Analysis of the Five Unimolecular Reaction Pathways of CD2ClCHFCl

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A: Kinetics, Dynamics, Photochemistry, and Excited States

Analysis of the Five Unimolecular Reaction Pathways of CDClCHFCl with Emphasis on CDCl(F)C: and CDCl(Cl)C: Formed by 1,1-HCl and 1,1-HF Elimination 2

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Timothy M. Brown, Blanton R. Gillespie, Caleb A. Smith, Matthew J. Nestler, George L. Heard, Donald W. Setser, and Bert E. Holmes J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b06680 • Publication Date (Web): 28 Sep 2018 Downloaded from http://pubs.acs.org on September 29, 2018

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The Journal of Physical Chemistry

Analysis of the Five Unimolecular Reaction Pathways of CD2ClCHFCl with Emphasis on CD2Cl(F)C: and CD2Cl(Cl)C: Formed by 1,1-HCl and 1,1-HF Elimination Timothy M. Brown‡, Blanton R. Gillespie‡, Caleb A. Smith‡, Matthew J. Nestler‡, 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, United States and †Kansas State University, Manhattan, Kansas 66506, United States ABSTRACT The five unimolecular HX and DX (X = F, Cl) elimination pathways of CD2ClCHFCl* were examined using the chemical-activation technique; the molecules were generated with 92 kcal mol-1 of vibrational energy in a room temperature bath gas by combination of CD2Cl and CHFCl radicals. The total unimolecular rate constant is 9.7 x 107 s-1 and branching fractions for each channel are 0.52 (2,1-DCl), 0.29 (1,1-HCl), 0.10 (2,1-DF), 0.07 (1,1-HF), and 0.02 (1,2-HCl). Comparison of the individual experimental rate constants to calculated statistical rate constants gave threshold energies for each process as 63, 72, 66, 73 and 70 kcal mol-1 listed in the same order as the branching fractions. The 1,1-HCl and 1,1-HF reactions give the carbenes, CD2Cl(F)C: and CD2Cl(Cl)C:, respectively, as products, which have hydrogen-bonded complexes with HCl or HF in the exit channel of the potentialenergy surface. These carbenes have energy in excess of the threshold energy for D-atom migration to give CDCl=CDF and CDCl=CDCl, and the subsequent cis-transisomerization rates of the dihaloethenes can provide information about energy disposal by the 1,1-HX elimination reactions. Electronic-structure calculations provide information for transition states of CD2ClCHFCl and hydrogen-bonded complexes of carbenes with HF and HCl. In addition, D-atom migration in both free carbenes and in complexes formed by the carbene hydrogen-bonding to HCl or HF are explored.

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1. INTRODUCTION Our laboratory has investigated unimolecular reactions of halogenated alkanes using the chemical-activation technique coupled with electronic-structure calculations; 1,2-HX (X = F, Cl, Br) elimination reactions1-3 and halogen interchange reactions4-7 have been characterized and threshold energies assigned for several systems. Our recent efforts have been directed toward 1,1-HX elimination reactions8-12 of 1,1-dihalo-methanes, -ethanes and -propanes. These reactions proceed with conservation of spin to give singlet halo-carbenes and HX. The dynamics in the exit channel can involve a post-transition state complex (PTSC) arising from hydrogen bonding between HX and the carbene. In some cases the 1,1-HX transition state may be submerged with respect to the enthalpy of reaction for formation of carbene + HX. Neither the PTSC or the submerged transition states are fully understood. Information about the decomposition pathways of halogenated alkanes is needed to understand the mechanisms for thermal degradation of HCFCs as they are recovered from industrial use and destroyed. We have selected CD2ClCHFCl to study by the chemical activation technique in order to characterize the 5 unimolecular decomposition pathways, 2,1-DCl, 1,1-HCl, 2,1-DF, 1,1-HF and 1,2-HCl elimination with a focus on the 1,1-HX reactions. The three-centered 1,1-HF elimination reaction, which is in competition with the four-centered 2,1-DF elimination reaction, has been reported for CD3CHF2, C2D5CHF28 , and CD3CHFCl.9 Electronic-structure calculations of transition states for 1,1-HF and 2,1-DF reactions were straightforward. Threshold energy assignments based upon matching calculated statistical rate constants with experimental values for 1,1-HF elimination were consistent with expectation of a small activation energy for addition of the carbene to HF, i.e., the threshold energy for elimination was in excess of the enthalpy of the reaction giving carbene plus HF. The low frequencies of the 1,1-HF transition state compensate for the high threshold energy, and 1,1-HF elimination can compete with 2,1-DF elimination for high temperatures or high excitation energies. For highly


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fluorinated molecules, such as CHF2CHF2 and CF3CHF2, 1,1-HF elimination can be an important component10,11 of the overall reaction for many experimental situations. According to electronic-structure calculations, a hydrogen-bonded adduct between carbene and HF exists in the exit cannel of 1,1-HF elimination reactions.11 The transition states for 1,1-HCl elimination9,12 are more difficult to characterize, because threshold energies are close to the enthalpy of reaction, and the hydrogen-bonded complexes12 may affect the stationary point on the potential energy surface associated with the transition state. In order to better understand the 1,1-HCl elimination process, we have selected CD2ClCHFCl for study using the gas-phase recombination reaction of CD2Cl and CHFCl to generate CD2ClCHFCl* molecules with 92 kcal mol-1 of vibrational energy, . This work allows direct comparison of the 1,1-HF and 1,1-HCl reactions to the 2,1-DF and 2,1–DCl reactions. The 1,2-HCl elimination reaction is a minor component of the overall unimolecular decomposition process. Previous studies7 of halogen-interchange reactions indicated that Cl/F interchange in CD2ClCHFCl* should not be important relative to HCl and HF elimination reactions, and this expectation was confirmed by experimental analysis for products expected from CD2FCHCl2*. Although the CD2ClCHFCl* system resembles previous work with CD3CHFCl*, the present study offers several advantages including the expected2 elevation of threshold energies by a β-Cl substituent for 2,1-DCl and 2,1-DF elimination, which makes 1,1-HCl and 1,1-HF reactions more competitive. The smaller rate constants associated with higher threshold energies and lower provides a more convenient experimental pressure range. Another advantage is the opportunity to observe the Z (cis) and E (trans) geometric isomers of CDCl=CDF and CDCl=CDCl following isomerization of CD2Cl(F)C: and CD2Cl(Cl)C:. In fact, the subsequent cis-trans isomerization of C2D2ClF and C2D2Cl2 alkenes can be used to discuss energy disposal from 1,1-HX elimination. A new aspect of this work is recognition of hydrogen-bonded complexes, CD2Cl(F)C:HCl and CD2Cl(Cl)C:HF, in exit channels of the 1,1-HCl and 1,1-HF elimination reactions, and their possible importance in subsequent evolution of carbenes to stable products.


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Experimental rate constants were measured for the five HX- and DX-elimination reactions and compared to calculated statistical rate constants to assign threshold energies for 2,1-DCl, 1,1-HCl, 2,1-DF, 1,1-HF and 1,2-HCl elimination processes. The vibrational frequencies and moments of inertia for the molecule and transition states that are required for the rate constant calculations plus information about the adducts and H-atom isomerization energies were obtained from electronic-structure calculations using B3PW91/6-311+G(2d,p) and M062X/aug-cc-pVTZ methods. The validity of transition states was confirmed by examination of intrinsic reaction coordinates (IRC). The stationary points of the exit-channel potential surface were investigated using CBS-QB3 and M06-2X methods. Calculations for several isodesmic reactions were also done to obtain certain thermochemical quantities that were not available in the literature. All electronic structure calculations used the Gaussian suite13 of codes. The energy profile for the CD2ClCHFCl* system of reactions is summarized in Figure 1. 2. EXPERIMENTAL and COMPUTATIONAL METHODS 2a. Chemical Reactions. The relevant chemical reactions are summarized in (1), (2) and (3). The CD2Cl and CHFCl radicals were produced by mercury sensitization of excess CD2ClI with CHFCl2. Some Cl atoms are produced by the interaction3 of Hg(3P1) atoms with CHFCl2, and CF3CH=CH2 was added to the mixture to control the Cl atom concentration. An asterisk denotes the species is vibrationally exited with an average energy, , acquired from the recombination reaction. CD2Cl + CHFCl → CD2ClCHFCl*

a.

→ CD2ClCD2Cl*

b.

→ CHFClCHFCl*

c.

(1)

Since [CD2Cl] > [CHFCl], the formation of meso- and d,l-CHFClCHFCl* is not important, and since the decomposition reaction of CD2ClCD2Cl* has been thoroughly studied2,5, attention can be focused on CD2ClCHFCl*. Fortunately, disproportionation reactions between CD2Cl and CHFCl radicals are much less


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important than recombination, and disproportionation can be ignored. The unimolecular decomposition and collisional stabilization processes for CD2ClCHFCl* are listed in (2). In principle, reaction (1a) is reversible, but threshold energies for elimination reactions are lower than the carbon-carbon bond dissociation energy and dissociation is not competitive. M represents the bath gas, comprised of CD2ClI, CHFCl2 and CF3CH=CH2. CD2ClCHFCl* → DCl + CDCl=CHF (Z and E)

+ M

a. (2,1-DCl)

→ HCl + CD2Cl(F)C:*

b. (1,1-HCl)

→ DF + CDCl=CHCl (Z and E)

c. (2,1-DF)

→ HF + CD2Cl(Cl)C:*

d. (1,1-HF)

→ HCl + CD2=CFCl

e. (1,2-HCl)

→ M + CD2ClCHFCl

f. (stabilization)

(2)

The five elimination reactions give unique products and each can be identified by gas-chromatographic analysis with mass-spectrometric detection (GCMS). Rate constants are obtained by comparing the ratio of the relative concentrations of a decomposition product, Di, to the stabilized molecule, S, at various pressures. The carbenes from (2b) and (2d) retain sufficient energy for isomerization by Datom migration; the threshold energies14-17 from the lowest energy isomer (cis) are calculated to be ≈ 9 (CD2Cl(F)C:) and ≈ 5 (CD2Cl(Cl)C:) kcal mol-1 for reaction (3). CD2Cl(F)C:* → CDCl=CDF* (Z and E)

a.

CD2Cl(Cl)C:* → CDCl=CDCl* (Z and E)

b.

(3)

The D-atom migration reactions of free carbenes strongly favor the Z (cis) CDCl=CDF and CDCl=CDCl isomers8,14-17, but these molecules have sufficient energy, see Figure 1, subsequently to undergo cis-trans isomerization and products of reaction (3) were monitored vs. pressure. The possible role of


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CD2Cl(F)C:HCl and CD2Cl(Cl)C:HF adducts in D-atom migration will be considered after the data are presented. The experimental methods are similar to previous reports from this laboratory.3,8,9 Reagents are loaded into quartz vessels and irradiated at room temperature with the 253.7 nm emission from a 15 W Hg germicidal lamp and the resulting mixture is transferred to the inlet of the gas chromatograph, a Shimadzu GC-17a with QP5000 mass spectrometer as detector and a 105 m RTX-1701 column, for analysis. 1,1,1-Trifluoropropene was added to the vessels to trap Cl-atoms, which would add to some of the decomposition products and affect the Di /S ratios, if Cl atoms are not removed. The starting ratios for experiments were CHFCl2:CD2ClI: CF3CH=CH2 = 1:3:4 plus a droplet of Hg. The vessels, which ranged in size from 11 to 475 cm3, were irradiated for about 5 minutes. Samples of pure reactants and products were available to verify retention times and mass spectra. Products from reaction (3) have two deuterium atoms, whereas those from (2a) and (2b) have only one, and the two sets of products can be distinguished by GCMS analysis. Calibration mixtures replicating the photolyzed mixtures were prepared from pure samples of the reactants and products in order to convert raw decomposition and stabilization product ratios to ratios of concentrations. Gas handling was done with grease-free vacuum systems. The rate constants were determined from plots of ratios of decomposition products to the stabilized product versus inverse pressure. In the limit of strong collisions, i.e., one collision reduces the vibrational energy sufficiently that subsequent decomposition of CD2ClCHFCl* is negligible, the slope of the Di /S plot is converted to a unimolecular rate constant after multiplying by the collision constant, kM. By analogy to chemical activation studies18-20 with similar bath gas molecules, CHFCl2, CD2ClI and CF3CH=CH2 should be efficient colliders at room temperature and the average energy removed per collision from CD2ClCHFCl* would be 6-8 kcal mol-1. Adjustment of the experimental rate constant to unit deactivation is less important than the uncertainty in collision cross sections used for kM (= πd2(8kT/πμ)1/2Ω(2,2)*). Collision diameters, d, and ε/k values required to obtain Ω(2,2)*are given in a footnote of Table 1.


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2b. Energy Profile and Models of Transition States. The energy profile for the 2,1-DX and 1,1-HX reactions of CD2ClCHFCl* is shown in Figure 1; all entries are for the lowest energy conformer. Obtaining the average energy, , of CD2ClCHFCl* requires a reliable value for the enthalpy of recombination of CD2Cl and CHFCl. The enthalpies of formation20 of CH2Cl and CHFCl at 298 K are 28.0 ± 0.7 and -14.5 ± 2.4 kcal mol-1, respectively. The enthalpy of formation of CH2ClCHFCl was obtained from the following isodesmic reaction. The average calculated enthalpy of reaction (4) from M06-2X and B3PW91 with the 6311+G(2d,p) basis set was -1.2 kcal mol-1. CH2ClCH2Cl + CH3CH2F → CH2ClCHFCl + C2H6

(4)

Combining this result with known enthalpies of formation22,23 of the other three molecules gave the enthalpy of formation of CH2ClCHFCl as -77.8 kcal mol-1 at 298 K. A second isodesmic reaction based on CHCl2CH2Cl + C2H5F with the CBS-Q3 method gave an enthalpy of formation of -76.8 kcal mol-1, which confirmed the more complete evaluation of reaction (4). The enthalpy for recombination of reaction 1a thus is 89.0 ± 2.4 kcal mol-1. The uncertainty of ± 2.4 kcal mol-1 comes from the enthalpy of formation21 of CHFCl. Converting to 0 K and adding the thermal energy of the radicals gives = 92 ± 2 kcal mol-1, which is 4 kcal mol-1 lower than the average energy9 of CD3CHFCl*. We have used experimentally based enthalpy of formation of CHFCl for both CD3CHFCl* and CD2ClCHFCl*. The most recent computationally derived value21 is -16.2 kcal mol-1, which would reduce of both CD3CHFCl* and CD2ClCHFCl* by 1.7 kcal mol-1. The E0 values for reaction 2 given in Figure 1 were determined from experimental data to be presented in the Experimental Results. In order to complete the energy profile in Figure 1, the enthalpies of formation of CHCl=CHF and CHCl=CHCl are needed. Since enthalpies of formation for cis-1,2C2H2Cl2 (-0.7 ± 0.5 kcal mol-1)22 and cis-1,2-C2H2F2 (-73.2 ± 0.5 kcal mol-1)23 are known, we used reaction (5) to find the enthalpy of formation of CHCl=CHF. cis-CHCl=CHCl + cis-CHF=CHF → 2 cis-CHCl=CHF


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(5)


The average energy change according to calculations by B3PW91/6-311+G(2d,p) and M06/6-311+G(2d,p) was -2.1 ± 0.5 kcal mol-1, which leads to an enthalpy of formation of -38.0 kcal mol-1 at 298 K. The enthalpies of formation of CH2Cl(F)C: and CH2Cl(Cl)C: were obtained from combining calculated energies of isomerization (53 and 57 kcal mol-1, respectively, from M06-2X/aug-cc-pVTZ with 0 K enthalpies of reaction (CBS-QB3) for the 2,1-HCl and 2,1-HF reactions , which are 16 and 11 kcal mol-1, respectively. Thus, the minimum energy required for formation of cis-CD2Cl(F)C: (+ HCl) and cis-CD2Cl(Cl)C: (+ HF) are 69 and 68 kcal mol-1, respectively, with uncertainties of ± 2 kcal mol-1. The cis-conformers have the lowest energy, and the profile in Figure 1 is based on the cis-conformers for carbenes, H-atom migration transition states and the cisalkene products. According to Hu’s BLYP/6-311G** calculations16, threshold energies for H-atom migration via the cis-transition states from the cisconformers are 7.9 and 10.4 kcal mol-1 for CD2Cl(Cl)C: and CD2Cl(F)C:, respectively. Our calculations from B3PW91/6-311+G(2d,p) [M062X/6-311+G(2d,p) and aug-ccpVTZ] gave 4.3 [5.3 and 6.9] and 7.2 [ 9.4 and 11.1] kcal mol-1. We adopted the higher range of values for Figure 1, the precise values do not affect our interpretations. According to calculations, trans-transition states are ≈ 5 - 6 kcal mol-1 higher in energy than cis-transition states, and H-atom migration favors formation of cis-CDCl=CDF and cis-CDCl=CDCl14-17. The last item needed for Figure 1 is threshold energies for cis-trans isomerization of CDCl=CDCl and CDCl=CDF. The thermal isomerization reactions24,25 have been studied by Jeffers24 using shock-tube methods, and reduction of the reported Arrhenius activation energies by 2 kcal mol-1 give E0 values of 54.9 ± 2 and 56.3 ± 2 kcal mol-1 for CDCl=CDCl and CDCl=CDF, respectively. The lower value for CDCl=CDCl should be noted. Vibrational frequencies and moments of inertia of the molecule and all transition states are needed for the calculation of the statistical rate constants. These were obtained from electronic-structure calculations using the Gaussian code13. The B3PW91/6-311+G(2d,p) method was initially selected for CD2ClCHClF and the 5 transition states. Since M06-2X/aug-cc-pVTZ was used to describe the hydrogen-


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bonded complexes, it also was extended to the 5 transition states. In the previous9 study of CD3CHFCl, we had selected MP2 methods for the 1,1-HF and HCl transition states; however, with experience the DFT method proved satisfactory for describing the 1,1-HCl transition state of CD2ClCHFCl. The MP2 method frequently gives unsatisfactorily high threshold energies for fourcentered HX elimination reactions when compared to experimental results. Density functional methods are not satisfactory26,27 for identification of the transition states for cis-trans isomerization of CDCl=CDCl and CDCl=CDF, and the CASSCF method was used to identify those transition states. The models for the molecules and transitions states are given in the Supporting Information. Additional details about the calculations for the rate constants are presented when the E0 values are assigned in section (3-C). Hydrogen-bonded complexes with carbenes are described in the Discussion section. 3. EXPERIMENTAL RESULTS 3a. Measurement of Rate Constants. Following photolysis at room temperature of CD2ICl:CHFCl2:CF3CH=CH2 mixtures with relative composition 1:3:4, the stabilization product (CD2ClCHFCl) and nine decomposition products (cis- and trans-1,2-dichloroethene-d1 and -d2, cis-and trans-1,2-fluorochloroethene-d1 and -d2, and 1,1-fluorochloroethene) were analyzed by GCMS. Data were collected from more than 30 experiments over a pressure range of 0.4 to 15 Torr by selecting quartz vessels of the desired volume while placing the same initial amount of mixture in each vessel. The rate constants for the combined HCl and DCl elimination products and the combined HF and DF elimination products were obtained from the two Di/S vs pressure plots presented in Figure 2. Individual rate constants for the five reactions, without distinction of cis- and trans-products, were obtained from plots of product branching ratios of Figure 3. Product branching ratios over a wide pressure range are more reliable than Di/S plots for the reactions with low yields at higher pressures. The cis- and trans-dichloroethene and cis- and transfluorochloroethene ratios are presented in Figure 4. Consider first the slopes of the plots from Figure 2. The least-squares slopes of the plots for the HCl + DCl and


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for HF + DF and combined decomposition processes, Di/S, vs pressure-1 are 5.00 ± 0.23 Torr with an intercept of 0.02 ± 0.06 and 1.11 ± 0.09 with an intercept of 0.02 ± 0.03. These data appear to be reliable and conversion of the slopes to a rate constant using the collision frequency given in the footnote of Table 1 gives rate constants of 7.8 x 107 and 1.7 x 107 for a total of 9.5 x 107 s-1. The DCl and HCl elimination reactions comprise 82% of the total reaction. Comparison with the rate constant (1.9 x 108 s-1)2 for CH2FCH2Cl* with 91 kcal mol-1 of energy is useful; the reduction for CD2ClCHFCl* is expected from substitution of three-H-atoms by a Cl-atom and two D-atoms, although the balance of decomposition channels are different for the two molecules. The branching ratios for 2,1-DCl versus 1,1-HCl elimination and for 2,1-DF/1,1-HF shown in Figure 3 give average values of 1.7 ± 0.2 and 1.4 ± 0.8. An important point from Figure 3 is the constant branching fraction for DCl/HCl, which confirms that contributions from the decomposition of CD2FCHCl2*, the Cl/F interchange product, is negligible. The branching ratio for 1,2-HCl/2,1-DCl shown in Figure 3 is just 0.03 ± 0.01 which corresponds to a rate constant of 0.16 x 107 s-1 with an uncertainty of 20%. The F- and Cl-atoms on the CH lower the rate constant for four-centered 1,2-HCl elimination because the threshold energy is elevated, vide infra. The branching fraction plot for the 2,1-DF(CDCl=CHCl) and 1,1-HF (CDCl=CDCl) product ratio seems to have an apparent pressure dependence. As pressure is increased, yields of the minor products become small and subject to distortion from secondary reactions in the complex chemical environment of mercury photosentization. For example, consider the CDCl=CDCl product, another source could be the CD2Cl recombination product, CD2ClCD2Cl*, and its decomposition product, CDCl=CD2. The half-quenching pressure2 for CD2ClCD2Cl* is 6.4 Torr and both CDCl=CD2 and CD2ClCD2Cl are present. The interaction of Clatoms with either molecule followed by disproportionation with another radical could generate CDCl=CDCl. We used the average branching ratio of 1.4 to separate the HF and DF components and 1.7 to separate the HCl and DCl components from the results of Figure 2 to obtain individual rate constants, in units of Torr, of 0.46, 0.65, 1.8 and 3.2 for 1,1-HF, 2,1-DF, 1,1-HCl and 2,1-DCl, respectively. The statistical uncertainty is 5-10%. Conversion to units of 107 s-1


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using the collision frequency gives 0.72, 1.0, 2.8 and 5.0 x 107 s-1 for the rate constants in Table 1. Although the statistical uncertainty is about 10%, the branching ratio for DF/HF reactions introduces more uncertainty for 1,1-HF and 2,1-DF rate constants. 3b. The cis/trans Ratios of Dichloroethene-d1 and -d2 and Chlorofluoroethene-d1 and -d2. Plots of cis/trans ratios for 2,1-DX and 1,1-HX reactions are shown in Figure 4. It should be remembered that for 1/P larger than 0.2, the majority of the CD2ClCHFCl* molecules have decomposed, but below 0.2 the yields of the individual cis and trans product yields are small and the data have uncertainty. The average ratio for 2,1-DCl (CDCl=CHF) elimination is 1.3 ± 0.2 with no obvious dependence on pressure. The cis/trans ratio for 2,1-DF(CDCl=CHCl) would be similar except for the cluster of points at higher pressure near and below 1.0. Giving less weight to points below 1/P = 0.2 gives a ratio of 1.2-1.1. Based upon numerous8-12 studies, product alkenes from four-centered elimination of HX from haloalkanes do not retain sufficient vibrational energy for cis/trans-isomerization, and ratios in Figure 4a are expected to be pressure independent. The cis-isomers of CDCl=CDF and CDCl=CDCl are lower in energy25 by about 0.8 kcal mol-1 with equilibrium constants (cis/trans) of about 1.6 at 600 K. Since the transition states for 2,1-DX elimination resemble the alkene products, a cis/ trans ratio of 1.4-1.2 would be expected for 2,1-DCl and 2,1-DF elimination. The initial (high pressure limit) cis/trans ratios for CDCl=CDF and CDCl=CDCl are determined by reaction (3), the D-atom migration reactions of CD2Cl(F)C: and CD2Cl(Cl)C:. The cis-isomers are favored because of lower threshold energies for cis-transition states. Inspection of Figure 4b shows that the ratio for CDCl=CDF declines with decreasing pressure from about 5.2 to 1.8, as might be expected by comparison to previous measurements28 of cis/trans-isomerization of CHF=CHF following H-atom migration from CH2F(F)C:. The higher apparent limit of 1.8 versus 1.3, the ratio from 2,1-DCl elimination from Figure 4a, implies that a small fraction of the CDCl=CDF energy distribution may lack sufficient energy to


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isomerize. The energy profile in Figure 1 for CD2Cl(F)C: supports this interpretation. In contrast to chlorofluoroethene, the cis/trans ratio for CDCl=CDCl in Figure 4b is 1.2 ± 0.2 over the entire range of pressure. The cis/trans-isomerization of CDCl=CDCl been observed previously following 1,1-DCl elimination from CH2ClCDCl2* with 86 kcal mol-1 of energy29. The initial (high pressure) cis/trans ratio of CDCl=CDCl was about 3; the ratio declined to about 1.5 at 10 Torr and it was approximately unity at 1 Torr. The highest pressure of our experiments was 15 Torr. The cis-CDCl=CDCl from D-atom migration from CD2Cl(Cl)C: will have about 5 kcal more energy than molecules from CH2ClCDCl2 -> CH2C(Cl)C: -> CHCl=CHCl. The cis/trans ratio for CDCl=CDCl in Figure 4b is nearly constant because the collisional deactivation rate is slower than the isomerization rate for pressures of our experiments. The near equivalence of the ratio with the low pressure (equilibrated) ratio of CHCl=CHCl from CH2Cl(Cl)C: isomerization29 supports this interpretation. Another factor could be modification of the H-atom migration rates by the hydrogen-bonded adduct in the exit channel of 1,1-HF elimination and this will be analyzed in the Discussion section. 3c. Assignment of Threshold Energies. Threshold energies were assigned by matching experimental rate constants of Table 1 to calculated statistical rate constants evaluated at the average energy . The statistical, RRKM, rate constants were obtained from equation 6. kE = (s‡/h)(I‡/I)1/2(ΣP‡(E-E0)/N*(E))

6.

The sums of states of the transition state, ΣP‡(E-E0), and the density of states of the molecule, N*(E), were calculated from the harmonic vibrational frequencies of the models of the transition states and the molecule using the Multiwell code of Barker.30 The (I‡/I) term is the ratio of the three overall rotational moments of inertia, which were considered to be inactive towards energy exchange with the vibrational modes. The reaction path degeneracy, s‡, is 2 for 2,1-DCl and -DF elimination (for the combined cis and trans pathways) and 1 for the other three reactions. The torsional vibrational modes of the molecule and 1,1-HCl and 1,1-HF


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transition states were treated as internal rotations for evaluation of the density and sums of states. The hindered rotor for CD2ClCHFCl was treated2 as a symmetric rotor with a reduced moment of inertia (27.4 amu Å2) equal to the average of the moments of the conformers with an average potential energy barrier of 5.8 kcal mol-1. The CD2Cl internal rotation in the 1,1-HCl and -HF transition states was treated as a free rotor, and the moments of inertia were 31.8 and 26.5 amu Å2, respectively. The vibrational frequencies and rotational moments of inertia used for equation 6 are given in the Supporting Information. Threshold energies, E0, were assigned for each reaction by matching kexp and k. Rate constants were calculated from models obtained from both M06-2X and B3PW91; the assignments from B3PW91 are used for Table 1. Experimental rate constants for 2,1-DCl and 1,1-HCl reactions are well defined, and E0 assignments of 63 and 72 kcal mol-1 should be reliable. These assignments are similar to those9 for CD3CHFCl*, although the presence of the Cl atom on CD did raise the threshold energy for 2,1-DCl elimination by 3 kcal mol-1, as expected. Within the combined uncertainties, E0 = 72 kcal mol-1 for 1,1-HCl elimination is similar to the enthalpy of reaction, and the energy barrier for addition of CD2Cl(F)C: (or CD3(F)C:)9 to HCl must be very small. The small rate constant for 1,2-HCl elimination requires a threshold energy of 70 kcal mol-1. This is one of the highest reported threshold energies for four-centered HCl elimination from a chloroalkane, and it is 6 kcal mol-1 higher than for CFCl2CH2F31 or CF2ClCH2F32. The 1,2-HCl elimination from CD2ClCHFCl* is another example that illustrates the elevation of the threshold energy for HCl elimination by halogen substitution on CH of the four-membered ring. The experimental rate constants for 2,1-DF and 1,1-HF elimination have more uncertainty than for the DCl and HCl processes, but a 50% change in rate constant only alters the E0 assignment by 1 kcal mol-1. The assigned values are 66 and 73 kcal mol-1 for 2,1-DF and 1,1-HF elimination, respectively. The threshold energy for 1,1-HF elimination is 5 kcal mol-1 larger than the enthalpy of the reaction, and the addition of CD2Cl(Cl)C: to HF would have an appreciable activation energy.


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An uncertainty of ± 2 kcal mol-1 is typically assigned1-8 to threshold energies determined from matching calculated rate constants to an experimental rate constant measured for a single energy, and this level of confidence should apply to four-centered reactions of CD2ClCHFCl. The sum of states for 1,1-HCl and 1,1HF transition states depend on the model selected to represent the torsional mode and one other low frequency. Therefore, these E0 values could have larger uncertainty than those of the four-centered HCl and HF elimination reactions. Rate constants calculated, for common threshold energies, from the M06-2X models were on average about 8% smaller for 1,1-HX reactions and 15% larger for 2,1-DF and 1,2-HCl reactions than the B3PW91 rate constants. Thus, the uncertainty in calculated rate constants associated with selection of the electronic-structure computational method is not serious. Since rate constants for the five competing reactions depend on the relative sums of states of the transition states, we provide a comparison of the sums at a common energy of 18 kcal mol-1 in Table 2. The sum of states for 1,1-HX reactions are about 25 times larger than for the 1,2-HX reactions for free-rotor treatment of the torsional motion in the 1,1-transition states. Table 2 also includes a comparison of the calculated Arrhenius rate constants at 1000 K; pre-exponential factors for the 1,1HX reactions are an order of magnitude larger than 2,1-HX reactions for the same reaction-path degeneracy. Threshold energies in kcal mol-1 calculated from B3PW91/6-311++G(2d,p) for the lowest energy conformer of CD2ClCHFCl are 54.9 (cis) and 55.3 (trans) for 2,1-DCl, 62.2 (cis) for 1,1-HCl, 60.2 (cis) and 60.3 (trans) for 2,1-DF, 67.1 (cis) for 1,1-HF, and 63.8 for 1,2-HCl. Comparison of the calculated values with the experimentally assigned thresholds energies from Table 1 shows the latter to be larger by 6-8 kcal mol-1. However, the calculations do show a significant difference between 1,1-HX and 2,1-DX elimination reactions. The threshold energies, in kcal mol-1, from the M06-2X calculation are 61.3 (cis) for 2,1-DCl, 66.5 (cis) for 1,1-HCl, 67.9 (cis) for 2,1-DF, 76.2 (cis) for 1,1-HF and 70.3 for 1,2-HCl. The cis and trans conformers for 1,1-HX reactions are considered in the Section 4.1. The calculated values for 1,1HF and 2,1-DF are somewhat too high, as is frequently the case11 but these values are closer to the experimental assignments than were the B3PW91 calculations.


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The principal discrepancy is the low value for 1,1-HCl elimination, and this question is more fully explored in the next section. 4. DISCUSSION 4.1 1,1-HX Branching Ratios, Transition States and Carbene-HX Complexes. Several examples (C2D5CHF2,8 CD3CHFCl,9 CHF2CHF210) demonstrate the relation between 2,1-DF and 1,1-HF elimination, and CD2ClCHFCl seems to fit the same pattern, even though HF and DF elimination reactions are only 10% of the total unimolecular reaction. The 1,1-HF/2,1-DF branching ratios are 0.3 to 0.4 for = 90-96 kcal mol-1. The difference between E0(1,1-HF) and E0(2,1-DF) is 7-10 kcal mol-1, and E0(1,1-HF) is larger than the enthalpy change for the reaction. The branching ratio for 1,1-HCl and 2,1-DCl elimination does depend on the parent molecule in question; it is only 0.16 for C2D5CHCl2, whereas it is 0.45 for CD3CHFCl and 0.63 for CD2ClCHFCl. The difference is mainly a consequence of the effect of the Cl-substituent on E0(2,1-DCl). The assigned E0(1,1-HCl) values are similar to the enthalpy change of the reaction, implying that addition of halo-carbenes to HCl may not have a significant activation energy. Calculated geometries of 1,1-HX transition states for CD2ClCHFCl and the corresponding hydrogen-bonded complexes are shown in Figure 5 for cis- and trans-conformers. Transition states for H-atom migration from the complexes also are shown in Figure 5. Calculated energies for stationary points of the exit channels are shown in Figure 6. Transition states for four-centered HX-elimination reactions are similar to many other examples and they need not be discussed. All 1,1- and 1,2-HX elimination transition state geometries were characterized by one imaginary frequency and verified by Intrinsic Reaction Coordinate calculations. The 1,1-HX transition states closely resemble those from CD3CHFCl, and the Clatom in the methyl group did not have a significant effect beyond adding three conformers associated with the CD2Cl group. The three-membered ring is clearly defined for HF elimination with an F-C-H angle of 33° in contrast to the much smaller 16° angle for H-C-Cl. The fractional extensions for the C-Cl and C-F bonds are 56 and 49%, and those for the HCl and HF bonds are 16 and 34%. The HCl moiety is much closer to a free molecule than is HF; this point is also evident from ACS Paragon Plus Environment

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the much longer, 0.21-0.29 Å, C-H distance for the 1,1-HCl transition states. The 1,1-HCl transition state and hydrogen-bonded complex share a strong resemblance and have similar energies; the principal differences are a longer C-H distance and migration of H-Cl to the plane defined by the F-C-C-Cl atoms. These structural features correlate with a calculated threshold energy that is below the enthalpy of reaction for the 1,1-HCl elimination, whereas the calculated E0 is larger than the enthalpy of reaction for 1,1-HF elimination. This tendency exists for all of the 1,1-HCl and -HF reactions that have been investigated.8-12 The potential energy barrier for the addition of HF to a carbene is a consequence of high D(H-F), even though the binding energy of the hydrogen-bonded complex is larger than for carbenes with HCl. There is a growing number of reports33-35 showing that carbenes can act as electron-pair donors in hydrogen-bonded complexes, and this certainly is the case for HF and HCl. The dynamics associated with a hydrogen-bonded complex in the exit channel for 1,1-HX elimination is considered in the next section. 4.2 Analysis of the Hydrogen-bonded Complexes with Carbenes. We first present results provided by a group with experience36,37 for calculations of properties of weakly bound complexes (we thank Professor S. Wategaonkar and Mr. S. Ghosh, of the Tata Research Institute). They used the ωB97X-D/6311++G** method for calculating the binding energies (or equivalent dissociation energies) of trans-CD2Cl(F)C:HCl and trans-CD2Cl(Cl)C:HF complexes relative to the trans-conformer of the free carbene; the values are 6.0 and 9.2 kcal mol-1, respectively. We subsequently calculated the binding energies, all in kcal mol-1, using the CBS-QB3 method for cis-and trans-conformers and obtained 4.9 (cis) and 4.9 (trans) for CD2Cl(F)C:HCl and 7.7 (cis) and 7.0 (trans) for CD2Cl(Cl)C:HF, relative to free carbenes of the corresponding conformation. The complexes with bonding to HF always have larger dissociation energies. The CBS-QB3 method, employing the geometry from a B3LYP optimization, gave an energy difference between cis- and trans-conformers of the free carbene as 2.4 and 2.5 kcal mol-1 for CD2Cl(F)C: and CD2Cl(Cl)C: respectively. These values are used for consistency in Figure 6. The M06-2X/aug-cc-pVTZ method was used to relate energies of complexes to the transition states for 1,1-elimination and to energies of the ACS Paragon Plus Environment

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transition states for H-atom migration. Energy profiles for cis- and transconformers of the exit channels are shown in Figure 6. All calculated energies include zero-point energy. The third conformer associated with the internal rotation of the CD2Cl group, which is ≈ 1.3 kcal mol-1 above the trans-conformer, is not shown in Figures 5 and 6. The potential barriers to internal rotation of the CD2Cl group are low; all conformers will be present and the cis-profile can serve as the reference for discussion. The relationship between the 1,1-HX transition states and their respective hydrogen-bonded complexes are very different for HCl and HF elimination reactions. The calculated energy separation is 14 kcal mol-1 for CD2Cl(Cl)C:HF with rather different structures (see Figure 5), whereas the energy separation is only 1.2 kcal mol-1 for CD2Cl(F)C:HCl. How reliable is the difference between the calculated M06-2X threshold energy and CBS-QB3 adduct energy? The experimentally assigned E0(1,1-HCl) (72 kcal mol-1) from a free-rotor model adjusted to an estimate for the cis-conformer, (about 70 kcal mol-1) is 3 kcal mol-1 higher than the calculated value. Furthermore, the calculated value is less than the dissociation limit. Thus, a higher level calculation was initiated to improve the M06-2X calculation for the difference in energy of the transition state and adduct Coordinates from the M06-2X calculated geometries were used for fixed-point calculations with CCSD(T)/aug-cc-pVTZ to obtain differences in energies between the cis conformers of 1,1-HX transition states and the adducts. The resulting threshold energies were E0(1,1-HCl) = 66.8 and E0(1,1-HF) = 72.7 kcal mol-1. The energies of the HCl and HF adducts were 64.1 and 60.5 kcal mol-1 relative to CD2ClCHFCl, which are 0.9 and 1.7 kcal mol-1 lower than the M06-2X values. The CCSD(T) calculation improved E0(1,1-HF), but hardly changed E0(1,1-HCl); these E0s are 2.7 (HCl) and 12.1 (HF) kcal mol-1 above the energies of the complexes. Finally, we did a CBS extrapolation of the CCSD(T) energies using the mixed Gaussian/Exponential scheme designed by Peterson.38 The extrapolated results are 2.9 (HCl) and 13.6 (HF) kcal mol-1 for the differences in energy between the adducts and the transition state; nearly the same as the M06-2X/aug-cc-pVTZ and CCSD(T) values. We conclude that the calculated E0(1,1-HCl) for the cis conformer


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must be about 3 kcal mol-1 above the adduct and submerged by approximately 2 kcal mol-1 relative to the enthalpy of reaction. The 1,1-HCl elimination reaction seems to be a prime example for which recrossing of the dividing plane of the potential energy surface defining the transition state could be important. If allowance is made for conformers and a recrossing factor of two in assignment of the experimental threshold energy, the disparity between the experimental E0 and the M06-2X/CCSD(T) value is not serious (69-70 ± 2) versus 67 (cis-conformer) kcal mol-1. A more precise analysis of the thermochemistry and the model for 1,1-HCl elimination rate constant will be required if the threshold energy of the 1,1-HCl transition state actually is comparable to the energy of the free carbene + HCl. The concept of roaming39 between transition states and the complex also may be applicable. Hydrogenbonded complexes also are important for understanding addition reactions of carbenes to HF and HCl, and experimental studies of those reactions could provide more insight. In addition to the vibrational energy released by formation of the hydrogen-bond, 8(HF) and 5(HCl) kcal mol-1, the complexes will have 24(HF) and 23(HCl) kcal mol-1 of excess vibrational energy for total energies of 32 and 28 kcal mol-1. Inspection of the IRC suggests that trajectories will follow the minimum energy path, and release of energy associated with formation of the adduct should assist in a statistical energy distribution for the carbene and HCl or HF products. A general feature is the similarity of the binding energy for each carbene complex and its transition state for H-atom migration, i.e., the threshold energy for H-atom migration in the free carbene is similar to that aided by the complex. The structures of the complex-aided, H-atom transition states are shown in Figure 5. The distances and orientation of HCl and HF are similar to those of the hydrogenbonded complexes, which emphases the importance of the empty p-orbital in the transfer of the H-atom. Although the complex-aided transition state for H-atom transfer from CH2Cl(F)C:HCl is 0.7 kcal mol-1 lower than the dissociation energy, the transition state for dissociation has lower vibrational frequencies than that for H-atom migration and the complex will dissociate before H-atom migration, occurs. Thus, H-atom migration will occur from free carbenes, and cis-CDCl=CDCl ACS Paragon Plus Environment

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and cis-CDCl=CDF should be the favored final products. Bonneau and co-workers40 mentioned the possibility of H-atom transfer in the entrance channel for reaction of CH2Cl(Cl) with olefins and complex assisted H-atom transfer may be important in reactions of carbenes with H-bond donors. 4.3 Isomerization of cis-CDCl=CFD and cis-CDCl=CDCl. Within the uncertainty of the thermochemistry, see Figure 1, both carbenes are 68 kcal mol-1 above the energy of CD2ClCHFCl. Assuming a statistical release of vibrational energy following 1,1-elimination, the carbenes would acquire approximately 2/3, or 16 kcal mol-1, of the 24 kcal mol-1 of excess energy. The half-width of the statistical distribution is ≈ 10 kcal mol-1, corresponding to 11-21 kcal mol-1, with an approximately symmetric distribution according to our calculations. Threshold energies for D-atom migration via the cis-transition state, see Figure 6, are 7.5 (CD2Cl(Cl)C:) and 11.6 (CD2Cl(F)C:) kcal mol-1, and the major part of the distributions will isomerize in favor of cis-CDCl=CDF and cis– CDCl=CDCl. Since the threshold energy for trans-H-atom migration is 17.5 kcal mol-1, CD2Cl(F)C: discrimination against formation of trans-CDCl=CDF is especially severe. A small fraction of the distribution for CD2Cl(F)C: could be below 11 kcal mol-1, but tunneling9 may aid migration of the D-atom. According to our preceding analysis, complex-assisted D-atom transfer is not important, but those processes also would favor cis-alkene products. The average vibrational energy of cis-CDCl=CDF is 69 +16 (-16, see Figure 1) = 69 kcal mol-1 with the same distribution as for CD2Cl(F)C:, and the threshold energy for cis/trans- isomerization is 56 kcal mol-1. According to our calculation for the cis/trans-rate constant, this average energy corresponds to a rate constant of 6-7 x 107 s-1. The pressure required for the same collision frequency would be 4-5 torr. This pressure corresponds to the pressure range for which the cis/trans-ratio of CDCl=CDF is changing in Figure 4b. Due to limitations of the Hg(3P1) sensitization technique, the data for CDCl=CDF for pressures ≥ 8 torr are not adequate for quantitative analysis, but the general change of the cis/trans-ratio with increasing pressure supports this analysis. The average vibrational energy of cis-CDCl=CDCl is about 6 kcal mol-1 higher and the threshold energy for


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isomerization is about 1.4 kcal lower than for CDCl=CDF. The combined effect is a 10-fold larger rate constant for isomerization and the cis/trans-ratio is close to its equilibrium value over the experimental pressure range. In addition, another source of CDCl=CDCl cannot be entirely excluded in this complex chemical system. The CO2 laser-induced decomposition and shock-tube pyrolysis studies of CH2ClCHFCl by Ogura and Yano41 reported cis/trans-ratios of ≈ 1.2 and ≈ 1.6 for CHCl=CHCl and CHF=CHCl, respectively. These ratios, which would be a combination from 1,1- and 2,1-elimination processes, are consistent with the cis/trans ratios reported in this work. 5. CONCLUSIONS The recombination of CD2Cl and CHFCl radicals was used to study the five competing unimolecular reactions of CD2ClCHFCl* with an energy of 92 kcal mol-1, The ratio of 1,1-HCl to 2,1-DCl elimination is 0.6; this ratio would decline to 0.4 for CH2ClCHFCl because of the lower threshold energy for 2,1-HCl elimination. The product ratio of 1,1-HF to 2,1-DF elimination is 0.7 from CD2ClCHFCl*. The 1,2-HCl elimination reaction has a low branching fraction due to elevation of the threshold energy by the F-and Cl-atoms on the carbon atom from which the Hatom leaves. For high temperatures and high energy experiments, the 1,1-HCl and 1,1-HF reactions should be included in the mechanism describing the decomposition of 1,1-dichloro-, 1,1-difluoro- and 1,1-chlorofluoro-alkanes. The exit channels of 1,1-HF and 1,1-HCl reactions have hydrogen-bonded complexes between the carbenes and HF or HCl with binding energies of 7.5-9.5 and 4.5-6.5 kcal mol-1, respectively. The hydrogen-bonded complex has a strong influence on the geometry and energy of the transition state for 1,1-HCl elimination. The M062X calculated results are preferred over those from B3PW91 for describing transition states and hydrogen-bonded complexes. Advanced level computations are needed to better understand the energies and dynamics for this part of the potential surface for CD2Cl(F)C:HCl. The CDCl=CDF and CDCl=CDCl molecules formed from isomerization of CD2Cl(F)C: and CD2Cl(Cl)C: retain enough energy to undergo cis/trans-isomerization. 6. AUTHOR INFORMATION ACS Paragon Plus Environment

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Corresponding Author. * BEH E-mail: [email protected] Telephone: 828-2325168 7. ACKNOWLEDGEMENTS Financial support from the National Science Foundation (CHE-1111546 and CHE1229406) is gratefully acknowledged and BRG thanks the North Carolina Space Grant program (NASA Grant NNX15AH81H) for funding a summer research fellowship. We thank Professor S. Wategaonkar and Mr. S. Gosh of the Tata Research Institute, Mumbai, India for assistance with calculations of the binding energies of the carbenes with HCl and HF. SUPPORTING INFORMATION. Supporting Information contains the vibrational frequencies and moments of inertia for the calculated molecular and transition state geometries using B3PW91/ 6-311+G(2d,p) and M06-2X/aug-cc-pVTZ for the unimolecular reactions of CD2ClCHFCl and using CASSCF for cis-trans isomerization of CDCl=CHF and CDCl=CHCl. Vibrational frequencies and moments of inertia calculated using M06-2X/aug-cc-pVTZ are given for the hydrogen-bonded complexes between the halo-carbenes and HCl or HF. This information is available free of charge via the Internet at http://pubs.acs.org.


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Table 1. Rate constants and threshold energies for CD2ClCHFCl* and CD3CHFCl*. Reactions

Experimental resultsa,b kexp, s-1

Slope of D/S plot

Calculated resultsc k, s-1

kcal mol-1

Torr CD2ClCHFCl* 2,1-DCl

1,1-HCl

E0 ,

= 92 kcal mol-1 3.2 ± 0.2

1.8 ± 0.1

5.0 ± 0.6 x 107

2.8 ± 0.3 x 107

6.0 x 107

63

8.7 x 107

62

2.6 x 107

72

4.1 x 107

71

2,1-DF

0.65 ± 0.06

1.0 ± 0.2 x 107

1.0 x 107

66

1,1-HF

0.46 ± 0.06

0.72 ± 0.15 x 107

0.72 x 107

73

1,2-HCl

0.10 ± 0.02

0.16 ± 0.03 x 107

0.21 x 107

70

CD3CHFCl*

=96 kcal mol-1

2,1-DCl

62

8.0 x 108

8.0 x 108

60

1,1-HCl

29

3.7 x 108

4.4 x 108

71

2,1-DF

9.4

1.2 x 108

1.3 x 108

64

1,1-HF

4.6

0.58 x 108

0.70 x 108

73

a. The units of the slopes of the D/S plots are Torr; these values are converted to rate constants with units of s-1 after multiplication by the collision rate constant, kM. The details needed to calculate kM are given in footnote (b). The rate constants for


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CD3CHFCl* are included for comparison to CD2ClCHFCl*; the experimental uncertainties for the rate constants of CD3CHFCl* are given in ref. 9. The uncertainty in the rate constants for CD2ClCHFCl was increased, at least to 10% to include the uncertainty in kM. b. The collision rate constants of the bath gases with CD2ClCHFCl* were calculated from the following collision diameters, Å, and ε/k ,o K, values: CF3CH=CH2 (4.90 and 250), CD2Cl I (5.0 and 410), CHFCl2 (4.6 and 405), CD2ClCHFCl (5.1 and 450). The collision rate constants were 4.82 x 10-10, 5.16 x 10-10, and 4.65 x 10-10cm3 molecule-1 s-1 for CD2ClCHFCl* with CHFCl2 , CF3CH=CH2 and CD2ClI, respectively, as bath gases. The effective collision frequency with CD2ClCHFCl* at 1 Torr pressure of the bath gas mixture is 1.56 x 107 s-1. c. Two rate constants are given for 2,1-DCl and 1,1-HCl to illustrate the dependence of kE on E0. All rate constants were obtained from the models defined by the B3PW91/6-311+G(2d,p) calculations. The E0’s are obtained by matching k to kexpt.


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Table 2. Sums of states of transition states (ΣP‡) and the Arrhenius rate constants for CD2ClCHFCl. Reaction

ΣP‡ (18 kcal mol-1)d

k(T)a,b , T = 1000 K

2,1-DCl

2.1 x 107 (2.0 x 107)

10.3 x 10 13 exp(-65,100/RT)

1,1-HClc

5.4 x 108 (5.1 x 108)

21.7 x 1014 exp(-74,600/RT)

1,2-HCl

2.2 x 107 (1.8 x 107)

6.8 x1013 exp(-72,600/RT)

2,1-DF

1.2 x 107 (1.0 x 107)

5.2 x 1013 exp(-68,100/RT)

1,1-HFc

3.0 x 108 (2.4 x 108)

9.2 x 1014 exp(-75,500/RT)

a. The reaction path degeneracies are 2 for 2,1-DCl and 2,1-DF, since both geometric isomers are included, and 1 for the other three reactions. Calculations of the Arrhenius rate constants are based on models from B3PW91/6311+G(2d,p). b. The threshold energies, or activation energies, should be lowered by 1 kcal for 2,1-HCl and 2,1-HF elimination reactions from CH2ClCHFCl. c. The CD2Cl torsional motion in 1,1-HX transition states has been treated as a free internal rotation. The sums of states and the pre-exponential factor of k(T) would be reduced for a hindered internal rotation. d. Calculations for the ΣP‡ are based on models from B3PW91/6-311+G(2d,p) and M06-2X/aug-cc-pVTZ (results in parenthesis).


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Figure Captions. Figure 1. Energy profile for reactions of CD2ClCHFCl. The structures that are shown represent transition states. The threshold energies in kcal mol-1 for 2,1-DCl (63), 1,1-HCl (72) 2,1-DF (66), 1,1-HF (73) and 1,2-HCl (70) are based on the experimental rate constants. The threshold energies from the M06-2X calculations are 2,1-DCl(61.3), 1,1-HCl(66.5), 2,1-DF(67.9), 1,1-HF(76.2) and 1,2HCl(70.3) kcal mol-1. The enthalpies (at 0 K) for 1,1-elimination reactions are 68 ± 2 and 69 ± 2 kcal mol-1 based on the energy of reaction for 2,1-elimination plus the calculated isomerization energy of the carbenes. The binding energies of the complexes are computed values; see the Discussion section. All computed values are for cis-conformers. Figure 2. Plots of decomposition to stabilization product ratios versus pressure-1. () DCl + HCl (CDCl=CHF + CDCl=CDF)/CD2ClCHFCl; slope = 5.00 ± 0.23 Torr with intercept = 0.02 ± 0.06; () DF + HF (CDCl=CHF + CDCl=CDF)/CD2ClCHFCl; slope = 1.11 ± 0.09 Torr with intercept = 0.03 ± 0.03. Figure 3. Plots of product branching ratios from CD2ClCHFCl* versus pressure-1 for () 2,1-DCl/1,1-HCl elimination, () for 2,1-DF/1,1-HF elimination, and for () 1,2-HCl/2,1-DCl elimination. Figure 4. Plots of cis/trans ratios of dichloroethene and chlorofluoroethene. (a) () 2,1-DCl elimination (cis-CDCl=CHF/trans-CDCl=CHF) and () 2,1-DF(cisCDCl=CHCl/trans-CDCl=CHCl) elimination (left plot) and (b) () 1,1-HCl (cisCDCl=CDF/trans-CDCl=CDF) and () 1,1-HF (cis-CDCl=CDCl/trans-CDCl=CDCl) elimination. Figure 5. Diagrams of the 1,1-HCl and 1,1-HF elimination transition states with their corresponding hydrogen-bonded complexes and complex-aided transition states for D-atom migration. These structures were obtained from M06-2X/augcc-pVTZ calculations; transition states satisfy the IRC test. The cis- and transconformers, as well as the third conformer, associated with the CD2Cl group rapidly interconvert; the trans- and cis-geometries are shown to illustrate the weak dependence on the conformation of the CD2Cl group. All heavy atoms of the


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complexes and the D-atom transfer transition states are in a common plane. For reference, bond lengths of the HCl and HF molecules are 1.274 and 0.917 Ǻ, respectively. Typical distances for C-Cl, C-F and C-H bonds are 1.79, 1.38 and 1.09 Ǻ, respectively. The abbreviation PTSC stands for post transition-state complex, which is used to emphasize the location of the hydrogen-bonded complex. Figure 6. Summary for energies for exit channels of 1,1-HCl and 1,1-HF reactions. Calculations were done for cis-and trans-conformers; the third conformer for the carbenes is about 2 kcal mol-1 higher in energy that the trans-conformer. Dissociation energies of the complexes are from CBS-QB3 calculations. The energies for transition states and complexes also were obtained from M06-2X calculations; these values were scaled to the energy of the H-bonded complexes. Comparison of the CBS-QB3 calculated dissociation energies (7.7 and 4.9 kcal mol-1 for cis conformers) and the CCSD(T)/aug-cc-pVTZ energies (9.5 and 5.9) kcal mol-1 obtained by subtraction of the energies of the complexes (relative to CD2ClCHFCl) from the enthalpy of the respective reactions suggests the uncertainty of the calculations.


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TOC Graphic


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REFERENCES. (1) 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 or DX Elimination (X=F, Cl) from Chemically Activated CF3CH2CH2Cl, C2H5CH2Cl and C2D5CH2Cl: Threshold Energies for HX Elimination. J. Phys. Chem. A 2005, 109, 4540-4551. (2) Duncan, J. R.; Solaka, S. A.; Setser, D. W.; Holmes, B. E. Unimolecular HCl and HF Elimination Reactions of 1,2-Dichloroethane, I,2-Difloroethane and 1,2Chlorofluoroethane; Assignment of Threshold Energies. J. Phys. Chem. A 2010, 114, 794-803. (3) Turpin, M. A.; Smith, K. C.; Heard, G. L.; Setser, D. W.; Holmes, B. E. Unimolecular Reactions of 1,1,1-Trichloroethane, 1,1,1-Trichloropropane, and 3,3,3-Trifluoro-1,1,1-trichloropropane: Determination of Threshold Energies by Chemical Activation. J. Phys. Chem. A 2014, 118, 9347-9356. (4) Burgin, M. O.; Simmons Jr., J. G.; Heard, G. L.; Setser, D. W.; Holmes, B. E. Unimolecular Reactions of Vibrationally Excited CF2ClCHFCH3, and CF2ClCHFCD3: Evidence for the 1,2-FCl Interchange Pathway. J. Phys. Chem. A 2007, 111, 22832292. (5) Tucker, M. K.; Rossabi, S. M.; McClintock, C. E.; Heard, G. L.; Setser, D. W.; Holmes, B. E. Unimolecular Isomerization of CH2FCD2Cl via the Interchange of F and Cl Atoms: Assignment of the Threshold Energy to the 1,2-Dyotropic Rearrangement. J. Phys. Chem. A 2013, 117, 6717-6723. (6) Friedrich, L.; Duncan; J. R.; Heard, G. L.; Setser, D. W.; Holmes, B. E. Unimolecular Reactions of CH2BrCH2Br, CH2BrCH2Cl, and CH2BrCD2Cl: Identification of the Cl-Br Interchange Reaction. J. Phys. Chem. A 2010, 114, 41384147. (7) Everett, W. C.; Holmes, B. E.; Heard, G. L. A Computational Study of the Threshold Energies of the 1,2-FCl Interchange Reaction of Chlorofluoroethanes. Can. J. Chem. 2010, 88, 1112-1117.


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(8) Wormack, Leah N.; McGreal. Meghan E.; McClintock, Corey E,; Heard, George L.; Setser, D. W.; Holmes, Bert E. Characterization of the 1,1-HF Elimination Reaction from the Competition between the 1,1-HF and 1,2-DF Unimolecular Elimination Reactions of CD3CD2CHF2. J. Phys. Chem. A 2015, 119, 3887-3896. (9) Brown, T. M.; Nestler, M. J.; Rossabi, S. M.; Heard, G. L.; Setser, D. W.; Holmes, B. E. Characterization of the 1,1-HCl Elimination Reaction of Vibrationally Excited CD3CHFCl Molecules and Assignment of Threshold Energies for 1,1-HCl, 2,1-DCl, 1,1-HF and 2,1-DF Elimination Reactions. J. Phys. Chem. A 2015, 119, 9441-9451. (10) Smith, C. A.; Heard, G. E.; Setser, D. W.; Holmes, B. E. Reinvestigation of the Unimolecular Reactions of CHF2CHF2: Identification of the 1,1-HF elimination Component from Addition of CHF2CF to Trans-2-Butene. J. Phys. Chem. A 2016, 120, 9357-9362. (11) Smith, C. A.; Gillespie, B. R.; Heard, G. L.; Setser, D. W.; Holmes, B. E. Unimolecular Reactions of CF3CHF2 Studied by Chemical Activation: Assignment of Rate Constants and Threshold Energies to 1,2-H-Atom Transfer, 1,1-HF Elimination and 1,2-HF Elimination Reactions and the Dependence of Threshold Energies on the Number of F-Atoms in Fluoroethane Molecules. J. Phys. Chem. A 2017, 121, 8746-8756. (12) Larkin, A.; Nestler, M. J.; Heard, G. L.; Setser, D. W.; Holmes, B. E. A Chemical Activation Study of the Unimolecular Reactions of C2D5CHCl2 with Analysis of the 1,1-HCl Elimination Pathway. J. Phys. Chem. A 2016, 120, 8244-8253 (13) Gaussian 16, Revision B.01, Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Petersson, G. A.; Nakatsuji, H.; et. al. Gaussian, Inc., Wallingford CT, 2016. (14) Keating, A. E.; Garcia- Garibay, M. A.; Houk, K. N. Origins of Steroselective Carbene 1,2-Shifts and Cycloaddition of 1,2-Dichloroethylidene: A Theoretical Model Based on CBS-Q and B3LYP Calculations. J. Am. Chem. Soc. 1997, 119, 10805-10809.


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(15). Keating, A. E.; Garcia- Garibay, M. A.; Houk, K. N. Influence of Bystander Substituents on the Rates of 1,2-H and 1,2-Ph Shifts in Singlet and Triplet Carbenes. J. Phys. Chem. A 1998, 102, 8467-8476. (16) Hu, C-H. Density Functional Study on the Reactivity of Carbenes Toward 1,2-H Shifts. J. Chin. Chem. Soc. 2001, 48, 5-12. (17) Shustov, G. V.; Liu. M. T. H.; Rauk, A. Origin of the Steroselectivity of the Intramolecular 1,1-Hydrogen Shift in Singlet Chlorocarbenes. A Theoretical study. J. Phys. Chem. A 1997, 101, 2509-2513. (18) Barker, J. R.; Yoder, L. M.; King, K. D. Vibrational Energy Transfer of Nonequilibrium Polyatomic Reaction Systems. J. Phys. Chem. A 2001, 105, 796809. (19) Richmond, G.; Setser, D. W. Vibrational Energy Transfer Probabilities of Highly Excited Fluoroethane and 1,2-Difluoroethane Molecules. J. Phys. Chem. 1980, 84, 2699-2705. (20) Marcoux, P. J.; Setser, D. W. Vibrational Energy Transfer Probabilities of Highly Vibrationally Excited CH3CF3. J. Phys. Chem. 1978, 82, 97-108. (21) Csontos, J.; Rolik, Z.; Das, S.; Kallay, M. High-Accuracy Thermochemistry of Atmospherically Important Fluorinated and Chlorinated Methane Derivatives. J. Phys. Chem. A 2010, 114, 13093-13103. (22) Manion, J. A. Evaluated Enthalpies of Formation of the Stable Closed Shell C1 and C2 Chlorinated Hydrocarbons. J. Phys. Chem. Ref. Data 2002, 31, 123-165. (23) Haworth, N. L.; Smith, M. H.; Bacskay, G. B.; Mackie, J. C. Heat of Formation of Hydrofluorocarbons Obtained by Gaussian-3 and Related Quantum Chemical Computations. J. Phys. Chem. A 2000, 104, 7600-7611. (24) Jeffers, P. M. Shock Tube Cis-Trans Isomerization Studies III. J. Phys. Chem. 1974, 78, 1469-1472.


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Page 30 of 38

Page 31 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(25) Craig N. C.; Piper, L. G.; Wheeler, V. L. Thermodynamics of cis-trans Isomerization. II. 1-Chloro-2-fluoroethene, 1,2-Difluorocyclopropanes and Related Molecules. J. Phys. Chem. 1971, 75, 1453-1460. (26) Riehl, J.-F.; Musaev, D.; Morokuma, K. Ab initio Molecular Orbital Study of the Unimolecular Dissociation Reactions of Di- and Trichloroethene. J. Chem. Phys. 1994, 101, 5942-5956. (27) Tarrazo-Antelo, T.; Martínez-Núñez, E.; Vázquez, S. A. Ab initio and RRKM Study of the Elimination of HF and HCl from Chlorofluoroethene. Chem. Phys. Lett. 2006, 435, 176-181. (28) Holmes, B. E.; Setser, D. W.; Pritchard, G. O. Energy Disposal in the ThreeCentered Elimination of DF from 1,1,2-Trifluoroethane-d1. Int. J. Chem. Kinet. 1976, 8, 215-234. (29) Kim, K. C.; Setser, D. W. Unimolecular Reactions and Energy Partitioning. Three- and Four-Centered Elimination Reactions of Chemically Activated 1,1,2Trichloroethane-d0,-d1 and -d2. J. Phys. Chem. 1974, 78, 2166-2179. (30) Barker, J. R. Multiple-Well, Multiple-Path Unimolecular Reaction Systems, Multi-Well Computer Suite. Int. J. Chem. Kinet. 2001, 33, 232-245. (31) 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 Unimolecular Reactions of both Molecules. ChemPhysChem. 2012, 13, 869-878. (32) Enstice, E. CF.; 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 2001, 115, 1054-1062. (33) Richter, G.; Mendez-Vega, E.; Sander, W. Singlet Halophenylcarbenes as Strong Hydrogen-Bond Acceptors. J. Phys. Chem. A 2016, 120, 3524-3532.


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(34) Xu, Z. F.; Xu, Kun; Lin, M. C. Thermal Decomposition of Ethanol. 4. Ab Initio Chemical Kinetics for Reactions of H Atoms with CH3CH2O and CH3CHOH Radicals. J. Phys. Chem. A 2011, 115, 3509-3522. (35) Zhang, M.; Lin, Z.; Song, C. Comprehensive Theoretical Studies on the CF3H Dissociation Mechanism and the Reactions of CF3H with OH and H Free Radicals. J. Chem. Phys. 2007, 126, 034307(1-8). (36) Ghosh, S.; Bhattacharyya, S.; Wategaonkar, S. Dissociation Energies of SulfurCentered Hydrogen-Bonded Complexes. J. Phys. Chem. A 2015, 119, 1086310870. (37) Bhattacharyya, S.; Prakash, V.; Wategaonkar, S. Acid-Base Formalism Extended to Excited States for OH-S Hydrogen Bonded Interactions, J. Phys. Chem. A 2016, 120, 6902-6916. (38) Peterson, K.A.; Woon, D. E.; Dunning, Jr., T. H. Benchmark Calculations with Correlated Molecular Wave Functions. IV. The Classical Barrier Height of the H + H2 → H2 + H Reaction, J. Chem. Phys. 1994, 100, 7410-7415. (39) Bowman, J. M.; Shepler, B. C. Roaming Radicals, Annu. Rev. Phys. Chem. 2011, 62, 531-553. (40) Bonneau, R.; Liu, M. T. H.; Kyu, C. K.; Goodman, J. L. Rearrangement of Alkylchlorocarbenes: 1,2-Shift in free Carbene, Carbene-Olefin Complex, and Excited States of Carbene Precursors. J. Am. Chem. Soc. 1996, 118, 3829-3837. (41) Ogura, H.; Yano, T. CO2 Laser-induced Decomposition of 1,2-Dichloro-1fluoroethane Bull. Chem. Soc. Jpn. 1985, 58, 1239-1259.


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Figure 1



Figure 2


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Figure 3



Figure 4b

Figure 4a 2.5

6

5

2

4 1.5

cis/trans

cis/trans

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

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1

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2 0.5

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1/P (1/torr)

1/P (1/torr)


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Figure 5



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Analysis of the Five Unimolecular Reaction Pathways of CD2ClCHFCl

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