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RESEARCH NOTES Experimental and Quantum Chemical Study of the Reaction CF2 + CH3 T CF2CH3 f CH2dCF2 + H: A Key Mechanism in the Reaction between Methane and Fluorocarbons Hai Yu, John C. Mackie, Eric M. Kennedy,* and Bogdan Z. Dlugogorski Process Safety and EnVironment Protection Research Group, School of Engineering, The UniVersity of Newcastle, Callaghan, NSW 2308, Australia
The reaction of CHClF2 with CH3Br was studied over the temperature range of 773-1123 K at atmospheric pressure in an alumina tubular reactor. At temperatures 99%, Actrol), CH3Cl (99.5%, Aldrich), and CH3Br (>99%, BOC Gases) in nitrogen. For other species, RMR values were estimated from published correlations.21 The reaction of CHClF2 with CH3Br was performed over the temperature range of 773-1123 K at atmospheric pressure. A reactant gas stream (CHClF2:CH3Br:N2 ) 1:1.07:18.6) passed through the reactor at a total flow rate of 1.04 cm3/s (STP). The volume of the reaction zone was maintained at 1.58 cm3, such that the residence time varied from 0.54 s to 0.37 s for 773 K to 1123 K. Results and Discussion (1) Quantum Chemical Study of the Reaction CF2 + CH3 T CF2CH3 f CF2dCH2 + H. Recombination between CF2 and CH3 leads to the initial formation of CF2CH3
CF2 + CH3 f CF2CH3
(R1)
in a well of depth of 57.9 kcal/mol below the reactants’ energy at 0 K, as calculated at the G3 level of theory. Results of the G3//B3LYP calculations, viz., the total energies (including zero point correction) (E0), the atomization energies (ΣD0), the enthalpies of formation at 0 and 298 K, the rotational constants, and the (scaled) vibrational frequencies of reactants, products, intermediates, and transition states for all the reactions studied in this work are summarized in Table 1, together with experimental values of the enthalpies of formation at 298 K. Computed enthalpies of reaction at 298 K are in good agreement with the experiment. A discrete transition state for the decomposition of CF2CH3,
CF2CH3 f CF2dCH2 + H
(R2)
was located using density functional methods. The optimized geometry is shown in Figure 1. As may be seen, the CF2dCH2 moiety in the transition state has a tight planar structure, similar to that of the product, and the departing H atom is located in a C-H bond of length 2.319 Å normal to the plane. At the G3 level of theory, the transition state CF2CH2-H TS is located at 39.3 kcal/mol above the CF2CH3 well (at 0 K). Also at the G3 level, the energy of the products, CF2dCH2 + H, was computed to be -31.1 kcal/mol, implying a reverse barrier of only 1.0 kcal/mol for the addition of hydrogen to CF2dCH2 at 0 K. A schematic PES for the reaction is shown in Figure 2. Using the G3 energies of the reactant and transition state, together with the optimized geometries obtained at the B3LYP//6-31G(d) level, with computed vibrational frequencies scaled by 0.96, the rate constant for the reaction CF2CH3 f CF2dCH2 + H has been calculated using standard statistical methods to be k2
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Table 1. G3//B3LYP Total Energies (E0), Atomization Energies (ΣD0), Heats of Formation (Computed and Experimental), Rotational Constants, and Scaled Harmonic Vibrational Frequencies of Reactants, Products, Intermediates, and Transition States rotational constant (GHz) CF2 CH3
84.88, 12.41, 10.83 284.8, 284.8 142.4
CF2CH3
10.00, 9.116, 5.046
CF2CH2-H TS
9.859, 8.761, 5.077
CH2dCF2
10.85, 10.39, 5.308
253.249 289.853
-46.47 35.02
-45.98 34.37
-43.499a 34.821a
-277.489965
601.041
-69.39
-71.20
-72.3 ( 2.0c
-277.427378
561.768
-30.12
-31.56
-276.927782
562.7032772
-82.68
-83.88
632.4, 1098.0, 1201.2 440.6, 1374.8, 1374.8, 3012.7, 3180.0, 3180.0 195.4,b 353.5, 433.6, 507.0, 829.7, 958.5, 1064.0, 1225.4, 1238.4, 1382.9, 1444.0, 1446.8, 2897.9, 2997.3, 3037.2 149.5,b 154.9, 178.0, 419.4, 522.0, 596.2, 724.8, 793.0, 908.5, 936.0, 1307.6, 1374.0, 1697.5, 3099.9, 3193.3 419.9, 520.9, 600.1, 694.8, 777.8, 909.6, 932.3, 1298.0, 1374.7, 1729.2, 3096.0, 3187.3
-237.60401 -39.793622
-0.501087 -37.828452 -99.68599 a
∆fH°298 (kcal/mol) computed
E0/Eh
H C F
ΣD0 (kcal/mol)
∆fH°0 (kcal/mol)
frequencies (cm-1)
51.63a 169.98a 18.4a
experimental
-82.2 ( 2.4d 52.10a 171.29a 18.97a
Data taken from ref 22. b Replaced by a hindered rotation in the kinetic analysis. See text. c Data taken from ref 23. d Data taken from ref 24.
Figure 1. Transition-state (CF2CH2-H TS) for CF2CH3 f CF2dCH2 + H. All bond lengths are given in angstroms.
Figure 2. Schematic potential energy surface of the reaction of CF2 and CH3.
) 5.2 × 1014e(-43.8 700-2000 K.
kcal/mol)/(RT)
s-1 in the temperature range of
Because the final products, CF2dCH2 + H, are significantly lower lying than the reactants, CF2 + CH3, it is necessary to determine what, if any, stabilization of the intermediate CF2CH3 might occur over the relevant range of temperatures (7002000 K) and pressures (1-10 000 Torr). This has been studied using the MultiWell suite of programs (J3). This requires, in addition to the rate constant for reaction R2, the high-pressure rate constant for the decomposition of the adduct CF2CH3
f CF2 + CH3 (-R1), together with the equilibrium constant for reaction R1 and the Lennard-Jones parameters for the adduct. The potential energy surface for recombination between CF2 and CH3 was investigated thoroughly via both density functional and MP2 methods. No discrete transition state for the recombination was located by either technique. The recombination is barrierless. To determine the limiting high-pressure rate constant for reaction -R1, the variational transition-state rate constant for CF2CH3 f CF2 + CH3 was evaluated over the temperature range of 700-2000 K. This was determined by calculating the minimum rate constant for varying C-C bond distances in CF2CH3, ranging from the equilibrium bond separation of 1.493 Å in CF2CH3 up to a C-C separation of 3.2 Å. The transition state for C-C fission was located at 2.65 Å at 700 K, 2.50 Å at 1100 K, and 2.40 Å at 1600 K. In the MultiWell simulations, the first transition state was used at temperatures in the range of 700-1000 K, the second in the range of 1100-1500 K, and the third at temperatures in the range of 1600-2000 K. At the elongated C-C distances in these variational transition states, Gaussian methods predict extremely weak torsions (∼15-30 cm-1) of the CF2 and CH3 moieties about the C-C bond axis. If these torsions in the transition state or in the adduct CF2CH3 were treated as vibrations, this would lead to significant errors in the rate constant for the reverse reaction R1 (-R1) and in the equilibrium constant between CF2CH3 and CF2 + CH3. Therefore, these torsions have been treated as hindered rotors in CF2CH3 and in the variational transition states. The barriers to internal rotation about the C-C axis have been evaluated at fixed torsional angles of the CH3 rotor around the axis. The barriers have been determined to be 3-fold with barriers to rotation of 2.38 kcal/mol in equilibrium with CF2CH3 and barriers of 0.13, 0.090, and 0.035 kcal/mol at C-C separations of 2.40, 2.50, and 2.65 Å, respectively. The MultiWell simulations used Lennard-Jones parameters of σ ) 4.5 Å and /kB ) 220 K, estimated from values for ethane, ethylene, and hexafluoroethane.20 A collisional energy transfer model derived by Hold et al.25 was also used in the simulations. From the MultiWell simulations over the entire temperature range and for all pressures in the range of 1-10 000 Torr, there is no significant stabilization of the CF2CH3 adduct. Reaction between CF2 and CH3 leads essentially only to the production of CF2dCH2 + H. The rate constant found for this reaction is
Ind. Eng. Chem. Res., Vol. 45, No. 10, 2006 3761
Figure 3. Conversion of CHClF2 and CH3Br and the rate of production of major products, each as a function of temperature.
independent of pressure and almost constant with temperature, ranging from 5.4 × 1012 cm3 mol-1 s-1 at 700 K to 4.3 × 1012 cm3 mol-1 s-1 at 2000 K. Over the temperature range of 7002000 K, the rate constant for CF2 + CH3 f CF2dCH2 + H can be approximated by the expression 2.1 × 1013T -0.207 cm3 mol-1 s-1. (2) Reaction of CHClF2 with CH3Br. In the previous studies of the reaction of CH4 with CBrClF2 or CCl2F2, it has been suggested that CH2dCF2 is formed via the following reaction pathways:15,16
CBrClF2/CCl2F2 f CClF2 + Br/Cl
(R3)
CClF2 + CH4 f CH3 + CHClF2
(methane activation) (R4) (methane activation) CH4 + Br/Cl f CH3 +HBr/HCl (R5) CBrClF2/CCl2F2 + CH3 f CClF2 + CH3Br/CH3Cl (R6) CClF2 + CH3 f CH3CClF2
(R7)
CH3CClF2 f CH2dCF2 + HCl
(R8)
A close analysis of the aforementioned reaction pathways suggests that the formation of CH2dCF2 via CH3CClF2 as an intermediate is unlikely. First, the amount of CClF2 formed via reaction R3 is very small, because of high energy barriers for the homolytic cleavage of C-Br and C-Cl bonds in CBrClF2 and CCl2F2 (>62.6 kcal/mol).3 The subsequent abstraction reactions R4-R6 proceed more rapidly, because of much lower activation energies (
