Kinetics and mechanism of the gas phase reaction of chlorine atoms with i-propanol

Article abstract of DOI:10.1039/b702933k

FTIR smog chamber techniques and ab initio calculations have been used to investigate the kinetics and mechanism of the reaction of Cl atoms with i-propanol in 700 Torr of N₂ at 296 K. The reaction is observed to proceed with a rate constant of k₁ = (8.28 ± 0.97) × 10^-11 cm³ molecule^-1 s^-1 and gives CH₃C(OH)CH₃ and CH₃CH(OH)CH₂ radicals in yields of 85 ± 7 and 15 ± 7%, respectively. Calculations indicate that abstraction of the secondary H can proceed through a lower energy pathway than the primary. Rapid decomposition of the chlorination product CH₃CCl(OH)CH₃ complicates its direct detection, likely due to heterogeneous chemistry. IR spectra for the chlorides CH ₃CCl(OH)CH₃ and CH₃CH(OH)CH₂Cl were inferred experimentally and assignments confirmed via comparison with ab initio computed spectra. the Owner Societies.

Full text of DOI:10.1039/b702933k

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www.rsc.org/pccp | Physical Chemistry Chemical Physics
Kinetics and mechanism of the gas phase reaction of chlorine atoms with
i-propanolw
T. Yamanaka,a M. Kawasaki,*a M. D. Hurley,b T. J. Wallington,*b W. F. Schneider*c
and J. Brucec
Received 26th February 2007, Accepted 24th May 2007
First published as an Advance Article on the web 15th June 2007
DOI: 10.1039/b702933k
FTIR smog chamber techniques and ab initio calculations have been used to investigate the
kinetics and mechanism of the reaction of Cl atoms with i-propanol in 700 Torr of N2 at 296 K.
The reaction is observed to proceed with a rate constant of k1 = (8.28 ꢀ 0.97) ꢁ 10ꢂ11 cm3
moleculeꢂ1 sꢂ1 and gives CH3C(OH)CH3 and CH3CH(OH)CH2 radicals in yields of 85 ꢀ 7 and
15 ꢀ 7%, respectively. Calculations indicate that abstraction of the secondary H can proceed
through a lower energy pathway than the primary. Rapid decomposition of the chlorination
product CH3CCl(OH)CH3 complicates its direct detection, likely due to heterogeneous chemistry.
IR spectra for the chlorides CH3CCl(OH)CH3 and CH3CH(OH)CH2Cl were inferred
experimentally and assignments confirmed via comparison with ab initio computed spectra.
mechanism of reaction (1).
1. Introduction
Cl þ CH3CHðOHÞCH3 ! CH3CðOHÞCH3 þ HCl ð1aÞ
Concerns over climate change and energy security have led to
increased interest in the use of renewable biofuels in the
Cl þ CH3CHðOHÞCH3 ! CH3CHðOHÞCH2 þ HCl ð1bÞ
Abstraction of the hydrogen in the –OH group in i-propanol is
endothermic by 6.3 kJ molꢂ1 4 and is not significant at ambient
C15H31C(O)OCH3 from palmitic acid, C17H35C(O)OCH3
automotive fuel pool. The principal biofuels under considera-
tion are methyl esters of long chain fatty acids (e.g.,
from stearic acid) for blending in diesel and alcohols (e.g.,
temperature. Experimental measurements of k1 and k1a/(k1a +
1b) at 296 K are reported and discussed with respect to the
C2H5OH, C3H7OH, and C4H9OH) for blending in gasoline.
Prior to the large scale use of such biofuels it is desirable to
have a detailed understanding of their environmental impact.
In smog chamber studies of the atmospheric oxidation me-
chanisms of organic compounds it is often advantageous to
employ Cl atom initiated oxidation in place of the OH radical
initiated oxidation which occurs in the atmosphere. The
mechanisms of the reactions of Cl atoms with organic com-
pounds are often similar to those of the analogous reactions of
OH radicals. It is generally more convenient in the laboratory
to generate Cl atoms than OH radicals.
k
literature data.
2. Experimental and computation methods
2.1 Experimental details
Experiments were performed in a 140 L Pyrex reactor inter-
faced to a Mattson Sirus 100 FTIR spectrometer.5 The reactor
was surrounded by 22 fluorescent blacklamps (GE F15T8-BL)
which were used to photochemically initiate the experiments.
Cl atoms were generated from the photolysis of molecular
chlorine in 700 Torr of N2 diluent at 296 ꢀ 2 K. The branching
ratio k1a/(k1a + k1b) was determined by measuring the for-
mation of 2-chloro-2-propanol following the UV irradiation
of CH3CH(OH)CH3–Cl2–N2 mixtures. Chloropropanols are
formed as the result of photochemical chain chlorination
reactions.
Facilitation of the design and interpretation of smog cham-
ber studies of the Cl atom initiated oxidation of biofuels
requires knowledge of the kinetics and mechanisms of the Cl
atom reactions. There is limited data concerning the kinetics
and mechanism of the reaction of Cl atoms with i-propanol.1–3
To improve our understanding of the reactions of Cl atoms
with alcohols in general and the atmospheric chemistry of
i-propanol in particular we have conducted a smog chamber
FTIR study and ab initio calculations of the kinetics and
Cl2 þ hv ! 2Cl
ð2Þ
Cl þ CH3CHðOHÞCH3 ! CH3CðOHÞCH3 þ HCl ð1aÞ
Cl þ CH3CHðOHÞCH3 ! CH3CHðOHÞCH2 þ HCl ð1bÞ
CH3CðOHÞCH3 þ Cl2 ! CH3CClðOHÞCH3 þ Cl ð3Þ
CH3CHðOHÞCH2 þ Cl2 ! CH3CHðOHÞCH2Cl þ Cl ð4Þ
a Department of Molecular Engineering, Kyoto University, Kyoto,
615-8510, Japan. E-mail: kawasaki@moleng.kyoto-u.ac.jp
b Ford Motor Company, P. O. Box 2053, Dearborn, MI 48121-2053,
USA. E-mail: twalling@ford.com
c Department of Chemical and Biomolecular Engineering and
Department of Chemistry and Biochemistry, University of Notre
Dame, Notre Dame, IN 46556. E-mail: wschneider@nd.edu
w The HTML version of this article has been enhanced with colour
images.
The rate of reaction (1), k1 (k1 = k1a + k1b), was studied using
relative rate techniques. In the relative rate experiments the
ꢃc
This journal is the Owner Societies 2007
Phys. Chem. Chem. Phys., 2007, 9, 4211–4217 | 4211

following reactions take place:
Cl þ CH3CHðOHÞCH3 ! products
3. Results
3.1 Relative rate study of k(Cl + CH3CH(OH)CH3)
ð1Þ
ð5Þ
The rate of reaction (1) was measured relative to reactions (7)
and (8):
Cl þ reference ! products
It can be shown that
Cl þ CH3CHðOHÞCH3 ! products
Cl þ C2H2 ! products
ð1Þ
ð7Þ
ð8Þ
ꢀ
ꢁ
ꢀ
ꢁ
½i-propanolꢄto
½i-propanolꢄt
k1
k5
½referenceꢄto
½referenceꢄt
ln
¼
ln
ð6Þ
Cl þ C2H4 ! products
Reaction mixtures consisted of 50.1–102 mTorr of reactant,
1.54–3.23 mTorr of reference, and 99.6–153 mTorr of Cl2 in
700 Torr total pressure of N2 diluent. The observed loss of
CH3CH(OH)CH3 versus those of the reference compounds is
shown in Fig. 1. Linear least squares analysis of the data in
Fig. 1 gives k1/k7 = 1.60 ꢀ 0.10 and k1/k8 = 0.91 ꢀ 0.07.
Using k7 = (5.07 ꢀ 0.34) ꢁ 10ꢂ11 and k8 = (9.29 ꢀ 0.51) ꢁ
10ꢂ11 cm3 moleculeꢂ1 sꢂ1 at 700 Torr9 gives k1 = (8.11 ꢀ 0.74)
where [i-propanol]to, [i-propanol]t, [reference]to and [referen-
ce]t are the concentrations of CH3CH(OH)CH3 and reference
compound at times to and t, and k1 and k5 are the rate
constants for reactions (1) and (5). Plots of
ln[CH3CH(OH)CH3]to/[CH3CH(OH)CH3]t) versus ln([refer-
ence]to/[reference]t) should be linear, pass through the origin,
and have a slope of k1/k5.
Concentrations of reactants and products were monitored
by FTIR spectroscopy using an analytical path length of 27 m.
In the kinetic experiments IR spectra were derived from 32 co-
added interferograms using a spectral resolution of 0.25 cmꢂ1
with a total data acquisition time of 90 s. In the mechanistic
experiments IR spectra were derived from 8 co-added inter-
ferograms using a spectral resolution of 1.0 cmꢂ1 with a total
data acquisition time of 11 s. Unless stated otherwise, the
quoted uncertainties are 2 standard deviations from least
squares regression analyses. Reagents were obtained from
commercial sources at purities 499%. The CH3CH(OH)CH3
sample was subjected to freeze–pump–thaw cycling to remove
volatile impurities below use. Ultrahigh purity (499.994%)
N2 diluent gas was used as received. In smog chamber experi-
ments unwanted loss of reactants and products via photolysis
and heterogeneous reactions has to be considered. Control
experiments were performed in which product mixtures
obtained after UV irradiation were allowed to stand in the
dark in the chamber for 1 h. With the exception of 2-chloro-
2-propanol, there was no observable (o2%) loss of reactants
or products, showing that unwanted heterogeneous reactions
are not a significant complication over the timescale of the
present experiments.
ꢁ 10ꢂ11 and (8.45 ꢀ 0.80) ꢁ 10ꢂ11 cm3 moleculeꢂ1 sꢂ1
.
Indistinguishable values of k1 were obtained using the two
different references. We choose to cite a final value which is the
average together with error limits which encompass the
extremes of the individual determinations; k1 = (8.28 ꢀ
0.97) ꢁ 10ꢂ11 cm3 moleculeꢂ1
s
ꢂ1. As shown in Table 1,
this result is in excellent agreement with previous relative1,2
and absolute3 rate studies; the rate of reaction (1) at room
temperature is well established.
3.2 Mechanism of the Cl + CH3CH(OH)CH3 reaction
The mechanism of the reaction of Cl atoms with
CH3CH(OH)CH3 was investigated by subjecting mixtures of
49.5–71.0 mTorr CH3CH(OH)CH3 and 100 mTorr Cl2 in
700 Torr of N2 diluent to UV irradiation. The photochemical
chain chlorination reactions (1)–(4) are expected to lead
to formation of the chlorides CH3CCl(OH)CH3 and
CH3CH(OH)CH2Cl. The yields of the chlorides provide infor-
mation on the branching ratio for reaction (1). Neither
2.2 Computational details
Initial molecular orbital calculations were performed using the
GAMESS system.6 Minimum energy structures and transition
states spectra were determined through gradient optimization
at the second-order Møller–Plesset level (MP2), using a
6-31G** basis. The identity of all minima and transition states
were confirmed through vibrational analysis using finite differ-
encing. Vibrational energies were scaled by 0.94 to correct for
known systematic overestimation.7 Simulated vibrational spec-
tra were generated using a Laplace distribution around each
peak with a variance of 98 I2 cmꢂ1, where I is the intensity, to
approximately represent the effects of line broadening. To
improve the energy estimates, fourth-order Møller–Plesset with
the core electrons frozen were performed at the MP2/6-31G**
derived geometries using a correlation-consistent triplet-zeta
basis (cc-pVTZ) and the Gaussian 03 code.8
Fig. 1 Loss of CH3CH(OH)CH3 versus C2H2 (triangles) and C2H4
(circles) in the presence of Cl atoms in 700 Torr of N2 at 296 ꢀ 2 K.
ꢃc
This journal is the Owner Societies 2007
4212 | Phys. Chem. Chem. Phys., 2007, 9, 4211–4217

Table 1 Reported values of k(Cl + CH3CH(OH)CH3)
k1/m3 moleculeꢂ1 sꢂ1 Temperature/K Techniquea Ref.
(8.4 ꢀ 0.4) ꢁ 10ꢂ11
298
RR
RR
RR
PLP-LIF
Nelson et al.1
Wu et al.2
Wu et al.2
Taketani
et al.3
(7.35 ꢀ 0.37) ꢁ 10ꢂ11 295
(8.27 ꢀ 0.74) ꢁ 10ꢂ11 295
(9.11 ꢀ 0.60) ꢁ 10ꢂ11 295
(8.28 ꢀ 0.97) ꢁ 10ꢂ11 296
RR (C2H2, This work
C2H4)
a
RR, relative rate; PLP-LIF, pulsed laser photolysis laser induced
fluorescence.
CH3CCl(OH)CH3 nor CH3CH(OH)CH2Cl are available com-
mercially. By analogy to other a-halogenated alcohols such as
CF3OH,10 CH2ClOH,11 CHCl2OH,11 CCl3OH,11 and
CH3CHClOH,12 itisexpectedthatCH3CCl(OH)CH3 willunder-
godecompositionwithinthesmogchambergivingCH3C(O)CH3
and HCl. The decomposition into CH3C(O)CH3 offers a means
to calibrate the IR spectrum of CH3CCl(OH)CH3.
In the initial experiments using CH3CH(OH)CH3–Cl2–N2
we were surprised by the large amount of CH3C(O)CH3
observed in the spectrum acquired after UV irradiation. In
addition to CH3C(O)CH3 there were some IR product features
which disappeared when the reaction mixtures were allowed to
stand in the dark and some which did not. We ascribe the
features that disappeared to CH3CCl(OH)CH3 and those that
did not to CH3CH(OH)CH2Cl. The observation of a substan-
tial yield of CH3C(O)CH3 in the spectrum taken after UV
irradiation indicates that for typical operating conditions of
our spectrometer (32 co-added interferograms, 0.25 cmꢂ1
resolution, data acquisition time 90 s) there is a significant
decomposition of CH3CCl(OH)CH3. To capture more of the
CH3CCl(OH)CH3 within our experimental observation time
we reduced the number of co-added interferograms to 8 and
reduced the spectral resolution to 1.0 cmꢂ1. This resulted in a
reduction in the data acquisition time from 90 to 11 s. The
results discussed below were obtained using this revised pro-
cedure. The rate of decomposition of CH3CCl(OH)CH3 in the
chamber was rapid (495% loss in 30 s, corresponding to a
pseudo first order loss rate 4 0.1 sꢂ1).
Fig. 2 Spectra acquired before (A) and after (B) a 14 s irradiation of a
mixture of 50.1 mTorr CH3CH(OH)CH3 and 101 mTorr Cl2 in 700
Torr of N2 diluent. Panel C shows the result of leaving the reaction
mixture to stand in the dark for 5 min. Comparison of the difference
spectrum (D) with a reference spectrum of CH3C(O)CH3 (E) shows
the formation of this species. Panel F shows the spectrum of
CH3CCl(OH)CH3 derived from panel D (see text for details).
Fig. 2 shows typical spectra before (A) and after (B) a 14 s
irradiation of a reaction mixture containing 50.1 mTorr
CH3CH(OH)CH3 and 101 mTorr Cl2 in 700 Torr of N2
diluent. The UV irradiation led to 65% consumption of the
CH3CH(OH)CH3. The result of allowing the product mixture
to stand for 5 min in the dark is shown in panel C. Panel D was
obtained by subtracting panel B from panel C. Positive
features in panel D are attributable to species which are
formed when the reaction mixture was left to sit in the dark
in the chamber. Negative features in panel D are attributable
to species which are lost when the reaction mixture was left to
sit in the dark in the chamber. Consistent with expectations
outlined above, comparison of panel D with the reference
spectrum of CH3C(O)CH3 given in panel E shows this species
was formed in the dark. The negative going features are then
assigned to CH3CCl(OH)CH3. Subtracting features attributa-
ble to CH3C(O)CH3 from panel D gives a spectrum of
CH3CCl(OH)CH3 which after inversion (multiplication by
ꢂ1) is displayed in panel F. Equating CH3C(O)CH3 formation
with CH3CCl(OH)CH3 loss we calculate the integrated cross
section for CH3CCl(OH)CH3 features in the range 700–
1600 cmꢂ1 to be (6.8 ꢀ 1.0) ꢁ 10ꢂ17 cm moleculeꢂ1. The
quoted error includes 10% random (reproducibility of
the measurements) and 10% systematic (calibration of
acetone reference spectrum and uncertainty in IR path
length13) uncertainties. After IR features attributable to
CH3CH(OH)CH3, CH3CCl(OH)CH3, and CH3C(O)CH3 were
subtracted from panel B there were residual features at 747,
1046, 1260, and 3619 cmꢂ1 which we attribute to the forma-
tion of CH3CH(OH)CH2Cl. In contrast to the behavior of
CH3CCl(OH)CH3, there was no evidence (o2%) for loss of
CH3CH(OH)CH2Cl when mixtures containing this compound
were left to stand in the dark. Fig. 3 shows the IR spectra for
CH3CCl(OH)CH3 and CH3CH(OH)CH2Cl derived in the
ꢃc
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Phys. Chem. Chem. Phys., 2007, 9, 4211–4217 | 4213

Fig. 4 Scaled MP2/6-31G** IR spectra of CH3CCl(OH)CH3 (top
panel) and CH3CH(OH)CH2Cl (bottom panel).
energy conformers are shown in Fig. 4. Table 2 reports the
most prominent features of each spectrum, approximate
assignments where possible, and infrared intensities scaled
relative to the calculated most intense modes. Agreement
between experiment and simulation is very good for both
isomers, confirming the experimental assignments.
Fig. 3 IR spectra of CH3CCl(OH)CH3 and CH3CH(OH)CH2Cl.
Fig. 5 shows the observed formation of CH3CH(OH)CH2Cl
and the sum of CH3CCl(OH)CH3 and CH3C(O)CH3 versus
loss of CH3CH(OH)CH3 following the UV irradiation of
CH3CH(OH)CH3–Cl2–N2 mixtures. The data plots are linear
indicating that with the exception of decomposition of
CH3CCl(OH)CH3 into CH3C(O)CH3, loss of the chloropro-
panols via unwanted secondary reactions is not a significant
complication in the present work. The lines through the data
are linear least squares fits. The regression to the combined
yield of CH3CCl(OH)CH3 and CH3C(O)CH3 gives k1a/
present work. The CH3CH(OH)CH2Cl spectrum was cali-
brated by assuming that this species accounts for the balance
of CH3CH(OH)CH3 loss after accounting for the formation of
CH3CCl(OH)CH3 and CH3C(O)CH3. The integrated cross
section for CH3CH(OH)CH2Cl features in the range
700–1600 cmꢂ1 was 4.0 ꢁ 10ꢂ17 cm moleculeꢂ1 (with an
uncertainty estimated to be ꢀ15%).
As further confirmation of these assignments, the vibra-
tional spectra of the chlorinated isomers were calculated at the
MP2/6-31G** level. Several conformations of each geometric
isomer were evaluated and produced equivalent vibrational
spectra. The structures and simulated spectra for the lowest
(k1a + k1b) = 0.85 ꢀ 0.07 and hence we infer k1b/(k1a
k1b) = 0.15 ꢀ 0.07.
+
Table 2 Observed and selected calculateda vibrational modes (cmꢂ1), calculated relative intensities (arbitrary units), and approximate assign-
ments for 1-, and 2-chloro-2-propanol
CH3CH(OH)CH2Cl
CH3CCl(OH)CH3
Expt’l
747
Calc’d
Rel. Int.
Approx. Desc.
C–Cl Stretch
Expt’l
Calc’d
Rel. Int.
Approx. Desc.
743
825
932
22
7
829
985
822
974
36
36
32
100
62
25
21
.3
1
3
2
26
C–Cl Stretch
C–C–C Bend
11
45
12
53
20
10
12
5
C–C Stretch
C–C Stretch
C-O Stretch
H–O–C Bend
CH2 Wag
HCC Bend
HCC Bend
CC Stretch
CH2 Scissor
O–H Stretch
1096
1139
1223
1320
1377
1444
1446
1459
1464
3502
1046
1260
1030
1136
1240
1276
1354
1357
1391
1433
3521
1144
1220
C–O Stretch
H–O–C Bend
H–C–H Bend
CH3 Bends
CH3 Bends
CH3 Bends
CH3 Bends
O–H Stretch
1387
3633
6
22
3619
a
MP2/6-31G** frequencies, scaled by 0.94.
ꢃc
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4214 | Phys. Chem. Chem. Phys., 2007, 9, 4211–4217

CH3C(O)CH3
following
the
UV
irradiation
of
CH3CH(OH)CH3–Cl2 mixtures in air diluent. The
CH3C(OH)CH3 radical formed in reaction (1a) reacts with
O2 to give CH3C(O)CH3 which then provides a measure of the
importance of reaction channel (1a). The CH3CH(OH)CH2
radicals formed in reaction (1b) react with O2 leading to
formation of CH3CHO, HCHO, and HO2 radicals. In the
present work we chose to conduct experiments in N2 diluent as
these give fewer products leading to a more simple analysis.
Ohta et al.14 studied the Cl atom initiated oxidation of
i-propanol in air in the presence of NO2 and observed
CH3C(O)CH3, CH3C(O)OONO2, CH3C(O)OH, HC(O)OH,
and HCHO as products. Ohta et al.14 conducted one experi-
ment and equated the formation of CH3C(O)CH3 and HCHO
to chemistry following reactions (1a) and (1b), respectively.
The plots of the observed formation of products versus
irradiation time were curved indicating the importance of
secondary reactions in the system. From the ratio of the initial
yields of CH3C(O)CH3 and HCHO the branching ratio k1a/k1b
= 5.7 was deduced. While Ohta et al.14 do not provide an
estimate of the uncertainty associated with this measurement,
based on the fact that the initial yields were based mainly on
one data point the uncertainty is likely to be significant. The
ratio k1a/k1b = 5.7 ratio corresponds to k1a/(k1a + k1b) = 0.85
which is in excellent agreement with the finding in the present
work.
Fig.
5 Formation of CH3CCl(OH)CH2Cl (triangles) and
CH3C(O)CH3 + CH3CCl(OH)CH3 (circles) versus CH3CH(OH)CH3
loss following the UV irradiation of CH3CH(OH)CH3–Cl2–N2
mixtures.
In the present work we use the combined yield of
CH3CCl(OH)CH3 and CH3C(O)CH3 following UV irradia-
tion of CH3CH(OH)CH3–Cl2 mixtures in N2 diluent as a
measure of k1a/(k1a + k1b). When the reaction mixtures were
left to stand in the dark for more than 1 min the decomposi-
tion of CH3CCl(OH)CH3 into CH3C(O)CH3 was essentially
complete and the method reduces to measurement of the
CH3C(O)CH3 yield. An alternative, but similar, method to
measure k1a/(k1a + k1b) is to monitor the formation of
3.3 Computed mechanism
To examine the origins of the more rapid rate of reaction 1a
over 1b, the reaction pathways for these two hydrogen ab-
stractions were calculated at the MP4/cc-pVTZ//MP2/
6-31G** level. Fig. 6 shows the computed potential energy
surfaces for reactions at the primary (11) and secondary (21)
Fig. 6 MP4/cc-pVTZ//MP2/6-31G** computed reaction pathways for primary (11, grey) and secondary (21, black) H abstraction by a Cl atom,
including zero-point energy corrections. Key distances are listed in A.
ꢃc
This journal is the Owner Societies 2007
Phys. Chem. Chem. Phys., 2007, 9, 4211–4217 | 4215

Fig. 7 MP4/cc-pVTZ//MP2/6-31G** computed reaction pathways for homogeneous, first-order HCl elimination from 2-chloro-isopropanol,
including zero-point energy corrections. Distances reported in A.
hydrogens. The features are similar to those noted for H
abstractions at the primary and secondary centers of etha-
nol.15 Abstraction of the secondary H atom is approximately
30 kJ molꢂ1 more exothermic than the primary H. Reaction
along the secondary path proceeds through a weakly bound
precursor state in which the Cl radical is associated primarily
with the alcohol oxygen. A low-energy transition state leads to
an exit adduct and ultimately products. At the transition state
the Cl–H–C angle is 174.31 and Cl–H and H–C distances 1.932
and 1.132 A, respectively, consistent with the analogous
ethanol path. In contrast, the primary H abstraction channel
proceeds directly via an H atom abstraction transition state
22 kJ molꢂ1 higher in energy than the secondary transition
state and leading to a weakly bound exit adduct, similar to
that noted for H abstraction from methanol.16 These results
are qualitatively consistent with the observed preferential
abstraction of the secondary hydrogen.
theory treatment,17 we obtain a first order rate constant for
homogeneous decomposition of 1.4 ꢁ 10ꢂ10 sꢂ1. Even allowing
for uncertainties in the barrier height and for contributions of
quantum mechanical tunneling, it is clear that homogeneous
decomposition of CH3CCl(OH)CH3 is much slower than the
rate of approximately 10ꢂ1 sꢂ1 implied by the lifetime observed
experimentally. We conclude that the decomposition of
CH3CCl(OH)CH3 observed in the chamber proceeds via a
heterogeneous mechanism likely similar to that identified for
other halogenated alcohols.18 The rapid heterogeneous decom-
position of CH3CCl(OH)CH3 observed in the chamber is
remarkable. In our previous studies we typically observed
decomposition rates of a-chloroalcohols of the order of 10ꢂ3
sꢂ1 11
Faster loss rates were observed in a chamber equipped
.
with a mixing fan which increased the exposure of the gas
mixtures to the chamber walls.11 We ascribe the rapid loss of
CH3CCl(OH)CH3 observed in the present set of experiments to
either (i) efficient eddy diffusion, or (ii) the presence of small
amounts of aerosol in the present experiments. While additional
experiments were not performed to distinguish between these
two possibilities it is very clear from the factor of 109 difference
between the experimentally observed decay rate and the com-
puted rate of homogeneous decay that the decomposition
observed in the chamber occurs via a heterogeneous mechan-
ism.
As noted above, both radicals are rapidly converted to the
corresponding chloro compounds, but while the 1-chloro-
isopropanol is stable in the reaction chamber, the 2-chloro
isomer decomposes to acetone and HCl. To determine the
likelihood of homogeneous decomposition, the reaction path
for direct HCl elimination was calculated and is shown in Fig. 7.
The reaction is predicted to be overall endothermic by 1.2 kJ
molꢂ1. Unlike the more exothermic HCl elimination from
CCl3OH,11 this reaction proceeds through a highly distorted
transition state in which the C–Cl–H–O fragment is non-planar
and the C–Cl and H–Cl distances are 2.665 and 2.037 A.
The large internal distortions in the transition state suggest a
high activation barrier for homogeneous decomposition, which
is borne out by the energetic results. Further, the activation
barrier is calculated to be 132.2 kJ molꢂ1. Combining the MP4/
cc-pVTZ estimate for the energy barrier with the MP2/6-31G**
structures and vibrational spectra in a standard transition state
4. Discussion
The results from the present work confirm the literature value
of k1 and provide a measurement of the branching ratio k1a/
(k1a + k1b) which is consistent with, but more direct than, the
single previous measurement. Using the data presented in the
present work we can calculate site specific rate constants of k1a
= (7.04 ꢀ 1.01) ꢁ 10ꢂ11 and k1b = (1.24 ꢀ 0.60) ꢁ 10ꢂ11 cm3
ꢃc
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4216 | Phys. Chem. Chem. Phys., 2007, 9, 4211–4217

moleculeꢂ1
s
ꢂ1. Dividing k1b by 2 provides a value for the
4 NIST Standard Reference Database 25, Structures and Properties,
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6 M. W. Schmidt, K. K. Baldridge, J. J. Boatz, S. T. Elbert, M. S.
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Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V.
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Johnson, W. Chen, M. W. Wong, C. Gonzalez and J. A. Pople,
GAUSSIAN 03, (Revision C.02), Gaussian, Inc., Wallingford, CT,
2004.
9 T. J. Wallington, J. M. Andino, I. M. Lorkovic, E. W. Kaiser and
G. Marston, J. Phys. Chem., 1990, 94, 3644.
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Smyth, J. Geophys. Res., 1995, 100, 23227.
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1996, 100, 6097.
reactivity of Cl atoms towards each –CH3 group in i-propanol
of (6.20 ꢀ 3.05) ꢁ 10ꢂ12 cm3 moleculeꢂ1 sꢂ1. We can compare
this with the reactivity of Cl atoms towards –CH3 groups in
C2H6, 2.95 ꢁ 10ꢂ11; C3H8, 3.05 ꢁ 10ꢂ11; and n-C4H10, 3.02 ꢁ
10ꢂ11 cm3 moleculeꢂ1 sꢂ1 19 The reactivity of a –CH3 group in
.
i-propanol is substantially (approximately a factor of 5) less
than that in an alkane. Combining the relative reactivity of
primary compared to tertiary C–H bonds in (CH3)3CH re-
ported by Cadman et al.20 of 1.82 (average of determinations
with/without hexafluoroethane diluent) with the average of
recent measurements21–25 of k(Cl + (CH3)3CH) = 1.4 ꢁ 10ꢂ10
we derive a reactivity of the tertiary C–H bond of 4.96 ꢁ 10ꢂ11
cm3 moleculeꢂ1 ꢂ1. The tertiary C–H bond in i-propanol is
s
approximately 40% more reactive than the tertiary C–H bond
in i-butane.
It is of interest to compare the results obtained in the present
work with those for the Cl + C2H5OH reaction. The Cl +
C2H5OH reaction proceeds with a rate constant of (9.5 ꢀ 1.9)
ꢁ 10ꢂ11 cm3 moleculeꢂ1 sꢂ1 with 0.075 ꢀ 0.025 of the reaction
proceeding via attack on the CH3– group.12 The reactivity of
the CH3– group in C2H5OH is (7.13 ꢀ 2.77) ꢁ 10ꢂ12 cm3
moleculeꢂ1 sꢂ1 which is indistinguishable from the reactivity
of each CH3– group in i-propanol. The reactivity of the –CH2–
group in C2H5OH is (8.79 ꢀ 1.77) ꢁ 10ꢂ11 and similar to the
reactivity of –CH2– groups in C3H6 (7.74 ꢁ 10ꢂ11) and
n-C4H10 (7.29 ꢁ 10ꢂ11) cm3 moleculeꢂ1 sꢂ1 19
.
The presence
of the –OH functional group in simple alcohols appears to
activate C–H bonds at the a position and deactivate C–H
bonds at the b position. Further studies of the site specific
reactivities of other simple alcohols would be of interest to
confirm or refute this finding. Finally, the substantial activa-
tion barrier and hence slow rate (o1.4 ꢁ 10ꢂ10
s
ꢂ1) of
homogeneous elimination of HCl elimination from
CH3CCl(OH)CH3 calculated in the present work is consistent
with the slow rates of HF elimination from CF3OH (o8 ꢁ
10ꢂ17
10ꢂ10
s
ꢂ1, ref. 18) and HCl elimination from CCl3OH (o2 ꢁ
s
ꢂ1, ref. 11). The homogeneous decomposition of
a-halo-alcohols appears to be negligibly slow in the gas phase
but can occur rapidly on contact with surfaces.
19 R. Atkinson, D. L. Baulch, R. A. Cox, J. N. Crowley, R. F.
Hampson, Jr, M. E. Jenkin, J. A. Kerr, M. J. Rossi and J. Troe,
IUPAC Subcommittee on Gas Kinetic Data Evaluation, http://
www.iupac-kinetic.ch.cam.ac.uk/index.html, downloaded August
2006.
20 P. Cadman, A. W. Kirk and A. F. Trotman-Dickenson, J. Chem.
Soc., Faraday Trans. 1, 1976, 72, 1027.
21 R. S. Lewis, S. P. Sander, S. Wagner and R. T. Watson, J. Phys.
Chem., 1980, 84, 2009.
Acknowledgements
WFS and JB gratefully acknowledge support from the Ford
Motor Company.
22 R. Atkinson and S. M. Aschmann, Int. J. Chem. Kinet., 1985,
17, 33.
23 T. J. Wallington, L. M. Skewes, W. O. Siegl, C.-H. Wu and S. M.
Japar, Int. J. Chem. Kinet., 1988, 20, 867.
24 P. Beichert, L. Wingen, J. Lee, R. Vogt, M. J. Ezell, M. Ragains,
R. Neavyn and B. J. Finlayson-Pitts, J. Phys. Chem., 1995,
99, 13156.
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Phys. Chem. Chem. Phys., 2007, 9, 4211–4217 | 4217