Cl2 molecular elimination reaction from 1,2-dichloroethane

Li Zhu, Joseph W. Bozzelli *

Department of Chemical Engineering, Chemistry and Environmental Science, New Jersey Institute of Technology,

Newark, NJ 07102, USA

Received 26 November 2001; in final form 11 March 2002

Abstract

The transition state energy for Cl2 molecular elimination from 1,2-dichloroethane is computed at compostite CBS-Q

(based on B3LYP/6-31++G(d) optimized structure) and G3MP2 (based on MP2/6-31++G(d) optimized structure)

levels of theory. Forward rate constants are calculated as 4:25E11T 0:826 � expð�93:4 kcal mol�1=RTÞ and

7:62E10T 0:892 � expð�98:5 kcal mol�1=RTÞ s�1, respectively for these two methods. Intrinsic reaction coordinate (IRC)

analysis is performed to verify the transition state structures. While the overall, elimination of Cl2 plus olefin formation

has a relative low DHrxn; the reaction with these barriers is evaluated to be not important in thermal reactions of

chlorocarbons. � 2002 Published by Elsevier Science B.V.

1. Introduction

The four-centered elimination reactions of 1,2-dichlorohydrocarbons to Cl2 plus correspondingolefins are of interest because the DHrxn is below theenergy of activation to the transition state of thecommonly accepted HCl elimination channel(s).The Cl2 elimination reactions have not been directlymeasured in experiments or calculated as we knowand the importance of this channel is, therefore,uncertain. The channels for the unimolecular dis-sociation of 1,2-dichloroethane are shown in Table1. The enthalpy changes for HC1 and Cl2 molecularelimination channels are well below the energy ofthe weakest bond (CACl). These HCl and Cl2 mo-lecular elimination reactions are only moderately

endothermic. If there is no barrier or a low barrierover that of DHrxn for the Cl2 elimination, this re-action may be important relative to the HCl elimi-nation and/or the CH2ClC �H2 þ Cl dissociationto radical channel(s). Cl2 elimination in other1,2-dichlorohydrocarbons may also be important;for example, Cl2 elimination reaction fromhexachloroethane (bottom of Table 1), which hasan even lower enthalpy of reaction than reaction(1e).

2. HCl elimination

Some HCl molecular elimination reactions aresummarized in Table 2. This table shows the trendof this type of reaction; it is not intended to becomplete but only serve to illustrate data for thesereactions. The activation energies for four-centeredHCl molecular elimination reaction from C2 species

3 May 2002

Chemical Physics Letters 357 (2002) 65–72

www.elsevier.com/locate/cplett

* Corresponding author. Fax: +1-973-642-7179.

E-mail address: bozzelli@njit.edu (J.W. Bozzelli).

0009-2614/02/$ - see front matter � 2002 Published by Elsevier Science B.V.

PII: S0009-2614 (02 )00442-6

are in the range of 40–65 kcal/mol, most of them arein the range of 50–60 kcal/mol. The HCl elimina-tions from heavier chlorocarbons tend to showsimilar values. The research group of Setser [1]

studied and reviewed the HF/HCl eliminations fromnine C1 to C3 fluorocarbons and chlorofluorohy-drocarbons. The HCl channel dominates the HFchannels for CHClF2 and CH2ClF because of its

Table 1

Dissociation reactions of 1,2-dichloroethane

DH 0rxn

Eaf

G2 G2MP2 BLYP/

6-311G**

B3LYP/

6-311G**

MP4/

6-311G**

Expt or

review

CH2ClCH2Cl ! CH2ClC �H2 þ Cl ð1aÞ 85.70a 86.85a 74.13a 75.11a 80.99b 81.7 DH 0rxn

! 2C �H2Cl ð1bÞ 89.87a 90.20a 79.59a 81.28a 89.82b 88.5c DH 0rxn

! CH2ClC �HClþH ð1cÞ 98.98a 99.06a 91.28a 94.24a 97.35b 98.0 DH 0rxn

! C2H3ClþHCl ð1dÞ 14.74d 50–60

! C2H4 þ Cl2 ð1eÞ 44.07d ?

C2Cl6 ! C2Cl4 þ Cl2 ð2Þ 29.64d ?

aRef. [21], by bond dissociation reactions.bRef. [22], by bond dissociation reactions.c Ref. [23].d From unpublished data of Manion, J.A. NIST.

Table 2

Literature on some HCl molecular elimination reactions

A Eaf References DHrxn Ear(s�1) (kcal/mol) (kcal/mol) (kcal/mol)

Four-centered

C2H5Cl 6.31E+13 57.4 [24] 17.24 40.2

CH3CHCl2 1.91E+13 53.54 [25] 14.86 38.7

CH2ClCH2Cl 6.46E+10 47 [20] 14.74 32.3

CHCl2CH2Cl 7.94E+13 59.5 [24] 13.18 46.3

1.26E+14 58.5 [24] 12.58 45.9

CH3CCl3 7.08E+13 54 [26] 13.06 40.9

CHCl2CHC12 1.00E+12 43.6 [27] 11.20 32.4

C2HCl5 3.98E+11 40 [27] 9.41 30.6

1.26E+14 59.7 [28] 9.41 50.3

CH3CFCl2 3.85E+11 47 [29] 17.14 29.9

CH3CF2Cl 5.00E+13 54.97 [30] 24.04 30.9

CHFClCF2Cl 2.00E+14 59.04 [30] 26.83 32.2

CH2@CHCl 1.00E+14 69.31 [31] 27.02 42.3

CHC1@CHCl 3.63E+12 52.70 [32] 32.16 20.5

Three-centered

CH2FCl 73.3 [1]

CHF2Cl 56.0 [1]

CHFC12 51.3 [1]

CH3C1 1.06E+14 102.7 [33]

CH2Cl2 1.42E+14 77.6 [34]

CHCl3a 1.72E+12*T 0:72 56

aUnpublished work in this group.

66 L. Zhu, J.W. Bozzelli / Chemical Physics Letters 357 (2002) 65–72

lower threshold energy. The possibility of boththree- and four-centered HCl elimination from C2chlorocarbons must be considered because thehalogen substitution lowers the threshold energy ofthree-centered elimination in some cases. Three-centered elimination followed by Cl migration/re-arrangement is also possible [2].

Barton and Howlett [3] studied the mechanism ofthe thermal decomposition of 1,2-dichloroethane ina flow reactor at 635–758 K. They obtained the rateof HCl elimination to be k1d ¼ 6:4 E10 � expð�47kcal mol�1=RTÞ s�1 from direct measurement,where DHrxn ¼ 14:7 kcal/mol. Buravtsev et al. [4]also measured k1d ¼ 1:26 E16 � expð�65 kcalmol�1=RTÞ s�1 from flow reactor at 920–1050 K.HCl is reported as the dominant channel for de-composition of each species in Table 2 except forCH3Cl and CH2Cl2.

3. Cl2 elimination

There are few studies or estimates on Cl2 mo-lecular elimination reactions for 1,2-dichlorocar-bons. Barton and Howlett [3] report k1e ¼ 1:0E13 � expð�72:0 kcal mol�1=RTÞ s�1 from fittingdata to a complex mechanism. Weissman andBenson [5] estimated the rate of Cl2 eliminationreaction from C2Cl6 to be 5:01 E13 � expð�54kcal mol�1=RTÞ s�1; which is some 18 kcal/moldifferent in Ea from that of Barton and Howlett.Huybrechts et al. [6] do not include molecular Cl2elimination channel in their CCl4=C2Cl6 pyrolysismechanism.

Hudgens [7] observed products of CF2Cl2 andCFCl3 decomposition from multiphoton CO2 laserirradiation with molecular beam mass spectro-metric detection; he showed 3% Cl2 for CF2Cl2and no Cl2 for CFCl3. These contrasted results ofDever and Grunwald [8] on CFCl2 who indicatedCl2 was the product in similar irradiation studies.King and Stephenson [9] studied CF2Cl2 dissoci-ation by 929 and 1082 cm�1 multiphoton CO2

laser irradiation, monitoring CF2 product, as anindicator of Cl2 elimination. While their studiesfocused on vibration energy distribution in CF2,the results suggested some Cl2 molecular elimina-tion.

Morrison and Grant [10] used Br2 as a scav-enger to identify products of CF2Cl2 dissociationfrom CO2 laser irradiation. Only two productswere found, CF2ClBr and CF2Br2, which wereconsidered products from CF2Cl and CF2, respec-tively. The fractional yield of CF2 passes through amaximum, 7%, and then decreases over a fluencerange of 500 mJ=cm2–6 J=cm2. Krajnovich et al.[11] studied CF2Cl2 multiphoton dissociation forCO2 laser irradiation in molecular beam experi-ments observing both Cl2 and CF2Cl. They re-ported 10% of the dissociation occurred by Cl2elimination.

Cameron and Bacskay [12] failed to find a tran-sition state for molecular elimination of Cl2 fromCF2Cl2 or Br2 from CF2Br2 at MP2/6-31G(d) andCASSCF/cc-pVDZ levels in their computationalstudies. They reported only that TST calculationswere indicative of abstraction of a Cl by Cl atom.Multireference CI calculation results at HF level ofLewerenz et al. [13] for CF2Cl2 indicates the barrierfor Cl2 molecular elimination is about 38 kcal/molabove the CF2Clþ Cl channel; using )117 [14] and)66 [15] kcal/mol for DH 0

f 298 of CF2Cl2 and CF2Cl,we place this Cl2 molecular elimination barrier at118 kcal/mol, well above the CF2ClACl bond en-ergy. More recently, Kumaran et al. [16] measuredCl atom yield from 100% CF2Cl2 decomposition at1446–2667 K in reflected shock waves using ARASdetection, ½Cl1=½CF2Cl20 ¼ 2:03; they concludemolecular elimination to CF2 þ Cl2 does not occurunder their conditions.

The Ea parameters for the Cl2 molecular elimi-nation from dichloromethanol and trichlorometh-anol have been calculated as �100 kcal/mol at theB3LYP/6-31G(d,p) level in this group [17], whereDHrxn are 91.7 and 69.4 kcal/mol, respectively.

The infrared multiphoton dissociation of tri-chloroethylene studies by Yokoyama et al. [18]show that the branching ratios are 0.28, 0.55, and0.17 for three- and four-centered HCl eliminationsand the Cl elimination, respectively, no Cl2 signal isobserved. The lack of Cl2 elimination is due to thehigh DHrxn of three-centered Cl2þ : C@HCl channel(calculated as 108.7 kcal/mol) or high barrier offour-centered Cl2 þ CHBCCl channel. Differentresults are found in similar experiments by Reiseret al. [19]; they find a small amount of CHBCCl in

L. Zhu, J.W. Bozzelli / Chemical Physics Letters 357 (2002) 65–72 67

Table 3

Transition state structure of reaction CH2ClCH2Cl ! C2H4 þ Cl2

Structure B3LYP/6-31++G(d) MP2/6-31++G(d)

Bond length Bond angle Dihedral angle Bond length Bond angle Dihedral angle

(�AA) (degree) (degree) (�AA) (degree) (degree)

r21 1.395 r21 1.389

r31 1.085 d312 119.97 r31 1.086 d312 120.65

r41 1.085 d412 119.97 d4123 )158.83 r41 1.086 d412 120.65 d4123 )163.43r51 2.353 d512 99.67 d5123 100.58 r51 2.363 d512 94.92 d5123 98.30

r62 1.083 d621 120.97 d6213 166.08 r62 1.084 d621 120.09 d6213 163.36

r72 1.083 d721 120.98 d7213 )7.23 r72 1.084 d721 120.09 d7213 0.13

r82 2.814 d821 106.25 d8213 )100.58 r82 2.613 d821 113.35 d8213 )98.28

Frequency

(cm�1, not

scaled)

)568 69 96 )1797 116 179

230 316 465 217 394 485

840 918 929 838 869 943

1005 1236 1239 949 1210 1267

1484 1558 3182 1508 1569 3212

3206 3271 3311 3223 3309 3339

Moments of

inertia

ð10�40 g cm2Þ

180.44 271.69 440.65 176.20 262.02 426.73

68

L.Zhu,J.W

.Bozzelli

/Chem

icalPhysics

Letters

357(2002)65–72

trichloroethylene system, but find no C2H2 fromCH2@CCl2 system.

No three- or four-centered Cl2 elimination reac-tions from chlorinated ethanes have been measuredor calculated that we know of. We investigate thistype of reaction using CH2ClCH2Cl as an exampleto determine the relative importance of four-cen-tered Cl2 molecular elimination reaction in chloro-carbons; the transition state structure and thecalculated Ea of this reaction are reported for thefirst time.

4. Calculation method

Density functional B3LYP/6-31++G(d) and abinitio MP2/6-31++G(d) methods in GAUSSIAN 98are carried out to locate the transition state of Cl2elimination from 1,2-dichloroethane, and the opti-mized geometry of 1,2-dichloroethane. The intrinsicreaction coordinate (IRC) method is performed toconfirm the TS structures. CBS-Q and G3MP2calculations are performed on the B3LYP/6-31++G(d) and MP2/6-31++G(d) geometries toobtain more accurate enthalpies of formation of thetransition states.

5. Results and discussion

The transition states found at two levels oftheory for reaction (1e) are shown in Table 3.Overall, The MP2 TS is similar to that at B3: theyare both non-symmetric; the CAC bond is �1.39�AA; and the distance of C2ACl8 (2.81 �AA at B3 and2.61 �AA at MP2) is further than the C1ACl5 (2.353�AA). The two planes (C2H4 and CACAClACl ring)are close to perpendicular (100�) in both TSs. Thebiggest differences between two are the angles ofC2AC1ACl5, the rhombus formed by the CCClClfour-membered-ring in B3 is shifted to the oneside by 5�–6� more than that in MP2 (seeTable 3).

The intrinsic reaction coordinate analyses at B3and MP2 levels are plotted in Fig. 1. An intrinsicreaction coordinate (IRC) is defined as the mini-mum energy path connecting the reactant to theproduct via the TS. The IRC calculation follows the

path both forward and backward from the TS whileoptimizing the geometry of the system at each pointalong the path. In Fig. 1, the dashed line corre-sponds to the TS of CH2ClCH2Cl ! C2H4 þ Cl2 asit approaches the products (right) and reactant(left). We calculated 20 points for the reactant di-rection and 12 points for the product direction witha step size of 0.1 amu1=2 Bohr ð0:6819 �10�21 g1=2 cmÞ. In Fig. 1, when rðC1ACl5Þ increasesfrom 1.82 to 2.71 �AA, rðC2ACl8Þ increases from 1.80to 3.61 �AA at B3 level (1.92 to 2.76 �AA at MP2). Incontrast, rðC1AC2Þ decreases from 1.53 to 1.34 �AA,and rðCl5ACl8Þ decreases from 3.03 to 2.15 �AA at B3level (3.14 to 2.01 A at MP2). This shows that bothof the CACl bonds break during the reaction, whilea ClACl bond forms and a CAC bond becomesC@C bond. Obvious major differences between B3and MP2 are the Cl5ACl8 and C2ACl8 curves.Cl5ACl8 is steep in MP2, and monotonous in B3,while C2ACl8 is steep in B3, but not in MP2.

Total energies are also obtained from higher le-vel calculations; composite CBS-Q based on theB3LYP/6-31++G(d) geometry, and composite

Fig. 1. Intrinsic reaction coordinates (IRC) analysis around TS

at MP2/6-31++G(d) (thin lines) and B3LYP/6-31++ G(d) (thick

lines).

L. Zhu, J.W. Bozzelli / Chemical Physics Letters 357 (2002) 65–72 69

G3MP2 based on the MP2/6-31++G(d) geometry,are listed in Table 4. Thermochemical properties,DH 0

f 298; S0298 and C0

pðT Þ ð300 < T =K < 1500Þ, forreactant, product, and TS of title reaction are listedin Table 5. The standard entropy and heat capaci-ties of TS are calculated from its frequencies andmoments of inertia using a ‘SMCPS’ program (un-published work in this group).

Table 6 shows the thermodynamic analysis using‘THERMKIN’ (unpublished work in this group)program on the title reaction based on two differentcalculation methods. The pre-exponential Afwd fac-

tors are 4:25 E11T 0:826 and 7:62 E10T 0:892 s�1 byB3 and MP2 calculations, respectively. Data inTable 6 show the density function A value are aboutthree times higher than the MP2 A value. Forwardactivation energies Eafwd are 93.4 and 98.5 kcal/molby CBS-Q//B3 and G3MP2//MP2 calculations, re-spectively. Our calculated Ea is 22–27 kcal/molhigher than previous fitting result by Barton andHowlett [3], and 40–45 kcal/mol higher than thevalue by Weissman and Benson [5].

The B3 or MP2 calculated Cl2 molecular elimi-nation channel is not important when compared

Table 4

Total electron energies at 298 K for reactant, TS, and products in the title reaction at two calculation levelsa

Species B3LYP/6-31++G(d) CBS-Q ZPVEb ;c H298–H0c

//B3LYP/6-31++G(d)

CH2Cl–CH2Cl )998.9609776 )997.9422611 36.03 3.67

TS )998.8152413 )997.7914800 32.74 4.22

Cl2 )920.3480719 )919.4559503 0.72 2.21

C2H4 )78.5392347 )78.4116395 31.44 2.50

Species MP2/6-31++G(d) G3MP2 ZPVEc ;d H298–H0c

//MP2/6-31++G(d)

CH2Cl–CH2Cl )997.4949619 )997.9365505 36.93 3.60

TS )997.3249974 )997.7797772 33.12 4.03

Cl2 )919.1703259 )919.4417331 0.76 2.20

C2H4 )78.2376789 )78.4244047 31.72 2.51

a Total energies (ZPVE and thermal corrections [35] are included) in hartree, 1 hartree¼ 627.51 kcal/mol.b Scaled by 0.9806 (from Scott and Radom, JPC 1996).c In kcal/mol.d Scaled by 0.9670 (from Scott and Radom, JPC 1996).

Table 5

Ideal gas-phase thermochemical propertiesa

DH 0f 298 S0

298 C0pðT Þ

300 K 400 K 500 K 600 K 800 K 1000 K 1500 K

Cl2 0.00 53.29 8.12 8.44 8.62 8.74 8.88 8.96 9.07

C2H4 12.55 52.41 10.31 12.55 14.86 16.96 20.10 22.27 26.36

//B3LYP/6-31++G(d)

CH2ClCH2Cl )31.05 75.35 18.68 22.04 24.74 26.89 30.14 32.54 36.32

TS 61.69b 73.83 19.10 22.03 24.52 26.57 29.72 32.08 35.92

//MP2/6-31++G(d)

CH2ClCH2Cl )31.05 74.95 18.18 21.35 24.08 26.33 29.76 32.27 36.19

TS 66.78c 75.82 18.70 21.71 24.24 26.30 29.45 31.81 35.68

aDH 0f 298 in kcal/mol, S0

298 and C0pðT Þ in cal/mol K.

b This work; CBS-Q // B3LYP/6-31++G(d).c This work, G3MP2 // MP2/6-31++G(d).

70 L. Zhu, J.W. Bozzelli / Chemical Physics Letters 357 (2002) 65–72

with the CACl bond cleavage path shown in Table1, because it is at least 10 kcal/mol above the Clradical elimination channel. This Cl2 eliminationchannel has similar barrier to the H radical elimi-nation channel. On the other hand, pre-exponentialA factor of molecular elimination is �3 orders ofmagnitude lower than that for dissociation channelsto radicals.

We estimate uncertainty of Cl2 elimination rateconstant from the error of DH 0

f 298 and S0298. The

DHTS-rectant (from DH 0f 298) is estimated to have 2

kcal/mol error, resulting expð�Ea=RTÞ has 270%error at 1000 K; DSTS-reactant (from S0

298) has 2 cal/mol K error, resulting A factor has 270% error; thisresults in an overall error up to 750%. Assuming therate constant of molecular Cl2 channel is 7.5 timesfaster than the current value shown in Table 6,which is 3.69E) 6 s�1 at B3 level or 8.17E) 8 s�1 at

MP2 level at 1000 K; both are still several orders ofmagnitude slower than the rate constant forClþ CH2CH2Cl at 1000 K (5E) 3 s�1 [20]), and sixto ten orders of magnitude slower than the vinylchloride + HCl channel (dominant channel forCH2ClCH2Cl decomposition) at this temperature.

6. Summary

The transition state for Cl2 molecular elimina-tion from 1,2-dichloroethane is identified byB3LYP/6-31++G(d) and MP2/6-31++G(d) calcu-lations. The Cl2 molecular elimination is not animportant channel in the unimolecular decomposi-tion of 1,2-dichloroethane and it is probably nottoo important in other chlorocarbon thermal dis-sociations.

Table 6

Outputs of THERMKIN calculations for Cl2 elimination from 1,2-dichloroethane at two different levels

(1) CH2ClCH2Cl¼TS // B3LYP/6-31++G(d)

DH 0f 298 (kcal/mol) )31.55 61.69

S0298 (cal/mol K) 75.35 78.83

T (K) DH (kcal/mol) DS (cal/mol K) A ðs�1Þ T n k ðs�1Þ300 93.24 3.483 3.608E+13 1.109E+02 4.223E) 55

400 93.27 3.557 4.993E+13 1.407E+02 5.448E) 38

500 93.25 3.530 6.157E+13 1.691E+02 1.060E) 27

600 93.22 3.469 7.165E+13 1.966E+02 7.895E) 21

800 93.13 3.341 8.957E+13 2.493E+02 3.222E) 12

1000 93.05 3.247 1.068E+14 2.997E+02 4.917E) 07

1500 92.86 3.097 1.485E+14 4.189E+02 4.375E+00

2000 92.63 2.965 1.853E+14 5.311E+02 1.396E+04

Three-parameter model equation: kðT Þ ¼ A0 T n � expð�Ea=RTÞA0 ¼ 4:254E þ 11 (s�1) n ¼ 0:826 Ea ¼ 93:40 (kcal/mol)

(2) CH2ClCH2Cl ¼TS // MP2/6-31++G(d)

DH 0f 298 (kcal/mol) )31.55 66.78

S0298 (cal/mol K) 74.95 75.82

T (K) DH (kcal/mol) DS (cal/mol K) A ðs�1Þ T n k ðs�1Þ300 98.33 0.874 9.705E+12 1.619E+02 2.222E) 59

400 98.38 1.013 1.388E+13 2.093E+02 2.433E) 41

500 98.40 1.055 1.771E+13 2.553E+02 1.720E) 30

600 98.39 1.050 2.121E+13 3.004E+02 3.044E) 23

800 98.35 0.988 2.741E+13 3.883E+02 3.695E) 14

1000 98.28 0.911 3.295E+13 4.738E+02 1.089E) 08

1500 98.05 0.729 4.510E+13 6.802E+02 2.326E) 01

2000 97.75 0.556 5.513E+13 8.792E+02 1.145E+03

Three-parameter model equation: kðT Þ ¼ A0 T n � expð�Ea=RTÞA0 ¼ 7:624E þ 10 (s�1) n ¼ 0:892 Ea ¼ 98:48 (kcal/mol)

A ¼ A0T n

L. Zhu, J.W. Bozzelli / Chemical Physics Letters 357 (2002) 65–72 71

Acknowledgements

We acknowledge the USEPA Northeast Re-gional Research Centers and the USEPA ResearchCenter on Airborne Organics for the funding. Wealso acknowledge Dr. Chad Sheng for the ‘SMCPS’and ‘THERMKIN’ programs.

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