FTIR and computational studies of gas-phase hydrogen atom abstraction kinetics by t-butoxy radical

Article abstract of DOI:10.1016/j.cplett.2006.06.100

By using Fourier-Transform Infrared (FTIR) absorption spectroscopy, rate coefficients in the range of 10^-16 to 10^-14 cm³ molecule^-1 s^-1 have been determined for the hydrogen atom abstraction reactions of several substrates including halogenated organic compounds and amines by t-butoxy radical generated from the uv photolysis of t-butyl nitrite in the gas phase. Arrhenius parameters for selected reactions have been measured in the temperature range 299-318 K. Transition states and activation barriers for such reactions have been computed with the help of Gaussian 03 software and found to match very well with the experimental values.

Full text of DOI:10.1016/j.cplett.2006.06.100

Chemical Physics Letters 427 (2006) 276–280
www.elsevier.com/locate/cplett
FTIR and computational studies of gas-phase hydrogen
atom abstraction kinetics by t-butoxy radical
*
Shuping Li, Wai Yip Fan
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore
Received 17 May 2006; in final form 26 June 2006
Available online 4 July 2006
Abstract
By using Fourier-Transform Infrared (FTIR) absorption spectroscopy, rate coefficients in the range of 10ꢀ16 to 10ꢀ14 cm3 mole-
culeꢀ1 sꢀ1 have been determined for the hydrogen atom abstraction reactions of several substrates including halogenated organic com-
pounds and amines by t-butoxy radical generated from the uv photolysis of t-butyl nitrite in the gas phase. Arrhenius parameters for
selected reactions have been measured in the temperature range 299–318 K. Transition states and activation barriers for such reactions
have been computed with the help of GAUSSIAN 03 software and found to match very well with the experimental values.
ꢀ 2006 Elsevier B.V. All rights reserved.
1. Introduction
In the gas phase, the rate coefficient and Arrhenius
parameters of the decomposition of t-butoxy radical rela-
tive to the alkoxy radical combination with NO have been
investigated [7–11]. These important parameters have also
been obtained by Choo [12–14] and Sway [15,16] by using
uv photolysis or by thermal decomposition of di-t-butyl
peroxide. However the substrates, RH studied were pri-
marily limited to a few alkanes and alkenes.
In this work, we have substantially explored further the
H-atom abstraction reactions of t-butoxy radicals with
amines, halogenated organic compounds and nitriles at
room temperature. FTIR spectroscopy was utilized to
probe the two main products from unimolecular decompo-
sition and hydrogen atom abstraction in a static cell [17].
The kinetic isotope effect and Arrhenius parameters for
some substrates were also explored in order to determine
activation barriers. Computational calculations using
GAUSSIAN 03 were performed to provide a correlation
between the experimental data with the various thermody-
namic and kinetic parameters [18].
Alkoxy radicals, ROÅ are key intermediates in the oxida-
tion of hydrocarbons in the atmosphere [1,2]. The reaction
pathways for alkoxy radicals vary according to their struc-
ture and much effort has been devoted to establishing the
relative importance of its unimolecular decomposition. In
addition, these radicals are favored for abstracting H atom
from a hydrocarbon because the formation of a strong
O–H bond allows the process to have appreciable rate coef-
ficients. In particular, the t-butoxy radical generated from
di-t-butyl peroxide has been used for H-atom abstraction
reactions in the liquid phase [3–6]. Since its unimolecular
reaction rate has been measured over a range of tempera-
tures, the rate of the abstraction reaction represented by
(2) below could be determined by measuring the ratio of
acetone to t-butanol which are the products of (1) and
(2), respectively.
ðCH3Þ CO þ ðMÞ ! ðCH3Þ CO þ CH3 þ ðMÞ
ðCH3Þ3CO þ RH ! ðCH3Þ 2COH þ R
ð1Þ
ð2Þ
3
3
2. Experimental section
*
The radical precursors, t-butyl nitrite, di-t-butyl perox-
Corresponding author. Fax: +65 67791691.
E-mail address: chmfanwy@nus.edu.sg (W.Y. Fan).
ide and the hydrogen-containing substrates were purchased
0009-2614/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2006.06.100

S. Li, W.Y. Fan / Chemical Physics Letters 427 (2006) 276–280
277
from Sigma–Aldrich. Distillation and free-pump–thaw
cycles were carried out to remove impurities before these
compounds were used in the kinetic runs. A static Pyrex
cell (15 cm pathlength) with a temperature controller was
used as the reaction vessel initially evacuated by a rotary
pump to a background pressure of 2 · 10ꢀ3 Torr. The pres-
sures used for the t-butyl nitrite and the substrates were 1–
2 Torr and 10–20 Torr, respectively. About 100–760 Torr
SF6 buffer gas was flowed into the cell from its gas cylinder
(Linde gas, 99.9%).
sures and hence the concentration of the two products
formed in the abstraction reaction can then be determined.
A few control experiments were carried out initially.
Firstly, upon uv photolysis of a sample containing t-butyl
nitrite, the production of only acetone was observed, indi-
cating that unimolecular decomposition of the t-butoxy
radical is dominant in the absence of any substrates. Sec-
ondly, it was only upon uv lamp photolysis in the presence
of both t-butyl nitrite and the substrate that IR signals of
acetone and t-butanol began to appear and increased in
intensity with respect to the irradiation time.
The t-butoxy radical was generated by 360–380 nm
broadband lamp photolysis of t-butyl nitrite. The infrared
spectrum (Nicolet Nexus 870) was scanned every 3 min
from 1000 to 4000 cmꢀ1 with a resolution of 1 cmꢀ1. The
relative rate of the IR signal changes of acetone
(1738 cmꢀ1) and t-butanol (1140 cmꢀ1) were monitored
with respect to the irradiation time. Initial rate measure-
ments were used to minimize contributions from secondary
reactions caused by product buildup. The experiment was
repeated five times for each reaction pair. During the first
few minutes of irradiation, there were little formation of
acetone and t-butanol (absorbances <20%) so the Beer–
Lambert’s Law could be used to form a linear relationship
between the absorbance and concentration. For experi-
ments using di-t-butyl peroxide as the precursor, a
254 nm mercury lamp was used as the photodissociation
source in a quartz cell of similar size (15 cm long, 2.5 cm
diameter) while maintaining all other conditions. For the
Arrhenius parameter measurements, a heating tape was
used to increase uniformly the temperature of the reaction
cell with a thermocouple installed inside the reaction cham-
ber for recording the gas temperature. Throughout the irra-
diation, only minimal temperature rise was observed and
hence the temperature range of the experiment was
recorded as 299 1 ꢁC.
Based on the simple reaction scheme below and assum-
ing a constant photolysis rate of t-butyl nitrite, the relation-
ship between t-butanol and acetone formation over the
irradiation period (Dt) is given by Eqs. (1a)–(1f) [19,20].
hm
ðCH3Þ3CONO !ðCH3Þ3CO þ NO
ð3Þ
ð4Þ
ð5Þ
kd
ðCH3Þ3CO þ M ! CH3COCH3 þ CH3
ka
ðCH3Þ3CO þ RH !ðCH3Þ3COH þ R
d½t ꢀ BuOHꢁ
¼ ka½ðCH3Þ3COꢁ½donorꢁ
ð1aÞ
dt
d½acetoneꢁ
¼ kd½ðCH3Þ3COꢁ
dt
ð1bÞ
½t ꢀ BuOHꢁ ¼ ka½ðCH3Þ3COꢁ½donorꢁDt
½acetoneꢁ ¼ kd½ðCH3Þ3COꢁDt
ð1cÞ
ð1dÞ
½t ꢀ BuOHꢁ ka½donorꢁ
¼
ð1eÞ
½acetoneꢁ
kd
kd½t ꢀ BuOHꢁ
Ka ¼
½acetoneꢁ
ð1fÞ
½donorꢁ
Under conditions where the t-butoxy radical and sub-
strate concentrations are constant which would occur dur-
ing the initial stages of irradiation, Eqs. (1a) and (1b) may
be integrated to give (1c) and (1d) which upon rearrange-
ment lead to an expression for the rate coefficient shown
in (1f). Eqs. (1a) and (1b) predict linear productions of t-
butanol and acetone and indeed this is shown by the results
obtained for the reactions as shown in Fig. 1. The linear
plot demonstrates constant rate of production of t-butoxy
radical concentration at low substrate consumption. From
the ratio of acetone to t-butanol, the absolute rate coeffi-
cients of the H-atom abstraction reactions can be deter-
mined since kd has already been well-documented [7–11].
As nitric oxide is the other product from nitrite photolysis,
it may have some effect on the abstraction reactions. Hence
another precursor (di-t-butylperoxide) was used to produce
t-butoxy radical but essentially the same H-atom abstrac-
tion reaction rate coefficients were obtained. From these
results, the presence of NO has essentially no effect or at
least negligible within the experimental error of the rate
coefficients.
The structures of the reactants, products and the transi-
tion states were optimized using density functional theory
at UB3LYP/6-31+G* level in GAUSSIAN 03 that the enthal-
pies, free energies and activation energies of the abstraction
processes could be determined. Harmonic frequencies were
calculated at the optimized geometries to characterize sta-
tionary points as equilibrium structures, with all real fre-
quencies, or transition states, with one imaginary
frequency, and to evaluate zero-point energy (ZPE)
correction.
3. Results and discussion
The Beer–Lambert’s Law, Abs = e c l where Abs = mea-
sured absorbance; l = path length (cm), c is the partial con-
centration of the absorbing species (mol lꢀ1) and e is the
molar absorptivity (cm2 molꢀ1), has been used towards
determining the ratio of the products. The absorptivity of
the vibrational bands of the products was estimated by cal-
ibrating known pressures of samples of acetone and t-buta-
nol added to the same total pressure (buffer gas and
reactants) used in the abstraction experiment. The pres-
The rate coefficients for the abstraction rate of various
substrates are shown in Table 1. The relative rates depend
on the number and type of hydrogen atoms of the sub-

278
S. Li, W.Y. Fan / Chemical Physics Letters 427 (2006) 276–280
The kinetic isotopic effect for H/D atoms was also exam-
acetone
t-butanol
3.0
2.5
2.0
1.5
1.0
0.5
0.0
ined for a few of the substrates here. However, when
CDCl3 and CD3CN were used as substrates, the signal of
deuterated t-butanol (O–D, 1042 cmꢀ1) was not observed
during the irradiation. This indicates that the isotope effect
has rendered the abstraction process too slow to be obser-
vable using our experimental setup. Since the smallest
measurable rate coefficient is about 6 · 10ꢀ16 cm3 mole-
culeꢀ1 sꢀ1 for CH3CN (see Table 1), the corresponding rate
coefficients for the deuterated substrates would be smaller
than this value. In addition, the abstraction rate coefficient
values which range from 10ꢀ16 to 10ꢀ14 cm3 moleculeꢀ1 sꢀ1
for all the substrates studied here indicate an activation
barrier to reaction; otherwise values in the region of
10ꢀ10 and 10ꢀ11 cm3 moleculeꢀ1 sꢀ1 would be expected
based on simple collision rates. Hence a temperature-
dependence study was carried out for a few selected sub-
strates in order to determine their activation barriers.
The Arrhenius plots for the abstraction reaction rate
coefficient ratios of triethylamine, chloroform, isobutyro-
nitrile and hexane are shown in Fig. 2. Based on the mea-
sured slopes and the accepted Arrhenius parameters for
kd(logA = 14.1 sꢀ1 and Ea = 64.0 kJ molꢀ1) [9], the activa-
tion energies Ea (kJ/mol) and the pre-exponential factors
logA (L molꢀ1 sꢀ1) of these reactions could be calculated
and shown in Table 1. The activation energy for hexane
is in good agreement with previously reported literature
values [14–16]. As expected the reaction involving an amine
has the lowest barrier while those with electron-withdraw-
ing groups such as Cl and CN tend to have more substan-
tial barriers.
0
5
10
15
20
25
Time (mins)
Fig. 1. Linear production of t-butanol and acetone during the photolysis
of t-butyl nitrite (2 Torr) and triethylamine (10 Torr) at 299 K. Experi-
mental error is denoted by a vertical bar.
strates in the following order; CH– > –CH2– > CH3 which
can be explained as due to the generation of the more sta-
ble tertiary carbon radicals after abstraction. As expected
the rate coefficients for reactions involving halogenated
substrates are slightly lower than those for the alkanes
because the halogen atoms tend to be electron-withdrawing
rendering the radicals less stable. In the case of amines
which are electron-donating groups, their reactions with
t-butoxy are much faster, in good agreement with the liquid
phase values [6]. Although it could not be inferred in our
experiments, hydrogen abstraction reactions from amine
preferably take place at the a-carbon attached to the nitro-
gen atom. Once formed, the a-aminoalkyl radical can be
stabilised by the conjugation between the unpaired electron
of the a-aminoalkyl radical and the lone paired electron of
nitrogen [4,6].
Computational studies were performed in order to fur-
ther understand and substantiate the experimental findings.
The calculated enthalpies and Gibbs free energies of
selected abstraction reactions at 298 K are shown in Table
1. A relationship between DG and the rate coefficients
could be clearly seen from the plot of the rate coefficients
Table 1
Rate coefficients of H-atom abstraction by t-butoxy radical and Arrhenius parameters, enthalpies and Gibbs free energies for selected abstraction reactions
Substrate
k (cm3 moleculeꢀ1 sꢀ1
)
Experimental
activation energy (kJ molꢀ1
Experimental logA
Calculated
DG (kJ/mol)
Calculated
DH (kJ/mol)
)
(L molꢀ1 sꢀ1
)
CHCl3 (chloroform)
CHBr3 (bromoform)
CH2Cl2 (dichloromethane)
(CH3)2CHCN
C6H14 (hexane)
CH3(CH2)2CN
C6H12 (1-hexene)
C6H5CH3 (toluene)
C2H2 (acetylene)
PhC2H (phenylacetylene)
CH3CN
(C2H5)3N (triethylamine)
(CH3)2NCH2CH2NH2
C6H11NH2 (cyclohexylamine)
CH3(CH2)3NH2
3.8 · 10ꢀ15
2.7 · 10ꢀ15
2.5 · 10ꢀ15
5.6 · 10ꢀ15
2.7 · 10ꢀ15
2.1 · 10ꢀ15
1.6 · 10ꢀ15
1.6 · 10ꢀ15
1.4 · 10ꢀ15
1.4 · 10ꢀ15
6.2 · 10ꢀ16
4.9 · 10ꢀ14
4.5 · 10ꢀ14
2.5 · 10ꢀ14
7.8 · 10ꢀ15
18.0 2.5
–
–
16.5 2.3
21.7 3.4
–
–
–
–
–
–
9.0 0.5
–
–
8.9 0.3
10.0 0.5
–
–
–
–
–
–
ꢀ48.5048
ꢀ49.9459
ꢀ46.7224
ꢀ48.2685
ꢀ20.1259
ꢀ19.4355
ꢀ94.4002
ꢀ87.2498
ꢀ54.9281
ꢀ47.9693
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
2.78 0.6
7.7 0.2
–
–
–
–
–
–
–
–
ꢀ75.831
ꢀ73.4344
(from a-carbon)

S. Li, W.Y. Fan / Chemical Physics Letters 427 (2006) 276–280
279
4.0
3.8
3.6
3.4
3.2
3.0
2.8
2.6
2.4
2.2
(CH3CH2)3N
CHCl3
(CH3)2CHCN
Hexane
3.15
3.20
3.25
3.30
3.35
3.40
1/T x 1000(K-1)
Fig. 2. Arrhenius plots of ka/kd for the reaction of t-butyl nitrite with
triethylamine, chloroform, isobutyronitrile and hexane.
(with the exception of the propylamine case) vs absolute
DG (which carry negative signs) in Fig. 3. Highly exogernic
reactions tend to have larger rate coefficients as well. Hence
this correlation appears to serve well in predicting rate
coefficients for the abstraction reaction. We are currently
unaware of the reason for the disagreement for the amine
case. We are also interested in finding out whether these
abstraction processes proceed via an elementary step or
through a series of reactions. Hence, the energy and geom-
etry of the transition states for the reactions between but-
oxy radical with CHCl3 and CDCl3 has been computed
and compare with the experimental values. These reactions
are ideal since the isotopic effect on the reaction rates can
also be examined. Fig. 4 shows such as a transition state
Fig. 4. Transition state structure for the reaction of t-butoxy radical with
CHCl3.
(for CHCl3 case) in which the main features lie in the reac-
tion coordinate (1861i cmꢀ1) which shows a gradual forma-
˚
tion of the O–H (1.22 A) bond and cleavage of the C–H
˚
(1.31 A) bond. These values could be compared to O–H
˚
˚
(0.97 A) and C–H (1.09 A) values of stable organic mole-
cules. The calculated activation barrier (20.7 kJ/mol) is just
outside the error associated with the experimentally
observed barrier (18 2.5 kJ/mol). Although the transition
state geometry for the CDCl3 case is similar, its activation
barrier shows a slightly higher value of 25.9 kJ/mol. The
difference of 5.2 kJ/mol can be loosely translated as a low-
ering of the rate coefficient by about 8 times assuming the
pre-exponential factors of the reactions remain the same
over a small temperature range. So both experimental
and computational data appear to favour a large kinetic
isotope effect operating during the abstraction reactions.
In addition, the computed activation energy of 13.1 kJ/
mol for the reaction between t-butoxy and (CH3)2CHCN
also agrees well with the experimental value of
16.5 2.3 kJ/mol. However similar computation could
not be performed for the triethylamine case presumably
due to the very low barrier as suggested by experiments
hence rendering the location of the transition state uncer-
tain. On the other hand, the calculations for the hexane
case involved extensive computational time and were not
included in this study. Nevertheless, it appears that it is suf-
ficient to consider these abstraction reactions as elementary
based on the good agreement between the experimental
and computed parameters.
9
8
7
6
5
4
3
y = 0.064x + 0.6277
2
R2 = 0.6146
1
0
0
25
50
75
100
Absolute ΔG
Fig. 3. Plot of the hydrogen abstraction rate coefficients vs absolute DG,
for six of the reactions for which their DGs have been calculated as listed in
Table 1.

280
S. Li, W.Y. Fan / Chemical Physics Letters 427 (2006) 276–280
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S.Li thanked the National University of Singapore for a
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