VOI. 42
NO.
SCIENCE IN CHINA (Series B)
5
October 1999
Theoretical studies of the reactions of 0 ( 3 with
~)
halogenated methyl ( I )
-Reaction
mechanism of the o ( ~ P
+ )CHzCl reaction
HOU Hua ( @
) , WANG Baoshan ( E% Lh ) and GU Yueshu
(ma?&)
(School of Chemistry, Shandong University, Jinan 250100, china)
Received April 1 3 , 1999
Abstract
The reaction of o('P) with CH2C1 radical has been studied using ab initio molecular orbital theo~y. G2
(MP.2) method is used to calculate the geometrical parameters, vibrational frequencies and energies of various stationary
points on the potential energy surface. The reaction mechanism is revealed. The addition of o ( ~ P )with CHzCl leads to
the formation of an energy rich intermediate OCH,Cl which can subsequently undergo decomposition or M e r i z a t i o n to
the final products. The calculated heat of reaction for each channel is in agreement with the experimental value. The
production of H + CHClO and C1 + CHzO are predicted to be the major channels. The overall rate constants are calculated using transition state theory on the basis of ab initio data. The rate constant is pressure independent and exhibits negative temperature dependence at lower temperatures, in accordance with the experimental results.
Keywords :
'
ab initio , potential energy surface, 0 ( P) atom, CH,Cl radical.
Incineration is an effective method for the management of halogenated hydrocarbons wastes''].
How to control secondary pollution during the incineration is an important subject. It is necessary to
understand the reaction mechanism of the combustion process. Halogenated methyl is highly reactive
when the hydrogen atoms in the methyl are substituted by halogen atoms. o ( ~ P )atoms can rapidly react with halogenated methyls. The reactions are highly exothermic and play an important role in the
combustion process. To date, there are few experimental and theoretical studies of this type of reactions.
The reaction of o ( ~ P )+ CHzCl is important in the incineration of the chlorinated hydrocarbons
wastes"] . It was not until 1997 that this reaction began to be examined. Overall rate constants were
obtained by Seetula et a1. 'I using a heatable tubular reactor coupled to a photoionization mass spectrometerand were set to k = ( 1 . 4 9
0.34) x 1 0 - ' ~ e x ~ [ ( l . 4 6 0.38) k~*mol-'/RT]
cm3m01euc1e- 's- over the temperature range 297-823 K . CHClO and CH20 were found to be the
major products. They indicated that CHClO and CH20 were generated from the decomposition of an
energy rich intermediate, OCH2C1. Until now, there has been no theoretical study of this reaction. In
this paper we examine this reaction using ab initw methods on the lowest-lying doublet potential energy surface. The reaction mechanism is illustrated in detail. The rate constants are calculated by transition state theory (TST) .
'
*
*
1 Computation methods
The geometries of reactants, ~roducts, intermediates and transition states are optimized at the
UMP2(fu11)/6-31G(d) level of theory. Based on the optimized structures, the high-level energies of
the stationary points are calculated at QCISD ( T) /6-3 11G( d ,p) and UMP2/6-3 11 + G(3df, 2p) lev-
SCIENCE IN CHINA (Series B)
480
Val. 42
els of theory, respectively. The total energies are obtained using the G2MP2 schemeL3]. To obtain reliable vibrational frequencies and zero-point energies, frequency analysis is carried out at the UMP2
.
(fu11)/6-3 1G( d ) theory. All calculations are performed using Gaussina 94 programs[41
2
Results and discussion
The optimized geometries of reactants, intermediates, transition states and products are shown in
fig. 1 . The corresponding frequencies and zero-point energies are listed in table 1 , with only one neg-
CHzO
OCHCl
H-
ClCO
HCO
0.128 Cl
0.098
HCl
0.074
CHCl
H-H
H'
TS5,C,
TS6, Cs
TS7, C,
Fig. I . The UM~2(fu11)/6-31G(d ) optimized geometries of the various forms of the reaction o ( ~ P+)CH2Cl. Bond
lengths in nrn and angles in degree.
No. 5
THEORETICAL STUDIES OF ATOMIC OXYGEN REACTIONS
481
ative frequency for each transition state. The total energies and relative energies at different levels of
theory are listed in table 2 . The overall energetic profile based on the G2MP2 energies (fig. 2) shows
the reaction mechanism of o ( ~ P )+ CHzCl reaction.
Table 1 UMPZ(fu11)/6-31G(d) harmonic vibrational frequencies of the reactants, products, adducts and transition states
.
Vibrational frequencies/cm - '
Species
CH2Cl
333
CH20
1 213
CHClO
47 1
HCO
1 136
ClCO
383
CHCl
919
HC1
3 048
OH
3 740
HZ
4 532
IM1
411
IM2
387
T S ~ 1 187ia'
TS2
735i
TS3
1 119i
TS4
2 315i
TS5
2 815i
TS6
1 139i
TS7
2 009i
884
1 297
774
2 020
648
1051
1 058
1 585
972
2 754
2 000
3 304
1 496
1792
1404
3 270
3 012
1811
3 420
3 084
3 153
739
470
437
267
488
422
299
199
407
749
754
633
775
608
662
498
361
512
1 122
889
688
1 070
757
795
749
625
661
1 133
1 207
745
1 267
847
988
817
1 160
699
1 384
1 309
1 185
1 447
966
1236
962
1 310
1128
ZPE/U rnol -
1 440
1 426
1 369
1 637
1 882
1352
1 408
1 581
1533
3 153
3 746
3 082
3 182
3 318
3204
3 203
3 089
2353
3 086
3 286
1 770
3 084
2 674
2621
1 788
2 003
1975
'
62.8
71.5
51.5
35.6
18.0
31.4
18.4
22.6
27.2
79.1
80.8
59.4
76.1
69.0
67.4
58.2
61.9
55.2
a) i represents imaginary frequency.
Table 2
Species
0 + CH2C1
C1+ CH20
H + CHCIO
H2 + ClCO
HC1+ HCO
OH + CHCl
IM1
IM2
TS1
TS2
TS3
TS4
TS5
TS6
TS7
The total energies (hartree) and relative energies/!d mol-' of various species
UMPZ
/6-311G(d,p)
573.679 37
- 573.820 18
- 573.808 25
- 573.820 90
- 573.837 57
- 573.663 09
- 573.818 76
- 573.939 71
- 573.775 77
- 573.788 25
- 573.714 24
- 573.767 62
- 573.767 60
- 573.768 27
- 573.706 60
-
UMP2
/6-311+ G(3df,2p)
- 573.793 31
- 573.939 54
- 573 .934 94
- 573.946 11
- 573.959 36
- 573.784 19
- 573 .944 27
- 573.969 19
- 573.904 98
- 573.915 06
- 573.848 22
- 573.897 51
- 573.897 19
- 573.894 48
- 573.840 41
UQCISD( T)
/6-311G(d,p)
- 573.732 50
89
- 573.843 43
- 573.867 35
- 573.879 94
- 573.714 41
- 573.868 37
- 573.881 73
- 573.828 66
- 573.848 48
- 573.758 44
- 573.813 99
- 573.819 74
- 573.814 66
- 573.748 23
- 573.893
G2MP2
AH
- 573.864 85
0.0
-364.0
-347.3
-400.0
-428.0
20.6
-384.1
-428.0
-307.9
- 337.2
- 126.8
-263.6
-286.2
-260.7
-112.1
-574.00348
-573.99712
-574.018 14
-574.02793
- 573.856 99
-574.01106
-574.02786
-573.98206
- 573.993 22
- 573.913 16
-573.96518
-573.97391
-573.96418
-573.90758
AH^
-358.2
-391.6
-420.9
o ( ~ P )atom attacks the radical center of CHzCl to form the rich energy intermediate OCH2Cl
(IM1) . This process is found to be a barrierless association. This is the nature of the radical-radical
reaction. The energy of IM1 is 384.1 kJ/mol lower than that of the reactants o ( ~ P+)CH2C1. This
SCIENCE IN CHINA (Series B)
482
1
Fig. 2 .
Vol. 42
CHCIOH
The overall profile of potential energy surface for the reaction of
HCl+HCO
o(")
with CHzCl calculated at
G2MP2 level.
implies that once IM1 is formed, it has enough energy to undergo rapid decomposition or isomerization. IM1 has Cs symmetry and 'A" ground state, which is different from the C H , O ( X ~ A ' ) ,CHC120
(x'A' ) and C C ~ , O ( X ~ )Aradicals.
'
2.2
The bond cleavage of IM 1
IM1 can undergo two bond cleavage reactions: IMl+H + CHClO via C-H cleavage transitian
. .
state (TS1) and I M l j C 1 + CH20 via C-C1 cleavage transition state (TS2). The barrier heights40f
TS1 and TS2 relative to IM1 are 76.1 kJ/mol and 46.9 kJ/mol, respectively. Compared with other
reaction channels of IM1, these two channels have the lowest barrier heights. CHClO and CH20 are
predicted to be the major products, which is in agreement with the experimental result. The breaking
C-H bond is elongated by 0.045 nm in TS1 and the breaking C-C1 bond is 0.035 nm longer in
TS2 than that of IM1. The C-0 bonds are shortened by 0.015 and 0 . 0 1 nm, respectively. Both
TS1 and TS2 have product-like structures.
2.3
Three-center decomposition of IM1
As is clear in fig. 2 , IM1 can decompose to H2 + CClO via the three-center transition state TS3.
TS3 has C, symmetry, 2 ~ electronic
"
ground state. Two C-H bonds are stretched simultaneously by
0.05 nm . The H-C-H angle is reduced to 29. lo. The forming H-H bond is 0.081 nm which is just
0.007 nm longer than that of the Hz molecule. TS3 is a later barrier. The reaction barrier height is
257.3 kJ/mol relative to IMl . Surprisingly, we cannot locate the transition state for another threecenter decomposition of IM1 leading to HC1+ HCO. Stretching C-H and C-C1 bonds simultaneously invariably leads to TS1 or TS2. HC1 and HCO have not been detected in the experiment yet.
2.4
The isomerization of IM1
IM1 can isomerize to another intermediate CHClOH (IM2) via the 1,2-H shift transition state
No. 5
THEORETICAL STUDIES OF ATOMIC OXYGEN REACTIONS
483
TS4. The barrier height for this process is 120.5 kJ/mol , which is 44 . 4 kJ/mol higher than that of
TS1 and 124.3 kJ mol-' lower than that of TS3. TS4 and IM2 have no symmetry. The energy of IM2
is 4 3 . 9 kJ/mol lower than that of IM1. Thus IM2 is a more stable intermediate. It can survive long
enough to further decomposition.
2.5
The decomposition of IM2
bond cleavage leading to H +
Four decomposition channels of IM2 have been found: 0-H
CHClO via transition state TS5 ; C-0 bond cleavage leading to the formation of CHCl + OH; the formation of HC1+ HCO and H2 + ClCO via four-center transition states TS6 and TS7, respectively. The
0 - H bond is 0.139 nm in TS5, 0.041 nm longer than that of IM2. The process of the formation of
CHCl + OH has no pronounced barrier and is the only endothermic reaction channel (AH = 2 0 . 6 kJ/
mol ) . TS6 has C, symmetry. C-C1 bond is stretched to 0.247 nm , 0-H bond is 0.110 nm and the
distance of the forming H-C1 bond is 0.179 nm, 0 . 0 5 nm longer than the equilibrium value of HC1
molecule. TS6 is an earlier barrier. This will lead to the highly vibrationally and rotationally excited
HC1 molecule. TS7 also has four-member ring structure. The 0-H and C-H bonds are elongated
by 0 . 0 4 8 nm and 0.019 nm , respectively. The H-H bond is 0.097 nm , 0 . 0 2 3 nm longer than the
equilibrium value of H2 molecule. The order of the barrier heights for the different decomposition
channels of IM2 is OH + CHCl > H2 + ClCO > HC1+ HCO > H + CHC10.
2.6
Rate constants
As is evident in table 2 , the heats of reaction for various channels calculated by G2MP2 theory
agree with the experimental values. This implies that the relative energies of transition states are reliable. The rate constants can be calculated by transition state theory (TST) The rate constants for the
addition-elimination reaction can be written as[51
.
ki
k = xa/h(Q: Q , ? / Q ~ Q ~ ) ~ - ~ - / ~ T +~ w
( ) P ( E ) ~ - E / R T / ( ~ - +,
x/ci+
w)dE.
(1)
For the o ( ~ P )+ CH2Cl reaction, the first step is a barrierless association, so E , = 0. The rate for the
redissociation of IMl to the reactants is very small because of the high exothermicity (384.1 kJ/mol)
of the formation of IM1 , e . g. k -,
<
,ki, so we can obtain the following equation:
The addition of o ( ~ P )with CH2C1 is the rate-determining step. The overall rate constants are pressure independent, in agreement with the experimental results.
Because the transition state for the association step is loose, the frequencies of the "loose transition state" can be assumed to be those of the reactant CH,Cl. Two active internal rotations are assumed and the three external moments of inertia are increased accordingly in order to interpret the
large rate constants. The internal and external rotational constants can be obtained through fitting the
. The stretching vibration of the C-0 bond in IM1 is taken as
experimental rate constant at 297
the reaction coordinate. The parameters used in the calculations are listed in tables 3 and 4 . We find
that the calculated results agree with the experimental results. Obviously, there is slightly negative
temperature dependence under the lower temperature.
According to eq. ( 2 ) , there are two reasons for the negative temperature dependence. One is
Vol. 42
SCIENCE IN CHINA (Series B )
484
the spin-orbital splitting of the o ( ~ P ) atom which is shown to be strongly temperature dependent on
the electronic partition function of the o ( ~ P )atom. The other is the quantum tunneling effect rc, along
the reaction coordinate. These two factors play an important role in the overall rate constants at lower
temperature . As the temperature increases, these two factors become less and less important. The rate
constants will increase with the elevated temperature and the negative temperature dependence will
disappear. The overall rate constants can be expressed as the following equation over a temperature
range of 250-2000
K:
k = 1.17 x 10-1°exp(239/ T ) + 2 . 2 4 x 1 0 - ' ~ e x ~ ( -1 2 4 1 / T ) .
This needs to be verified by experimental studies at higher temperatures.
Table 3
Parameten used in the TST calculations
Reaction coordinate (imaginary vibrational mode)/cmMoment of inertialg. mol -
- nm2
'
Free internal rotational constant/cm-'
Electronic partition function of 0 ( 3 ~ atom
)
Table 4
0.018, 0.319, 0.336, 0.2, 1.25, 1.92
5
1.82,1.82
T) + exp( -3261 T)
+ 3exp( -228/
Comparison of the calculated rate constants with the experimental values
T/ K
k,,,,/ x 10-
cm3 molecule'
' s-la)
/,k
x 10 - 1°cm3.molecule 's'
297
2 . 6 7 i 0 . 2 3 , 2.61 i 0 . 1 6
2.64
377
517
823
2.60k0.19, 2.28 i 0 . 3 0
2.31k0.28, 1.87k0.20, 2.11k0.14
1.91 k 0 . 2 8 , 1.53k0.28
2.28
2.06
2.05
a) from reference [ 2 ]
3
1 122.0
CH2C1 "loose transition state"
'
.
Conclusions
G2MP2 theory is used to investigate the reaction mechanism for the 0 ( 3 ~ +) CH2Cl reaction.
The addition of o ( ~ P ) with CH2Cl radical is the rate-determining step for the overall reaction. The
major products are H + HClCO and C1+ C H 2 0 . Transition state theory is used to calculate the overall
rate constants based on ab initio potential energy surface. The reaction of o ( ~ P ) with CH2C1 is found
to show negative temperature dependence and pressure independence. The calculated results are in
agreement with the experimental results.
References
1 Seetula,
, Slagle, I . R. , Gutman, D. et al. , Kinetics of the reaction of the CC!, radical with oxygen atoms, Chem.
, 1996, 252 : 299.
J . A. , Slagle , I . R . , Kinetics of the reaction of the CH2Cl radical with oxygen atoms, Chem . Phys . Lett. , 1997,
J.
A.
Phys . Lett.
2
Seetula,
277: 381.
3
Cuniss, L. A. , Raghavachari , K .
1993, 98 : 1293.
4
Frisch, M. J .
5
Hsu, D . S . Y . , Shaub, W . M . , Creamer, T. et al . , Kinetic modeling of CO production from the reaction of CH3 with O2 in
shock waves, Ber. Bunsen Phys. Chem., 1983, 87: 909.
, Truckd,
G. W.
, Pople, J.
A.
, Gaussian-2 theory using reduced
, Head-Gordon, M.
Moller-Plesset orders, J. Chem. Phys . ,
et al. , Caussiun 94, Reyiswn El, Pittsburgh PA: Gaussian Inc., 1994.