Article pubs.acs.org/JPCA
Unimolecular Reactions of 1,1,1-Trichloroethane, 1,1,1Trichloropropane, and 3,3,3-Trifluoro-1,1,1-trichloropropane: Determination of Threshold Energies by Chemical Activation Martha A. Turpin,† Kylie C. Smith,† George L. Heard,† D. W. Setser,‡ and Bert E. Holmes*,† †
Department of Chemistry, University of North Carolina Asheville, One University Heights, Asheville, North Carolina 28804-8511, United States ‡ Kansas State University, Manhattan, Kansas 66506, United States S Supporting Information *
ABSTRACT: The recombination of CCl3 radicals with CH3, CH3CH2, and CF3CH2 radicals was used to generate CH3CCl3, CH3CH2CCl3, and CF3CH2CCl3 molecules with approximately 87 kcal mol−1 of vibrational energy in a bath gas at room temperature. The competition between collisional deactivation and unimolecular reaction by HCl elimination was used to obtain the experimental rate constants for each molecule. These experimental rate constants were matched to calculated statistical unimolecular rate constants to assign threshold energies to the three HCl elimination reactions. The models needed for the calculations of the rate constants were obtained from molecular structure calculations using density functional theory (DFT) with the hybrid density-functional MO62X recommended by Truhlar for transition states. The assigned threshold energies are 52 ± 2, 50 ± 2, and 52 ± 2 kcal mol−1 for CH3CCl3, CH3CH2CCl3, and CF3CH2CCl3, respectively, and the CH3 and CF3 groups have only a minor effect on the threshold energies for HCl elimination. The DFT calculated threshold energies are in agreement with the experimentally assigned values. The addition of Cl atoms to the same carbon atom lowers the threshold energy for HCl elimination in the CH3CH2Cl, CH3CHCl2, and CH3CCl3 series. This trend, which is the opposite of that for CH3CH2F, CH3CHF2, and CH3CF3, is discussed in terms of transition-state structure and correlated with the relative stabilities of CH3CH2+, CH3CHCl+, and CH3CCl2+ ions; the relative stabilities are based on the hydride affinities obtained from calculations. Comparison of the reactions of CH3CCl3 and CH2ClCHCl2 shows that the threshold energy is much higher for the isomer with chlorine atoms on both carbon atoms. associated with the mesomeric effect4,5 of the Cl atom on CCl. The effect of the second chlorine atom on the E0 for 1,1,1trichloroalkanes is an important question for the development of transition-state models for these HCl elimination reactions. The CH3CCl3 molecule also occupies a pivotal position in understanding the energy disposal in HCl and HF elimination reactions, because the translational energy released to HCl and CH2CCl2 has been experimentally measured.10 The selection of a satisfactory method to calculate the potential energy surface for the CH3CCl3 reaction would benefit from having a reliable E0 value as one test of the computational method for the potential energy surface.11−13 The principal goal of the present work is to assign reliable E0 values for three representative 1,1,1-trichloroalkane molecules using the chemical-activation technique to circumvent the radical-induced chain reactions present in thermal-activation experiments.
I. INTRODUCTION Conventional studies of the unimolecular reactions of 1,1,1trichloroalkanes by thermal-activation methods have been limited by free-radical initiated chain reactions associated with the decomposition of RCHCl3 radicals to RCHCCl2 + Cl atoms. The single report1 dating from 1950 of the Arrhenius parameters for CH3CCl3 gives k(T) = 1014 exp(−54000/RT) s−1 for a temperature range of 636−708 K. It was necessary to use added propene to suppress the chain reaction caused by the presence of Cl atoms. This activation energy corresponds to a threshold energy, E0, of 52.5 kcal mol−1. However, the uncertainty in this value for E0 must be 2−3 kcal mol−1 considering the experimental problems of the pyrolysis experiments. The threshold energies for the reactions of CH3CH2Cl2,3 and CH3CHCl24−6 reactions are established as 55 ± 1 and 52 ± 1 kcal mol−1, respectively, and the addition of a Cl atom on the carbon atom (CCl) that is attached to the Cl atom in the four-member ring of the transition state definitely lowers the threshold energy. The same trend7−9 also exists for C2H5CH2Cl and C2H5CHCl2, although the pyrolysis experiments for C2H5CHCl2 are not conclusive.9 This trend has been © 2014 American Chemical Society
Received: August 1, 2014 Revised: September 8, 2014 Published: September 9, 2014 9347
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112 kcal mol−1 of energy with halocarbon molecules is useful as a background. The total quenching cross sections of Hg(3P1) atoms, which have traditionally been defined as the quenching rate constant divided by (π8kT/μHg,Q)1/2, are large for halocarbons; the reported values for CCl4 and CH3I are 6620,21 and 5920 Å2, respectively, and those for C2H5I and CF3CH2I would be of similar magnitude. The quenching mechanism is dominated by interaction with the Cl or I atom, but the balance between forming mercury halide plus the radical or mercury atom plus a halogen atom and the radical is not established. Because Cl atoms will initiate secondary reactions in our systems, this is an important point. Because of an interest in chemical lasers based upon direct formation of HgX(B 2Σ+), X = F, Cl, Br, and I, the reactions of all three triplet states (3P2, 3P1, 3P0) from Hg(6s,6p) excitation have been studied with molecular halogens and with CCl4.22−26 The Hg(3P2) state has nearly unity branching fraction for the formation of HgX(B,2Σ+) in reactions with the molecular halogens; however, the branching fraction declines for reactions with the two lower energy Hg* states. The branching fraction for formation of HgX(B) for reactions of Hg(3P2) with CCl4 and CH2I2 is less than 0.3 and the reaction of Hg(3P1) with CCl4 or CBr4 gave no discernible yield of HgX(B).21−23 Because the HgX(A) state is dissociative, the question for our molecules is the importance of HgX(X2Σ+) formation. The bond dissociation energies of HgCl(X) and HgI(X) are 22 and 8 kcal mol−1, respectively, which is much less than the available energy (112 kcal mol−1 − D(R−X)). Gunning and coworkers27 investigated the direct formation of HgCl by excitation of 202Hg(3P1) with measurement of the enhancement of 202Hg in the solid Hg2Cl2 product. They found considerable enhancement for the CH3Cl reaction, but not for the CCl4 reaction. They interpreted the difference as evidence that the presence of hydrogen in the reagent molecule was necessary to aid in the formation of HgCl. However, given the large cross section and the fact that Hg(3P2) reacts directly to give HgCl(B), an alternative explanation is that secondary reactions in the 202Hg(3P1) + CCl4 system, such as Cl + 202HgCl giving Cl2 + 202Hg, causes loss of isotopic specificity. In summary, it seems that both Hg + Cl and HgCl are products from Hg(3P1) + CCl4 and that precautions are needed to control the Cl atom concentration. This certainly was the case for other experiments that used mercury sensitization to generate radicals.14,15,28 We selected CF3CHCH2 as a scavenger for the Cl atoms. Because HgI is weakly bound, the reactions with CH3I, C2H5I, and CF3CH2I probably give mainly I atoms. However, the I atoms do not initiate secondary reaction and they are scavenged by excess mercury in the photolysis vessel. The experiments consisted of the irradiation of quartz vessels containing the prepared mixtures and a droplet of mercury with the 253.7 nm photons from a low-pressure germicidal lamp (Spectroline XX-15G). An irradiation time of just a few minutes was usually sufficient to generate enough products for analysis, although up to 20 min were needed for the experiments at high pressure with CH3CCl3. The gas handling was done in an allglass, grease-free, low-pressure, vacuum line. Pressures were measured with a MKS-270 electronic manometer. Photolysis mixtures were prepared by measuring each component in a calibrated volume followed by cryogenic transfer to the quartz photolysis vessel. After photolysis, the contents of the vessels were transferred to the inlet vacuum line of a gas chromatograph for analysis. Pure compounds were available for preparation of mixtures used for calibration of the response
We selected CH3CCl3, CH3CH2CCl3, and CF3CH2CCl3 molecules as the systems to examine by the chemical-activation technique using the recombination reactions of CCl3 radicals with CH3, CH3CH2, and CF3CH2 radicals at room temperature. The E0 for HF elimination is sufficiently high that HCl elimination was the only observed reaction for CF3CH2CCl3. The relevant reactions are summarized below. The vibrational energy of these molecules, which is approximately 87 kcal mol−1, is denoted by the asterisk. CH3CCl3* →HCl + CH 2CCl 2 + M →CH3CCl3 + M
(1)
CH3CH 2CCl3* →HCl + CH3CHCCl 2 + M →CH3CH 2CCl3 + M
(2)
CF3CH 2CCl3* →HCl + CF3CHCCl 2 + M →CF3CH 2CCl3 + M
(3)
The radicals were generated by the mercury photosensitization14−16 of CCl4 mixtures with CH3I, C2H5I, or CF3CH2I. The experimental rate constants were obtained from measurement of the ratio of the decomposition product (CH2CCl2, CH3CHCCl2, or CF3CHCCl2) to the collisionally stabilized molecule for a range of pressure of the bath gas, M. The self-recombination products of the radicals, C2Cl6, C2H6, C4H10, and CF3CH2CH2CF3 were observed, but they are not of interest. The experimental rate constants are matched to calculated RRKM statistical rate constants to assign the E0 values. The models of the molecules and transition states needed for RRKM calculations were obtained from electronic-structure calculations using Density-Functional Theory. One of the density functionals recommended by Truhlar and co-workers17 for transition states was selected for the electronic-structure calculations. The results from this method (M06-2X) were compared to the results from the B3PW91 method, which we have used for several previous studies of the unimolecular reactions of chlorofluoroalkanes.3,6,18,19 In fact, the calculated vibrational frequencies and moments of inertia are similar for both methods; however, the calculated threshold energies for HCl elimination by M06-2X seem to be somewhat closer to the experimental values than those from the B3PW91 method. The electronic structures calculated by the DFT methods provide information that aids in the examination of the nature of the transition states. This information is used to discuss and analyze the opposing trends for the threshold energies in the CH3CH2Cl, CH3CHCl2, CH3CCl3 and CH3CH2F, CH3CHF2, CH3CF3 series. Comparison with CH2ClCHCl2 provides another illustration6 of the difference in threshold energies for HCl elimination from substitution of halogen atoms on the CX vs the CH atoms in the transition state; CX and CH denote the carbon atoms with halogen and hydrogen atoms as leaving groups in the transition state. The results for CH3CH2CCl3 and CF3CH2CCl3 permit the evaluation of the effect of CH3 and CF3 groups on the threshold energy for HCl elimination.
II. EXPERIMENTAL METHODS To study the chemically activated CH3CCl3, CH3CH2CCl3, and CF3CH2CCl3 molecules, a source of CCl3 radicals is needed. We decided to use the mercury photosensitized reaction of CCl4 in mixtures of CH3I, C2H5I, and CF3CH2I. A short review of the reactions of the Hg(6s,6p;3P1) excited-state atoms with 9348
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of the gas chromatograph. Because the experiments for each molecule required a different mixture and a different pressure region, each series of experiments is summarized separately. The purity of starting materials was ascertained by gas chromatography using flame-ionization and mass-spectrometric detectors. The CH3CH2I and CCl4 used for the generation of CH3CH2CCl3 were obtained from Aldrich and Arcos, respectively. The quartz vessels, which ranged in size from 1.44 to 10.94 cm3, typically contained CCl4, CH3CH2I, and CF3CHCH2 in the ratio 1:2.5:2. Some experiments were done with a reduced level of CF3CHCH2, i.e., a ratio of 1:2.5:1, but the CH3CHCCl2/CH3CH2CCl3 ratio was reduced, suggesting that some of the Cl atoms had reacted with CH3CHCCl2. Therefore, the D/S vs pressure data were collected from experiments with the higher level of CF3CH CH2. The analysis was done with a Shimadzu gas chromatograph GC-14A equipped with a Mxt-1 column and a flameionization detector. Commercial samples of CH3CH2CCl3 and CH 3 CHCCl 2 from Synquest and Chem Samp Co., respectively, were used to prepare calibration mixtures. The calibration factor for [CH3CHCCl2]/[CH3CH2CCl3] was 1.43 ± 0.18. The CF3CH2I used to generate CF3CH2CCl3 was obtained from SynQuest. The photolysis vessels ranged in size from 6.51 to 478.3 cm3, and the mixture composition of CCl4/CF3CH2I/ CF3CHCH2 was 1:10:2. The higher mole fraction of CF3CH2I needed to obtain good yields, relative to the CH3CH2I system, implies that the quenching cross section of CF3CH2I may be smaller than for CH3CH2I. Analysis was done with the Shimadzu gas chromatograph and flame-ionization detector with a Mxt-624 column. The CF3CH2CH2CF3 and C2Cl6 recombination products with retention times of 8.5 and 63 min did not interfere with the analysis of CF3CHCCl2 and CF3CH2CCl3 with retention times of 23 and 39 min, respectively. Commercial samples of CF3CH2CCl3 and CF3CHCCl2, which were obtained from Princeton BioMolecular Research Inc. and Matrix Scientific, respectively, were used to prepare mixtures for calibration. The calibration factor for the [CF3CHCCl2]/[CF3CH2CCl3] ratio was 1.30 ± 0.11. The large rate constant for CH3CCl3 required the addition of an inert gas to attain high pressure. Sulfur hexafluoride was selected because it has relatively good efficiency for vibrational deactivation and an expected20 small quenching cross section for Hg(3P1) atoms. The experiments utilized CCl4/CH3I/ CF3CHCH2/SF6 mixtures with a composition of 1:2.5:2.5:25 in vessels ranging from 1.44 to 4.15 cm3, giving pressures from 1060 to 360 Torr. Analyses were done with the Shimadzu gas chromatograph with flame-ionization detector and a Mxt-624 column. Samples of CH3CCl3 and CH2CCl2, from American Custom Chemical and Aldrich, respectively, were used to prepare mixtures for calibration. The calibration factor for the CH2CCl2/CH3CCl3 ratio was 1.30 ± 0.09.
Figure 1. Plot of decomposition/stabilization ratios versus inverse pressure for CH3CCl3*. The least-squares slope and intercept values are 577 ± 55 Torr and −0.28 ± 0.10, respectively.
Figure 2. Plot of decomposition/stabilization ratios versus inverse pressure for CH3CH2CCl3*. The least-squares slope and intercept values are 13.4 ± 0.4 Torr and −0.01 ± 0.03, respectively.
from effects of inefficient collisional deactivation. In the high pressure limit, the experimental rate constant is the average rate constant for the ensemble of molecules. Multiplication of the slopes by the collision rate constant, kM, gives the average unimolecular rate constant in s−1 units for efficient deactivating bath gases. The collision diameters and intermolecular well depths used to calculate kM are given in the footnote of Table 1. On the basis of comparison with previous work,29,30 the alkyl iodides, CCl4 and CF3CHCH are efficient deactivating bath gas molecules.6,16,18 The deactivating efficiency by SF6 has been measured29,30 for highly excited CF3CH3*, CH2FCH2F*, CH3CH2F*, and CH2ClCH2Cl* molecules and, in principle, a small adjustment to the apparent rate constant for CH3CCl3 could be made. The low frequencies in CH3CCl3 may increase the deactivation efficiency, the collision diameters used to calculate rate constant have uncertainty and the experimental measurement (note the nonzero intercept in Figure 1) has uncertainty. Thus, in fact, no adjustment to the experimental rate constant was made. The slopes of the D/S plots also are
III. EXPERIMENTAL RESULTS III-A. Rate Constants. The unimolecular rate constant for each molecule is obtained from the slope of the plot of the ratio of the decomposition product (D) to the collisionally stabilized molecule (S) vs pressure−1. The plots of D/S vs pressure−1 are shown in Figures 1−3 for the three molecules. Data were collected in the high-pressure regime, i.e., for D/S less than 1.5, to obtain the limiting high-pressure slope that is largely free 9349
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CH3CCl3 were more difficult because of the need to prepare photolysis samples with added SF6 to have high pressures; the highest pressure was 1000 Torr. The plot in Figure 1 is linear, but the intercept is −0.28 ± 0.10 rather than zero. As a result, the uncertainty in the rate constant for CH3CCl3 is 15−20%. The decline in the rate constants for the series of molecules is expected for substitution of CH3 and CF3 groups for an H atom in CH3CCl3. It is instructive to compare the rate constants (halfquenching pressures) for CH3CH2CH2Cl (5.4 Torr)3 and CF 3CH3CH2Cl (0.17)3 to those of the corresponding trichloride molecules. Exchanging a CH3 group by a CF3 group lowers the rate constants by a factor of 32 for CH3CH2CH2Cl and a factor of 22 for CH3CH2CCl3, because of the enhanced density of vibrational states provided by the lower frequencies of the CF3 group. The half-quenching pressure for C2H5Cl is 310 Torr,28 and the three 1,1,1trichloride molecules have 2−3 times larger rate constants than the corresponding monochloride molecules. Because the CCl3 group will increase the density of states even more than the CF3 group, this comparison implies that the threshold energy for the 1,1,1-trichloride molecules must be lower than for the corresponding monochloride molecules to have such large rate constants. A more precise assignment requires comparison of the experimental rate constants in s−1 units to the calculated RRKM rate constants. III-B. Reaction of Cl Atoms with CF3CHCH2. It is of general interest to compare the reactions of Cl atoms with propene and CF3CHCH2. The reaction with propene31 proceeds without a significant barrier to give 1-chloropropyl and 2-chloropropyl radicals in an approximate 9:1 ratio. The CF3 group is expected to reduce the electron density on the central carbon atom in CF3CHCH2 so that terminal addition is favored and there is no possibility of H-abstraction at the allylic carbon. Under our experimental conditions the dominant fate of the chlorotrifluoropropyl radicals is abstraction of an Iatom from the alkyl iodides. On the basis of the yield of CF3CHICH2Cl versus CF3CHClCH2I found in the photolysis experiments with CCl4 and CH3I, addition of Cl to the terminal carbon is favored by a factor of approximately 15. Two minor products, CF3CH(CCl3)CH2Cl and CF3CHCHCl, that were detected likely arose from Cl atom addition to CF3CHCH2. The latter product is formed by the disproportionation reaction of the 1-chloropropyl radical with either CCl3 or CH3 radicals.
Figure 3. Plot of decomposition/stabilization ratios versus inverse pressure for CF3CH2CCl3*. The least-squares slope and intercept values are 0.62 ± 0.03 Torr and 0.009 ± 0.01, respectively,.
Table 1. Summary of Rate Constants and Threshold Energies CH3CCl3a
property average energy, kcal/mol slope of D/S plot, Torr rate constant,d s−1 assigned E0, kcal/mol calculated k⟨E⟩, s−1
CH3CH2CCl3b
CF3CH2CCl3c
88
88
86
577 ± 15
13.4 ± 0.4
0.62 ± 0.03
8.0 ± 2.0 × 109
2.1 ± 0.2 × 108
8.3 ± 0.8 × 106
52
50
52
8.9 × 109
1.9 × 107
8.5 × 106 (9.0 × 106)e
a The collision rate constant for the gas mixture, which was largely SF6, was 4.31 × 10−10 cm3 molecule−1 s−1 based on the collision diameters and ε/k values given in footnote d. bThe collision rate constant for the gas mixture, which was a 1.0:2.5:1.0 mixture of CCl4/C2H5I/ CF3CHCH2, was 4.92 × 10−10 cm3 molecule−1 s−1 based on the collision diameters and ε/k values given in footnote d. cThe collision rate constant for the gas mixture, which was mainly CF3CH2I, was 4.20 × 10−10 cm3 molecule−1 s−1 based on the collision diameters and ε/k values given in footnote d. dThe following collision diameters (Angstrom) and ε/k (K) values were used to calculate the collision rate constants. The ε/k values for the trichlo molecules and the iodides are large. CCl4 (5.6 Å, 415); CH3CCl3 (5.2 Å, 490 K); CH3CH2CCl3 (5.4 Å, 490 K); CF3CH2CCl3 (5.5 Å, 450 K); CH3I (4.7 Å, 406 K); CH3CH2I (4.8 Å, 503 K); CF3CH2CH2I (5.3 Å, 350 K); CF3CH CH2 (4.9 Å, 250 K); SF6 (5.2 Å, 212 K). eThis is the value obtained by averaging kE over the thermal distribution function.
IV. CALCULATED RESULTS IV-A. Thermochemistry. The enthalpy of reaction associated with the formation of the 1,1,1-trichloride molecules is needed to assign the energy of each molecule. Our standard procedure3,16,18,19 to obtain the average vibrational energy for molecules formed by radical recombination at 298 K has been to add the thermal energy contributed by the two radicals (3RT + the vibrational energy of each radical) to the enthalpy change for the recombination reaction at 0 K. Because the low vibrational frequencies associated with the CCl3 group broaden the thermal energy distribution, we decided to average the RRKM rate constant over the thermal distribution function, f(E), to obtain ⟨kE⟩, as well as use the more convenient ⟨E⟩ defined above to find k⟨E⟩ for comparison with the experimental rate constant. The energy distribution functions were obtained in the usual way using the properties of the transition states for dissociation to the radicals.15 Both methods are based on the assumption that the activation energy for radical recombination
equivalent to the half-quenching pressure, which are convenient laboratory indications of the rate constants. The least-squares slopes of the plots in Figures 1, 2 and 3, which are tabulated in Table 1, are 577 ± 60, 13.4 ± 0.4, and 0.62 ± 0.03 Torr for CH3CCl3, C2H5CCl3, and CF3CH2CCl3, respectively. The rate constants in s−1 units also are given in Table 1. Test were made to ensure adequate protection of the olefin product from reaction with Cl atoms in each series of experiments by varying the amount of CF3CHCH2 needed to reach a maximum D/S ratio at a given pressure. The data for CH3CH2CCl3 and CF3CH2CCl3 are satisfactory; the plots are linear with the origin as the intercept and the standard deviation of the slope is less than 5%. The experiments with 9350
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calculate the sums of states and densities of states for the transition states and molecules, respectively, were obtained by DFT calculations from one of the methods recommended by Truhlar,17 MO6-2X, with the 6-311+G(2d,p) basis set using the Gaussian37 codes. The vibrational frequencies and moments of inertia for the molecules and transition states from these calculations were then inserted into the DENSUMS code of Barker38 to evaluate the sums and densities. The molecular frequencies from the MO6-2X calculations are similar to other DFT methods, but the calculated threshold energies for HCl elimination better match experimental values (vide inf ra). The torsional motion associated with the CCl3 group in the molecules was treated as a hindered internal rotation. For CH3CCl3 the internal rotation can be considered as that of the methyl group, because CCl3 is so heavy, and the reduced moment and potential barrier were calculated to be 3.17 amu Å2 and 5.2 kcal mol−1, respectively. The CH3 torsional motion in CH3CH2CCl3 and its transition state were treated as vibrations, because the mode (and the frequency) is similar in both structures. The reduced moment and the potential barrier for the internal rotation of the (CH3)CH2− group in CH3CH2CCl3 were calculated to be 23.7 amu Å2 and 6.3 kcal mol−1, respectively. Care is needed in treating the CF3 and CCl3 torsional motions in CF3CH2CCl3, because the two torsional vibrations are coupled. Therefore, both motions were treated as hindered internal rotors in the molecule, and the CF3 torsion in the transition state also was treated as an internal rotation. The potential energy barriers in the molecule were calculated to be 4.4 and 1.5 kcal mol−1 for CCl3 and CF3, respectively; the corresponding reduced moments are 88.9 and 67.9 amu Å2. The barrier for the CF3 group in the transition state was assumed to be 3.0 kcal mol−1, which is more typical for CF3 groups; the reduced moment was 71.4 amu Å2. The barriers to internal rotation were calculated by B3PW91/6311+G(2d,p), because it has been calibrated for energetics of ground-state molecules. Some of the bending frequencies associated with the CCl3 group are actually lower than the torsional frequency in the molecule and such frequencies in the transition state are even lower. For example, the MO6-2X/6-311+G(2d,p) calculation with CH3CCl3 gave 240 and 241 cm−1 as the two lowest frequencies, whereas the torsional frequency was 315 cm−1 and the four lowest frequencies of the transition state were 101, 153, 254, and 274 cm−1. Because these low frequencies strongly affect the rate constant, other basis sets and other DFT methods were examined to evaluate the sensitivity of the frequencies to the computational method. Some results are tabulated in the Supporting Information, and some comments are in the Discussion. The rate constants were evaluated using the standard RRKM expression of eq 7.
are effectively zero. Thus, we need to evaluate the enthalpy change for the recombination reactions at 0 K plus the small thermal contribution to the energy for each molecule. The enthalpy of formation for CH3,32 CCl3,33 and CH3CCl334 at 298 K are +35.1, +17.0, and −34.5 kcal mol−1, respectively, which give the enthalpy of reaction as −86.6 kcal mol−1. Converting to 0 K gives −84.7 kcal mol−1; the uncertainty in this number should be ±1 kcal mol−1. The estimate for ⟨E⟩ of CH3CCl3 using 84.7 kcal mol−1 plus 3RT plus the thermal vibrational energy of each radical gives 87.7 kcal mol−1. The average energy calculated using the distribution function is 88.5 kcal mol−1. The enthalpy of formation of C2H5 and CF3CH2 are known; 32 however, the enthalpies of formation for CH3CH2CCl3 and CF3CH2CCl3 are not available. Therefore, we employed the isodesmic reactions 4 and 5 with computation of the enthalpies of reaction by DFT with the B3PW91 method and the 6-31G(d′,p′) and 6-311G(2d,p) basis sets to evaluate the enthalpies of formation at 298 K. CH3CCl3 + CH3CH 2CH3 → CH3CH 2CCl3 + CH3CH3 (4)
CH3CCl3 + CF3CH 2CH3 → CF3CH 2CCl3 + CH3CH3 (5)
The calculated enthalpies of reaction were −0.2 and −0.1 kcal mol−1 for reaction 4 and 9.1 and 10.0 kcal mol−1 for reaction 5 from the 6-31G(d′,p′) and 6-311G(2d,p) basis sets, respectively. Combining these values with the enthalpies of formation of CH3CCl3 (−34.5 kcal mol−1),33 C3H8(−24.8 kcal mol −1 ), 36 C 2 H 6 (−20.1 kcal mol −1 ), 33 and CF 3 CH 2 CH 3 (−185.5 kcal mol−1)35 give −39.4 and −190.4 kcal mol−1 as the enthalpies of formation at 298 K for CH3CH2CCl3 and CF3CH2CCl3, respectively. Combining these values with the enthalpies of formation32 of the radicals gives the enthalpies for the recombination reactions as −85.1 and −81.6 kcal mol−1 for CH3CH2CCl3 and CF3CH2CCl3, respectively. The 81.6 kcal mol−1 value seems surprisingly low, because a CF3 group normally increases the bond dissociation energy of the adjacent carbon−carbon bond. The enthalpy of formation of CF3CH2CH3 from Khursan35 is based on a comprehensive computational analysis of several fluoropropanes and his value is supported by earlier36 computations, which gave −183.1 kcal mol−1. A test was made for the enthalpy of formation of CF3CH2CCl3 using the MO6-2X method for reaction 5, and a similar result was obtained for the enthalpy of reaction. Finally, reaction 6 was chosen as a more direct test using the previously determined enthalpy of formation of CH3CH2CCl3. CH3CH 2CCl3 + CH3CF3 → CF3CH 2CCl3 + C2H6
(6)
The enthalpy change was 10.1 kcal mol−1 as calculated from B3PW91, which leads to −189 kcal mol−1 as the enthalpy of formation for CF3CH2CCl3. Apparently, the carbon−carbon bond energies in CF3CH2CCl3 are not as large as in many comparable molecules. Adjusting the enthalpies of recombination to 0 K gives 83.8 and 80.9 kcal mol−1; we assign an uncertainty of ±2 kcal mol−1 to these values. Including the average thermal energy of the radicals raises the average energies to 87 and 85 kcal mol−1. If the thermal distribution function method is used, the average energies become 88 and 86 for CH3CH2CCl3 and CF3CH2CCl3, respectively. IV-B. Computation of Rate Constants and Assignment of Threshold Energies. The molecular models needed to
kE = (s†/h)(I †/I )1/2
∑ P†(E − E0)/NE
(7)
The sum of states of the transition state is given by the ΣP†(E − E0) term; the density of states of the molecule is NE, s† is the reaction path degeneracy, h is Planck’s constant, and (I†/I) is the ratio of moments of inertia for the three overall rotations. The reaction path degeneracy was 3 for CH3CCl3 and 2 for CH3CH2CCl3 and CF3CH2CCl3; the symmetry number 3 was included with the internal rotation of the CCl3 group. The experimental rate constants were compared to the calculated k⟨E⟩ with ⟨E⟩ defined by the average with the thermal 9351
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the E0 values reflect the relative energies of the three isomeric transition states of C2H3Cl3. In marked contrast with the trends mentioned above for chloro- and bromoalkanes, the threshold energies for HF elimination are virtually independent of whether the out-of-ring F atoms are on CH or CF. The E0 values systematically increase with the number of F atoms on the CH and CF atoms.6,41 Before attempting to explain these trends in threshold energies from substitution of halogen atoms on CH and CX in H−X elimination transition states, rate constants from some different DFT methods will be summarized. V-B. Comparison of DFT Methods and Basis Sets. In our recent work we have combined DFT calculations of electronic structures, mainly with the B3PW91 and B3LYP methods, with chemical-activation rate constants to assign threshold energies for several unimolecular reactions. The critical factor for the calculations in this approach to assigning E0 values is the reliability of the low vibrational frequencies calculated for the molecule and transition state. The experimentally assigned E0 values for HF elimination from fluoroalkanes were usually in close agreement with the DFT calculated values; however, the calculated E0’s for HCl elimination from chloroalkanes were often 2−5 kcal mol−1 lower than the experimentally favored values.3,6,16,18,19,41 For that reason, we decided to use the MO6-2X method in the present work and, indeed, the calculated E0 are higher than from the B3PW91 method as shown in Table 2. Calculations
distribution function, which is only slightly larger than the value from the simple method to estimate the average energy. The only unknown is the threshold energy, and the results are summarized in Table 1 for calculations with the MO6-2X/6311+G(2d,p) method. Threshold energies of 52, 50, and 52 kcal mol−1 were assigned to CH3CCl3, CH3CH2CCl3 and CF3CH2CCl3, respectively. The values of k⟨E⟩ and ⟨kE⟩ were compared for CF3CH2CCl3, and the latter was only 6% larger; the difference is even smaller for the other two molecules. Thus, the comparison of k⟨E⟩ to the experimental rate constant is adequate for assignment of E0 values. On the basis of the results in Table 1, it is evident that the E0 values for CH3CCl3, CH3CH2CCl3, and CF3CH2CCl3 are smaller than those for the monochloride series3 of analogous molecules, which are 55, 54, and 58 kcal mol−1. In all six cases, the state counting for eq 7 was done without consideration of anharmonic effects. The inclusion of anharmonicity would lower the rate constants somewhat and require lower threshold energies. Nevertheless, the same methodology was used in both series and the general reduction in E0 for the HCl elimination for the 1,1,1-trichloride molecules is established. The uncertainty in the E0 values assigned by the chemical-activation method is usually 1−2 kcal mol−1 depending on the quality of the experimental data, the reliability of ⟨E⟩, and the confidence in the properties of the transition state. The latter will be addressed in the Discussion. The threshold energies for CH3CCl3, CH3CH2CCl3, and CF3CH2 CCl3 were 53.8, 52.7, and 53.8 kcal mol −1, respectively, from the MO6-2X/6-311+G(2d,p) calculations. As a point of reference, the calculated value for CH3CH2Cl was 56.8 kcal mol−1.
Table 2. Comparison of E0 from Different DFT Methods and Basis Setsa
V. DISCUSSION V-A. Threshold Energies for HCl Elimination. The results of this chemical-activation study clearly show that the threshold energies for HCl elimination from 1,1,1-trichloroalkane molecules are in the 50−52 kcal mol−1 range. The chemical-activation method is able to suppress the interference of the Cl atom chain reaction that makes the thermal-activation studies of 1,1,1-trichloroalkanes so difficult. These results, the thermal-activation study1 of CH3CCl3, and less extensive results for 1,1-dichloroalkanes4−6 indicate a systematic decline in threshold energies when Cl atoms are bound to the CCl atom of the transition state. This trend for CH3CH2Cl, CH3CHCl2, and CH3CCl3 is supported by experimental data and by the DFT calculations. Fewer experimental data are available for the 1,1dibromo and 1,1,1-tribromo compounds, but calculations39 suggest that the same trend exists. In contrast to the reduction in threshold energy with additional Cl atoms on CCl, the threshold energy is elevated if a Cl atom is bound to the CH. This trend can be illustrated by comparing the chemicalactivation data 40 for CH 2 ClCHCl 2 formed by radical recombination with that from CH3CCl3. The total rate constant40 for CH2ClCHCl2 is 34 times smaller than for CH3CCl3. A simple comparison using the rate constants calculated for CH3CCl3 suggests that the threshold energies are 8−10 kcal mol−1 higher for CH2ClCHCl2. Calculations by MO6-2X/6-311+(2d,p) gave 58 and 64 kcal mol−1 as the E0 for the CHClCHCl and CH2CCl2 channels, respectively. Numerous other examples6,18,19 can be cited to illustrate the contravening effects on E0 for HCl elimination by substitution of Cl atoms on CH and CCl atoms, but the isomers of trichloroethane are especially illustrative. Because the enthalpies of formation of CH3CCl3 and CH2ClCHCl2 are nearly equal,33
a
method
CH3CH2Cl
CH3CHCl2
CH3CCl3
CH3CH2CCl3
MO6-2X/(2d,p) B3PW91/(2d,p) MO6-2X/cc-pDVZ B3PW91/cc-pDVZ
56.8 52.9 57.5 53.0
55.4 49.6 56.3 49.9
53.8 47.0 55.7 48.5
52.7 45.9 54.9 47.6
E0 are in units of kcal mol−1.
also were done with the cc-pVDZ basis set. The trend in E0 was the same as for the 6-311+G(2d,p) basis set, but the values are 1−2 kcal mol−1 higher and the difference between C2H5Cl and CH3CCl3 may be underestimated. In general, the calculated vibrational frequencies of the molecule and transition state are not very sensitive to the specific DFT method or basis set.3,42,43 However, given the very low frequencies of the 1,1,1-trichloro systems, this point needs to be explicitly examined. The first test was to compare the rate constants for the 6-311+G(2d,p) and 6-31G(d′,p′) basis sets from the MO6-2X method. The rate constant from the (d′,p′) basis state at the same energy was smaller by a factor of 1.26; the principal reason is the higher frequencies in the transition state. The calculated threshold energy for CH3CCl3 from the (d′,p′) basis set increased to 56.3 kcal mol−1. We also compared results from a MO6-2X calculation with the ccpVDZ basis set, because prior work42 with CH3CF3 suggested that this basis set may give higher frequencies in the transition state. This rate constant was smaller than the 6-311+G(2d,p) result by a factor of 1.45. Finally, we compared the results from a calculation with B3PW91/6-311+G(2d,p) with MO6-2X/6311+G(2d,p). The B3PW91 rate constant also was smaller than that from MO6-2X by a factor of 1.28. These reductions are comparable to the decrease (1.35) in the rate constant for raising E0 by 1 kcal mol−1 or lowering ⟨E⟩ by 2 kcal mol−1. 9352
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order of the calculated46 E0 (which reflects the relative stabilities of the transition states.) associated with CH 2 ClCHCl2 and CH 3 CCl 3 . The stabilization of the CH 3CH 2+-bridged structure, which is the most stable structure,44 by an α-CH3 group is 18 kcal mol−1, whereas that for an α-Cl atom is 10 kcal mol−1. Thus, the effect upon E0 for substitution by a methyl group on CCl would be expected to be larger than for a chlorine atom, which is the case. The 27 kcal mol−1 difference between CH3CHCl+ and CH2ClCH2+ correlates with the 8−10 kcal mol−1 increase6 in E0 of CH2ClCH2Cl relative to CH3CH2Cl. The relative stabilities of several such pairs of cations correlate with elevated E0 for HCl elimination reactions with halogen substitution at CH.18,19 The structures of the transition states for HCl elimination shown in Figure 4 illustrate the effect that Cl atoms have on the geometry around CCl, as well as other aspects of the structure. The most important point is the similarity of the planar structure around CCl to the structure of the carbocations.44 The CCl−Cl bond length is actually shorter than that in either the reactant or product; however, it is similar to the length in CH3CHCl+, which has some double bond character. The structure around the CH end of the transition states becomes more similar to sp3 geometry as Cl atoms are added to CCl. Note the longer in-ring C−Cl and Cl−H distances for CH3CCl3; i.e., the Cl atom is more isolated. When two H atoms are attached to CCl, the ∠C−C−Cl angle in the fourmembered ring exceeds 90° and the CH end is more nearly planar; i.e., the dihedral angle is approximately 20° (see the structures on the right half of Figure 4). However, when one or more Cl atoms is attached to CCl, the ∠C−C−Cl angle is less than 90° and the CH end is less planar; i.e., the dihedral angle is between 30 and 40°. Furthermore, the diagrams of the transition states show that the effect is cumulative when two Cl atoms are attached to CCl. These same effects also are demonstrated by the two transition states derived from CH2ClCHCl2. In summary, the analogy with both the stabilization energies and the structures of the transition states with those properties of the carbocations provides an explanation of the trend in threshold energies of 1,1-dichloro and 1,1,1-trichloroalkane (and probably the analogous bromoalkane molecules39). The large destabilization (23.5 kcal mol−1) of CH3CH2+ by an α-CF3 group44 also correlates with the elevation of E0 with substitution of CF3 at CCl (see CF3CFClCH3 as an example).47 Fluorine atoms have a smaller effect on the stability of the carbocations than chlorine atoms44 and CH3CHF+ and CH3CF2+ have nearly the same hydride affinity. There is no correlation between threshold energies for HF elimination and hydride affinities, except for the reduction in threshold energy with α-CH3 substitution at CF. The calculated transition-state structures6,19 with F atoms added to CF do not exhibit the trends of geometry shown in Figure 4. The carbocation character seems not to be as fully developed in the transition state for HF elimination as for HCl elimination. The general increase in threshold energy as F atoms are added correlates with the reduction in D(CC) of the fluorinated ethenes. Because the transition states have some C−C double bond nature, this trend may dominate the carbocation effects. Other differences between the transition states for HF and HCl elimination are the greater extension of the in-ring C−Cl length (about 50%) relative to the C−F extension (about 35%) and the longer C−H in-ring distance for
Although MO6-2X/6-311+G(2d,p) gives calculated threshold energies that are in close agreement with the experimental values, the frequencies for the transition state may be somewhat too low. This possibility is reinforced by comparison with the experimental Arrhenius pre-exponential factor,1 which was reported as 1 × 1014 s−1 without error limits (the activation energy was 54 kcal mol−1). Inspection of the data suggests that the pre-exponential factor could be as large as 3 × 1014 s−1. However, the transition-state model from MO6-2X for CH3CCl3 gives an Arrhenius expression of 4.5 × 1014exp(−53.4 kcal mol−1/RT) at 690 K for E0 = 52 kcal mol−1, and this comparison also suggests that some of the frequencies of the transition state are too low. A comprehensive computational study43 of HCl elimination from CH3CClCH2 using a variety of modern functionals and a high level basis set found that the computed frequencies of the transition state were very similar for all of the methods. The low frequencies of the transition state for the trichloroalkanes seem to provide a specific challenge in addition to the trend noted in earlier studies3,6 for computed pre-exponential factors to be slightly larger than experimental values for HCl elimination from chloroalkanes. Nevertheless, different basis sets and DFT methods give very similar transition-state structures, and these are examined in the next section to identify the effects of Cl substitution on CCl and CH. V-C. Correlation of E0 with the Stability of Carbocations. The long known reduction of the threshold energy for HCl elimination with addition of CH3 groups to to CCl has been explained as a consequence of the stabilization of the positive charge on CCl. This trend can be explicitly correlated7 with the relative stabilities of the positive ions CH3CH2+, (CH3)2CH+, and (CH3)3C+. Heydtmann and co-workers7 noted the similar stabilities of the RCH2CH2+ (R = CH3, C2H5) ions, which correlated with nearly constant E0 of primary chloroalkanes. We wish to test this correlation with the positive ions derived from HCl elimination of the di- and trichloroethanes. The hydride affinities were calculated by following the strategy outlined in ref 44 on the basis of the recommended CBS-Q method,44,45 and the results are given in Table 3. The increased stability in the CH3CH2+, CH3CHCl+, CH3CCl2+ series, which amounts to a total of 22 kcal mol−1, correlates with the decline in threshold energies of the CH3CH2Cl, CH3CHCl2, and CH3CCl3 series. The relative stabilities of the CH3CCl2+, CH2ClCHCl+, and CHCl2CH2+ ions follow the Table 3. Hydride Affinities Calculated by the CBS-Q Method44 ion CH3CH2+ CH3CHCl+ CH2ClCH2+ CH3CCl2+ CH2ClCHCl+ CHCl2CH2+ c CH2FCHCl+ c CH3CFCl+
hydride affinitya 276.1 (270.2) 260.8 287.7 254.4 267.4 294.4 274.5 258.1
b
hydride affinitya
ion +
CH3CHF CH2FCH2+ c CH3CF2+ CH2FCHF+ CF2HCH2+ c CHFClCH2+ c CH2ClCHF+
263.9 289.8 263.4 280.2 319.0 314.6 268.8
The enthalpy change for H− + R+ → RH in units of kcal mol−1; the experimental energy (331.0 kcal mol−1) of H− was used in the computation of the enthalpy. bThe energy of the bridged structure. The energy for the CH3CH2+ structure was obtained by constraining the geometry. cThe energy of these structures was obtained by constraining the geometry for the calculation; see ref 44. a
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Figure 4. Diagrams of transition states for HCl elimination from CH3CH2Cl, CH2ClCH2Cl, CH3CHCl2, CH3CCl3, and CH2ClCHCl2. The structures and E0 values were obtained from MO6-2X/6-311+G(2d,p) calculations. Typical C−Cl bond lengths in chloroalkanes and chloroalkenes are 1.79 and 1.72 Å; the C−Cl distance for CCl is less than either. The C−Cl bond length in CH3CHCl+ is 1.61 Å. 9354
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(2) Rajakumar, B.; Arunan, E. Ab Initio, DFT, and Transition State Theory Calculations on 1,2-HF, HCl and ClF Elimination Reactions from CH2FCH2Cl. Phys. Chem. Chem. Phys. 2003, 5, 3897−3904. This paper has a compilation of thermal rate constant measurements for C2H5Cl. (3) Ferguson, J. D.; Johnson, N. L.; Kekenes-Huskey, P. M.; Everett, W. C.; Heard, G. L.; Setser, D. W.; Holmes, B. E. Unimolecular Rate Constants for HX and DX Elimination (X = F, Cl) from Chemically Activated CF3CH2CH2Cl, C2H5CH2Cl and C2D5CH2Cl: Threshold Energies for HF and HCl Elimination. J. Phys. Chem. A 2005, 109, 4540−4551. This paper shows that the E0 assigned from thermal and chemical-activation studies of C2H5Cl are compatible. (4) Hartmann, H.; Heydtmann, H.; Rinck, G. Die Kinetik des Thermischen Zerfalls von 1,1-Dichloräthan. Z. Phys. Chem. N.F. 1961, 28, 71−84. (5) Jonas, R.; Heydtmann, H. The Thermal Unimolecular Decomposition of CH3CD2Cl, CD3CD2Cl and CH3CHCl2. Ber. Bunsen-Ges. Phys. Chem. 1978, 82, 823−827. (6) Duncan, J. R.; Solaka, S. A.; Setser, D. W.; Holmes, B. E. Unimolecular HCl and HF Elimination Reactions of 1,2-Dichloroethane, 1,2-Difluoroethane, and 1,2-Chlorofluoroethane: Assignment of Threshold Energies. J. Phys. Chem. A 2010, 114, 794−803. (7) Hartmann, H.; Bosche, H. G.; Heydtmann, H. Ü ber den Einflußder Kettenlänge auf die Kinetik des Thermischen Zerfalls von Primären Chloralkanen. Z. Phys. Chem. N. F. 1964, 42, 329−344. (8) Howlett, K. E. Studies on Unimolecular Chlorohyrdocarbon Decomposition Part IV. J. Chem. Soc. 1953, 945−947. (9) Holbrook, K. A.; Parry, K. A. W. Gas-Phase Pyrolysis of 1,1Dichlorobutane and 1,1-Dichloropropane. J. Chem. Soc. (B) 1971, 1762−1765. The large pre-exponential factor for the rate constant for CH3CH2CHCl2 suggests that the reported activation energy may be too large. (10) Sudbo, A. S.; Schulz, P. A.; Shen, Y. R.; Lee, Y. T. Three and Four Center Elimination of HCl in the Multiphoton Dissociation of Halogenated Hydrocarbons. J. Chem. Phys. 1978, 69, 2312−2322. (11) Sun, L.; Park, K.; Song, K.; Setser, D. W.; Hase, W. L. Use of a Single Trajectory to Study Product Energy Partitioning in Unimolecular Dissociation: Mass Effects for Halogenated Alkanes. J. Chem. Phys. 2006, 124, 064313(1−9). (12) Dong, E.; Setser, D. W.; Hase, W. L.; Song, K. Comparison of Electronic Structure Theory in Direct Dynamics Simulations of C2H5F→HF + C2H4 Product Energy Partitioning. J. Phys. Chem. A 2006, 110, 1484−1490. (13) Park, K.; Sun, R.; Hase, W. L. Direct Dynamics Simulation of CH3CCl3→CH2CCl2 + HCl Product Energy Partitioning. Comparison with Experiment. Manuscript to be published. (14) Setser, D. W. Vibrationally Excited 1,2-Dichloroethane Produced by the Mercury Photosensitization of Dichloromethane. J. Am. Chem. Soc. 1968, 90, 582−587. (15) Chang, H. W.; Setser, D. W. Unimolecular Reaction and Collisional Deactivation of Chemically Activated 1,2-Difluoroethane Produced by Mercury Photosensitization of Chlorofluoromethane at 300 and 475 K. J. Am. Chem. Soc. 1969, 91, 7648−7657. (16) McClintock, C. E.; Smith, K. C.; Tucker, M. K.; Heard, G. L.; Setser, D. W.; Holmes, B. E. The Effects of CF3 and CH3 Groups on the Threshold Energy for the Unimolecular Interchange Reaction of Cl- and F-Atoms in CF3CHFCH2Cl and CH3CHFCH2Cl. J. Phys. Chem. A 2014, 118, 6717−6723. (17) Xu, X.; Alecu, I. M.; Truhlar, D. G. How Well Can Modern Density Functionals Predict Internuclear Distances at Transition States. J. Chem. Theory Comput. 2011, 7, 1667−1676. (18) Enstice, E. C.; Duncan, J. R.; Setser, D. W.; Holmes, B. E. Unimolecular Reactions in the CF3CH2Cl ↔CF2ClCH2F System: Isomerization by Interchange of Cl and F Atoms. J. Phys. Chem. A 2011, 115, 1054−1062. (19) Solaka, S. A.; Boshamer, S. E.; Parworth, C. L.; Heard, G. L.; Setser, D. W.; Holmes, B. E. Isomerization of CF2ClCH2Cl and CFCl2CH2F by Interchange of Cl and F Atoms with Analysis of the
HCl elimination than for H−F elimination. Further work is needed to quantify these ideas. Inspection of the structures in Figure 4 suggests that changes in the structure of the transition states, as well as changes in mass, should be included when building models to describe the energy disposal in HCl elimination reactions.11−13
VI. CONCLUSIONS The mercury photosensitization technique with CF3CHCH2 as the scavenger of Cl atoms was used to generate CCl3 and CH3 (or C2H5 or CF3CH2) radicals, which subsequently recombined to form CH3CCl3*, C2H5Cl3*, and CF3CH2CCl3* molecules with about 87 kcal mol−1 of vibrational energy. The experimentally measured rate constants for CH 3 CCl 3 , C2H5CCl3, and CF3CH2CCl3 are 8.0 × 109, 2.1 × 108, and 8.3 × 106 s−1, respectively. The reduction in rate constants is a consequence of the increase in density of states from the CH3 and CF3 groups, because the threshold energies for the reactions were similar, i.e., 52, 50, and 52 kcal mol−1. The reduction in threshold energy for the 1,1,1-trichloride molecules, relative to C2H5Cl, with an E0 = 55 ± 1 kcal mol−1 , correlates with the stabilization energy of the carbocations in the CH3CH2+, CH3CHCl+, and CH3CCl2+ series. Furthermore, the geometry of the transition state for HCl elimination with Cl atoms on the CCl closely resembles the geometry of the analogous carbocation. The analogy between the structures and relative stabilization energies of the carbocations with the transition states for HCl elimination also can explain the elevation of the E0 for CH2ClCH2Cl (E0 = 61 ± 2 kcal mol−1) relative to C2H5Cl and CH3CHCl2. The electronic structure calculations were done with MO6-2X/6311+G(2p,d). The threshold energies were assigned by matching the experimental rate constants of CH3CCl3, CH3CH2CCl3, and CF3CH2CCl3 to statistical calculated rate constants.
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ASSOCIATED CONTENT
S Supporting Information *
Supporting Information contains calculated vibrational frequencies, cm−1, and moments of inertia (Ix, Iy, and Iz), amu Å2, using M06-2X/6-311+G(2d,p) for the molecule and the transition states for CF3CH2CCl3 and for CH3CH2CCl3. Identical information is presented for CH3CCl3 using three computational approaches: M06-2X/6-311+G(2d,p), B3PW91.cc-pVDZ, and P3PW91/6-31G(d′,p′). This material is available free of charge via the Internet at http://pubs.acs.org
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AUTHOR INFORMATION
Corresponding Author
*B. E. Holmes. Phone: 828-232-5168. E-mail: bholmes@unca. edu. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from the National Science Foundation (CHE-1111546 and MRI-1229406) is gratefully acknowledged.
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REFERENCES
(1) Barton, D. H.; Onyon, P. F. The Pyrolysis of 1,1,1-Trichloroethane. J. Am. Chem. Soc. 1950, 72, 988−995. 9355
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