ), solid line is the linear least-squares
3
fit to the data.
(
in the order of k2) and its concentration is 100 times lower
escape rate out of the reactor; thus, a steady-state concentration
of Cl atoms is maintained, expressed as
than that of CH2Br2.
Derivation of the Kinetic Equations. In the absence of any
reaction, the flow of Cl atoms in the reactor must equal their
F /V ) kesc,Cl[Cl]o
Cl cell
(II)

8500 J. Phys. Chem. A, Vol. 101, No. 45, 1997
Kambanis et al.
a,b
In the presence of a species RH capable of reacting with Cl
atoms, the flow of Cl atoms in the reactor equals their escape
rate plus their reaction rate k, expressed as
TABLE 4: Optimized Structural Parameters, Vibrational
a,c
d
3
Frequencies, and Zero-Point Energies (ZPE) for CH ,
CH
3
Br, CH
2
2
Br, CH Br
2
, CHBr
2
, CHBr
3 3
, and CBr
species
geometrical parameters
vibrational frequencies ZPE
F /V ) kesc,Cl[Cl] + k[Cl] [RH]
(III)
Cl cell
r
r
CH
CH
C-H, 1.074
523.2, 1504.5,
1507.1, 3253.3,
3437.9, 3460.9
586.1, 972.7,
973.5, 1359.4,
1483.1, 1484.3,
3173.1, 3296.9,
72.86
88.60
3
3
∠
H-C-H, 120.00
where subscripts o and r refer to absence and presence of RH,
respectively. Since the flow of chlorine atoms was always kept
constant, application of the steady-state approximation for Cl
atoms and reaction 1 leads to
τ H-C(-H)-H, 180.00
Br C-Br, 1.992; C-H, 1.083
∠
∠
H-C-H, 111.87
H-C-Br, 106.95
τ H-C(-Br)-H, 120.00
3
314.5
133.8, 677.0,
969.9, 1462.3,
[
Cl]ok
) [Cl]rk
+ k [Cl] [CH Br]
(IV)
esc,Cl
esc,Cl
1
r
3
CH
CH
2
2
Br C-Br, 1.889; C-H, 1.069
53.97
66.97
∠
∠
H-C-H, 125.40
H-C-Br, 117.31
3356.3, 3538.4
After minor rearrangement, and expressing the difference ([Cl]o
[Cl]r) as ∆[Cl], the above equation becomes
τ H-C(-Br)-H, 180.00
C-Br, 1.969; C-H, 1.080
-
Br
2
168.0, 569.4,
630.5, 805.1,
1149.5, 1251.0,
1428.1, 3234.1,
3344.0
∠
H-C-H, 114.04
∠ H-C-Br, 107.83
Br-C-Br, 111.55
∆
[Cl] kesc,Cl ) k [Cl] [CH Br]
(V)
1
r
3
∠
which is transformed into the final equation, considering the
linear dependence of the mass spectrometric intensity on the
steady-state concentration (expression I)
τ H-C(-Br)-H, 123.57
τ H-C(-Br)-Br, 118.21
C-Br, 1.883; C-H, 1.070
CHBr
CHBr
2
3
186.6, 392.2,
610.2, 780.4,
1256.2, 3459.3
35.59
43.03
∠
∠
H-C-Br, 117.36
Br-C-Br, 119.32
(
R - 1)kesc,Cl ) k [CH Br]
(VI)
1
3
τ H-C(-Br)-Br, 152.17
C-Br, 1.960; C-H, 1.079
∠ H-C-Br, 107.90
151.2, 151.7,
215.6, 519.2,
655.8, 658.3,
1201.1, 1202.0
3329.1
164.3, 164.8,
183.0, 302.3,
818.3, 821.7
where R denotes the mass spectrometric intensity ratio ICl,o/
ICl,r.
∠ Br-C-Br, 111.00
τ H-C(-Br)-Br, 118.03
τ Br-C(-Br)-Br, 123.94
C-Br, 1.884
Kinetic Results. The experimental runs for reaction 1 were
performed by monitoring the drop in Cl atoms signal intensity
as CH3Br reactant was alternately added or withheld. The peak
CBr
3
13.07
∠
Br-C-Br, 118.10
+
intensity at m/e 94 (CH3Br ) was simultaneously measured and
τ Br-C(-Br)-Br, 152.95
was correlated with the steady-state concentration [CH3Br] via
the calibration plots. The experiments were performed at four
temperatures of 273, 303, 333, and 363 K. Similar experimental
procedures were performed for the corresponding reactions of
CH2Br2 and CHBr3. The steady-state Cl atoms concentrations
a
At the 3-21++G(2d,2p) level of theory, including second-order
Br, CH Br
Bond lengths in angstroms, bond (∠ ) and dihedral angles
Møller-Plesset perturbation (MP2, frozen core) for CH
and CHBr
(τ) in degrees. Scaled by 0.89; in cm
scaled by 0.89; in kJ mol
3
2
2
,
b
3
.
c
-1 d
.
Using vibrational frequencies
-
1
.
11
-3
11
ranged from 1.5 × 10 molecules cm to 5.5 × 10 molecules
-
3
12
Application of the steady-state approximation for Cl atoms and
BrCl molecules leads to
cm , while those of bromomethanes ranged from 1.0 × 10
-
3
13
-3
molecules cm to 2.5 × 10 molecules cm . A typical plot
of (R - 1)kesc,Cl vs [CH2Br2] at 303 K is shown in Figure 1.
The slopes of the linear least-squares fits to the data provided
the rate constants, and the results obtained for the three reactions
are listed in Table 3. The corresponding Arrhenius plots in the
temperature range 273-363 K are shown in Figures 2, 3, and
∆
[Cl]kesc,Cl ) [BrCl]kesc,BrCl
Using expression I, the above equation becomes
∆I R ) IBrCl RCl
(VII)
Cl BrCl
4
, respectively, and the rate constants derived are (2σ uncertain-
ties, in cm molecule s )
3
-1 -1
Thus, the ratio RBrCl/RCl can be determined by the ratio of the
BrCl product intensity at m/e 114 to the corresponding drop in
Cl atoms intensity at m/e 35. An average of five measurements
gave a RBrCl/RCl ratio of 1.29 ( 0.09. In addition, the mass
spectrum of BrCl at an electron energy of 19 eV was determined
to be (relative intensities in parentheses): 35 (3.9), 37 (1.2), 79
(12.1), 81 (12.1), 114 (82.7), 116 (100), and 118 (22.4).
-11
k1 ) (1.66 ( 0.14) × 10 exp(-1072 ( 46/T)
-11
k2 ) (0.84 ( 0.15) × 10 exp(-911 ( 101/T)
-11
k3 ) (0.43 ( 0.11) × 10 exp(-809 ( 142/T)
Discussion
Measurement of the Calibration Factor of BrCl at m/e
14. Separate experiments were performed in order to measure
The rate parameters of the reactions of Cl atoms with
CH3Br and CH2Br2 are in very good agreement with the results
of three recent experimental studies,
agreement (especially for CH2Br2) with an earlier study, as can
be seen in Table 1. The results show a systematic trend toward
a decrease of the activation energy, the preexponential A factor,
and the rate constant as a function of increasing degree of
bromine substitution in bromomethanes. A similar decline of
the A factor and the activation energy is observed for the
corresponding OH radical reactions, although the rate constants
1
the mass spectrometric calibration factor of BrCl at m/e 114
relative to that of Cl atoms, at an electron energy of 19 eV, by
using the fast reaction of Cl atoms with Br2 molecules (k4 )
9
-11
and in rather poor
8
1.20 ( 0.15) × 10-10 cm3 molecule
-1 -1
13
(
s
at 298 K ):
Br + Cl f BrCl + Br
(4)
2
A great excess of Br2 was used in order to minimize the
contribution of the slower secondary reaction (k5 ) (1.45 (
-
11
3
-1 -1
13
14
0
.20) × 10
cm molecule
s
at 298 K ):
show an inverse trend. The decrease of the preexponential
factors as the number of bromine atoms is increasing can be
explained by the smaller entropic differences between reactants
BrCl + Cl f Cl + Br
(5)
2

Reactions of Cl Atoms with Bromomethanes
J. Phys. Chem. A, Vol. 101, No. 45, 1997 8501
0
) and Total Enthalpies at 0.0 and 298.15 K (H , H298) of All Species, Calculated at
TABLE 5: Total Electronic Energies (E
0
-
1
MP2/3-21++G(2d,2p) and MP2/6-31++G(2d,2p) Levels of Theory (in hartrees; 1 hartree ) 2625.5 kJ mol
)
total energy
MP2/3-21++G(2d,2p)
MP2/6-31++G(2d,2p)
species
E
0
H
0
H
298
E
0
H
0
H
298
H
Br
-0.497 800
-2560.500 308
-39.474 648
-2600.085 232
-2599.416 225
-5160.026 944
-5159.358 743
-7719.967 449
-7719.301 906
-0.497 800
-2560.500 308
-39.446 897
-2600.051 486
-2599.395 670
-5160.001 437
-5159.345 187
-7719.951 057
-7719.296 929
-0.495 440
-2560.497 948
-39.442 850
-2600.047 341
-2599.390 963
-5159.996 439
-5159.340 101
-7719.944 721
-7719.290 569
-0.498 801
-2570.138 216
-39.708 159
-2609.969 580
-2609.301 844
-5179.561 064
-5178.897 731
-7749.154 037
-7748.495 150
-0.498 801
-2570.138 216
-39.680 408
-2609.935 834
-2609.281 290
-5179.535 556
-5178.884 176
-7749.137 646
-7748.490 173
-0.496 441
-2570.135 856
-39.676 361
-2609.931 689
-2699.276 582
-5179.530 559
-5178.879 089
-7749.131 309
-7748.483 813
CH
CH
CH
CH
CHBr
CHBr
3
3
2
2
Br
Br
Br
2
2
3
CBr
3
and transition states, mainly determined by their external
rotational degrees of freedom. The concurrent decrease of the
activation energy most possibly reflects the weakening of the
C-H bond strengths. The ratios kCl/kOH of their rate constants
with Cl atoms to those with OH radicals show a decline from
more than an order of magnitude for CH3Br to a factor of almost
TABLE 6: Ab Initio C-H and C-Br Bond Dissociation
-1
3 2 2 3
Energies (in kJ mol ) for CH Br, CH Br , and CHBr ,
Calculated at 0.0 and 298.15 K at the MP2/3-21++G(2d,2p)
and MP2/6-31++G(2d,2p) Levels of Theory. The
Experimental Values Are Calculated from Thermochemical
Data Available (in ref 20)
dissociation energy
2
for CHBr3. Thus, as the number of bromine atoms increases,
the rate constants are becoming less sensitive to the nature of
the attacking radical species. However, these correlations should
also consider the possibility of intermediate adducts formation
between bromomethanes and the incoming Cl atom or OH
radical. The formation of weakly bound adducts between
CH3Br molecule and Cl atom has been proposed on the basis
MP2/3-21++G(2d,2p) MP2/6-31++G(2d,2p)
exptl
bond
0 K
298.15 K
0 K
298.15 K
298 K
BrH2C-H 414.870
Br2HC-H 416.007
422.543
422.438
416.699
279.728
282.315
280.069
408.903
400.598
390.336
307.734
304.690
302.598
416.576 420.5 ( 8.4
407.028 416.7 ( 12
396.595 392.5 ( 12
313.672 293.4 ( 0.8
310.125 289.7 ( 12
305.514 274.6 ( 12
Br3C-H
H3C-Br
410.440
273.791
11
BrH2C-Br 276.880
Br2HC-Br 277.152
of experimental results and is also supported by ab initio
calculations.1
1,15
The experimental CH3Br-Cl bond strength
-
1 11
at 298 K was reported to be 24.5 kJ mol , in accordance
(2d,2p) level of theory, while those of the radical species were
performed at the UHF/3-21++G(2d,2p) level. The calculated
vibrational frequencies were scaled down by the factor 0.89 in
-
1 15
with our ab initio estimation of 28.49 kJ mol . Since the
CH3Br-Cl bond is very weak, collisional stabilization of the
CH3BrCl adduct is likely to be an inefficient process, with
negligible effects on the kinetics of the reaction of Cl atoms
with CH3Br. Indeed, this reaction has been performed at total
pressures ranging from 1 mTorr to almost 1 atm, with almost
no pressure effect on the activation energy.9,10 However, in
the reaction of Cl atoms with CH3I, an increase of the apparent
activation energy with pressure was observed,1 and this was
1
9
order to compensate for anharmonicity effects. Structural
parameters, vibrational frequencies, and zero-point energies of
CH3, CH3Br, CH2Br, CH2Br2, CHBr2, CHBr3, and CBr3 are
shown in Table 4. Single-point energy calculations of all species
at the MP2/3-21++G(2d,2p) and MP2/6-31++G(2d,2p) levels
of theory were performed using their optimized geometries. The
total enthalpies and the C-H and C-Br bond strengths were
calculated at two temperatures (0.0 and 298.15 K), assuming
the rigid-rotor and harmonic oscillator approximations, and the
results are shown in Tables 5 and 6, respectively. The
theoretical results indicate a reduction in C-H bond strength
with increasing bromine substitution, especially with the larger
and more reliable basis set of 6-31++G(2d,2p). Thus, at
standard temperature (298.15 K), the C-H bond strengths of
CH3Br, CH2Br2, and CHBr3 were calculated to be 416.58,
1,16
16
attributed to collisional stabilization of the CH3ICl adduct. The
CH3I-Cl bond strength was calculated by ab initio theoretical
-1 17
methods to be 52.41 kJ mol , almost twice the corresponding
CH3Br-Cl bond strength. Thus, in the CH3BrCl adduct, the
pathway of dissociation back to its constituents is much faster
than its forward pathway of decomposition to CH2Br and HCl
products, considering that a higher energy barrier must be
surmounted in the forward direction and significant molecular
rearrangements must take place. Furthermore, due to the very
low energy barrier in the backward direction, any effect of
collisional stabilization of the adduct should be canceled out
by its collisional energization; thus, the rate back to reactants
should be independent of pressure. As a result, the kinetics of
the overall reaction are not affected by the total pressure.
To examine the effects of bromine substitution on the strength
of the C-H and C-Br bonds, we have performed ab initio
theoretical calculations, using the GAMESS computational
programs package.18 Restricted Hartree-Fock (RHF) and
unrestricted Hartree-Fock (UHF) wave functions were used
for closed and open shell species, respectively. The 3-21G and
-
1
407.03, and 396.60 kJ mol , respectively. Considering the
currently accepted heats of formation of bromomethanes and
20
bromomethyl radicals, the C-H bond strengths derived (420.5
-
1
( 8.4, 416.7 ( 12, and 392.5 ( 12 kJ mol , respectively) are
in very good agreement with the theoretical estimates of the
present study.
The calculated C-Br bond strengths at the MP2/6-31++G-
(2d,2p) level of theory show a similar effect of bond weakening
upon increasing bromine substititution, unlike the results at the
lower MP2/3-21++G(2d,2p) level, which show no particular
trend. Using the higher level of MP2/6-31++G(2d,2p), the
computed C-Br bond strengths at 298.15 K were calculated to
-
1
6-31G basis sets were employed, augmented by adding two sets
be 313.67, 310.13, and 305.51 kJ mol for CH3Br, CH2Br2,
and CHBr3, respectively. In comparison, the corresponding
of polarization functions and diffuse functions to all atoms.
Second-order Møller-Plesset perturbation theory (MP2, frozen
core) was used in order to take correlation effects into account.
No symmetry constraints were imposed in all calculations.
Geometry optimizations and vibrational frequency calculations
of bromomethanes were performed at the MP2/RHF/3-21++G-
2
0
C-Br bond strengths, derived from thermochemical data
-
1
(293.4 ( 0.8, 289.7 ( 12, and 274.6 ( 12 kJ mol ,
-
1
respectively) are ∼20 kJ mol lower. However, the relatively
small deviation of the theoretical predictions from the experi-
mentally determined values should be considered acceptable

8502 J. Phys. Chem. A, Vol. 101, No. 45, 1997
Kambanis et al.
within the chemical accuracy of ab initio molecular mechanics
calculations at these levels of theory. In conclusion, the
theoretical results of this study are providing strong support for
the currently available thermochemical data of bromine-contain-
ing compounds.
(7) Keene, W. C.; Jacob, D. J.; Fan, S.-M. Atmos. EnViron. 1996, 30,
i-iii.
(
8) Tschuikow-Roux, E.; Faraji, F.; Paddison, S.; Niedzielski, J.;
Miyokawa, K. J. Phys. Chem. 1988, 92, 1488-1495.
9) Gierczak, T.; Goldfarb, L.; Sueper, D.; Ravishankara, A. R. Int. J.
(
Chem. Kinet. 1994, 26, 719-728.
The tropospheric degradation of CH3Br and CH2Br2 via their
reactions with Cl atoms may contribute significantly only at
the marine boundary layer, considering the possibility of ele-
vated Cl atom concentrations near the sea surface, as it has
already been discussed.9 In the case of CHBr3, its reaction with
the more abundant OH radicals is only a factor of 2 slower
than with Cl atoms, and its photolysis is probably an important
tropospheric loss process. Therefore, the contibution of its
reaction with Cl atoms to the tropospheric chemistry of CHBr3
is expected to be small, since the competing pathways (reaction
with OH radicals, photolysis) are much more effective.
(10) Orlando, J. J.; Tyndall, G. S.; Wallington, T. J.; Dill, M. Int. J.
Chem. Kinet. 1996, 28, 433-442.
(11) Piety, C. A.; Nicovich, J. M.; Ayhens, Y. V.; Estupinan, E. G.;
Soller, R.; McKee, M. L.; Wine, P. H. Proceedings of the 14th International
Symposium on Gas Kinetics; University of Leeds: Leeds, U.K., 1996.
(12) Lazarou, Y. G.; Michael, C.; Papagiannakopoulos, P. J. Phys. Chem.
1992, 96, 1705-1708.
14
(13) Clyne, M. A. A.; Cruse, H. W. J. Chem. Soc., Faraday Trans. 2
972, 68, 1377-1387.
1
(14) DeMore, W. B. J. Phys. Chem. 1996, 100, 5813-5820.
(15) Lazarou, Y. G.; Kambanis, K. G.; Papagiannakopoulos, P. To be
submitted to J. Phys. Chem.
(16) Kambanis, K. G.; Lazarou, Y. G.; Papagiannakopoulos, P. Chem.
Phys. Lett. 1997, 268, 498-504.
(17) Lazarou, Y. G.; Kambanis, K. G.; Papagiannakopoulos, P. Chem.
Phys. Lett. 1997, 271, 280-286.
References and Notes
(1) World Meteorological Organization (WMO), Scientific Assessment
(18) Schmidt, M. W., Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.;
of Ozone Depletion, 1994. Report No. 37; WMO: Geneva, 1995.
Gordon, M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.;
Su, S. J.; Windus, T. L.; Dupuis, M; Montgomery, J. A. J. Comput. Chem.
(
2) Tang, T.; McConnell, J. C. Geophys. Res. Lett. 1996, 23, 2633-
2
636.
1993, 14, 1347-1363.
(
(
3) Butler, J. H. Geophys. Res. Lett. 1994, 21, 185-188.
4) Schall C.; Heumann, K. G. Fresenius J. Anal. Chem. 1993, 346,
(19) Hehre, W. J.; Radom, L.; Schleyer, P.v. R.; Pople, J. A. Ab-initio
Molecular Orbital Theory; Wiley-Interscience: New York, 1986.
7
3
3
17-722.
(
5) Rudolph, J.; Koppmann, R.; Plass-D u¨ lmer, C. Atmos. EnViron. 1996,
(20) DeMore, W. B.; Sander, S. P.; Golden, D. M.; Hampson, R. F.;
Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R.; Kolb, C. E.; Molina,
M. J. Chemical Kinetics and Photochemical Data for Use in Stratospheric
Modelling. JPL Publication 94-26; JPL: Pasadena, CA, 1994.
0, 1887-1894.
(6) Finlayson-Pitts, B. J.; Ezell, M. J.; Pitts, J. N., Jr. Nature, 1989,
37, 241-244.