CHEM. RES. CHINESE UNIVERSITIES 2012, 28(1), 147—152
Theoretical Study of CH3CH=CH2+O(1D) Reaction:
Mechanism and Kinetics
WU Nan-nan, LIU Hong-xia, DUAN Xue-mei and LIU Jing-yao*
State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry,
Jilin University, Changchun 130021, P. R. China
Abstract The mechanism and kinetics for the reaction of propene(CH3CH=CH2) molecule with O(1D) atom were
investigated theoretically. The electronic structure information of the potential energy surface(PES) was obtained at
the B3LYP/6-311+G(d,p) level, and the single-point energies were refined by the multi-level MCG3-MPWB method.
The calculated results show that O(1D) atom can attack CH3CH=CH2 via the barrierless insertion mechanism to
form four energy-riched intermediates CH3C(OH)CH2(IM1), CH3CHCHOH(IM2), CH2OHCHCH2(IM3) and cycloCH2OCHCH3(IM4), respectively, on the singlet PES. The branching ratios as well as the pressure- and temperaturedependence of various product channels for this multi-well reaction were predicted by variational transition-state and
Rice-Ramsperger-Kassel-Marcus(RRKM) theories. The present results will be useful to gain a deep insight into the
reaction mechanism and kinetics of CH3CH=CH2+O(1D) reaction.
Keywords Propene; O(1D); Mechanism; Kinetics
Article ID 1005-9040(2012)-01-147-06
1
Introduction
Propene(CH3CH=CH2) as a hazardous gas can cause
undesirable effects on the atmosphere such as photochemical
smog due to its strong photochemical reactivity, and as a prototype alkene fuel it would contribute to soot production and
other pollutant formation in combustion processes. On the other
hand, singlet oxygen atom O(1D) is very important in both
combustion process and atmospheric chemistry because of its
high reactivity with other molecules. The reactions of O(1D)
with saturated hydrocarbons[1―10] have been extensively investigated experimentally and theoretically, while less attention
has been paid to the reactions between O(1D) and each of unsaturated hydrocarbons[11―14]. Once propene is emitted into the
stratosphere, the oxidation of CH3CH=CH2 by O(1D) may
play an important role in its degradation processes:
CH3CH=CH2+O(1D) → Products
Two experimental studies have been reported for this reaction. The rate constant was determined to be (5.99±1.2)×10–10
cm3·molecule¯1·s–1 at 298 K by Kajimoto et al.[12], later they
revisited the reaction and estimated the branching ratios of
some possible oxides, such as 0.22 for propylene oxide and
propionaldehyde, 0.11 for acetone, 0.20 for allylalcohol, 0.02
for acrolein and 0.06 for acetaldehyde under high pressure condition(1.50×107 Pa)[13]. In their work, they proposed the possible reaction mechanism, that is, the CH3CH=CH2+O(1D)
reaction proceeds through the insertion into C―H bonds mechanism or addition to the double bond mechanism in which
the long-lived intermediates are formed. However, for this
complex reaction with multi-well and multi-channel, the details
of the mechanism as well as the pressure- and temperaturedependence of the fragmentation product channels and product
distributions are still not completely clear in experiments. To
the best of our knowledge, there is hitherto no theoretical study
regarding the title reaction. In consideration of the importance
of this reaction in combustion and atmosphere, a comprehensive theoretical study is thus highly desirable to shed light on
the reaction mechanism and kinetics. In the present work, we
explored the potential energy surface(PES) by performing
quantum chemistry calculations and did the kinetic calculations
using microcanonical Rice-Ramsperger-Kassel-Marcus(RRKM)
unimolecular rate theory[15]. The PES information was obtained
at the MCG3-MPWB//B3LYP/6-311+G(d,p) level of the theory,
and the branching ratio variations of the various product fragments and intermediates were discussed with respect to temperature and pressure.
2
Calculation Methods
In this work, we employed hybrid density functional
B3LYP method in conjunction with 6-311+G(d,p) basis set to
perform the optimization calculations of all the stationary
points(including reactants, products, intermediates, and transition states) involved in CH3CH=CH2+O(1D) reaction. It is
known that B3LYP method has been widely used in the studies
of many systems[16―18] and has been shown to provide a good
compromise between computational time and accuracy. In the
previous researches[7,8] B3LYP method is proved to be efficient
———————————
*Corresponding author. E-mail: [email protected]
Received February 15, 2011; accepted March 23, 2011.
Supported by the National Natural Science Foundation of China(Nos.20303007, 20333050, 20973077) and the Program for
New Century Excellent Talents(NCET) in Universities of China.
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CHEM. RES. CHINESE UNIVERSITIES
and reliable enough for providing accurate electronic structure
information, while it is prone to underestimating energy barriers. Thus, to obtain more reliable energeties, the single-point
energies were refined with a newly developed multi-level method MCG3-MPWB, which has proved to be one of the powerful methods for achieving higher-accuracy energies with
less computational cost[19]. To characterize the nature of each
stationary point, harmonic vibrational frequency calculations
were performed at the same level. The local minima possess all
real frequencies, whereas the transition state has one and only
one imaginary frequency. The reaction path was calculated by
means of intrinsic reaction coordinate(IRC) to confirm that the
transition states connect designated intermediates. Unless noted,
the MCG3-MPWB energies with inclusion of B3LYP zeropoint vibrational energies(ZPE) were used throughout. All the
calculations were carried out via the Gaussian 03 program
packages[20].
According to the variational transition-state and RRKM
theories, the kinetic calculations for this multi-channel and
multi-well reaction were carried out via the MultiWell 2008.3
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[21,22]
program
on the basis of the PES obtained above in order to
identify the likely mechanism and the branching ratios of various product channels.
3
Results and Discussion
3.1 Potential Energy Surface and Reaction Mechanism
The optimized geometries of the reactants, products, intermediates and transition states for CH3CH=CH2+O(1D)
reaction are shown in Figs.1―3, respectively, along with the
avai- lable experimental data from the literature[23―31]. It is
found that when comparison is available, the agreement between theoretical and experimental results is good, with the
largest discrepancy within a factor of 1.6%. The schematic
profile of the singlet PES is depicted in Fig.4. The total energy
of the reactant R[CH3CH=CH2+O(1D)] is set to be zero for
reference, and the values in parentheses are relative energies in
kJ/mol with reference to that of R.
Fig.1 B3LYP/6-311+G(d,p) optimized geometries for the reactants and products of CH3CH=CH2+O(1D) reaction
The values in parentheses are the pertinent experimental data from the literature[23―31] and a―i represent refs.[23]―[31], respectively. Bond
lengths are in nm and bond angles are in degree.
Fig.2 B3LYP/6-311+G(d,p) optimized geometries for the intermediates(IM) of the CH3CH=CH2+O(1D) reaction
The values in parentheses are the pertinent experimental data from the literature. Bond lengths are in nm and angles are in degree. a. Ref.[ 27].
WU Nan-nan et al.
No.1
Fig.3
149
B3LYP/6-311+G(d,p) optimized geometries for the corresponding transition states(TS)
of CH3CH=CH2+O(1D) reaction
Bond lengths are in nm and bond angles are in degree.
Fig.4
Schematic singlet potential energy surface of primary product channels for CH3CH=CH2+O(1D) reaction
at the MCG3-MPWB//B3LYP/6-311+G(d,p)+ZPE level
The products P1 to P14 are CH3COCH2+H, CH3CCH+H2O, CH3CHCO+H2, CH3CHCHO+H, CH2OCHCH2+H, CH2CCH2+H2O, CH4+CH2CO, CH2CHCHO+
H2, CH3O+CH2CH, CH3+CH2CHO, C2H4+CH2O, CH3CHCH+OH, CH3CCH2+OH and CH2CHCH2+OH, respectively.
3.1.1
Entrance Channels
1
The reaction of CH3CH=CH2 molecule with the O( D)
atom may proceed barrierlessly via the insertion into C―H
bonds or addition to the double bond, leading to four entrance
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CHEM. RES. CHINESE UNIVERSITIES
intermediates, labeled IM1―IM4 in the text. Intermediates
CH3C(OH)CH2(IM1), CH3CHCHOH(IM2) and CH2OHCHCH2
(IM3) are produced by the O(1D) atom insertion into central
vinylic C―H bond, terminal vinylic C―H bond, and alkylic
C―H bond in ―CH3 group, with the stabilization energies of
–646.5, –634.0 and –603.5 kJ/mol, respectively. A threemembered ring intermediate cyclo-CH2OCHCH3(IM4, –574.6
kJ/mol) is formed by the O(1D) atom addition to the C=C
double bond. These processes make intermediates IM1―IM4
highly activated so that further isomerization or dissociation
from them is possible. In addition, IM3 and IM4 can interconvert to each other via the 1,4 H-shift five-membered ring transition state TS9 with a barrier of 270.2 kJ/mol.
3.1.2
Isomerization and Dissociation
If start from the energy-rich intermediate IM1, there are
five possible dissociation and isomerization pathways(Fig.4):
(1) direct O―H bond rupture to give P1 CH3COCH2+H(–301.9
kJ/mol) with no distinct barriers; (2) 1,3 H-shift to form IM6
CH3C(O)CH3(–696.7 kJ/mol) via TS1 after surmounting a barrier of 217.5 kJ/mol, then IM6 can undergo a concerted 1,3
H-migration and C―C bond fission process to yield P7
CH4+CH2CO(–616.8 kJ/mol) via TS17(–341.3 kJ/mol); alternatively, IM6 can dissociate to P10 CH3+CH2CHO(–327.5
kJ/mol) via a high-lying transition state TS18(–67.3 kJ/mol);
(3) concerted 1,3 H-shift and C―C bond rupture via a
four-membered ring transition state TS3(–301.9 kJ/mol) to
produce P7; (4) 1,2 H2O-elimination through a four-membered
ring transition state TS2(–321.2 kJ/mol) to form P2
CH3CCH+H2O(–544.9 kJ/mol); (5) OH-extrusion to form P13
CH3CCH2+OH(–200.3 kJ/mol) without any barriers. It is seen
that owing to much higher energies required in the formations
of P10(IM1→IM8→P10) and P13(IM1→P13), these two
channels are energetically unfeasible and not considered in the
later kinetic calculations.
If start from IM2, six kinds of pathways are identified: (1)
H-extrusion of the H―O bond to form P4 CH3CHCHO+H
(–299.0 kJ/mol) with no distinct barriers(Fig.4); (2) 1,4
H2-elimination to yield P8 CH2CHCHO+H2(–555.4 kJ/mol) via
a six-membered ring transition state TS5(–390.2 kJ/mol); (3)
1,2-H2O-elimination leading to P2 via TS4 after surmounting a
barrier of 328.3 kJ/mol; (4) 1,2 H2-elimination process through
TS20(–284.0 kJ/mol) to give P3 CH3CHCO+H2(–558.3
kJ/mol); (5) direct C―O bond fission to produce P12
CH3CHCH+OH(–183.4 kJ/mol) barrierlessly; (6) isomerization
to IM5 CH3CH2CHO(–663.3 kJ/mol) via a 1,3 H-shift transition state TS6(–390.6 kJ/mol). Once isomer IM5 is formed, two
possible reaction pathways could take place, as shown in Fig.4:
(i) via a four-membered ring transition state TS21(–341.3
kJ/mol) to produce P11 C2H4+CH2CO(–545.8 kJ/mol), and (ii)
via a 1,3-CH3 migration transition state TS13(–292.3 kJ/mol) to
form IM7 CH3OCHCH2(–590.1 kJ/mol), followed by two direct C―O bond rupture processes to produce P9 CH3O+
CH2CH(–174.8 kJ/mol) and P10, respectively. It is easily seen
that the formation of IM7 from IM5 can not compete with the
other fragmentation pathway (i) because the energies of TS13
and IM7 are relatively high in pathway (ii). Moreover, since
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P12 has much less thermodynamic stability, path (5) is much
less probable. Therefore, we have neglected these two pathways in the kinetic calculations.
As seen from Fig.4, there are six possible transformation
and dissociation pathways starting from IM3 CH2OHCHCH2.
IM3 can directly decompose into different products such as P5
CH2OCHCH2+H(–176.5 kJ/mol) and P14 CH2CHCH2+OH
–222.4 kJ/mol) via loose, variational transition states without
any barriers. IM3 could undergo 1,2 H2O-elimination via
TS10(–292.7 kJ/mol) to yield P6 CH2CCH2+H2O(–547.0
kJ/mol), or via the 1,2 H2-elimination transition state
TS7(–251.8 kJ/mol) or TS11(–247.6 kJ/mol) to give the same
product P8. And IM3 can undergo a concerted 1,3H-shift and
C―C bond rupture process leading to P11 via TS8(–248.8
kJ/mol); it can also isomerize by 1,3H-shift transition state
TS12 to form a four-membered ring intermediate IM8
OCH2CH2CH2(–559.1 kJ/mol), with a barrier of 392.3 kJ/mol.
Clearly, this isomerization process to IM8 and two direct dissociation routes from IM3 involve higher energies than the
other pathways, and as a result, they would make negligeable
contribution to the title reaction and will not be taken into account in the kinetic calculations.
For the evolution of the addition entrance intermediate
IM4, three isomerization pathways take place via three different concerted 1,2-H-migration and ring-open processes to
form isomers IM5(via TS19), IM9 CH3CCH2OH(–332.9
kJ/mol, via TS15) and IM10 CH3CH(OH)CH(–362.6 kJ/mol,
via TS14), respectively. These processes require overcoming
barriers of 325.4, 280.2 and 282.7 kJ/mol, respectively. Also,
IM4 could take a 1,2 H-shift accompanied by C―C bond and
C―O bond ruptures process to give P11 via TS16(–258.0
kJ/mol). Because isomers IM9 and IM10 lie relatively shallow,
the formations of them are thermodynamically unfeasible.
In summary, just from the energetic point of view, the
formations of six intermediates IM1―IM6 and eight product
fragments P1, P2, P3, P4, P6, P7, P8 and P11 are most likely
accessible, whereas those unfavorable pathways can be neglected for the reaction of O(1D)+CH3CH=CH2. However,
because the difference among energies of the rate-limiting steps
of these pathways is relatively small, it is difficult to determine
which are the probably reaction channels and feasible products
at different temperatures and pressures solely on the basis of
energies. To provide the product branching ratios of all these
competitive channels, there is need to perform kinetic RRKM
calculations.
3.2
Kinetic Calculations
Partial product distributions were calculated on the basis
of the PES obtained above via the MultiWell 2008.3 program[21,22]. In the kinetic simulations, the inverse Laplace
transform(ILT) method[32,33] was used for the barrier-free entrance association processes. The Arrhenius parameters(needed
for the ILT method) were obtained from the equilibrium constant and the measured rate constant[11].
Fig.5 shows the branching ratios of various reaction
channels at the experimental temperature 123 K in a pressure
WU Nan-nan et al.
No.1
7
range from 10 Pa to 1.5×10 Pa. One can see that at 123 K, the
fragmentation making bimolecular products is dominant at low
pressures below 1.5×105 Pa with negative pressure dependence.
The dominant product is P6 CH2CCH2+H2O with the yield
changing from 0.39 at 10 Pa to 0.22 at 1.5×105 Pa; and P8
CH2CHCHO+H2 and P7 CH4+CH2CO are the second feasible
products with the yields of 0.16 and 0.15 at the collisionless
limit(10 Pa), and P11 C2H4+CH2CO(yield 0.10), P1
CH3COCH2+H(yield 0.09) and P2 CH3CCH+H2O(yield 0.07)
are the minor products, while the yields of P3 CH3CHCO+H2
and P4 CH3CHCHO+H have negligible fraction(yield<0.05)
even at the extreme pressure 10 Pa. With increasing pressure(>1.5×105 Pa), the effective stabilization becomes more
important and begins to take over gradually. The yields of IM3
CH2OHCHCH2, IM5 CH3CH2CHO and IM6 CH3C(O)CH3
rapidly increase with the increasing pressure and reach a maximum of 0.37 for IM3 at 1.0×106 Pa, 0.07 for IM5 at 1.0×106
Pa and 0.17 for IM6 at 3.0×105 Pa. However, the yields of IM3,
IM5 and IM6 turn drops with the persistently increasing pressure and the branching ratios are 0.27, 0.02 and 0.01 at 1.5×107
Pa, respectively. The yields of IM1 CH3C(OH)CH2, IM2
CH3CHCHOH and IM4 cyclo-CH2OCHCH3 keep a gradual
increase over the whole considered pressure from 10 Pa to
1.5×107 Pa, with a maximum value of each of them being about
0.23 at 1.5×107 Pa. In experiment, the estimated branching
ratios are 0.20 for IM3(allylalcohol) and 0.22 for IM4 (propylene oxide), which are in agreement with our theoretical results,
while the yields of IM6(acetone, 0.11) and IM5(propionaldehyde, 0.22) are overestimated compared to our calculated results. Based on the calculated PES, the isomerizations of IM1
to IM6 and IM2 to IM5 need to overcome much higher energy
barriers, hence the yields of IM5 and IM6 are predicted to be
considerably small at this temperature in our kinetic calculations.
151
studied pressures. The branching ratios of fragmentation products are in the following order: P6>P1≈P11>P2>P8>P7>P4>
P3.
Fig.6
Product distributions at various pressures of
10(A), 1.0×105(B) and 1.5 × 107 Pa(C) in a
temperature range of 123―2000 K
Branching ratios of IM1―IM6, P1―P4, P6―P8 and P11 denoted by
curves a―n.
4
Fig.5
Branching ratios of various reaction channels
at 123 K in a pressure range from 10 Pa to
1.5×107 Pa
Branching ratios of IM1―IM6, P1―P4, P6―P8 and P11, denoted
by curves a―n.
To gain more kinetic information, the product distributions
in a temperature range of 123―2000 K at various pressures(10,
1×105 and 1.5×107 Pa) are shown in Fig.6(A―C), respectively.
It is clear to see that the yields of the collisionally stabilized
intermediates are always rapidly decreased when the temperature increases from 123 K to 2000 K, while the bimolecular
products become dominant at high temperatures at three
Conclusions
The kinetics and mechanism of CH3CH=CH2+O(1D)
reaction on singlet potential energy surface were investigated
at the MCG3-MPWB//B3LYP/6-311+G(d,p) level. The branching ratios for the major product channels in 123―2000 K
were predicted. The primary intermediates IM3 CH2OHCHCH2,
IM1 CH3C(OH)CH2, IM4 cyclo-CH2OCHCH3 and IM2
CH3CHCHOH are stable in the low-temperature range and at
high pressures, while at higher temperature or lower pressure
condition, the fragmentation to bimolecular products become
predominant, and P6 CH2CCH2+H2O is the primary product,
whereas P11 C2H4+CH2CO, P1 CH3COCH2+H and P2
CH3CCH+H2O are the minor products; the yields of P3
CH3CHCO+H2 and P4 CH3CHCHO+H are negligibly small.
The present study may be helpful for deeply understanding the
mechanism and identifying the product distributions of this
reaction in the future experiments.
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CHEM. RES. CHINESE UNIVERSITIES
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D., Kudin K. N., Strain M. C., Farkas O., Tomasi J., Barone V., Cossi
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