Unimolecular Isomerization of CH2FCD2Cl via the Interchange of Cl

Jul 10, 2013 - of vibrational energy. Three unimolecular processes are in competition with collisional deactivation of CH2FCD2Cl; HCl and DF eliminati...
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Unimolecular Isomerization of CH2FCD2Cl via the Interchange of Cl and F Atoms: Assignment of the Threshold Energy to the 1,2Dyotropic Rearrangement Mary K. Tucker, Samuel M. Rossabi, Corey E. McClintock, 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, and Kansas State University, Manhattan, Kansas 66506, United States S Supporting Information *

ABSTRACT: The room-temperature gas-phase recombination of CH2F and CD2Cl radicals was used to prepare CH2FCD2Cl molecules with 91 kcal mol−1 of vibrational energy. Three unimolecular processes are in competition with collisional deactivation of CH2FCD2Cl; HCl and DF elimination to give CHFCD2 and CH2CDCl plus isomerization to give CH2ClCD2F by the interchange of F and Cl atoms. The Cl/F interchange reaction was observed, and the rate constant was assigned from measurement of CHClCD2 as a product, which is formed by HF elimination from CH2ClCD2F. These experiments plus previously published results from chemically activated CH2ClCH2F and electronic structure and RRKM calculations for the kinetic-isotope effects permit assignment of the three rate constants for CH2FCD2Cl (and for CH2ClCD2F). The product branching ratio for the interchange reaction versus elimination is 0.24 ± 0.04. Comparison of the experimental rate constant with the RRKM calculated rate constant permitted the assignment of a threshold energy of 62 ± 3 kcal mol−1 for this type-1 dyotropic rearrangement. On the basis of electronic structure calculations, the nature of the transition state for the rearrangement reaction is discussed. The radical recombination reactions in the chemical system also generate vibrationally excited CD2ClCD2Cl and CH2FCH2F molecules, and the rate constants for DCl and HF elimination were measured in order to confirm that the photolysis of CD2ClI and (CH2F)2CO mixtures was giving reliable data for CH2FCD2Cl.

I. INTRODUCTION The unimolecular reaction involving the interchange of halogen atoms on adjacent carbon atoms has been demonstrated in the gas phase for several chlorofluoroethane1−5 and chlorofluoropropane6−11 molecules, plus 1,2-bromochloroethane12 and 1bromo-1,1-difluoro-2,2-difluoropropane13 molecules. In each case, the molecules were prepared with ∼90 kcal mol−1 of vibrational energy by chemical activation reactions. The exchange of a Cl atom and a methyl group in neopentyl chloride is another example14 of such interchange processes. These 1,2-halogen interchange reactions belong to the type-1 dyotropic rearrangement class of reactions, which are defined as the unimolecular 1,2-shift of two migrating groups that interchange their relative positions on a stationary scaffold.15 The experimentally characterized examples mentioned above have significantly expanded the number of simple, gas phase, type-1 dyotropic rearrangement reactions on a carbon backbone. The interchange reaction has not been experimentally demonstrated for the simplest chlorofluoroethane, CH2FCH2Cl, although electronic structure calculations3,16a suggest a threshold energy, E0(Cl/F), of ∼60 kcal mol−1, and interchange may be competitive with HCl and HF elimination. Experimental assignment of a reliable E0(Cl/F) would be useful © 2013 American Chemical Society

because CH2FCH2Cl could serve as a reference to identify changes in the threshold energies for type-1 rearrangements in more complex molecules. For example, in the future we intend to illustrate the effect of electron-density donating or electrondensity withdrawing groups (CH3 and CF3, respectively) on E 0 (Cl/F) by studying the rearrangement reactions in CH3CHFCH2Cl and CF3CHFCH2Cl. In order to identify the rearrangement process and measure the rate constant for 1,2-chlorofluoroethane, we have employed the same strategy that previously was used to identify interchange of Cl and Br atoms in 1,2-bromochloroethane.12 In those experiments, CH2BrCD2Cl* molecules were generated by the recombination of CH2Br and CD2Cl radicals, and the formation of CH2ClCD2Br* was identified from the HBr (+CHClCD2) and DCl (+CH2CDBr) products. In the current experiments, chemically activated CH2FCD2Cl* was generated with 91 kcal mol−1 of vibrational energy by the recombination of CH2F and CD2Cl radicals. The energy can be assigned from the previous analysis for CH2FCH2Cl.16 These Received: April 2, 2013 Revised: July 3, 2013 Published: July 10, 2013 6717

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radicals were obtained by the cophotolysis of CD2ClI with (CH2F)2CO at room temperature. The reaction mechanism is summarized by eqs 1 and 2 below; M represents a bath gas molecule, CD2ClI or (CH2F)2CO, and an asterisk denotes vibrational excitation. The interchange process can be identified from either the HF or DCl elimination products from CH2ClCD2F*.

Electronic structure calculations were done to obtain frequencies and moments of inertia for the molecules and their transition states. Calculations were done using density functional theory (DFT) with the B3PW91 method and the 631G(d′,p′) basis set. This information was used to calculate the statistical unimolecular rate constants. On the basis of a large number of examples,1−14,16a the threshold energies obtained from these electronic−structure calculations tend to be more reliable for E0(HF) than for E0(HCl).

II. EXPERIMENTAL AND COMPUTATIONAL METHODS The CH2FCD2Cl* experiments consisted of photolysis of 10 to 1 mixtures of (CH2F)2CO and CD2ClI in quartz vessels by the output of a Oriel high pressure 200-W Hg lamp at room temperature. The experimental procedures used in the present work are the same as those used for the study of CH2FCH2Cl, and ref 16a can be consulted for more experimental detail. The same photolysis procedure was used for the CH2FCH2F* experiments but (CH2F)2CO was photolyzed in the presence of a 10-fold excess of CFCl2CH3 as a bath gas to increase the pressure. After photolysis, analysis for the products was accomplished by gas chromatography with mass spectrometric (GC−MS) detection. Two different photolysis methods were used to produce CD2Cl radicals from 1:10 mixtures of CD2ClI and SF6, respectively, for the CD2ClCD2Cl* experiments. One method was the photolysis with the Oriel high-pressure Hg lamp in Pyrex vessels containing small amounts of mercury(I) iodide. In this case, a red solid was observed that was assumed to be HgI2 formed as HgI removed iodine from the CD2ClI. The second method was mercury-photosensitization using a low-pressure Hg germicidal lamp to photolyze mixtures in quartz vessels containing a small Hg droplet. A yellow solid was observed during the Hg photosensitization that was assumed to be Hg2I2 formed by the dimerization of HgI that was produced as the Hg(3P1) removed iodine from the CD2ClI. For both photolysis methods, the GC analysis used FID detection. The CD2ClI was purchased from CDN isotopes with a stated isotopic purity of 99.9%. Products were identified by comparison to true samples. Preprepared mixtures were used for calibration of the response needed for the decomposition (D) and stabilization (S) measurements to obtain the D/S ratios of CH2CHF/ CH2FCH2F and CH2CHCl/CH2ClCH2Cl. It was assumed that the FID response was the same for CH2CHCl/ CH2ClCH2Cl and for CD2CDCl/CD2ClCD2Cl. No calibration was needed for the ratio of CHClCD2/CH2CDCl. The chemical activation rate constants are obtained from plots of the ratio of the decomposition to stabilization products vs pressure−1. Since pressure is proportional to the collisional frequency (kM[M], with kM being the rate constant for collisional stabilization), it is convenient to discuss the experimental rate constant in units of Torr, as derived from the slope of the D/S vs pressure−1 plot. Assignment of the collision diameters and ε/k values to the collision partners provides the conversion factor from Torr to s−1 using the standard collision rate constant, k M = πd A,M 2 (8kT/ πμA,M)1/2Ω2,2(T). The Lennard-Jones well depths of the collision pairs, ε/k, are used to obtain the values of the Ω2,2 integrals; see ref 17 for more detail. Electronic structure calculations were done with the Gaussian21 code using the B3PW91 method and the 631G(d′,p′) basis set in order to match the calculations of the previous study.16a True transition states were identified by

Since the rate constants for 1c and 2c are the same, the unimolecular reaction system is defined by five rate constants plus collisional deactivation. The HCl and HF elimination rate constants for CH2FCH2Cl have been characterized both by experiments and RRKM calculations.16a For activation by recombination of CH2F and CH2Cl, the rate constants for HF and HCl elimination were (1.1 ± 0.3) × 108 and (0.82 ± 0.25) × 108 s−1, respectively, and the threshold energies were assigned as 62 and 64 kcal mol−1, respectively.16b Since the entropy of the HCl transition state is larger than that for HF elimination, a higher threshold energy for HCl elimination is required to have the same rate constant for HCl as for HF elimination. In the current study, no attempt was made to measure the rate constants for DF and HCl elimination from CH2FCD2Cl. Instead, the four elimination rate constants for CH2FCD2Cl and CH2ClCD2F were obtained from calculated kinetic-isotope effects and the experimental rate constants16a for CH2FCH2Cl; these kinetic-isotope effects were based upon RRKM calculations using models obtained from electronic− structure calculations. Since the CH2FCD2Cl and CH2ClCD2F molecules are not separated by gas chromatography and since their mass spectra are similar, the interchange process must be detected from either the CHClCD2/CH2CDCl or the CH2CDF/ CHFCD2 ratios, which can be measured from mass spectrometry because the masses of the vinyl chloride or vinyl fluoride in the ratios differ by one deuterium atom. In practice, we selected the ratio of vinyl chloride-d2 vs vinyl chloride-d1 because at the selected ionization energy the vinyl fluoride ion gave some loss of H and D atoms, and the 47/48 mass ratio could not be used in a simple way to measure the CH2CDF/CD2CHF ratio. In addition, for the chosen experimental conditions, the concentration of CH2F greatly exceeded that of CD2Cl. Thus, much more C2H4F2 (and CH2CHF) was present than C2D4Cl2 (and CD2CDCl), and interference from CD2CDCl was minimal. In order to verify that the experimental system was well-behaved, experiments were done with photolysis of only (CH2F)2CO to observe the HF elimination reaction from CH2FCH2F* and with only CD 2 ClI to observe DCl elimination from CD2ClCD2Cl*. These rate constants can be compared to prior measurements.17−20 6718

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standard methods described in earlier work. The calculated vibrational frequencies and moments of inertia were used as input data for RRKM calculations of the rate constants using the Multiwell code supplied by Professor Barker.22 The torsional mode of CH2FCD2Cl is best treated as an asymmetric internal rotation for computation of the density of states at 91 kcal mol−1 of energy. The two barriers to internal rotation are 4.5 and 7.8 kcal mol−1, and treating the internal rotation as a symmetric rotor with a barrier equal to the average of the two barriers gave a density of states for CH2FCH2Cl that was within 3% of the density calculated for the asymmetric rotor.16a Therefore, the symmetric rotor approximation can be used to calculate the density of states for CH 2 FCD 2 Cl (and CH2ClCD2F). The standard RRKM rate constant equation at energy E is given below. In comparison with experimental results, the average energy, ⟨E⟩, of the molecules is equated to E. In the present work, our use of eq 3 is mainly to obtain rate constant ratios, and the approximations in eq 3 itself, as well as in the evaluation of state densities and sums from harmonic frequencies, will not be very important. kE = (s†/h)(I †/I )1/2

∑ P†(E − E0)/N *(E)

Figure 1. Plot of the ratio of the decomposition product (CH2 CHF) to the stabilization product (C2H4F2) versus inverse pressure (Torr−1) for CH2FCH2F* in CFCl2CH3 bath gas. The least-squares slope of the plot is 17.1 ± 0.9 Torr with an intercept of 0.05 ± 0.06; the correlation coefficient is 0.97. The collision diameters of C2H4F2, CFCl2CH3, and (CH2F)2CO are 5.0, 5.3, and 5.0 Å; the ε/k values in the same order are 250, 420, and 450 K. These parameters were used to calculate the collision rate constant, which converts the slope to a unimolecular rate constant (3.0 × 108) in s−1 units.

(3) †

The sums of the states of the transition state, ΣP (E − E0), and the density of states of the molecule, N*(E), were calculated in the harmonic approximation; (I†/I) is the ratio of the overall moments of inertia, and s† is the reaction path degeneracy. The main factor in comparing the rate constants for HCl vs DCl and HF vs DF elimination is the difference in threshold energies from CH2FCD2Cl and CH2ClCD2F, which arises from changes in zero-point energies23 of the transition states vs that of the molecule. These zero-point energies were obtained from the calculated frequencies of the molecule and transition states.

III. EXPERIMENTAL RESULTS Since the DCl and HF elimination rate constants for CH2FCD2Cl* could not be measured directly from D/S vs pressure−1 plots, we decided to test the experimental system by measuring the rate constants for CH 2 FCH 2 F* and CD2ClCD2Cl*. The ratio of CHFCH2 to CH2FCH2F was measured by GC−MS via comparison of masses 46 to 33. Data were collected over the 8 to 100 Torr pressure range, and the results are shown in Figure 1. The linear least-squares fit to the data give a slope of 17 ± 1 Torr, which becomes a rate constant of 3.1 ± 0.2 × 108 s−1 when the collision diameters mentioned in the figure caption are used to calculate the collision rate constant, kM. The rate constant derived from the data in Figure 1 is in excellent agreement with several previous measurements,17,18 which give an average rate constant of 3.0 ± 0.5 × 108 s−1. Although (CH2F)2CO and CFCl2CH3 are expected to be efficient collisional deactivation partners, the linear fit to the data give an upper limit to the unit deactivation rate constant. By comparison with deactivation measurements 17 for CH2FCH2F* in several bath gases, as well as inspection of the present data, the average energy removed per collision, ⟨ΔEd⟩, should be 8−10 kcal mol−1, and the unit deactivation rate constant would be reduced slightly to ∼2.5 ± 0.5 × 108 s−1. Experiments were also done with a 1:10 mixture of just CD2ClI in SF6 bath gas to generate the CD2Cl radicals that combined to form CD2ClCD2Cl* using the two different photolysis methods described in the previous section. The D/S ratio of CD2CDCl/CD2ClCD2Cl was measured by GC-FID, and the ratios are plotted vs pressure−1 in Figure 2. The data in

Figure 2. Plot of the ratio of the decomposition product (CD2 CDCl) to the stabilization product (C2D4Cl2) versus pressure−1 (Torr−1) for CD2ClCD2Cl* in SF6 bath gas using the high-pressure Hg lamp and Pyrex reaction vessels containing Hg2I2 (○) and the Hg germicidal (■) lamp with clean quartz vessels. The least-squares slope of the plot is 6.4 ± 0.3 Torr with an intercept of −0.002 ± 0.019; the correlation coefficient is 0.998. The collision diameters of C2D4Cl2, CD2ICl, and SF6 are 5.5, 5.1, and 5.0 Å and the ε/k are 350, 400, and 259 K in the same order. These parameters were used to convert the slope of the plot to a unimolecular rate constant (0.94 ± 0.05 × 108) in units of s−1. The collision diameter and ε/k for CH2FCH2Cl are 5.5 Å and 350 K.

Figure 2 from either method are in excellent agreement illustrating the utility of both photolysis procedures for producing chloromethyl radicals. The slope of the plot is 6.4 ± 0.3 Torr. Two previous measurements for CD2ClCD2Cl*, which employed the reaction of CD2 with CD3Cl to obtain the CD2Cl radicals, gave 5.1 ± 0.2 and 4.9 ± 0.5 Torr as slopes of the D/S plots. The collisional deactivation of CH2ClCH2Cl* has been studied19,20 in CH3Cl and SF6 bath gases,19,20 and the limiting high pressure rate constants (in Torr units) were 16 ± 2 and 21 ± 3. Thus, the 6.4 Torr value in SF6 bath gas for CD2ClCD2Cl* is within the range of the expected values based 6719

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lowest pressures and the large number of points at the lower pressures should be noted. A steady-state analysis for CH2ClCD2F* based on the mechanism shown in eqs 1 and 2 gives the equations below.

upon the expected kinetic-isotope effect and the results for C2H4Cl2* in SF6 and CH3Cl bath gases. Converting the slope of Figure 2 to a rate constant using the cross-sections given in the figure caption give 0.94 ± 0.05 × 108 s−1. The collisional deactivation for CH3Cl and SF6 has been assigned19 to a stepladder model with ⟨ΔEd⟩ as 8 kcal mol−1. Thus, the unit deactivation rate constant for C2D4Cl2* would be 0.78 × 108 s−1 after reduction of the experimental rate constant by a factor of 1.2. On the basis of the rate constant measurements for C2H4F2* and C2D4Cl2*, the (CH2F)2CO and CD2ICl photolytic systems at room temperature provide a reliable source of CH2F and CD2Cl radicals without complications. Since the recombination rate constants are large at room temperature,23−25 the radical recombination reactions are the dominant processes. Therefore, we can proceed to use the mixed photolytic system at room temperature with confidence for preparation of CH2FCD2Cl* molecules. The experiments designed to observe the interchange reaction employed photolysis of a 10:1 mixture of (CH2F)2CO to CD2ClI. Both photodissociation of CD2ClI and abstraction of the I atom from CD2ClI by CH2F radicals give CD2Cl radicals. Under these conditions the main radical recombination products are C2H4F2* and CH2FCD2Cl*. In the analysis, attention was focused upon vinyl chloride-d1 and vinyl chloride-d2 products from the DF and HF elimination from CH2FCD2Cl* and CH2ClCD2F*, respectively. The relevant masses are 63 and 65 for vinyl chloride-d1 and 64 and 66 for vinyl chloride-d2. The ratio of the masses 63 and 64 were used to obtain the molecular ratio because the 35Cl isotope is more abundant. The experimental results for a range of pressure are shown in Figure 3. As a point of reference, D/S is ∼1.0 at ∼6

0 = d[CH 2ClCD2 F*]/dt = kI[CH 2FCD2 Cl*] − [CH 2ClCD2 F*](kDCl + kHF + kI + kM[M])

(4)

The ratio for the rates of formation of CH2CDCl and CHClCD2 is given by eq 5 in which eq 4 is used to obtain the ratio of CH2FCD2Cl* and CH2ClCD2F* concentrations. The kI is the rate constant for the interconversion of CH2FCD2Cl* and CH2ClCD2F*. kDF[CH 2FCD2 Cl*]/kHF[CH 2ClCD2 F*] = (kDF/kHF)(kDCl + kHF + kI + kM[M])/kI

(5)

In the limit of low pressure, the kM[M] term is not important, and the ratio of CH2CDCl to CHClCD2 becomes eq 6. If kDF from CH2FCD2Cl* and kDCl and kHF from CH2ClCD2F* [CH 2CDCl]/[CHClCD2 ] = kDF/kHF(1.0 + kDCl /kI + kHF/kI)

(6)

are assigned from the calculated rate constant ratios for CH2FCD2Cl and CH2ClCD2F vs CH2FCH2Cl and the experimental rate constants16a for CH2ClCH2F, then kI can be evaluated from the data in Figure 3. The calculated ratio of rate constants for kHCl(CH2FCD2Cl) and kHF(CH2ClCD2F) versus kHCl and kHF from CH2FCH2Cl is 1.5. The calculated, intramolecular, primary kinetic-isotope effect for CH2FCD2Cl and CH2ClCD2F is also 1.5 for kHF/kDF and kHCl/kDCl, which corresponds to a combined kinetic-isotope effect of 2.3 relative to CH2FCH2Cl. On the basis of the experimental16a HF and HCl elimination rate constants of 7.0 ± 0.9 and 5.3 ± 0.5 Torr, the rate constants become 4.7 and 3.0 for kHF and kDF for CH2FCD2Cl and 3.5 and 2.3 for kHCl and kDCl for CH2ClCD2F. Substitution into eq 6 gives kI = 1.7 Torr. The calculated ratio of kDF/kHF should be reliable, so the main uncertainty in kI is in the values for kDCl and kHF. The cumulative estimated uncertainty is 20−30%, and kI = 1.7 ± 0.5 Torr; conversion to units of s−1 using the collision diameters given in the captions of Figures 1 and 2 gives kI = 0.38 × 108 s−1. In Figure 3, the ratio of CH2CDCl/CHClCD2 begins to increase for pressures above ∼2 Torr, because the collisional deactivation rate begins to compete with the elimination rate for the C2H2D2FCl* molecules, and kM[M] must be added to the analysis. The value for kI corresponds to a branching ratio of 0.24 relative to the combined DF and HCl elimination pathways for CH2FCD2Cl and 0.23 for the combined HF and DCl pathways of CH2ClCD2F. Since kHF(C2H4ClF) is thought to be the most reliable experimental rate constant, the most useful comparison (see below) is probably with kHF(CH2ClCD2F) and that branching ratio is 0.36. The kinetic-isotope effect for kI(CH2ClCD2F) and kHF(CH2ClCD2F) relative to C2H4ClF would be the same; therefore, kI/kHF for CH2FCH2Cl also would be 0.36.

Figure 3. Plot of the ratio of the decomposition product (CH2 CDCl) from CH2FCD2Cl* to the decomposition product (CHCl CD2) from CH2ClCD2F* versus the inverse pressure (Torr−1). The average ratio at low pressure, which is 3.3, is based on 16 experiments for pressures less than 1 Torr.

Torr for CH2FCD2Cl*. The CH2CDCl/CHClCD2 ratio decreases from about 7 to 3.3 as the pressure is lowered from 7 to 0.2 Torr. Obviously the interchange reaction does compete with HCl and DF elimination from CH2FCD2Cl* because CHClCD2 product yield is 0.3 of the CH2CDCl product yield at the lowest pressures. The interchange rate constant will be assigned using the low pressure limit of the ratio shown in Figure 3, i.e., CH2CDCl/CHClCD2 equals 3.3 at the

IV. DISCUSSION The threshold energy for the Cl/F interchange reaction was assigned by comparing the experimental kI/kHF ratio to the calculated ratio for ⟨E⟩ = 91 kcal mol−1. The threshold energy 6720

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Two other laboratories26,27 have used DFT calculations to investigate substituent effects on threshold energies in type-1 dyotropic rearrangements using 1,2-C2H4Cl227 and 1,2C2H4Br226 as reference molecules. Both studies found that electron-density withdrawing groups raised the threshold energy for rearrangement and that electron-density donating groups lowered the threshold energy. The DFT calculation27 for 1,2-C2H4Cl2, which used the B3LYP/6-311++G(d,p) method, reported a threshold energy of 41.9 kcal mol−1 for the rearrangement. Thus, the threshold energy for interchange of two Cl atoms rather than one Cl and one F atom is 20 kcal mol−1 lower. Zou and Yu27 also did a calculation for the rearrangement of CH2ClCH2Br; they found a threshold energy of 37.2 kcal mol−1, which compares favorably with our12 calculated value of 41.8 kcal mol−1 by the B3PW91/631G(d′,p′) method. The experimental assignment12 for the threshold energy of the Cl/Br rearrangement was 43 kcal mol−1. The threshold energy for interchange of Cl and Br atoms is more than 10 kcal mol−1 lower than for HBr elimination, and rearrangement will be faster than the elimination reactions for 1,2-bromochloroalkanes. Fernández, Sierra, and Cossió 26 found a threshold energy of just 28.2 kcal mol−1 for the rearrangement of two Br atoms in 1,2-C2H4Br2, and dyotropic rearrangements seem to have lower threshold energies if both halogen atoms are equivalent. These authors developed an interesting molecular orbital model for the bonding of the Br atoms in the transition state for rearrangement, which has a planar structure for the C2H4 scaffold with the two Br atoms located above and below the C2H4 plane. Four electrons fill the two bonding molecular orbitals formed by combining two p-orbitals of the Br atoms with the π-orbital of ethene and two additional p-orbitals of the Br atoms with the π*-orbital of ethene. Substitution of a Cl atom for a Br atom reduces the bonding energy of the molecular orbitals of the transition state and hence raises the threshold energy for isomerization. The same argument applies for substitution of a F atom for a Cl atom in the bridge for the CH2ClCH2Cl case. This trend is expected since the energy of the atomic orbitals of the halogen atoms better match the energy of the π-orbital as the halogen becomes heavier, which facilitates the formation of the bonding orbitals. The increased bonding of the molecular orbitals probably complements the reduction of the carbon−halogen bond energy of the molecules in the CH2XCH2X (X = F, Cl, Br) series to explain the decrease of E0(X/Y) for the 1,2-dihaloethanes. As a final point, we wish to use the threshold energies for HCl and HF elimination from CH2FCH2Cl and CH3CHClF to comment on the reverse addition reactions of HCl and HF with CH2CHF and CH2CHCl, respectively. The transition state for the interchange of Cl and F atoms connects the two forms of the isomerized molecules, as confirmed by the intrinsic reaction coordinate calculation mentioned in ref 12. Thus, that part of the potential energy surface does not influence the addition reaction of HX molecules to monohaloalkenes. In contrast, the threshold energies for HCl and HF elimination directly relate to the HX addition reaction. For example, consider the trends in the threshold energies for HCl and HF elimination for CH2FCH2Cl and CH3CHClF and the microscopic reverse reactions of the addition of HCl and HF to CH2CHF and CH2CHCl, respectively. The thermochemistry of all the molecules and transition states was summarized in ref 16a. The threshold energies for formation of CH3CHClF, i.e., Markovnikov-type addition, are 34 and 44 kcal mol−1 for

for HF elimination from CH2ClCD2F can be taken to be the same as for CH2ClCH2F because the changes in the zero-point energies in the molecule and transition state for the CH2ClCD2F are the same. Thus, the E0(HF) for CH2ClCD2F is 62 ± 2 kcal mol−1. The reaction path degeneracies are unity and 2 for isomerization and for DF and HCl elimination, respectively. The E0(Cl/F) must be equal to E0(HF) for kI to be 0.36 of kHF; therefore, E0(Cl/F) = 62 kcal mol−1. In addition to the factor of 2 in reaction path degeneracy, the sums of states and I† are larger for the HF elimination transition state than for the interchange transition state. Given the somewhat indirect method for obtaining the experimental rate constant and the assumptions in the model for the calculated rate constant, the uncertainty in our assigned E0(Cl/F) is 2−3 kcal mol−1. It should be noted that the assignment of E0(Cl/F) is dependent on the experimental values of kHF and kHCl of CH2ClCH2F. If these should change, then kI and E0(Cl/F) also would have to be modified. The electronic-structure calculations gave E0(Cl/ F) = 60 kcal mol−1 for the 6-31G(d′,p′) basis set, 56.4 kcal mol−1 for the 6-311+G(2d,p) basis set,3,16a and 58.0 kcal mol−1 for the 6-311++G(2d,p) basis set,3,16a and the agreement between the DFT calculated and experimentally assigned threshold energy is modest and seems better for the smaller basis set. The results for other methods and basis sets are given in ref 3. The calculated E0(HF) was 61 kcal mol−1, which also is in good agreement with the experimental assignment.16a As previously discussed,16a the agreement for the experimental (64 ± 2 kcal mol−1) and calculated (58 kcal mol−1) values for E0(HCl) are not in good agreement; other basis sets and methods do not provide any improvement. The calculated results for CH2FCD2Cl and the experimental results for C2D4Cl2 can be used to summarize the secondary and primary kinetic-isotope effects using eq 7. The ratios of overall moments of inertia have been dropped because the effect of the isotopic substitution is negligible. The density of states for CH2FCD2Cl and CH2ClCD2F are the same, but the ratio of densities for C2H2D2ClF versus C2H4ClF is 4.03. The different kinetic-isotope effects are determined by the ratio of the sums of states for the transition states. kH/kD = [∑ P†(E − E0H)H /∑ P†(E − E0D)D ][N *(E)D /N *(E)H ]

(7)

The isotope effect for Cl/F interchange, HCl elimination, and HF elimination for CH2FCD2Cl relative to CH2FCH2Cl were all close to 1.5; these ratios were nearly constant because E0H and E0D are equal, i.e., these are purely secondary isotope effects. Furthermore, the actual values of the threshold energies are similar, which causes the ratio of sums of states of the transition states to be similar. The kinetic-isotope effects for DCl and DF elimination were 2.33 and 2.31 for CH2FCD2Cl and CD2FCH2Cl relative to CH2FCH2Cl. The increased isotope effect is a consequence of the 1.0 kcal mol−1 increase in E0D for DCl or DF elimination because the D atom is in the four-membered ring of the transition state. The larger E0D reduces the value for ΣP†(E − E0D). The kinetic-isotope effect (3.3 ± 0.3)19 for C2D4Cl2 is the cumulative effect of the secondary effect of three deuterium atoms plus the primary effect from the D atom in the four-membered ring of the transition state. These isotope effects for 1,2-dihaloethanes are somewhat larger than for many other examples of DX elimination reactions because ⟨E⟩ is lower and E0 is higher than normal, as discussed in ref 23. 6721

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The Journal of Physical Chemistry A the HCl and HF addition reactions, respectively. In contrast, the threshold energies for formation of CH2FCH2Cl are 51 and 50 kcal mol−1 for HCl and HF addition to CH2CHF and CH2CHCl, respectively. The same trend is found in favor of Markovnikov addition of HCl to CH2CHCl (37 versus 48 kcal mol−1) and HF to CH2CHF (40 versus 55 kcal mol−1). Clearly Markovnikov-type addition is favored, even though Cl and F atoms are electron-density withdrawing groups according to the Hammett σρ criteria. In their study of substituent effects on HX addition reactions, Suresh, Koga, and Gadre28 found that electron-density donating groups greatly favored Markovnikov-type addition, but that electron-density withdrawing groups exhibited less regiospecificity. The strong preference for Markovnikov-type addition of HF and HCl to vinyl halides is a consequence of the greater stability associated with electronic effects when both halogen are located on the same carbon atom of the transition state.16 The mesomeric (resonance) effect associated with the low E0(HCl) for elimination reactions from CH3CHCl2 and CH3CHClF corresponds to a low threshold energy for Markovnikov addition as noted above.

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REFERENCES

(1) Solaka, S. A.; Boshamer, S. E.; Parworth, C. E.; Heard, G. L.; Setser, D. W.; Holmes, B. E. Isomerisation of CF2ClCH2Cl and CFCl2CH2F Molecules by Interchange of Cl and F Atoms with Analysis of the Unimolecular Reactions of Both Molecules. ChemPhysChem 2012, 13 (3), 869−878. (2) Enstice, E.; Duncan, J. R.; Setser, D. W.; Holmes, B. E. Unimolecular Reaction in the CF3CH2Cl ↔ CF2ClCH2F System: Isomerization by Interchange of Cl and F Atoms. J. Phys. Chem. A 2011, 115, 1054−1062. (3) 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. 2011, 88, 1112−1117. (4) Beaver, M. R.; Heard, G. L.; Holmes, B. E. Theoretical Calculations of Product Percentage Yields for the Thermal Decomposition of 2-Chloro-1,1-difluoroethane. Tetrahedron Lett. 2003, 44, 7265−7268. (5) Dolbier, W. R.; Romelaer, R.; Baker, J. M. Anomalous Elimination of HCl from 2-Chloro-1,1-difluoroethane: Likely Involvement of a 1,2-FCl Interchange Mechanism. Tetrahedron Lett. 2002, 43, 8075−8077. (6) Burgin, M. O.; Heard, G. L.; Martell, J. M.; Holmes, B. E. Unimolecular Reaction Kinetics of CF2ClCF2CH3 and CF2ClCF2CD3: Experimental Evidence for a Novel 1,2-FCl Rearrangement Pathway. J. Phys. Chem. A 2001, 105, 1615−1621. (7) Heard, G. L.; Holmes, B. E. A 1,2-FCl Rearrangement As an Intermediate Step in the Unimolecular 1,3-HCl Elimination from Chlorofluoropropanes. J. Phys. Chem. A 2001, 105, 1622−1625. (8) Burgin, M. O.; Simmons, J. G., Jr.; 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, 2283−2292. (9) Beaver, M. R.; Simmons, J. G., Jr.; Heard, G. L.; Setser, D. W.; Holmes, B. E. Unimolecular Reactions Including ClF Interchange of Vibrationally Excited CF2ClCHFCH2CH3 and CF2ClCHFCD2CD3. J. Phys. Chem. A 2007, 111, 8445−8455. (10) Zaluzhna, O.; Simmons, J. G., Jr.; Setser, D. W.; Holmes, B. E. Unimolecular Reactions of CF2ClCFClCH2F and CF2ClCF2CH2Cl: Observation of FCl Interchange. J. Phys. Chem. A 2008, 112, 12117− 12124. (11) Zaluzhna, O.; Simmons, J. G., Jr.; Heard, G. L.; Setser, D. W.; Holmes, B. E. Unimolecular Elimination of HF and HCl from Chemically Activated CF3CFClCH2Cl. J. Phys. Chem. A 2008, 112, 6090−6097. (12) Friederich, L.; Duncan, J. R.; Heard, G. E.; Setser, D. W.; Holmes, B. E. Unimolecular Reactions of CH 2 BrCH 2 Br, CH2BrCH2Cl, and CH2BrCD2Cl: Identification of the Cl−Br Interchange Reaction. J. Phys. Chem. A 2010, 114, 4138−4147. (13) Lisowski, C. E.; Duncan, J. R.; Heard, G. L.; Setser, D. W.; Holmes, B. E. Unimolecular Reactions of Chemically Activated CF2BrCF2CH3 and CF2BrCF2CD3: Evidence for 1,2-FBr Interchange. J. Phys. Chem. A 2008, 112, 441−447. (14) Lisowski, C. E.; Duncan, J. R.; Ranieri, A. J.; Heard, G. L.; Setser, D. W.; Holmes, B. E. Isomerization of Neopentyl Chloride and Neopentyl Bromide by a 1,2-Interchange of a Halogen Atom and a Methyl Group. J. Phys. Chem. A 2010, 114, 10395−10402. (15) (a) Fernández, I.; Cossio, F. P.; Miguel, A. S. Dyotropic Reactions: Mechanisms and Synthetic Applications. Chem. Rev. 2009, 109, 6687−6711. (b) Damrauer, R.; Stanton, J. F. Studies of 1,2Dihalo Shifts in Carbon−Carbon, Carbon−Silicon, and Silicon− Silicon Systems: A Computational Study. Organometallics 2012, 31, 8426−8436. (c) Gutiierrez, O.; Tantillo, D. J. Analogies between Synthetic and Biosynthetic Reactions in Which [1,2]-Alkyl Shifts Are

ASSOCIATED CONTENT

S Supporting Information *

Table of the molecular and transition state structure vibrational frequencies, overall moments of inertia, and the reduced moments of inertia for the internal rotors calculated using B3PW91/6-31G(d′,p′) for CD2ClCH2F and CH2FCH2Cl. This material is available free of charge via the Internet at http:// pubs.acs.org.

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ACKNOWLEDGMENTS

Financial support from the National Science Foundation (CHE-1111546) is acknowledged.

V. CONCLUSIONS The Cl/F interchange reaction has been observed and the rate constant measured for CH2FCD2Cl formed with 91 kcal mol−1 from the recombination of CD2Cl and CH2F radicals at room temperature. The interchange reaction was identified from measurement of the DCl (CH2CDF) and HF (CHCl CD2) products formed from the elimination reactions of CH2ClCD2F. The product branching ratio for interchange vs DF plus HCl elimination was 0.24. The threshold energy for Cl/F interchange was assigned as 62 kcal mol−1 based upon the threshold energy for HF elimination as a reference. The Cl/F interchange in 1,2-chlorofluoroethane is one of the simplest examples of a 1,2-dyotropic type-1 rearrangement,15 and 62 kcal mol−1 can serve as a reference for comparison to the threshold energies of other type-1 dyotropic rearrangements of Cl and F atoms on a carbon scaffold. Previous work12 with CH2BrCD2Cl established 43 kcal mol−1 as the reference threshold energy for interchange of Cl and Br atoms. DFT calculations provide modest agreement with the experimental threshold energies for these rearrangement reactions. As part of the study for CH2FCD2Cl, the experimental rate constants in the literature for chemically activated CH2FCH2F and CD2ClCD2Cl were confirmed.

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AUTHOR INFORMATION

Corresponding Author

*(B.E.H.) Phone: 828-232-5168. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 6722

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Combined with Other Events: Dyotropic, Schmidt, and Carbocation Rearrangements. J. Org. Chem. 2012, 77, 8845−8850. (16) (a) Duncan, J. R.; Solaka, S.; 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 and Pre-Exponential Factors. J. Phys. Chem. A 2010, 114, 794−803. (b) In ref 16a, the RRKM rate constants were matched to the experimental rate constants for threshold energies of 63 ± 2 and 65 ± 2 kcal mol−1 for HF and HCl elimination, respectively. In the present work, we have selected 62 ± 2 and 64 ± 2 kcal mol−1 because slightly smaller RRKM rate constants would be obtained if anharmonicity had been included in the state counting of eq 3. (17) Richmond, G.; Setser, D. W. Vibrational Energy Transfer Probabilities of Highly Vibrationally Excited Fluoroethane and 1,2Difluoroethane Molecules. J. Phys. Chem. 1980, 84, 2699−2705. (18) 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. J. Am. Chem. Soc. 1969, 91, 7648−7657. (19) Setser, D. W.; Siefert, E. E. Cascade Deactivation and Pressure Dependent Nonequilibrium Kinetic Isotope Effects for Collisional Deactivation of Dichloroethane-d0 and -d2 Molecules 3613−3622 and Vibrational Energy Transfer Probabilities of Higfhly Vibrationally Excited Dichloroethane with Ar, Kr, Xe, and Sulfur Hexafluoride. J. Chem. Phys. 1972, 57, 3623−3628. (20) Dees, K.; Setser, D. W. Kinetic Isotope Effects in the Unimolecular Reactions of Chemically Activated Chloroethane-d0 and -d5 and 1,2-Dichloroethane-d0 and -d4 Molecules. J. Chem. Phys. 1968, 49, 1193−1206. (21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.A. ; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; et al. Gaussian 03; Gaussian, Inc.: Wallingford, CT, 2004. (22) Barker, J. R. Multiple-Well, Multiple-Path Unimolecular Reaction Systems. I. MultiWell Computer Program Suite. Int. J. Chem. Kinet. 2001, 33, 232−245. (23) Zhu, L.; Simmons, J. G., Jr.; Burgin, M. O.; Setser, D. W.; Holmes, B. E. Rate Constants and Kinetic Isotope Effects for Unimolecular 1,2-HX or DX (X = F or Cl) Elimination from Chemically Activated CF3CFClCH3-d0, -d1, -d2, and -d3. J. Phys. Chem. A 2006, 110, 1506−1517. (24) Cobos, C. J.; Croce, A. E.; Luther, K.; Troe, J. Temperature and Pressure Dependence of the Reaction 2CF3 (+M) ↔ C2F6 (+M). J. Phys. Chem. A 2010, 114, 4748−4754. (25) Roussel, P. B.; Lightfoot, P. D.; Caralp, F.; Catoire, V.; Lesclaux, R.; Forst, W. Ultraviolet Absorption Spectra of the CH2Cl and CHCl2 Radicals and the Kinetics of Their Self-Recombination Reaction from 273 to 686 K. J. Chem. Soc., Faraday Trans. 1991, 87, 2367−2377. (26) Fernández, I.; Sierra, M. A.; Cossio, F. P. Steroelectronic Effects on Type I 1,2-Dyotropic Rearrangements in Vicinal Dibromides. Chem.Eur. J. 2006, 12, 6323−6330. (27) Zou, J.-W.; Yu, C.-H. Dyotropic Rearrangements of Dihalogenated Hydrocarbons: A Density Functional Theory Study. J. Phys. Chem. A 2004, 108, 5649−5654. (28) Suresh, C. H.; Koga, N.; Gadre, S. R. Revisiting Markovnikov Addition to Alkenes via Molecular Electrostatic Potential. J. Org. Chem. 2001, 66, 6883−6890.

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