Ab Initio Study of the Catalytic Decomposition of Tetrafluoroethylene

The reaction of tetrafluoroethylene with molecular oxygen was considered as the ... in the 3A state, was determined and found to belong to point group...
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11276

J. Phys. Chem. 1996, 100, 11276-11281

Ab Initio Study of the Catalytic Decomposition of Tetrafluoroethylene by Molecular Oxygen Steven R. Davis* and Baoan Yu Department of Chemistry, UniVersity of Mississippi, UniVersity, Mississippi 38677 ReceiVed: January 19, 1996; In Final Form: March 13, 1996X

The reaction of tetrafluoroethylene with molecular oxygen was considered as the following three-step process: TFE + O2 f C2F4OO f CF2 + CF2OO f 2CF2 + O2, with the overall reaction being C2F4 + O2 f 2CF2 + O2. The equilibrium geometries and vibrational frequencies were determined for all reactants, products, and transition states, as well as activation energies and reaction enthalpies. Geometries were obtained at the MP2/6-311G(d) level while energies were computed at the MP4SDTQ/6-311G(d)||MP2/6-311G(d) computational level. The •CF2-CF2OO• biradical was characterized for the first time and found to belong to the Cs point group and the 3A′′ ground electronic state. The equilibrium geometry for the •CF2OO• biradical, in the 3A state, was determined and found to belong to point group C1. The second step was found to be rate determining with an activation energy of 54.0 kcal/mol. The overall reaction enthalpy of 62.7 kcal/mol agrees well with the experimental value of 66 kcal/mol.

Introduction There have been a number of studies published in the literature concerning the oxidation of tetrafluoroethylene (TFE) under various conditions. Modica and LaGraff1 studied the kinetics of the oxidation of TFE by molecular oxygen behind incident shock waves. They proposed that the reaction occurs in two steps. First, the decomposition of TFE, according to C2F4 f 2CF2, occurs with subsequent oxidation. They also state that at the temperatures studied (1200-2400 K) O2 does not react with TFE directly, but with the CF2 formed from the previous dissociation step. The reaction appeared to be first order in both CF2 and O2. Keating and Matula2 reported results for the oxidation of TFE also behind reflected shock waves in the temperature range 1670-2500 K. The experimental data support the reaction CF2 + O2 f CF2O + O for oxygen-lean conditions, but in oxygen-rich experiments complicating reactions were evident. Heicklen and Knight3-4 have studied the reaction of TFE with O atoms in the presence of molecular oxygen. They proposed the formation of the C2F4O epoxide from the reaction between TFE and O. A proposed intermediate is the CF2O2 radical from the reaction of 3CF2 and O2 as well as that for C2F4O* + O2 f CF2O + CF2O2. The major end products found were CF2O, c-C3F6, and the epoxide. The thermal oxidation of TFE using a plug flow reactor has been studied previously by Heicklen and Knight.4 They allowed a mixture of TFE and O2 to flow through a Pyrex tube heated to temperatures between 500 and 900 K and monitored the effluent gas by IR spectroscopy. The major product was found to be CF2O at all temperatures, while small amounts of c-C3F6, C3F6, and an unidentified C3 fluorocarbon were present depending on the furnace temperature. Since neither TFE nor O2 decomposes at these temperatures, the reaction was postulated as proceeding via a TFE-O2 encounter as either a gas-phase or wall reaction. The nature of this TFE-O2 intermediate was further studied by Chowdhury et al.5 and Pola and Ludvik.6 In the first paper,5 TFE was vibrationally excited by collisions with SF6 which had been heated by laser radiation. The excited TFE was reported to react with O2 through two mechanisms. One was through a dioxetane intermediate to yield CF2O in the X

Abstract published in AdVance ACS Abstracts, May 15, 1996.

S0022-3654(96)00206-7 CCC: $12.00

reaction TFE + O2 f OCF2-CF2O f 2CF2O. The other was from the direct oxidation of CF2. In a companion study,6 the addition of O2(3Σ) to the double bond of TFE proceeded to form the •CF2-CF2OO• triplet biradical, which reportedly undergoes triplet to singlet intersystem crossing and then reacts intramolecularly to yield the dioxetane. The dioxetane can then undergo a unimolecular decomposition reaction to yield CF2O as above. Summarizing the important details of the studies mentioned above, the initial shock tube studies for the reaction of TFE with O2, at high temperature, assumed a C2F4 a 2CF2 equilibrium with subsequent reaction of CF2 and O2 and direct production of carbonyl fluoride. At temperatures below the dissociation energies of TFE or O2, the production of carbonyl fluoride was postulated to proceed through a TFE-O2 intermediate. However, in a recent paper from this laboratory,7 the reaction of TFE and O2 was studied using the matrix isolation technique. Separate gas streams of Ar/TFE and Ar/O2 were allowed to mix briefly before condensation onto a 10 K matrix support. It was found that when an Ar/TFE stream was heated, in a plug flow reactor (above 900 K), prior to mixing with an O2 stream, the main product trapped in the matrix was CF2. There was a conspicuous absence of oxidation products, especially that of carbonyl fluoride. Due to the absence of oxidation products, and the fact that with the absence of O2 in the second stream no CF2 was detected, it was postulated that O2 catalyzed the C2F4 a 2CF2 reaction according to the following set of reactions:

•

C2F4(1Ag) + O2(3Σg-) f •CF2CF2OO•(3A′′)

(1)

CF2CF2OO•(3A′′) f :CF2(1A1) + •CF2OO•(3A)

(2)

CF2OO•(3A) f :CF2(1A1) + O2(3Σg-)

(3)

•

The sum of these three reactions is just the catalytic decomposition of TFE by molecular oxygen:

C2F4(1Ag) + O2(3Σg-) f 2:CF2(1A1) + O2(3Σg-)

(4)

One feature of this mechanism is that it is spin conservative for each step, occurring on the triplet potential surface, and © 1996 American Chemical Society

Catalytic Decomposition of TFE by O2

J. Phys. Chem., Vol. 100, No. 27, 1996 11277

triplet f singlet conversion is not necessary as in the previously proposed mechanisms.5-6 Computational Methods The ab initio calculations were performed using the Gaussian 948 suite of programs. Initially, the standard 6-31G(d,p) basis was used at the Hartree-Fock level to determine the minima and transition states. All SCF calculations were performed using the unrestricted methodology for open-shell species and the restricted for closed shells. The equilibrium geometries were obtained by analytic gradient techniques using the Berny algorithm.9 The stationary points were characterized as a minimum or a transition state (one imaginary frequency) by determination of harmonic vibrational frequencies using analytic second derivatives.10 Electron correlation effects were included by using MøllerPlesset perturbation theory through second and fourth order,11 including all single, double, triple, and quadruple excitations (MP4SDTQ). Geometries were also optimized at the MP2 level using analytic gradients12 with only valence orbitals active. The basis set used in the MP2 and MP4 calculations was the standard 6-311G(d).13 Vibrational frequencies were also determined at the MP2 level of theory, with valence orbitals active either analytically or by numerical differentiation of analytic gradients14 within the 6-31G(d) basis. Perturbation theory through fourth order, MP4SDTQ, was used at the MP2/6-311G(d) optimized geometries to calculate relative energies. Single point energy evaluations were performed at the level of spin-projected Møller-Plesset theory through second (PMP2) and fourth order (PMP4SDTQ). All activation and reaction energies reported include zero-point energy corrections. The zero-point energy was determined by using the calculated harmonic frequencies scaled by 0.94. Results and Discussion C2F4(1Ag) + O2(3Σg-) f CF2CF2OO(3A′′) Reaction. The existence of a complex formed between tetrafluoroethylene and molecular oxygen oxygen has been postulated by several different groups to be the first step in the oxidation of TFE to give carbonyl fluoride. As set forth in the Introduction, experimental evidence points to a triplet state TFE-O2 complex. Therefore, the reaction of tetrafluoroethylene with molecular oxygen was studied on the triplet electronic potential energy surface. There are two different geometries for the TFE-O2 complex which give rise to a triplet state. First, an O atom can be added to each carbon with an unpaired electron residing on each oxygen atom. This would correspond to the triplet state arising from the dioxetane. Second, the O2 molecule can be added to one carbon, producing a triplet composed of an unpaired electron on the opposite carbon and on the terminal oxygen. This corresponds to the peroxide in which the opposite carbon is unsaturated. Drawing from the experimental results, we chose to pursue the second structure. One supporting piece of evidence is that carbonyl fluoride was conspicuously absent in the experimental studies which rules against the dioxetane triplet biradical which could form CF2O simply by fission of the C-C bond. In addition, the formation of the dioxetane triplet would necessitate breaking both the CdC π bond and the OdO double bond while formation of the peroxide triplet requires breaking of the CdC π bond and only the OdO π bond, leaving the O-O single bond intact. Also, the peroxide triplet is more in line with the proposed experimental mechanism7 giving rise to CF2 as a final product of the CF2CF2O2 intermediate.

Figure 1. Optimized geometries of CF2CF2O2, CF2CF2- -O2 transition state (TS1) and CF2- -CF2O2 transition state (TS2) at the MP2/6-311G(d) level.

TABLE 1: Equilibrium Geometries for CF2CF2O2, CF2CF2- -O2 (TS1), and CF2- -CF2O2 (TS2) Calculated at the MP2/6-311G(d) Levela parameter R(C1-C2) R(C1-F3) R(C1-F4) R(C2-F5) R(C2-F6) R(C2-O7) R(O7-O8) A(C2-C1-F3) A(C2-C1-F4) A(F3-C1-F4) A(C1-C2-F5) A(C1-C2-F6) A(F5-C2-F6) A(C1-C2-O7) A(C2-O7-O8) A(O7-C2-F5) A(O7-C2-F6) D(C1-C2-O7-O8) D(F3-C1-C2-F5) D(F3-C1-C2-F4) D(F5-C2-C1-F6) D(F3-C2-C1-F6) a

CF2CF2O2 CF2CF2- -O2 TS1 CF2- -CF2O2 TS2 1.5120 1.3182 1.3182 1.3320 1.3320 1.4212 1.3164 118.83 113.83 112.50 110.25 110.25 109.25 107.42 110.54 109.82 109.82 0.00 -54.28 130.73 120.71 -174.99

1.4026 1.3142 1.3133 1.3118 1.3068 1.8289 1.2134 118.82 118.87 113.55 117.61 117.26 113.72 107.52 112.48 97.37 98.94 101.40 -36.98 145.50 141.28 -178.26

2.6726 1.2960 1.2948 1.3077 1.3174 1.4057 1.3196 109.41 105.09 104.95 110.28 100.74 112.06 113.29 108.62 108.15 112.27 36.14 -95.28 112.22 110.53 146.20

Distances (R) in angstroms; angles (A) and dihedrals (D) in degrees.

The experimental or theoretical characterization of 1-peroxydifluoromethyl-2-difluoromethyldiyl biradical (•CF2CF2OO•) has not been reported in the literature; therefore, we computed the equilibrium geometry and vibrational frequencies at the MP2/ 6-31G(d) level, with an energy calculation performed at the MP4SDTQ/6-311G(d)||2MP2/6-311G(d) level. The structure is given pictorially in Figure 1 while the geometrical parameters are listed in Table 1. The vibrational frequencies calculated at this geometry are each real, verifying the structure as a minimum on the potential surface, and are listed in Table 2. The geometric parameters for C2F4, CF2, and O2 are also given in Table 3 for comparsion. The main difference between CF2CF2O2 and the TFE reactant is the C-C bond length, which is lengthened by 0.1844 Å due to the breaking of the CdC π bond. It is interesting to note that the C-C single bond is slightly shorter than the average C-C bond in alkanes due to the electronwithdrawing effect of the fluorine atoms, which slightly shrink the carbon MO’s resulting in a shortening of the bond.

11278 J. Phys. Chem., Vol. 100, No. 27, 1996

Davis and Yu

TABLE 2: Vibrational Frequencies and Assignments for CF2CF2O2 and Vibrational Frequencies for CF2CF2- -O2 TS1 and CF2- -CF2O2 TS2 Calculated at the MP2/6-31G(d) Levela CF2CF2O2

a

freq

assignment

CF2CF2- -O2 (TS1) freq

CF2- -CF2O2 (TS2) freq

63 a′′ 114 a′′ 171 a′′ 209 a′′ 332 a′ 365 a′ 413 a′′ 538 a′ 571 a′′ 627 a′ 727 a′ 829 a′ 1074 a′ 1165 a′ 1234 a′ 1280 a′′ 1336 a′′ 1463 a′

C2F2 libration O-O libration C2F2 wag C1F2, C2F2 out-of-phase rock C-O-O bend C1F2, C2F2 in-phase scissor C2F2 rock C1F2, C2F2 out-of-phase scissor O-C1-F bend C1F2; C2F2 deformation O-C-C bend C-O stretch O-O; C-O stretch O-O str; C1F2, C2F2 out-of-phase sym stretch C1F2, C2F2 out-of-phase sym stretch C1F2 asym stretch C2F2 asym stretch C-C stretch

1385i 65 103 141 204 265 333 386 455 503 565 592 772 1218 1374 1380 1400 1688

93i 20 34 53 76 83 137 319 467 540 653 675 1034 1170 1184 1250 1283 1343

Frequencies in cm-1.

TABLE 3: Equilibrium Geometries for C2F4, CF2, and O2 Calculated at the MP2/6-311G(d) Levela parameter

C2F4

CF2

R(C-C) R(C-F) A(F-C-F) R(O-O) A(F-C-C) D(F3-C-C-F4) D(F3-C-C-F5)

1.3276 1.3173 113.72

1.3003 104.92

a

O2

1.2240 123.14 180.00 0.00

Distances (R) in angstroms; angles (A) and dihedrals (D) in degrees.

The other major difference is the F-C-C angles which are much closer to tetrahedral. The values for the F-C-C bond angles for the radical carbon are still several degrees higher than those for C2 due to the smaller amount of space required for the single electron as opposed to a bond pair. This is also manifest in the larger F3-C1-C2-F4 dihedral angle (130.7°) as opposed to that on the saturated carbon atom which is 120.7°. The C-F bonds in the product also increase slightly over the C2F4 values which can be explained by the change in hybridization of the carbon atoms from sp2 to sp3. An additional result of the radical carbon is the C-F bonds on C1 are 0.0138 Å longer than those on C2. One explanation is that C2 has three electron-withdrawing groups while C1 has only two; therefore, the amount of shrinkage the MO’s experience on C2 should be greater than on C1. The O-O bond distance in the product is substantially longer than in the O2 reactant due to its single bond nature. The structure belongs to point group Cs with C1C2-O7-O8 all lying in a single plane and a ground electronic state of 3A′′. A quick perusal of the vibrational frequencies in Table 2 shows two which are very low in magnitude at 63 and 115 cm-1. These two frequencies can be attributed to very shallow parts of the molecular potential energy surface. A look at the normal modes shows that the 63 cm-1 band is due mainly to rotation of the CF2 group of the radical carbon about the C-C bond, while the 115 cm-1 mode is due to rotation of the O2 moiety about the C-O bond. In the high-frequency region, there are six modes close together in magnitude. The highest frequency mode, at 1463 cm-1, corresponds to the C-C stretch, while the 1336 and 1280 cm-1 modes are the C1F2 and C2F2 antisymmetric stretching modes, respectively. The two CF2 symmetric stretches are not isolated on separate carbon atoms

and are spanned by the 1234 cm-1 mode and the 1165 cm-1 band. In addition, the 1165 cm-1 mode has substantial O-O stretching character as well. We are interested in characterizing the transition state for the C2F4 + O2 reaction which consists of bond formation between carbon and oxygen. In order to locate the transition state on the potential energy surface, we assumed the reaction coordinate to be composed mainly of the C-O bond. Starting from the minimum CF2CF2O2 geometry, the C-O bond was incrementally increased and the geometry optimized at each C-O value. The maximum on this surface was used as a starting point for a transition state search. Verification of the proper transition state included calculation of the harmonic frequencies (1 imaginary value) and the intrinsic reaction coordinate. Following the intrinsic reaction coordinate on each side of the transition state gave the correct CF2CF2O2(3A′′) product and the TFE(1Ag) and O2(3Σg-) reactants and is composed almost completely of the C-O bond distance coordinate. The transition state belongs to point group C1 therefore the symmetry plane of the product is not present. The geometrical parameters for the transition state are given in Table 1, and it is shown pictorially in Figure 1 (labeled TS1), while vibrational frequencies are given in Table 2. The C-O bond length in TS1 is 1.8289 Å, which is 29% longer than in the product. The C-C bond length is midway between the C2F4 reactant and CF2CF2O2 product, signifying partial double bond character in the transition state. This is also reflected in the combination C-C and O-O stretching fundamental at 1688 cm-1, which is 225 cm-1 higher in the transition state than in the product. Relative to the reactant C2F4, the C-F bonds are actually slightly shorter in the transition state while longer in the product. Within the transition state itself, the C-F bonds on the radical carbon are shorter than those on C1. CF2CF2OO(3A′′) f CF2(1A1) + CF2OO(3A) Reaction. The second reaction considered is that of C-C bond fission in the CF2CF2O2 intermediate to produce CF2O2 and CF2. This reaction is also spin conservative and results in CF2 being in the (1A1) ground state while the CF2O2 moiety is in the (3A) excited state. We employed the same technique of locating the transition state, and the intrinsic reaction coordinate was followed which verified the connection between the appropriate reactant and products. The transition state structure is shown pictorially in Figure 1 (labeled TS2), while geometrical param-

Catalytic Decomposition of TFE by O2

J. Phys. Chem., Vol. 100, No. 27, 1996 11279 TABLE 5: Vibrational Frequenciesa and Assignments for CF2O2 and Vibrational Frequencies for CF2- -O2 (TS3) Calculated at the MP2/6-31G(d) Level CF2O2

Figure 2. Optimized geometries of CF2O2 and CF2- -O2 transition state (TS3) at the MP2/6-311G(d) level.

TABLE 4: Equilibrium Geometries for CF2O2 and CF2- -O2 (TS3) Calculated at the MP2/6-311G(d) Levela

a

parameter

CF2O2

CF2- -O2 TS3

R(C1-F2) R(C1-F3) R(C1-O4) R(O4-O5) A(F2-C1-O4) A(F3-C1-O4) A(F2-C1-F3) A(O5-O4-C1) D(F2-C1-O4-O5) D(F3-C1-O4-O5) D(F2-C1-O4-F3)

1.3106 1.3164 1.4013 1.3228 107.88 112.85 112.09 108.62 164.58 -71.03 -124.39

1.3037 1.3043 1.7733 1.2070 102.32 104.50 108.09 114.84 -147.95 -35.33 -112.63

Distances (R) in angstroms; angles (A) and dihedrals (D) in degrees.

eters are listed in Table 1 and vibrational frequencies given in Table 2. The C-C bond length in TS2 has increased by 77% to 2.6726 Å, illustrating that the C-C bond is essentially broken at this point in the reaction. Due to this fact, the transition state closely resembles the products of the reaction, which are the separated CF2 and CF2OO moieties. Regarding the position of TS2 along the reaction coordinate, the F3-C1-C2 and F4C1-C2 angles are 4.4° and 8.7° less than in the reactant and are each 8.9° less in the final CF2 product. Also, the bond distances for F3-C1 and F4-C1 are much closer to that of CF2. The symmetry plane of the reactant is not present in the transition state with the CF2O2 moiety belonging to point group C1. The product CF2 is well characterized both experimentally and theoretically. It is found to be in the 1A1 ground state as opposed to the CH2 molecule, which is a ground state triplet. The CF2OO(3A) product has not been reported in the literature, although the (3A′) state of CF2O2 has been reported by Rahman et al.15 with the geometry computed at the 3-21G level. Our computed structure differs only in that the molecule was not constrained to belong to point groups Cs. From the vibrational analysis of CF2O2 we have found that the potential energy surface for rotation about the C-O bond is very shallow, but the global minimum for the structure is without the symmetry plane. The geometry computed at the MP2/6-311G(d) level is shown in Figure 2 with the geometrical parameters given in Table 4. It is interesting to compare the CF2OO molecule with the analogous part of the CF2CF2OO moiety. The C-F bonds in CF2OO are slightly shorter than in the CF2CF2O2 structure, as is the C-O bond length. The O-O length is only very slightly elongated in the CF2OO molecule. An explanation of this phenomenon is the fact that the carbon atom is a radical and therefore the hybridization of the C-F bonding MO’s is between sp2 and sp3, whereas in the CF2CF2O2 molecule the carbon atom bonded to oxygen does not have an unpaired electron and the MO’s are sp3 hybrids. An additional factor is that the second carbon in the CF2CF2O2 molecule partially lessens the electronwithdrawing effect of the two fluorines since it is has a partial

a

freq

assignment

CF2- -O2 (TS3) freq

100 306 467 558 659 1065 1163 1253 1341

O-O libration C-O-O bend F-C-F bend F-C-O bend F-C-F scissor C-O stretch O-O stretch CF2 sym stretch CF2 asym stretch

1464i 70 218 319 508 627 1179 1251 1485

Frequencies in cm-1.

positive charge. This shrinks the MO’s on the carbon in CF2OO relatively more, also shortening the C-F bonds in this structure. The vibrational frequencies calculated for CF2O2 are given in Table 5. There is a very low frequency at 99 cm-1 which corresponds mainly to rotation of O2 about the C-O bond indicative of a shallow potential for this degree of freedom. The antisymmetric C-F stretch is the highest frequency mode at 1341 cm-1 while the symmetric stretch is at 1253 cm-1. The next mode at 1163 cm-1 is composed mainly of an O-O stretch and the 1065 cm-1 band is largely the C-O stretching motion. The 659 cm-1 frequency is due to the CF2 scissor, and the next three modes are delocalized bending motions. CF2OO(3A) f CF2(1A1) + O2(3Σg-) Reaction. The last reaction to be studied is the decomposition of CF2O2(3A) to give the final products, CF2 and molecular oxygen, each in their ground electronic states. The optimized parameters and harmonic frequencies for the transition state are listed in Tables 4 and 5, respectively, while the structure is shown in Figure 2 (labeled TS3). The transition state is characterized by a single imaginary frequency, while the intrinsic reaction coordinate calculation confirms the correct reactant and products. The intrinsic reaction coordinate is mainly composed of the C-O internal coordinate as would be expected from the bond cleavage in the proposed reaction mechanism. Reaction Energetics. One of the main feature of this set of reactions is that each step is totally spin conservative with the entire reaction sequence contained on the triplet electronic potential energy surface. There is no need for intersystem crossing to explain the formation of products, as is the case for dioxetane formation. Each reactant and product are in their ground electronic states throughout the reaction sequence except for the CF2O2 biradical. Single point energy calculations at the MP4SDTQ/6-311G(d)||MP2/6-311G(d) level show that the 3A state is 17.2 kcal/mol higher than the 1A′ state, including zeropoint energies. For this reaction sequence to occur with spin conservation, the dissociation of the CF2O2 biradical would need to occur on a time frame that is shorter than intersystem crossing to the singlet electronic ground state. After relaxation of CF2O2 to the singlet state though, the dissociation reaction can occur with ground state CF2 and excited O2(1Σg+) as products. A comparison of the activation energies can shed some light on the relative rates of the CF2O2 f CF2 + O2 reaction on both the triplet and singlet potential energy surfaces. As reported above, the activation barrier for the unimolecular dissociation of CF2O2 on the triplet surface is only 12.9 kcal/mol while the reaction on the singlet surface is much higher at 44.9 kcal/mol. The reaction rate on the triplet surface is much higher due to the lower Ea, but we cannot comment on the relative lifetime of the triplet state of CF2O2.

11280 J. Phys. Chem., Vol. 100, No. 27, 1996

Davis and Yu

TABLE 6: Total Energiesa and Zero-Point Energiesb for Reactants and Products molecule

PMP2/ 6-311G*

PMP4SDTQ/ 6-311G*

ZPE

〈S2〉c

O2 CF2 C2F4 CF2O2 CF2O2 (TS3) CF2CF2O2 CF2CF2O2 (TS1) CF2CF2O2 (TS2)

-150.029 946 5 -237.248 673 5 -474.615 105 1 -387.255 915 8 -387.229 001 5 -624.602 627 5 -624.580 123 6 -624.507 552 9

-150.044 645 -237.278 108 6 -474.662 928 7 -387.302 454 -387.279 135 -624.672 960 -624.650 931 -624.582 350

1.9 4.2 12.7 9.3 7.6 16.8 15.4 13.9

2.038 0 0 2.014 2.211 2.016 2.352 2.228

a Energy in hartrees. b Energy in kcal/mol. c Reported values for 〈S2〉 calculated at the Hartree-Fock level without spin annihilation.

TABLE 7: Reaction Energetics for Each Elementary Reaction Stepa reaction

Ea(PMP2/ 6-311G*)

Ea(PMP4SDTQ/ 6-311G*)

C2F4 + O2 f CF2CF2O2 CF2CF2O2 f CF2O2 + CF2 CF2O2 f CF2 + O2

41.5 56.8 15.2

36.3 54.0 12.9

reaction

∆H(PMP2/ 6-311G*)

∆H(PMP4SDTQ/ 6-311G*)

C2F4 + O2 f CF2CF2O2 CF2CF2O2 f CF2O2 + CF2 CF2O2 f CF2 + O2 sum (C2F4 f 2 CF2)

28.8 58.2 -17.5 69.5

23.9 54.7 -15.9 62.7

expb

66.0

a

Energies are in kcal/mol and include zero-point vibrational corrections. Enthalpies are at 0 K. b Reference 18.

The shape of the intrinsic reaction coordinate surfaces provide insight into the energy of the bond rearrangements ongoing for each reaction step. For steps 1 and 3, there is a substantial activation barrier for both the forward and reverse directions, while step 2 is more like a classical bond separation with an activation energy only in the forward direction. The activation barrier in the forward direction in step 1 is due to the fissures of the CdC and OdO π bonds, while that for the reverse direction results from breaking the C-O linkage. The forward reaction is endothermic since the C-O single bond energy is less than the sum of the CdC and OdO π bond energies. In the forward direction of reaction 3, a C-O bond is broken while in the reverse direction a OdO π bond is cleaved. This forward reaction is exothermic, reflecting that the OdO double bond formation plus the products is lower in energy than the reactant. In step 2 no new bonds are formed, yielding a barrier only in the forward direction. The total energy of each molecule, and associated zero-point energy, are listed in Table 6 while the activation and reaction energies for each step are given in Table 7. The largest activation energy, at 54.0 kcal/mol, is for the second step which is due to the fissure of the C-C bond in the TFE-O2 intermediate and as such should be the rate-limiting step of the reaction sequence. It is also interesting to note in Table 7 that the reverse step for reaction 2 appears to occur with a negative barrier. This anomaly is due to the fact that the sum of the energies of the separated CF2O2 and CF2 products is slightly higher than the transition state. However, the IRC calculation shows that when the separate moieties are within approximately 3 Å of each other, their total energy is less than the transition state. This is apparently due to basis set superpositions error (BSSE) in the calculation for the two products in close proximity. Since both moieties have two fluorine atoms, the ratio of electrons to basis functions is rather high for each. One method to correct for BSSE is to use the counterpoise technique. The MP2 energies

for each separate moiety in reaction 2 were calculated in the presence of the basis set of the other. The energy of the CF2OO molecule was calculated in the presence of the CF2 basis functions at a C-C distance of 2.7 Å, which is that of TS2. Conversely, the energy of CF2 was calculated in the presence of the basis functions on CF2OO. The resulting BSSE was found to be 3.5 kcal/mol. The activation energy for the reverse direction was found to be -2.7 kcal/mol, which would give a corrected value of 0.8 kcal/mol. This is in harmony with either a very small or an absence of an activation barrier for the bond formation reaction. The overall sum of the three reactions is simply the catalytic decomposition of TFE or the fission of the CdC double bond. Therefore, the overall enthalpy calculated for the three reaction steps should be that of the C2F4 a 2CF2 reaction. If the overall enthalpy matches the experimental value for the CdC bond energy, the computed values for the activation barriers should also be described adequately. Our calculated value for the CdC bond energy is ∆H0 K ) 62.7 kcal/mol at the MP4 level. The experimental value ∆H0 K ) 66.0 kcal/mol, as reported by Carlson,18 is in good agreement with our calculated value, suggesting that the current level of theory is adequate. Concluding Remarks The catalytic decomposition of tetrafluoroethylene by molecular oxygen has been studied using ab initio quantum chemical methods. The geometries of each reactant, product, and transition state was determined at the MP2/6-311G(d) level with energies calculated at the MP4(SDTQ)/6-311G(d) level, using the MP2 geometries. The geometry of the CF2CF2O2 peroxide biradical was determined and found to belong to point group Cs. Calculations of the CF2CF2O2 molecule in the singlet electronic state found it to be dissociative at the Hartree-Fock level. The vibrational frequencies were determined for this molecule and found to contain two very low-energy modes corresponding to librational motions of the F2C or the O2 groups. The reaction sequence was divided into three elementary steps, each being spin conservative and occurring on the triplet potential energy surface. However, for step 3 the reactant CF2O2 is in an excited electronic state (3A), which is 17.2 kcal/mol over the 1A′ ground state. The activation energy for this step is 32 kcal/mol lower on the triplet surface than on the singlet surface, however. The overall reaction (4) is that for fission of the CdC bond with O2 acting as a catalyst. The activation barrier for the rate-determining step (2) was found to be 54.0 kcal/mol, which results in a decrease of 8.7 kcal/mol over the simple fission of the CdC bond. The experimental enthalpy of reaction 4 is very close to the sum of the calculated values for reactions 1-3, with a difference of only 3.3 kcal/mol. This provides support that the current level of theory used is adequate to determine reaction energetics for the C2F2 + O2 system. Acknowledgment. The financial support of the Office of Naval Research (N00014-93-1-0019) and the National Science Foundation (CHE-9512473) and computer time through the Mississippi Center for Supercomputer Research are gratefully acknowledged. References and Notes (1) Modica, A. P.; LeGraff, J. E. J. Chem. Phys. 1965, 43, 3383. (2) Keating, E. L.; Matula, R. A. J. Chem. Phys. 1977, 66, 1237. (3) Heicklen, J.; Knight, V. J. Phys. Chem. 1966, 70, 3893. (4) Heicklen, J.; Knight, V. Aerospace Report No. TDR-469, 1969; p 5250. (5) Chowdhury, P. K.; Pola, J.; Ram Rao, K. V. S.; Mittal, J. P. Chem. Phys. Lett. 1987, 142, 252.

Catalytic Decomposition of TFE by O2 (6) Pola, J.; Ludvik, J. J. Chem. Soc., Perkin Trans. 2 1987, 1727. (7) Liu, L.; Davis, S. R. J. Phys. Chem. 1992, 96, 9719. (8) Gaussian 94, Revision C.2: Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Anders, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian, Inc., Pittsburgh, PA, 1995. (9) No reference given. (10) Schlegel, H. B. J. Comput. Chem. 1982, 3, 214.

J. Phys. Chem., Vol. 100, No. 27, 1996 11281 (11) Pople, J. A.; Krishnan, R.; Schlegel, H. B.; Binkley, J. S. Int. J. Quantum. Chem., Quantum Chem. Symp. 1979, No. 13, 325. (12) Krishnan, R.; Pople, J. A. Int. J. Quantum Chem. 1978, 14, 91. (13) Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon, M. S.; Defrees, D. J.; Pople, J. A. J. Chem. Phys. 1982, 77, 3654. (14) Krishnan, R.; Schlegel, H. B.; Pople, J. A. J. Chem. Phys. 1980, 72, 4654. (15) Rahman, M.; McKee, M. L.; Shevlin, P. B.; Sztyrbicka, R. J. Am. Chem. Soc. 1988, 110, 4002. (16) Backsay, G. B. Chem. Phys. 1981, 61, 385. (17) Seeger, R.; Pople, J. A. J. Chem. Phys. 1976, 65, 265. (18) Carlson, G. A. J. Phys. Chem. 1971, 75, 1625.

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