Journal of Fluorine Chemistry 159 (2014) 57–64

A DFT study on kinetics of the gas phase reactions of CH3CH2OCF3 withOH radicals and Cl atoms

Bhupesh Kumar Mishra a, Makroni Lily b, Arup Kumar Chakrabartty a,Ramesh Chandra Deka a,*, Asit K. Chandra b,**a Department of Chemical Sciences, Tezpur University, Tezpur, Assam 784 028, Indiab Department of Chemistry, North-Eastern Hill University, Shillong 793 022, India

A R T I C L E I N F O

Article history:

Received 3 September 2013

Received in revised form 18 November 2013

Accepted 4 December 2013

Available online 12 December 2013

Keywords:

H-abstraction

Hydrofluoroethers

Enthalpy of formation

G2(MP2)

Canonical transition state theory

A B S T R A C T

A theoretical study on the mechanism and kinetics of the gas phase reactions of CH3CH2OCF3 (HFE-263)

with the OH radicals and Cl atoms have been performed using meta-hybrid density functional MPWB1K

method and 6-31+G(d,p) basis set. Energetics are further refined by calculating the energy of the species

with a high level G2(MP2) method. Reaction profiles are modeled including the formation of pre-reactive

and post-reactive complexes at entrance and exit channels. The hydrogen abstraction from –CH2 group is

found to be the dominant reaction channel for reaction with OH radicals, whereas hydrogen abstraction

from CH3 group is the dominant channel for Cl atoms, especially at higher temperature. Using group-

balanced isodesmic reactions, the standard enthalpies of formation for CH3CH2OCF3 and radicals generated

by hydrogen abstraction, CH3CHOCF3 and CH2CH2OCF3 are reported for the first time. The calculated bond

dissociation energies for C–H bonds are in good agreement with experimental results. The rate constants of

the two reactions are determined for the first time in a wide temperature range of 250–1000 K. The

G2(MP2) calculated rate constant values are 0.52 � 10�13 and 0.77 � 10�12 cm3 molecule�1 s�1,

respectively for reactions with OH radicals and Cl atoms at 298 K.

� 2013 Elsevier B.V. All rights reserved.

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Journal of Fluorine Chemistry

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1. Introduction

In order to reduce the adverse effect of chlorofluorocarbons(CFCs) toward Earth’s stratospheric ozone layer their commercialproduction is banned as per the Montreal protocol and aninternational effort has gone into finding environmentally moreacceptable alternatives to replace chlorofluorocarbons [1,2]. In thiscontext, a number of CFCs replaceable compound such ashydrofluorocarbon (HFC) and hydrochlorofluorocarbon (HCFC)have been developed for short term use because of their non-reliability toward protection of ozone layer. Recently, volatileorganic compound especially hydrofluoroethers (HFEs) is designedand widely recommended as a third generation replacement forCFCs, HFCs and HCFCs in applications such as cleaning of electronicequipments, heat transfer fluid in refrigerators, lubricant deposi-tion and foam blowing agents [3–6]. Also HFEs are believed to bemore reactive in the troposphere due to the presence of –O– etherlinkage [7]. Although HFEs do not contain Cl atom and have zeroozone depletion potential, they are potential greenhouse gases

* Corresponding author. Tel.: +91 3712267008.** Corresponding author. Tel.: +91 3642722622.

E-mail addresses: ramesh@tezu.ernet.in (R.C. Deka), akchandra@nehu.ac.in

(A.K. Chandra).

0022-1139/$ – see front matter � 2013 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.jfluchem.2013.12.002

because of their strong absorption in the range of 1000–3000 cm�1

[8,9]. Reactions of HFEs with OH radicals constitute the maindegradation channel of HFEs in the troposphere [10]. The majordegradation pathways of HFEs in atmosphere may be initiated byphotolytic degradation with OH radicals. Although the reactionwith OH radicals constitutes the main tropospheric sink fordegradation of halogenated ethers, the chlorine atom plays animportant role in the atmospheric chemistry [11]. In fact, chlorineatoms have been monitored in concentrations in the order of104 molecule cm�3 over the marine boundary layer [12]. Thus, it isimportant to study the kinetics and mechanistic degradationpathways of HFEs for complete assessment of atmosphericchemistry as well as to explore the impact of HFEs on theenvironment. Therefore, considerable attention has been paid inrecent years to perform experimental and theoretical studies onthe decomposition kinetics of HFEs [13–30]. In the present work,we have investigated the hydrogen abstraction reactions betweenCH3CH2OCF3 (HFE-263) and OH radicals and Cl atoms by usingquantum chemical methods. To the best of our knowledge, verylittle attention has been paid to this reaction and this is the firstdetail theoretical study for this important reaction. Recently, thisreaction has been studied experimentally by Oyaro et al. [31] bythe relative rate method at 298 K using gas chromatography–massspectroscopy (GC–MS) detection and reported a rate constants as

B.K. Mishra et al. / Journal of Fluorine Chemistry 159 (2014) 57–6458

k(OH + CH3CH2OCF3) = 1.55 � 10�13 and k(Cl + CH3CH2OCF3)= 2.2 � 10�12 cm3 molecule�1 s�1, respectively. Our calculationindicates that one reaction channel from –CH2 and two reactionchannels from –CH3 groups are feasible for the CH3CH2OCF3 + OH/Cl reactions as given below:

CH3CH2OCF3þ OH ! CH3CHOCF3þ H2O (R1)

CH3CH2OCF3þ OH ! CH2CH2OCF3þ H2O (R2)

CH3CH2OCF3þ Cl ! CH3CHOCF3þ HCl (R3)

CH3CH2OCF3þ Cl ! CH2CH2OCF3þ HCl (R4)

It has been reported from experimental findings that –CH2 ismore reactive than the hydrogen atoms at the –CH3 group [31].However, experimental studies provided only the total rateconstant and it is difficult to predict the detailed mechanism andthermo chemistry. Thus, for better understanding of mechanisticpathways, kinetics and thermochemistry we must rely onquantum chemical methods. The aim of the present paper isto have a more accurate thermo chemical data using G2(MP2)method. Canonical Transition State Theory (CTST) is also utilizedto predict the rate constant of the title reactions on the basis ofab initio data obtained during the present investigation. To thebest of our knowledge this is the first detailed theoretical studyof the above mentioned H-abstraction reactions of HFE-263 withOH radicals and Cl atoms. Bond dissociation energies (BDEs) ofthe breaking C–H bonds are known to be strongly correlatedwith the observed reactivity trend for the hydrogen abstractionreaction, and the ether linkage (–O–) is important for thereactivity of the haloethers. Thus, we present BDE of the twotypes of C–H and C–O bonds in CH3CH2OCF3. In addition, theknowledge of accurate enthalpy of formation (DfH2988) forCH3CH2OCF3 and radicals generated CH3CHOCF3 andCH2CH2OCF3 is of vital importance for determining thethermodynamic properties and atmospheric modeling. Howev-er, no theoretical or experimental study on standard enthalpy offormation has been reported so far for these species. Here, wepredict the enthalpies of formation using isodesmic reactions byperforming single-point energy calculation at high level oftheory, G2(MP2) with geometry parameters obtained at theMPWB1K/6-31+G(d,p) level. We also report here the rateconstant for the reactions ((R1)–(R4)) in a wide temperaturerange of 250–1000 K.

2. Results and discussion

The calculated enthalpy of reactions (DrH8) and reaction freeenergies (DrG8) at 298 K for the reaction of CH3CH2OCF3 with OHradicals and Cl atoms are recorded in Table 1. Thermal correctionsto the energy at 298 K were included in the determination of thesethermodynamic functions. The free energy values show that allreaction channels are exergonic (DG < 0) and therefore should bespontaneous in nature. The DrG8 values for CH3CH2OCF3 + OH

Table 1Thermochemical data for the H abstraction reaction channels of CH3CH2OCF3

calculated at G2(MP2) and MPWB1K/6-31+G(d,p) (within parenthesis) level of

theories. All values are in kcal mol�1.

Reaction channels DrH8 DrG8

Reaction (R1) �19.39 (�17.05) �20.80 (�18.45)

Reaction (R2) �16.34 (�11.80) �18.09 (�13.55)

Reaction (R3) �4.01 (�2.89) �6.65 (�5.53)

Reaction (R4) �0.97 (2.35) �3.94 (�0.62)

reaction is much more negative than that for the CH3CH2OCF3 + Clreaction indicating the former reaction is thermodynamically morefavorable than the latter. The enthalpy of reaction (DrH2988) valuesgiven in Table 1 for (R1) and (R2) shows that both the reactions aresignificantly exothermic in nature.

There are two potential hydrogen abstraction sites ofCH3CH2OCF3, namely the –CH2 and –CH3 group. However, ascan be seen from the geometrical parameters and stereogra-phical orientation, the hydrogen atoms in the –CH3 group arenot equivalent. One H-atom is different from the other two inthe –CH3 group. Three transition states (TS) are thereforelocated for each of the two reactions CH3CH2OCF3 + OH andCH3CH2OCF3 + Cl; one TS for H-abstraction from the –CH2 group(TS1 for reaction with OH and TS3 for reaction with Cl atom) andtwo TSs for the same from the –CH3 group (TS2a and TS2b forreaction with OH and TS4a and TS4b for reaction with Cl).Therefore three H-abstraction reaction channels exist for boththe reactions studied here. In the entrance channel for reaction(R1) and one of the channels for (R2) (through TS2b and definedas R2b), pre-reactive complexes ((R1) and (R2)) have been foundin the present work. In the exit channels, there are also productcomplexes occurring before the release of the final products,which are labeled as PC1 and PC2. In pre-reactive complexes(R1) and (R2), hydrogen bonds are formed between the oxygenatom of hydroxyl radical with the hydrogen atom inCH3CH2OCF3 with bond distances of 2.71 and 2.79 A, respec-tively while the other bond lengths are very close to those inequilibrium structures. At the same time, the post-reactionhydrogen bonded complexes (PC1 and PC2) with energy lessthan the corresponding products are located at the exits of thereaction channels (R1) and (R2) for reactions with OH radicalswhich can be identified with relatively strong C–H� � �O and O–H� � �F bonds, as shown in Fig. 1. So it is clear that the reactionchannels (R1) and (R2) may proceed via indirect mechanisms.The search was made along the minimum energy path on arelaxed potential energy surface.

The optimized geometries of reactants, reactive complex,transition states, product complexes and products obtained atMPWB1K/6-31+G(d,p) level with available limited experimentalvalues for OH, H2O and HCl are shown in Fig. 1. Our calculatedvalues are in very good agreement with the correspondingexperimental values (given in parentheses) in Fig. 1 [32]. Duringthe formation of transition states, the important structuralparameters have to be observed are one of the C–H bond of theleaving hydrogen and the newly formed bond between H and Oatoms in the OH radical. It can be observed from Fig. 1 that thelength of the breaking C–H bond (C3-H5) increases from 1.087 to1.186 A (almost 9% increase) in the optimized structure of TS1,whereas the length of the newly formed H–O bond (H5-O2:1.349 A) is longer by about 40% from the normal O–H bondlength (0.953 A) in H2O. Similarly, for transition states TS2a andTS2b for reactions (R2), the length of the breaking C–H bondincreases by almost 12% whereas the newly formed H–O bondsare longer by almost 33% from the O–H bond length in H2O. Thefact that the elongation of forming bond is larger than that of thebreaking bond indicates that the barrier of the reaction is nearthe corresponding reactants. This means that the reactions (R1)and (R2) will proceed via early transition state. In the optimizedstructures of TS3, TS4a and TS4b, the reactive C–H bonds that willbe broken, increases by 24%, 35% and 34%, respectivelycompared to the C–H equilibrium bond length in CH3CH2OCF3,and the forming H–Cl bonds are about 16%, 9% and 10%,respectively longer than the regular bond length of the isolatedHCl. This implies that the barrier of the reactions (R3) and (R4) iscloser to the products, and that the reactions with Cl atomsproceed via a late transition state.

Fig. 1. Optimized geometries of reactants, reactant complex, transition states, product complex and products involved in the H atom abstraction reactions of

CH3CH2OCF3 + OH/Cl at MPWB1K/6-31+G(d,p) method. The experimental values are given in parentheses. Bond lengths are given in A.

B.K. Mishra et al. / Journal of Fluorine Chemistry 159 (2014) 57–64 59

Results obtained from frequency calculations for speciesinvolved in reactions (R1)–(R4) are recorded in Table 2. All thereactants, reactant complexes, product complexes and productswere identified as stationary points with zero imaginary frequen-cy, while transition states (TS1, TS2a, TS2b, TS3, TS4a and TS4b) wereidentified as first order saddle points with only one imaginaryfrequency. Visualization of the vibration corresponding to thecalculated imaginary frequencies shows a well defined transitionstate geometry connecting reactants and products during transi-tion. The existence of transition states, pre- and post-reaction

complexes on the potential energy surface is further ascertained byintrinsic reaction coordinate (IRC) calculation performed at thesame level of theory using the Gonzalez–Schlegel steepest descentpath in the mass-weighted Cartesian coordinates with a step sizeof 0.01 (amu1/2-bohr) [33].

The energy barriers calculated at the G2(MP2) level for thereactions ((R1)–(R4)) are recorded in Table 3. These results show thatenergy barriers for H atom abstraction by OH radical and Cl atomfrom the –CH2 group of CH3CH2OCF3 ((R1) and (R3)) are 1.60 and1.34 kcal mol�1, respectively at G2(MP2)//MPWB1K/6-31G+(d,p)

Table 2Harmonic vibrational frequencies of reactants, reactant complexes, transition states, product complexes and products at MPWB1K/6-31+G(d,p) level of theory.

Species Vibrational frequencies (cm�1)

CH3CH2OCF3 44, 115, 195, 253, 360, 442, 443, 607, 634, 698, 838, 882, 1002, 1142, 1166, 1210, 1230, 1322, 1361, 1383,

1440, 1493, 1514, 1534, 1561, 3136, 3141, 3196, 3226, 3234

OH 3868

RC1 44, 103, 113, 131, 191, 202, 211, 282, 363, 416, 441, 450, 605, 636, 696, 854, 878, 1001, 1134, 1165, 1211,

1217, 1324, 1358, 1388, 1442, 1493, 1517, 1536, 1574, 3137, 3158, 3216, 3227, 3242, 3866

RC2 24, 80, 86, 134, 143, 214, 248, 360, 366, 425, 443, 450, 609, 634, 701, 844, 886, 1003, 1135, 1165, 1209,

1250, 1328, 1366, 1370, 1446, 1495, 1524, 1534, 1560, 3134, 3153, 3208, 3223, 3234, 3824

TS1 923i, 30, 100, 134, 146, 176, 191, 249, 360, 440, 449, 607, 634, 698, 753, 826, 886, 953, 1017, 1153,

1182, 1192, 1238, 1331, 1363, 1382, 1406, 1434, 1476, 1509, 1522, 3132, 3183, 3220, 3240, 3907

TS2a 1415i, 36, 47, 64, 92, 136, 200, 313, 405, 443, 445, 609, 633, 662, 706, 828, 843, 929, 1063, 1136,

1152, 1212, 1236, 1270, 1321, 1355, 1374, 1403, 1468, 1518, 1556, 3155, 3184, 3216, 3272, 3908

TS2b 1469i, 32, 71, 122, 154, 192, 291, 359, 411, 444, 445, 604, 634, 669, 709, 864, 890, 931, 1009, 1144,

1161, 1244, 1289, 1328, 1337, 1377, 1462, 1475, 1498, 1543, 3130, 3184, 3200, 3273, 3895

TS3 780i, 28, 60, 127, 147, 192, 222, 367, 420, 449, 488, 619, 635, 701, 828, 899, 1011, 1073, 1107,

1177, 1242, 1250, 1277, 1313, 1379, 1414, 1467, 1503, 1511, 3121, 3198, 3212, 3246

TS4a 614i, 20, 30, 55, 117, 188, 328, 384, 442, 445, 609, 633, 658, 699, 809, 824, 881, 927, 1108, 1130,

1204, 1242, 1255, 1321, 1339, 1373, 1456, 1512, 1554, 3152, 3210, 3215, 3320

TS4b 733i, 26, 36, 100, 142, 191, 366, 432, 442, 448, 599, 610, 633, 704, 820, 875, 940, 956, 1007,

1157, 1227, 1241, 1314, 1331, 1381, 1451, 1493, 1524, 3086, 3165, 3210, 3320

PC1 41, 68, 81, 107, 150, 177, 185, 199, 219, 369, 440, 441, 512, 584, 612, 644, 700, 893, 1017, 1044,

1176, 1234, 1248, 1324, 1372, 1409, 1467, 1519, 1667, 3077, 3178, 3229, 3278, 3965, 4084

PC2 30, 27, 99, 115, 136, 177, 194, 231, 331, 359, 377, 441, 450, 584, 636, 652, 704, 855, 905, 1045,

1140, 1208, 1239, 1315, 1337, 1360, 1453, 1514, 1663, 3160, 3236, 3252, 3377, 3944, 4080

CH3CHOCF3 53, 96, 184, 187, 364, 440, 442, 562, 612, 638, 702, 895, 1019, 1037, 1175, 1250, 1258, 1326,

1377, 1418, 1476, 1496, 1515, 3075, 3167, 3225, 3281

CH2CH2OCF3 40, 106, 136, 195, 372, 436, 441, 455, 607, 633, 700, 858, 954, 1028, 1157, 1166, 1231, 1292,

1323, 1380, 1455, 1496, 1530, 3071, 3144, 3259, 3381

H2O 1636, 3973, 4100

HCl 3084

TS2b

6.23

TS2a

B.K. Mishra et al. / Journal of Fluorine Chemistry 159 (2014) 57–6460

level of theory. On the other hand, the same from the –CH3 group arefound to be 6.23 and 3.34 kcal mol�1 for H-abstraction by OH radicaland 2.51 and 1.60 kcal mol�1 for H-abstraction by Cl atom. Thebarrier height values show that hydrogen abstraction by OH radicaland Cl atom from the –CH2 group of CH3CH2OCF3 is more facile thanthat from the –CH3 group. This finding is in line with theexperimental observation [31], as well as to the fact that thecalculated C–H bond dissociation energy from –CH2 group(98.6 kcal mol�1) is much lower than that for the –CH3 group(103.2 kcal mol�1). The barrier heights also reveal that H-abstractionby Cl atom is kinetically more favorable than that by OH radical,whereas reaction enthalpies predict that the latter reaction isthermodynamically more favorable. A potential energy diagrams ofthe title reactions are constructed with the results obtained at theG2(MP2)//MPWB1K/6-31+G(d,p) level and are shown in Figs. 2 and

Table 3Zero-point energy corrected associated energy barriers, DE in kcal mol�1 at

G2(MP2)//MPWB1K/6-31+G(d,p) level of theory for the reactants, reactant

complexes, transition states, product complexes and products.

Species DE

CH3CH2OCF3 + OH 0.00

RC1 �1.83

RC2 �2.16

TS1 1.60

TS2a 6.23

TS2b 3.34

PC1 �20.80

PC2 �16.92

CH3CHOCF3 + H2O �19.79

CH2CH2OCF3 + H2O �16.83

CH3CH2OCF3 + Cl 0.00

TS3 1.34

TS4a 2.51

TS4b 1.60

CH3CHOCF3 + HCl �4.52

CH2CH2OCF3 + HCl �1.57

3. In the construction of energy diagram zero-point energy correctedtotal energy data as recorded in Table 3 are utilized. These energiesare plotted with respect to the ground state energy of CH3CH2OC-F3 + OH/Cl including ZPE arbitrarily taken as zero.

Table 4 lists the calculated bond-dissociation energies, BDE(D298

0) of the C–H and C–O bonds of CH3CH2OCF3 molecule alongwith the available experimental data. The D298

0 value obtainedfrom the G2(MP2) results for the C–H bonds in the –CH2 and –CH3

3.34

-16.92

PC2

-2.16

RC2

-1.80

RC1

-20.80

PC1

P2 = CH2CH

2OCF

3

P1 = CH3CHOCF

3

R = CH3CH

2OCF

3

-16.83

P2 + H2O

P1 + H2O

-19 .79

1.60

0.00

R + OH

TS1

En

erg

y +

ZP

E (

kcal m

ol-1

)

Fig. 2. Schematic potential energy (including ZPE) profiles for the CH3CH2OCF3 + OH

reaction at the G2(MP2)//MPWB1K/6-31+G(d,p) level.

-4.52

-1.57

P2 + HC l

P1 + HC l

2.51

1.34

TS4b

TS3

P2 = CH2CH

2OCF

3

P1 = CH3CHOCF

3

R = CH3CH

2OCF

3

1.60

0.00

R + Cl

TS4a

En

erg

y +

ZP

E (

kcal m

ol-1

)

Fig. 3. Schematic potential energy (including ZPE) profiles for the CH3CH2OCF3 + Cl

reaction at the G2(MP2)//MPWB1K/6-31+G(d,p) level.

B.K. Mishra et al. / Journal of Fluorine Chemistry 159 (2014) 57–64 61

sites of CH3CH2OCF3 amount to 98.65 and 103.17 kcal mol�1,respectively. These values for D0

298 conform well to theexperimental findings of Oyaro et al. [31] for the C–H bondenergies at two different carbon center [CH3CH(–H)OCF3

(96.88 kcal mol�1)] and [CH2(–H)CH2OCF3 (102.87 kcal mol�1)]of CH3CH2OCF3. The good agreement between the theoreticaland experimental above-mentioned results implies that theG2(MP2)//MPWB1 K/6-31 + G(d,p) level is a suitable method tocompute the bond dissociation energies. Moreover, owing to thelower C–H bond dissociation energy, –CH2 group is more reactivetoward hydrogen abstraction than –CH3 group. This is reflected inthe calculated barrier height for hydrogen abstraction from –CH2

and –CH3 groups.The standard enthalpy of formation (DfH2988) at 298 K for

CH3CH2OCF3 and the two radicals generated from hydrogenabstraction, CH3CHOCF3 and CH2CH2OCF3 can be valuableinformation for understanding the mechanism and thermochemi-cal properties of their reactions and most importantly foratmospheric modeling, but these values are not yet reported.The group-balanced isodesmic reactions, in which the number andtypes of bonds are conserved, are used as working chemicalreactions herein to calculate the DfH2988. Here, three isodesmic

Table 4Calculated bond dissociation energy (D298

0) (kcal mol�1) for species at 298 K at

G2(MP2)//MPWB1K/6-31+G(d,p) level of theory.

Bond dissociation type G2(MP2) Exp. valuesa

C–H bond

CH3CH2OCF3! CH3CHOCF3 + H 98.65 96.88

CH3CH2OCF3! CH2CH2OCF3 + H 103.17 102.87

C–O bond

H3CH2OCF3! CH3CH2O + CF3 104.81

CH3CH2OCF3! CH3CH2 + OCF3 98.95

a Experimental values from Ref. [31].

reactions are used to estimate the enthalpies of formation of theCH3CH2OCF3. The used isodesmic reactions are as follows:

a. For CH3CH2OCF3

CH3CH2OCF3þ CH4 ! CH3OCH3þ CH3CF3 (R5)

CH3CH2OCF3þ CH2F2 ! CF3OCHF2þ CH3CH3 (R6)

CH3CH2OCF3þ CHF3 ! CH3OCH3þ CF3CF3 (R7)

b. For CH3CHOCF3

CH3CHOCF3þ CH4 ! CH3CH2OCF3þ CH3 (R8)

CH3CHOCF3þ CF2Cl2 ! CF3OCHF2þ CH3CCl2 (R9)

CH3CHOCF3þ CH2FCl ! CH3OCH3þ CF3CFCl (R10)

c. For CH2CH2OCF3

H2CH2OCF3þ CH4 ! CH3CH2OCF3þ CH3 (R11)

CH2CH2OCF3þ CHF2Cl ! CF3OCHF2þ CH3CHCl (R12)

CH2CH2OCF3þ CH3Cl ! CH3OCH3 þ CF3CHCl (R13)

All geometrical parameters of the species involved in theisodesmic reactions ((R5)–(R13)) were first optimized at theMPWB1K/6-31+G(d,p) level and then energies of the species werefurther refined by performing single point calculations at thesophisticated G2(MP2) level of theory. At first we have calculatedthe reaction enthalpies (DrH2988) of the isodesmic reactions ((R5)–(R13)) as mentioned above using total energies of the speciesobtained at G2(MP2) level and including thermal correction toenthalpy estimated at MPWB1K/6-31+G(d,p) level. Since, the(DrH2988) value corresponds to the difference of the enthalpy offormation (DfH2988) values between the products and thereactants, the (DfH2988) values of the reactant and product speciesare easily evaluated by combining them with the known enthalpiesof formation of the reference compounds involved in our isodesmicreaction schemes. The experimental DfH2988 values for CH4:�17.91 kcal mol�1, CH3OCH3: �44.0 kcal mol�1, CH3CF3:�179.1 kcal mol�1, CH3CH3: �20.01 kcal mol�1, CF3OCHF2:�312.30 kcal mol�1 and CH3: 34.85 kcal mol�1 are taken fromRef. [24], CH2F2: �107.77 kcal mol�1, CHF3: �166.24 kcal mol�1,CF2Cl2: �117.72 kcal mol�1, CH3Cl: �19.76 kcal mol�1, CH2Cl2:�22.41 kcal mol�1, CHF2Cl: �115.35 kcal mol�1 and CH2FCl:�63.13 kcal mol�1 are taken from Ref. [34], CF3CF3:�321.20 kcal mol�1 [35], CF3CHCl: �131.93 kcal mol�1 [36],CF3CFCl: �173.41 kcal mol�1 [37], CH3CCl2: 10.16 kcal mol�1

[38] and CH3CHCl: 18.30 kcal mol�1 [38] to evaluate the requiredenthalpies of formation.

Table 5Enthalpies of formation (DfH298

0) (kcal mol�1) at 298 K from the isodesmic

reactions.

Species Isodesmic reaction schemes G2(MP2) Average value

CH3CH2OCF3 (R5) �219.69 �219.53

(R6) �219.71

(R7) �219.20

CH3CHOCF3 (R8) �172.32 �173.02

(R9) �173.88

(R10) �172.86

CH2CH2OCF3 (R11) �168.01 �168.49

(R12) �168.58

(R13) �168.89

Table 6Rate constant values (in cm3 molecule�1 s�1) for hydrogen abstraction reactions of

CH3CH2OCF3 with OH radicals and total rate constant (kOH) values as calculated

using G2(MP2) barrier height.

Temp (K) kR1 kR2a kR2b kOH (ktotal)

250.0 0.3495E�13 0.4910E�16 0.6366E�14 0.4136E�13

298.0 0.4500E�13 0.1599E�15 0.7603E�14 0.5276E�13

350.0 0.5992E�13 0.4853E�15 0.1032E�13 0.7072E�13

450.0 0.1008E�12 0.2649E�14 0.1965E�13 0.1231E�12

550.0 0.1603E�12 0.9533E�14 0.3578E�13 0.2057E�12

650.0 0.2424E�12 0.2609E�13 0.6088E�13 0.3294E�12

750.0 0.3512E�12 0.5925E�13 0.9737E�13 0.5079E�12

850.0 0.4911E�12 0.1177E�12 0.1479E�12 0.7567E�12

950.0 0.6663E�12 0.2118E�12 0.2150E�12 0.1093E�11

1000.0 0.7684E�12 0.2758E�12 0.2557E�12 0.1300E�11

Table 7Rate constant values (in cm3 molecule�1 s�1) for hydrogen abstraction reactions of

CH3CH2OCF3 with Cl atoms and total rate constant (kCl) values as calculated using

G2(MP2) barrier height.

Temp kR3 kR4a kR4b kCl (ktotal)

250.0 0.1934E�12 0.4391E�13 0.2111E�12 0.4484E�12

298.0 0.3125E�12 0.1081E�12 0.3578E�12 0.7783E�12

350.0 0.4851E�12 0.2331E�12 0.5761E�12 0.1294E�11

450.0 0.9638E�12 0.7085E�12 0.1203E�11 0.2876E�11

550.0 0.1664E�11 0.1614E�11 0.2159E�11 0.5437E�11

650.0 0.2617E�11 0.3088E�11 0.3504E�11 0.9208E�11

750.0 0.3847E�11 0.5256E�11 0.5291E�11 0.1439E�10

850.0 0.5374E�11 0.8231E�11 0.7565E�11 0.2117E�10

950.0 0.7213E�11 0.1210E�10 0.1036E�10 0.2968E�10

1000.0 0.8253E�11 0.1440E�10 0.1196E�10 0.3462E�10

B.K. Mishra et al. / Journal of Fluorine Chemistry 159 (2014) 57–6462

The calculated values of enthalpies of formation are listed inTable 5. As can be seen from Table 5, the values of DfH2988 for thespecies obtained by the three working chemical reactions arereasonably consistent with each other. The DfH2988 for CH3CH2OCF3,CH3CHOCF3 and CH2CH2OCF3 species calculated from G2(MP2)results are �219.53, �173.02 and �168.49 kcal mol�1, respectively.TheDfH2988 values for CH3CHOCF3 and CH2CH2OCF3 radicals can alsobe easily calculated from the reported DrH2988 values for reactions(R1) and (R2) in Table 1, the calculated DfH2988 value for CH3CH2OCF3

and the experimental DfH2988 values for H2O (�57.8 kcal mol�1) andOH (8.93 kcal mol�1) radical [28]. The DfH2988 values for CH3CHOCF3

and CH2CH2OCF3 radicals calculated from the G2(MP2) results are�172.19 and �169.14 kcal mol�1, respectively which is in goodagreement with the calculated values from isodesmic reactions. Theexperimental heat of reactions for (R1) and (R2) calculated fromDfH2988 values for CH3CHOCF3 and CH2CH2OCF3 radicals as reportedin Table 5 are found to be �20.22 and �15.69 kcal mol�1,respectively which are close to our G2(MP2) values as reported inTable 1 [32].

Although no comparison between theory and experiment canbe made due to the lack of the experimental results, the heat offormation (DfH2988) values can be expected to provide goodreference information for upcoming laboratory investigationsbecause G2(MP2) method is known to produce reliable thermo-chemical data.

The rate constant for title reactions are calculated by usingCanonical Transition State Theory [39,40] given by the followingexpression:

k ¼ sG ðTÞ kBT

h

QTS

QA:QBexp�DE

RT(1)

where s is the number of equivalent H-atoms, G(T) is the tunnelingcorrection factor at temperature T. QTS, QA and QB are the totalpartition functions (per unit volume) for the transition states andreactants, respectively. DE# is the barrier height including zeropoint energy correction, kB is the Boltzmann constant, h is thePlanck’s constant and R represents the universal gas constant. Thetunneling correction was estimated by using the Eckart’sunsymmetrical barrier method [41]. In the calculation of reactantelectronic partition function, the excited state of the OH radicals isincluded, with a 140 cm�1 splitting: the 2P3/2 and 2P1/2 electronicstates of Cl atoms are also included with 881 cm�1 splitting due tospin-orbit coupling. The partition functions for the respectivetransition states and reactants at any temperature are calculatedwith rigid rotor and harmonic oscillator approximations and thevibrational frequencies obtained from at the MPWB1K/6-31+G(d,p) level are used. As discussed before, the H-abstractionby OH radicals from the –CH2 group and also for one channel (R2b)from the –CH3 group proceeds via two step mechanism. The firststep involves a fast pre-equilibrium (Keq) between the reactantsand the hydrogen bonded reaction complex (RC) and the secondstep is the hydrogen abstraction with the rate constant Ky2. Theoverall rate constant including equilibrium constant (Keq) and rateconstant (Ky2) are given by,

Keq ¼QRC

QA � QB

eðER � ERCÞRT

(2)

and Ky2 can be obtained from TST in the from

Ky2 ¼ sG ðTÞ kBT

h

QTS

QRC

e�ðETS � ERCÞRT

(3)

The rate constant for H-abstraction from CH3CH2OCF3 viareaction (R1) is then obtained by the following expression,

k ¼ Keq � ky2 ¼ sG ðTÞ kBT

h

QTS

QA � QB

e�ðETS � ERÞRT

(4)

where QA, QB, QRC and QTS represent the total partition functions(per unit volume) of the reactants, reaction complex and transitionstates, respectively. ETS, ERC and ER are the total energies (ZPEcorrected) of transition state, reaction complex and reactants,respectively. Thus, it seems that the final expression (Eq. (4)) forestimating rate constant and barrier height turns out to be theusual CTST expression (Eq. (1)) used for the determination of rateconstant and barrier height of a direct reaction, irrespective of theenergy of pre-reactive hydrogen bonded complex (RC). However,the tunneling factor G(T) will obviously be different due to pre- andpost-reaction complex formation and as a result will affect thetotal rate constant.

The calculated total rate constant (kOH and kCl) values forhydrogen abstraction reactions of CH3CH2OCF3 with OH radicalsand Cl atoms within a range of 250–1000 K are presented in Tables6 and 7. The overall rate constant for H atom abstraction reactionsof CH3CH2OCF3 by OH radicals and Cl atoms as given by reactions((R1)–(R4)) are calculated as kOH = 0.52 � 10�13 cm3

molecule�1 s�1 and kCl = 0.77 � 10�12 cm3 molecule�1 s�1 at298 K. Our calculated rate constant values for H atom abstractionreaction of CH3CH2OCF3 by OH radicals and Cl atoms are inreasonable agreement with the experimental values ofkOH = 1.55 � 10�13 and kCl = 2.2 � 10�12 cm3 molecule�1 s�1, re-spectively reported by Oyaro et al. [31]. The rate constants valuesfor different reaction channels and total rate constant value forCH3CH2OCF3 + OH/Cl reactions are also shown in Figs. 4 and 5.Significant non-Arrhenius behavior can be seen from these plots,which is of course expected for multichannel reaction and effect oftunneling.

The temperature variation of the hydrogen abstraction rateconstant for this CH3CH2OCF3 + OH/Cl reaction can be described bya three parameter model equation developed by Truhlar et al. [42]:

k ¼ C exp�ðD1 � ðD2=TÞÞ

RT

� �(5)

1.0 1.5 2.0 2.5 3.0 3.5 4.0

-39

-38

-37

-36

-35

-34

-33

-32

-31

-30

-29

-28

-27

lnk (

cm

3 m

ole

cu

le-1 s

-1)

1000/T( K-1)

kOH

kR1

kR2a

kR2b

expa

Fig. 4. Rate constants for hydrogen abstraction reactions of CH3CH2OCF3 with OH

radicals and total rate constant (kOH) for the CH3CH2OCF3 + OH reactions. aRef. [31].

B.K. Mishra et al. / Journal of Fluorine Chemistry 159 (2014) 57–64 63

where C, D1 and D2 are fitting parameters (D1 > 0 and D2 > 0), and R

is gas constant. The units of C, D1 and D2 in the present case arecm3 molecule�1 s�1, kcal mol�1 and K kcal mol�1, respectively

From above Eq. (5), the activation energy, Ea is given by:

Ea ¼ D1 �2D2

T

� �(6)

The G2(MP2) calculated rate constant values for the reactionbetween CH3CH2OCF3 with OH radical and Cl atom in thetemperature range of 250–1000 K is fitted in the model Eq. (5)and found to be well described by the following equations:

kOH ¼ 1:82 � 10�11exp�ð3204:1 � 427; 476=TÞ

T

� �ðR2

¼ 0:994Þ (7)

1.0 1.5 2.0 2.5 3. 0 3.5 4. 0

-31

-30

-29

-28

-27

-26

-25

-24

lnk (

cm

3 m

ole

cu

le-1 s

-1)

1000/T (K-1)

kCl

kR3

kR4a

kR4b

ex pa

Fig. 5. Rate constants for hydrogen abstraction reactions of CH3CH2OCF3 with Cl

atoms and total rate constant (kCl) for the CH3CH2OCF3 + Cl reactions. aRef [31].

(8)kCl ¼ 8:6 � 10�10exp �ð3731:8�501;828=TÞT

h iðR2 ¼ 0:995Þ

The correlation coefficient values for both the equations areclosed to 1 indicating the good fitting of the rate constant values inthe model equation. The value of activation energy, Ea ofCH3CH2OCF3 with OH and Cl, calculated from Eq. (6) is 0.67 and0.72 kcal mol�1, respectively at 298 K. The rate constant values inthis temperature range can be useful for atmospheric modelingcalculations that can help to assess the atmospheric lifetimes.

3. Conclusions

The potential energy surface and reaction kinetics of the H atomabstraction reactions of CH3CH2OCF3 with OH radicals and Clatoms are investigated at G2(MP2)//MPWB1K/6-31+G(d,p) level oftheory. The barrier heights for dominant reaction pathways (R1)and (R3) calculated at G2(MP2) level are found to be 1.60 and1.34 kcal mol�1, respectively. The thermal rate constant for the Hatom abstraction of CH3CH2OCF3 by OH radicals and Cl atoms arefound to be 0.52 � 10�13 and 0.77 � 10�12 cm3 molecule�1 s�1,respectively at 298 K which are in reasonable agreement with theavailable experimental data. The rate constants are provided forthe first time in a wide temperature range of 250–1000 K. To thisend, three parameter model equations kOH = 1.82 � 10�11 exp[�(3204.1 � 427,476/T)/T] and kCl = 8.6 � 10�10 exp [�(3731.78 �501,828/T)/T] have been proposed to describe the rate constants inthis temperature range. Our calculations suggest that the H-abstraction from the –CH2 group is kinetically and thermodynam-ically more favorable than that from the –CH3 group forCH3CH2OCF3 + OH reaction. On the other hand, rate constantsfor H-abstraction from the –CH3 group is found to be slightlyhigher than that from the –CH2 group for CH3CH2OCF3 + Clreaction. Of course the H-abstraction from the –CH2 group isthermodynamically more favorable even for CH3CH2OCF3 + Clreactions because of its more exothermic nature. The DfH2988 forCH3CH2OCF3 and corresponding product species CH3CHOCF3 andCH2CH2OCF3 are predicted to be �219.53, �173.02 and�168.49 kcal mol�1, respectively at G2(MP2) level.

4. Computational methods

Ab initio quantum mechanical calculations were performedwith the Gaussian 09 suite of program [43]. Geometry optimiza-tion of the reactants, products and transition states were made atthe MPWB1K level of theory [44] using 6-31+G(d,p) basis set. Thehybrid meta-density functional, MPWB1K has been found to givereliable results for thermo chemistry and kinetics [45]. In order todetermine the nature of different stationary points on thepotential energy surface, vibrational frequencies calculationswere performed using the same level of theory at which theoptimization was made. All the stationary points had beenidentified to correspond to stable minima by ascertaining that allthe vibrational frequencies had real positive values. The transitionstates were characterized by the presence of only one imaginaryfrequency. To ascertain that the identified transition statesconnect reactants and products smoothly, intrinsic reactioncoordinate (IRC) calculations [33] were performed at theMPWB1K/6-31+G(d,p) level. As the reaction energy barriers arevery much sensitive to the theoretical levels, the higher-ordercorrelation corrected relative energies along with the densityfunctional energies are necessary to obtain theoretically consis-tent reaction energies. Therefore, a potentially high-level methodsuch as G2(MP2) has been used for single-point energy calcula-tions. The G2(MP2) [46] energy is calculated in the followingmanner:

B.K. Mishra et al. / Journal of Fluorine Chemistry 159 (2014) 57–6464

E½G2ðMP2Þ� ¼ Ebase þ DEðMP2Þ þ HLC þ ZPE

where,

Ebase ¼ E½QCISDðTÞ=6-311Gðd; pÞ�;

DEðMP2Þ ¼ E½MP2=6-311 þ Gð3df; 2pÞ�

� E½MP2=6-311Gðd; pÞ�; and

HLC (high level correction) = �0.00481nb � 0.00019na (na andnb are the number of a and b valence electrons with na � nb) andZPE = zero-point energy.

In this method the geometry and frequency calculations wereperformed at MPWB1K/6-31+G(d,p) level. The ZPE thus, obtainedwas corrected with a scale factor of 0.951 to partly eliminate thesystematic errors [44]. This dual level calculation (G2(MP2)//MPWB1K/6-31+G(d,p)) is known to produce reliable kinetic data[47–54]. Spin contamination was not important for the open shellradicals CH3CHOCF3, CH2CH2OCF3 and transition states because<S2> value never exceeded 0.76 at MPWB1K/6-31+G(d,p) levelbefore annihilation; that is only slightly larger than the expectedvalue of <S2> = 0.75 for doublets.

Acknowledgments

The authors acknowledge financial support from the Depart-ment of Science and Technology, New Delhi in the form of a project(SR/NM.NS-1023/2011(G)). BKM is thankful to University GrantsCommission, New Delhi for providing Dr. D. S. Kothari Post doctoralFellowship. AKC acknowledges CSIR, New Delhi, for financialassistance through project no. 01(2494)/11/EMR-II.

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