Experimental and Computational Studies of Unimolecular 1,1-HX (X

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A: Kinetics, Dynamics, Photochemistry, and Excited States

Experimental and Computational Studies of Unimolecular 1,1-HX (X = F, Cl) Elimination Reactions of CDCHFCl: The Role of Carbene:HF and HCl Adducts in the Exit Channel of RCHFCl and RCHCl Reactions 2

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Blanton R. Gillespie, Chaitanya A. Patel, Mallory M. Rothrock, George L. Heard, Donald W. Setser, and Bert E. Holmes J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b00779 • Publication Date (Web): 06 Mar 2019 Downloaded from http://pubs.acs.org on March 10, 2019

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The Journal of Physical Chemistry

Experimental and Computational Studies of Unimolecular 1,1-HX (X = F, Cl) Elimination Reactions of C2D5CHFCl: The Role of Carbene:HF and HCl Adducts in the Exit Channel of RCHFCl and RCHCl2 Reactions Blanton R. Gillespie,‡ Chaitanya A. Patel,‡ Mallory M. Rothrock,‡ George L. Heard,‡ D. W. Setser,† and Bert E. Holmes*,‡ ‡Department

of Chemistry, University of North Carolina-Asheville. One University Heights, Asheville, North Carolina 28804-8511, United States. †Kansas State University, Manhattan, Kansas 66506, United States

ABSTRACT The gas-phase unimolecular reactions of C2D5CHFCl molecules with 94 kcal mol-1 of vibrational energy have been studied by the chemical-activation experimental technique and by electronic-structure computations. Products from the reaction of C2D5CHFCl molecules, formed by the recombination of C2D5 and CHFCl radicals in a room temperature bath gas, were measured by gas chromatography-mass spectrometry. The 2,1-DCl (81%) and 1,1-HCl (17%) elimination reactions are the principal processes, but 2,1-DF and 1,1-HF elimination reactions also are observed. Comparison of experimental rate constants to calculated statistical rate constants provides threshold energies. The potential surfaces associated with C2D5(F)C: + HCl and C2D5(Cl)C: + HF reactions are of special interest, because hydrogen-bonded adducts with HCl and HF with dissociation energies of 6.4 and 9.3 kcal mol-1, respectively, are predicted by calculations. The relationship between the geometries and threshold energies of transition states for 1,1-HCl elimination and carbene:HCl adducts is complex and previous studies of related molecules, such as CD3CHFCl, CD2ClCHFCl, C2D5CHCl2, and halogenated methanes are included in the computational analysis. Extensive calculations for CH3CHFCl as a model for 1,1-HCl reactions illustrate properties of the exit-channel potential energy surface. Since the 1,1-HCl transition state is submerged relative to dissociation of the adduct, inner and outer transition states should be considered for analysis of rate constants describing 1,1-HCl elimination and addition reactions of carbenes to HCl.

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1. INTRODUCTION. Gas-phase, unimolecular, 1,1-HX (X=F, Cl) elimination reactions of 1,1dihaloalkanes give singlet-state halocarbenes + HX as products1-10. The halogen atom stabilizes the singlet halocarbene, which is the ground state, and the enthalpy of these reactions is typically about 70 kcal mol-1. In addition to interest in kinetics of these reactions,1-10 adducts between carbenes and HX molecules provide examples of hydrogen bonding to a carbon atom, and such adducts are beginning to receive attention11-13 in the hydrogen-bond literature. The current study of C2D5CHFCl provides additional examples of carbene:HX adducts. Low vibrational frequencies of the transition state enable 1,1-HX elimination reactions to compete with traditional 2,1-HX elimination reactions at high temperatures or high vibrational energies.1-10 For most chemical activation systems, the carbene product has sufficient energy to isomerize to an olefin. Several examples of 1,1-HF elimination reactions have been successfully characterized by experimental and computational studies.1-8 According to computational results7,8, hydrogen-bonded carbene:HF adducts with dissociation energies between 6-10 kcal mol-1 exist in the exit channels. Calculated threshold energies for 1,1-HF elimination are a few kcal mol-1 higher than the enthalpy of reaction, and those calculated values are in accord with threshold energies derived from comparing experimental rate constants with statistical rate constants obtained from models of transition states. In contrast to 1,1-HF reactions, 1,1-HCl elimination is more difficult to understand, although dissociation energies of carbene:HCl adducts are smaller than those for carbene:HF.7-10 Calculated threshold energies of 1,1-HCl elimination transition states are similar to energies of their corresponding adducts, as are the geometries at the maximum and minimum positions on the potential. Thus, calculated 1,1-HCl threshold energies are below the enthalpy of reaction. On the other hand, experimental threshold energies for 1,1-HCl elimination obtained by comparing experimental rate constants to RRKM rate constants based on properties of HCl elimination transition states are higher than calculated values. In fact, the experimental values usually are close to the enthalpy of reaction.7,8,10 One objective of this work is to resolve this discrepancy. A transition state for D-atom migration in the exit channel, which can give olefin + HCl or HF product without intermediate formation of the carbene, is another complication6,8 in the exit channels. Adduct-assisted transition states for migration have calculated threshold energies that can be above or below the enthalpy of reaction. Such a reaction pathway has been postulated in liquid-phase experiments to explain the competition between addition of CH3(Cl)C: to olefins versus isomerization.14


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In the present work, we provide another example of 1,1-HCl elimination in competition with 2,1-DCl elimination by a gas-phase chemical-activation study of C2D5CHFCl*. The 2,1-DCl elimination reaction also is of interest because the effect on the threshold energy when the F-atom is on the carbon that is losing a Clatom7,15,16 remains to be defined. The competing 2,1-DF and 1,1-HF reactions are less important, but they are observed. Electronic-structure calculations of the stationary points on the potential surface complement the experimental results. We also have taken this opportunity to review other examples, including fluorochloromethanes, of 1,1-HCl reactions with systematic examination of exitchannel properties using higher level theory than was available in our previous studies. The 1,1-HCl elimination reactions of CH3CHFCl and CH3CHCl2 were investigated by computations as representative models. The Gaussian-16 suite of codes17 was employed to characterize the stationary points of the potential surface and enthalpies of reaction. The M06-2X/aug-ccpVTZ method was used to define the geometry and energy of the molecules and the transition states. Enthalpies of reaction, dissociation energies of adducts and the isomerization energies of carbenes were evaluated with CBS-QB3 and G3-B3 methods. For some reactions, energies of certain fixed points on the potential surface were improved by CCSD(T) calculations. Dissociation energies of RC(X)C:HX adducts are of importance for understanding the role of potential wells in exit channels of 1,1-HX reactions, or conversely entrance channels for reactions of carbenes with HX or other electron-pair acceptors.11-14 The effect of substituents on the calculated dissociation energy of carbene:HCl adducts also was monitored. Vibrational frequencies and moments of inertia of the molecule and transition states are used to calculate statistical rate constants, which are compared to experimental rate constants to assign experimental threshold energies, E0, to the four unimolecular reactions of C2D5CHFCl. Experiments involve mercury sensitization of C2D5I and CHFCl2 mixtures at room temperature for a range of pressures followed by analysis of products by gas chromatography. The recombination of C2D5 and CHFCl radicals generate C2D5CHFCl* with 94 kcal mol-1 of vibrational energy as shown in reaction (1). C2D5 + CHFCl

→ C2D5CHFCl* = 94 kcal mol-1 → C2D5H + :CFCl → C2D5Cl + :CFH

(1)

The self-combination and disproportionation reactions of CHFCl and C2D5 radicals were observed, but they are not of interest. The majority of :CHF and :CFCl reacts ACS Paragon Plus Environment

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with CF3CH=CH2, which is added as a scavenger for Cl-atoms. The competitive unimolecular reactions and collisional deactivation by collisions with bath gas molecules, M, are listed in (2). The ratio of the decomposition products and collisionally stabilized molecule gives the rate constant, kexp. C2D5CHFCl* → → → → +M →

DCl + CD3CD=CHF (E and Z) 2,1-DCl HCl + CD3CD2(F)C: 1,1-HCl DF + CD3CD=CHCl (E and Z) 2,1-DF HF + CD3CD2(Cl)C: 1,1-HF CD3CD2CHFCl + M (deactivation)

a. b. c. (2) d. e.

The carbenes from (2b) and (2d) retain sufficient vibrational energy such that Datom migration will occur at the pressures of our experiments and the 1fluoropropene-d5 and 1-chloropropene-d5 are used to identify 1,1-HCl and 1,1-HF elimination, respectively. CD3CD2(F)C:

→ CD3CD=CDF (E and Z)

CD3CD2(Cl)C: →

CD3CD=CDCl (E and Z)

(3) (4)

Reactions 2a, 2c, 3 and 4 tend to favor the Z(cis)-isomers of 1-chloro- and 1fluoropropene because the threshold energy is slightly lower than for the E(trans)isomers. The cis-trans isomerization of chloro- and fluoropropene-d5 following reactions (3) and (4) was not observed and the question of energy disposal from 1,1-HX reactions3,8,10 is not of interest. Yields of the Z- and E-isomers were combined for analysis of the reactions of C2D5CHFCl*. 2. ENERGY PROFILE FOR THE C2D5CHFCl SYSTEM. The average vibrational energy of C2D5CHFCl*, the enthalpies of reaction at 0 K, dissociation energies of C2D5(Cl)C:HF and C2D5(F)C:HCl, and threshold energies of transition states for the C2D5CHFCl system are shown in Figure 1. The diagram for C2D5CHFCl is similar to energy profiles of other RCHFCl systems, and it can serve as a basis for other 1,1-HX elimination systems to be discussed later. The enthalpies of reaction and formation are based on standard thermochemical sources18-22 and electronic-structure calculations. The basis for the numerical values given in Figure 1 are described below. If conformers exist, the values in Figure 1 are for the cis-conformer.


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The average vibrational energy, , of C2D5CHFCl* was obtained from enthalpies of formation of C2H5, CHFCl, and C2H5CHFCl; the latter was obtained from an isodesmic calculation for reaction (5). C2H5CHF2 + C2H5Cl → C2H5CHFCl + C2H5F

(5)

Three computational methods, CBS-QB3, M06-2X/aug-cc-pVTZ and B3PW91/aug-cc-pVTZ gave 6.9, 7.4 and 8.2 kcal mol-1, respectively, for the enthalpy of reaction (5). We adopted 7.4 kcal mol-1, which when combined with the known enthalpies of formation of C2H5Cl (-26.8)18, C2H5F (-65.2)19 and C2H5CHF2 (-124.4)20 gave an enthalpy of formation at 298 K for C2H5CHFCl of 78.6 kcal mol-1. Combining -78.6 with the enthalpies of formation of C2H5 (28.7)18 and CHFCl (-14.5)21 gives -92.8 kcal mol-1 for the enthalpy of reaction at 298 K. Converting -92.8 to 0 K and adding the thermal energy that contributes to vibrational excitation gives = 94 ± 2 kcal mol-1 for C2H5CHFCl, which is the same as for C2D5CHFCl. The uncertainty of ± 2 kcal mol-1 is mainly associated with the enthalpy of formation21 of CHFCl and we continue to use the experimental result, -14.5 kcal mol-1, rather than the computed result of -16.2 kcal mol-1. The latter would lower by 1.7 kcal mol-1. The enthalpy of reaction for cis-conformers of (2a) and (2c) were obtained in two ways. The CBS-QB3 (and G3-B3) calculations gave 16.7 (15.9) and 11.4 (10.9) kcal mol-1, respectively, for C2H5CHFCl. The CBS-QB3 calculation for C2D5CHFCl gave 16.1 and 11.9 kcal mol-1 for (2a) and (2c), and these results are in Figure 1. These enthalpies of reactions also can be obtained from 298 K enthalpies of formation [cis-CH3CH=CHCl (-3.1)22, cis-CH3CH=CHF (-40.8) and C2H5CHFCl (-78.6) together with HCl (-22.0) and HF (-65.1) kcal mol-1], which give 15.8 and 10.5 kcal mol-1. Converting to 0 K gives 14.6 and 9.3 kcal mol-1, which are close to the results from the G3-B3 calculation. The enthalpy of formation of CH3CH=CHF is based on an isodesmic calculation for propene + vinyl fluoride using B3PW91/6-311+G(2d,p)10f6d as well as G3-B3 calculations, which gave 0.93 and 0.57 kcal mol-1, respectively, for average of 0.8 kcal mol-1 as the enthalpy of reaction. Combining this value with the known enthalpies of formation gave -40.8 kcal mol-1 as the enthalpy of formation of cis-CH3CH=CHF. Energies of the trans-conformers of 1-fluoro- and 1-chloropropene are about 0.5 kcal mol-1 higher than the cis-isomers.22 The enthalpies for reactions (2b) and (2d) were obtained for cis-conformers by adding the calculated energy of isomerization for reactions (3) and (4) to the energies from (2a) and (2c). The isomerization energies for (3) and (4) were 53.7 ACS Paragon Plus Environment

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and 59.4 kcal mol-1, respectively, from M06-2X/aug-cc-pVTZ calculations, which give 69.8 and 71.3 kcal mol-1 as enthalpies of reaction. The isomerization energy for the trans-conformers is about 1 kcal mol-1 larger than for the cis-conformer. These isomerization energies were confirmed (for hydrogenated carbenes) with G3-B3 and CBS-QB3 methods. The dissociation enthalpies of the carbene-HX adducts obtained from CBS-QB3 calculations were 6.4 and 9.3 kcal mol-1 for HCl and HF, respectively. These calculated values are similar to the dissociation enthalpies of CD2Cl(Cl)C:HF and CD2Cl(F)C:HCl.8 The calculated threshold energies of transition states for reactions (2), (3), (4) and the D-atom adduct-assisted transfer for C2D5(Cl)C:HF and C2D5(F)C:HCl were obtained from M06-2X/aug-cc-pVTZ calculations. IRC calculations verified each transition state. Further detail about the transition states is presented in the section describing calculations of unimolecular rate constants. Parentheses identify experimentally assigned threshold energies in Figure 1. In viewing Figure 1, it should be remembered that C2D5CHFCl* molecules have 24 kcal mol-1 of energy in excess of the enthalpy of the 1,1-HX elimination reactions. 3.

EXPERIMENTAL METHODS.

Experiments consist of preparing mixtures of C2D5I, CHFCl2, and CF3CH=CH2 with a (2:1:2) ratio plus a droplet of Hg in quartz vessels with volume between 4 and 135 mL followed by irradiation at 253.7 nm with a 15 W mercury germicidal lamp at room temperature. Typical photolysis times were 5 minutes and the germicidal lamp irradiated the entire reaction vessel. The Hg(3P1) atoms abstract Iatoms and Cl-atoms to generate the desired radicals.23 Some free Cl-atoms also are produced and CF3CH=CH2 serves to control the Cl-atom concentration. Following irradiation, the contents of the vessels are transferred to a vacuum system and injected into a gas chromatograph with mass spectrometer as the detector (Shimadzu 2010 GC-MS mass spectrometer and 105m RTX-200 column). These experiments are similar to previous studies from this laboratory and additional details are given in those references.7,8,10,23 The desired measurements are ratios of the concentrations of the 5 products from reaction 1 at various pressures. The experimental rate constants, kexp, are defined by the relation kexp /kM[M] = Di/S, where kM is the collision rate constant, [M] is the bath gas concentration, and Di/S is the ratio of a decomposition product to stabilized C2D5CHFCl. The information used to calculate kM is in a footnote of Table 1. This simple relation for the rate constant is true for efficient collision, i.e., one collision removes enough


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vibrational energy so that C2D5CHFCl* does not decompose further at the given pressure. To obtain the Di/S ratios, calibration of the GC-MS is required to convert raw data to ratios of concentrations of products from each experiment. 4.

EXPERIMENTAL RESULTS.

The measurements consist of recording the GC-MS signals from cis- and transCD3CD=CDF, cis- and trans-CD3CD=CDCl, cis- and trans-CD3CD=CHF, cis- and trans-CD3CD=CHCl and C2D5CHFCl following irradiation. The d4 and d5 isotopes are identified by different masses, and cis- and trans-isomers are separated by GC. The 1-fluoropropene and 1-chloropropene isomers were identified by comparison to pure hydrogenated samples (SynQuest). A sample of C2H5CHFCl was not available because no commercial source exists; however, identification was straightforward from the library of mass spectra. Experimental rate constants are obtained from plots of decomposition product (1-fluoropropene and 1chloropropene) divided by the stabilization product (C2D5CHFCl). Experiments from 1 to 10 Torr gave D/S ratios suitable for defining rate constants and the results are shown in Figure 2. It is evident that products from DCl and HCl elimination account for the majority of the products. Experiments usually were done in triplicate to ensure reliability. Plots of branching ratios of various products are shown in Figures 3 and 4; these plots extend to lower pressures than for measurement of rate constants. Before summarizing experimental results, calibration of the GC-MS must be considered. As noted, pure samples of cis- and trans-1-fluoropropene-d0 and 1chloropropene-d0 were available. Calibration mixtures were prepared of these four components plus the starting materials (C2D5I, CHFCl2 and CF3CH=CH2) to obtain calibration factors for product ratios. The branching ratio plots in Figures 3 and 4 do not require calibration, except for comparison of total HF + DF to total HCl + DCl products, since parent-ion masses were measured [64-CD3CD=CHF; 65CD3C=CDF and 80-CD3CD=CHCl; 81-CD3CD=CDCl]; these data are considered first. Primary data are cis- and trans-isomers of CD3CD=CHF and CD3CD=CDF from DCl or HCl elimination plus isomers of CD3CD=CHCl and CD3CD=CDCl from DF or HF elimination. The data consist of two sets of experiments separated in time by 10 months. The first set was done at higher pressures (1/P ≤ 2) and those average cis/trans ratios are 3.0 for 1,1-HCl elimination and 1.6 for 2,1-DCl; the second set, which has points at lower pressure, gives ratios of 2.4 and 1.3. Neither set has a definitive pressure dependence. The average of the combined sets would be cis/trans = 2.9 and 1.5, and the 1,1-HCl reaction followed by D-atom migration favors the cis-isomer more than does the 2,1-DCl reaction. It should be


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remembered that C2D5CHFCl* is ≥ 80% decomposed at 1 Torr. The cis/trans ratios for 2,1-DF and 1,1-HF elimination have nearly the same values and data with 1/P ≤ 2 have an average ratio of about 2.6. The apparent dependence of the cis/trans ratios of 1-chloropropene-d5 in Figure 3A on pressure is probably associated with side-reactions, such as addition of Cl-atoms to 1-chloropropene followed by dissociation of the radical. The low yield of 1-chloropropene-d4 and d5 contribute to the scatter of the points. The next step is to combine the cis- and trans-yields and compare ratios of 2,1DCl/1,1-HCl (CD3CD=CHF/CD3CD=CDF) and 2,1-DF/1,1-HF (CD3CD=CHCl/CD3CD=CDCl); see Figure 4. The DF/HF ratio is constant at about 3.4. However, the DCl/HCl ratio seems to be pressure dependent declining from 5 ± 1 to 3.8. The 1-fluoropropenes are major products and the ratio should be reliable. The probable explanation for the pressure dependence is the high threshold energy, ≈ 10 kcal mol-1, for isomerization of CD3CD2(F)C:. At higher pressure, CD3CD2(F)C: collides with the bath gas rather than isomerize and hence the ratio increases. The threshold energy for isomerization of CD3CD2(Cl)C: is 6 kcal mol-1 and CD3CD2(Cl)C: isomerizes more readily than CD3CD2(F)C:. Branching ratios of 2-5 are typical for 2,1-DX/1,1-HX from molecules with ≈ 95 kcal mol-1 of vibrational energy. The ratios of (HCl + DCl)/(HF + DF) products also are shown in Figure 4. The average ratio is quite large, ≈ 70, and 98% of the reaction is either DCl or HCl elimination. This ratio does depend on the calibration for 1-fluoropropene and 1chloropropene, but the relative response was measured several times and the calibration should be reliable. The reduction (0.15 to 0.02) in importance of the branching fraction (HF + DF) versus (HCl + DCl) relative to the CD3CHFCl system, see Table 2, is pronounced. We confirmed the previous calibrations for CH2=CHF and CH2=CHCl, and the difference between CD3CHFCl and C2D5CHFCl is not experimental error. The most difficult experimental measurement was the calibration factor for C2D5CHFCl, because a pure sample could not be obtained. We finally purchased CH3CFClCH3 (SynQuest) as a surrogate molecule and assumed that the ionization cross sections of the isomers would be the same. Based on total ion currents for CH3CFClCH3 and m/z for the parent ion of the halopropene, the calibration factors for 1-fluoropropene/2,2-fluorochloropropane and 1-chloropropene/2,2fluorochloropropane were 5.35 ± 0.10 and 2.47 ± 0.15. These factors were used to obtain the plots shown in Figure 2. The slopes of the plots in units of Torr are 2.5 ± 0.1, 0.53 ± 0.04, 0.039 ± 0.003, and 0.011 ± 0.001. Multiplying these numbers by ACS Paragon Plus Environment

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the collision rate constant gives the rate constants in Table 2. The sum of the DCl + HCl and DF + HF rate constants can be compared to those of C2D5CHCl2 and C2D5CHF2, which are provided in Table 1. The rate constants from C2D5CHFCl are 1.4 times larger for DCl + HCl relative to C2D5CHCl210 and 2.1 times smaller for DF + HF reactions relative to C2D5CHF24. The difference is mainly a consequence of the difference in , 94 versus 88 kcal mol-1 and 94 vs. 96 kcal mol-1 for the three molecules, and the twofold larger reaction path degeneracy for dichloro- and difluoropropanes. 5. Calculation of RRKM Rate Constants and Assignment of E0 Values. Assignment of E0 values from comparison of calculated rate constants, k, to experimental rate constants, kexp, begins with the assumption that the appropriate dividing surface between reactant and product is located at the transition state. This assumption may not be valid for 1,1-HCl elimination, and a more realistic model will be examined in the Discussion section. The stationary points corresponding to the transition states were located by the M06-2X/aug-cc-pVTZ method, and the geometries of the 1,1-HX transition states together with that of the adducts are shown in Figure 5. Diagrams of the 2,1-DX transition states are shown in the Supporting Information. Although transition-state geometries and vibrational frequencies for 2,1-DX and 1,1-HX reactions are similar from various computational methods, M06-2X seems to provide better threshold energies than MP2 or other DFT methods. The moments of inertia and vibrational frequencies of transition states, adducts and the molecule are in Supporting Information. The lower frequencies of 1,1-HX transition states relative to 2,1-DX transition states has been discussed previously.4-8,10 The statistical rate constant at energy E was obtained in the usual way from equation 6. kE = (s‡/h)(I‡/I)½ (ΣP‡(E-E0)/N*(E))

(6)

The sum of vibrational states for the transition state, ΣP‡(E-E0), and the density of states for the molecule, N*(E), were obtained with the MultiWell code of Barker24 for harmonic frequencies. The reaction path degeneracy is s‡ and I‡/I is the ratio of the three overall moments of rotation. The torsion associated with the methyl group is nearly the same in all structures, and it was treated as a vibration for the molecule and transition states. However, the C2D5 torsion of C2D5CHFCl becomes a ring mode in the 2,1-DX transition state, and N*(E) was evaluated by treating this torsion as a hindered, asymmetric internal rotation.25 The three conformers of C2D5CHFCl differ in energy by less than 0.2 kcal mol-1 with barriers to internal rotation of 4.1., 4.9 and 5.1 kcal mol-1; the average reduced moment is 24.0 amu ACS Paragon Plus Environment

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Å2. The C2D5 torsion in the 1,1-HX transition states was treated as a free internal rotation with reduced moments of 25.4 and 27.0 amu Å2 for 1,1-HF and 1,1-HCl, respectively. Since anharmonicity was not included in the calculation of sums and density of states, calculated kE may be upper limits, because E-E0 is much smaller than E, providing there are no compensating factors. The assigned threshold energies for C2D5CHFCl are listed in Table 1 and comparison is provided to reactions of related molecules. Absolute values of E0 have more uncertainty than relative values, and it can be noted that E0(1,1-HX) is usually about 10 kcal mol-1 larger than E0(2,1-HX). A change in kexp by a factor of 1.5 is necessary to alter the assigned E0 by 1 kcal mol-1. The combined uncertainty in kE and experimental rate constants implies an uncertainty in E0 assigned to individual reactions of ± 2 kcal mol-1. This estimate would not include systematic changes, such as a revision in . 6. DISCUSSION 6-A. Summary for reactions of C2D5CHFCl. The reactions of the C2D5CHFCl system resembles those of CD3CHFCl7 and CD2ClCHFCl.8 For all three cases 1,1-HCl and 1,1-HF processes compete with 2,1-DCl and 2,1-DF at high energies or high temperatures. The main difference between C2D5CHFCl and the other two molecules is the much smaller branching fraction for 2,1-DF (and 1,1-HF) reactions. It seems that exchanging a H-atom by a methyl group lowers E0(2,1-DCl) more than E0(2,1-DF) relative to CD3CHFCl. Of course, substitution of a Cl-atom for a H-atom in CD3CHFCl was expected25 to lower both E0(2,1-HX) and increase the branching for 1,1-HX processes for CD2ClCHFCl. The effect upon E0(2,1-HCl) upon substituting a F-atom for a Hatom on the CCl atom is not fully understood.7,15,16,25 The current study can be compared to C2D5CH2Cl26, which has a rate constant of 2.2 x 108 s-1 for = 91 kcal mol-1 and E0 = 54 kcal mol-1. These results suggest that the F-atom slightly increased the threshold energy for C2D5CHFCl in contradistinction to CD3CHFCl, which favored a larger increase in E0 relative to CH3CH2Cl. Based on the experimental evidence, including refs. 15 and 16, substitution of a F-atom for a Hatom on the CCl atom of the transition state seems to increase E0 slightly. Although the data seem reliable, the 2,1-DCl threshold energy for CD3CHFCl seems anomalous; see Supporting Information for more detail. Substituting a Cl-atom for a H-atom on CF has about the same effect as a F-atom on E0(2,1-HF), i.e., the threshold energies for 2,1-DF are similar for C2D5CHF2and C2D5CHFCl. The M06-2X calculated threshold energies for C2D5CHFCl are 54.7, 63.0, 64.7 and 73.8 kcal mol-1 for 2,1-DCl, 1,1-HCl, 2,1-DF and 1,1-HF, respectively, and the


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values are similar for CD3CHFCl. The M06-2X method gives better energy thresholds than other DFT methods7,15,16,25 for 1,1-dihaloethane reactions; but, higher level calculations can provide modest improvement. The 2,1-DX and 1,1-HF rate constants were successfully treated by conventional transition-state theory. Although 1,1-HF elimination reactions have ≈ 10 kcal mol-1 wells in the exit channel, E0(1,1-HF) are above the dissociation limit and crossing the 1,1-HF transition-state barrier is the rate limiting step. This is not true for 1,1HCl elimination, and a detailed examination of other 1,1-HCl reactions with discussion of an improve model is provided in the next section. An expanded view of exit-channel energetics from M06-2X calculations for 1,1-HCl and 1,1-HF elimination from C2D5CHFCl is given in Figure 6. The difference in energy of cisand trans-conformers is small. The calculated E0(1,1-HF) is about 2 kcal mol-1 above the carbene + HF thermochemical limit, but E0(1,1-HCl) is about 7 kcal mol1 below the carbene + HCl limit. Calculated energies of the 1,1-HCl transition state and adduct are very similar, but the geometries differ significantly as shown in Figure 5. The distance between the carbene C-atom and the H-atom is 0.28 Å longer in the adduct, and the Cl-atom is above the F-C-H plane in the transition state. The zero-point energy is larger for an adduct than its transition state, and the energy of C2D5(F)C:HCl is 0.5 kcal mol-1 below the transition state, if zero-point energy is removed from the M06-2X calculation. We also used the M06-2X geometry for a single-point calculation with the CCSD(T) method; the difference in the transition state and adduct electronic energy increased slightly to 1.6 kcal mol-1 The adduct-assisted transition state for D-atom migration of C2D5(F)C:HCl is 2.5 kcal mol-1 above the dissociation limit and should not play a role in formation of CD3CD=CDF. However, for C2D5(Cl)C:HF the adduct-assisted transition state is 3.9 kcal mol-1 below the dissociation limit, and some CD3CD=CDCl may have been formed directly rather than via isomerization of CD3CD2(Cl)C:. Such adductassisted processes may be of importance for bimolecular reactions of halocarbenes with certain reagents14 that can serve as H-bond donors. The D0 of 6.4 and 9.3 kcal mol-1 for the HCl and HF adducts, respectively, are much larger than the classical hydrogen-bond dissociation energies27 of 3.0 kcal mol-1 for (H2O)2 or (HF)2; however, the dissociation energy of the H2O:HF adduct is 6.3 kcal mol-1. Sosulin et. al.13 showed that a weak H-bonded complex existed from the interaction of a Fatom of :CF2 and HF, as well as the much stronger bond from the interaction with the electron pair of :CF2. We confirmed Sosulin's result with a M06-2X calculation. Since HCl is a weaker H-bond acceptor than HF, such interactions will be even less important in the exit channel for 1,1-HCl elimination. ACS Paragon Plus Environment

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. 6-B. Analysis of 1,1-HCl elimination reactions. Conventional transition-state analysis of 1,1-HCl elimination reactions has several defects. A major one is that threshold energies assigned from matching k to kexp are always larger than the theoretical E0, which is usually less than the enthalpy of reaction. The expanded energy diagram in Figure 6 for C2D5CHFCl demonstrates that the calculated E0(1,1-HCl) is submerged relative to the dissociation limit. The summary of calculations for several 1,1-HCl adducts and transition states in Table 2 demonstrates that this is a systemic characteristic. A second difficulty is understanding the relationship between the geometries and energies (see Figures 57) of transition states and adducts; this is especially troublesome for 1,1dichloroethane, 1,1-dichloropropane and other 1,1-dichloromolecules. Comparison of energies of adducts and transition states required M06-2X calculations with the molecule as the reference energy. Agreement between the energy of the adduct from the M06-2X and CBS-QB3 calculations was usually satisfactory, even though the reference energies were different. Dissociation energies given in Table 2 are from CBS-QB3 calculations with carbene + HCl as the reference, which we take as the best calculation. The dissociation energies range from 4-8 kcal mol-1 except for lower values for chlorofluoromethanes, CF3CHFCl and CF3CHFCHFCl. The uncertainty31 associated with CBS-QB3 results typically is ± 1 kcal mol-1. Our G3B3 calculations for carbene:HCl dissociation energies of Table 2 were within ± 1 kcal mol-1 of those obtained with CBS-QB3. Dissociation energies in Table 2 include the zero-point energy; the De values would be about 2 kcal mol-1 larger. The vibrational frequency associated with the HCl stretch mode in the adduct is included in Table 2 as another measure of the carbene:HCl interaction. As expected from other studies32 of H-bonded adducts, a linear correlation exists between the HCl frequency and the dissociation energy. A general trend is an increase in dissociation energy upon replacement of the H-atom of HClC: with a methyl or ethyl group. Substitution of F-atoms for H-atoms in the methyl group reduces the dissociation energy. The calculated threshold energy of 1,1-HCl elimination approaches the enthalpy of reaction as the dissociation energy of the adduct become smaller. Conversely, as adducts becomes more strongly bound, greater effort is required to locate adducts and transition states. This difficulty becomes especially noticeable for HCl adducts with chlorocarbenes. The CBSQB3 method, which uses B3LYP/6-311G(2d,p) for geometry optimization, failed to locate adducts for CH3(Cl)C:HCl and C2H5(Cl)C:HCl, and those D0 were estimated by subtraction of the adduct energy (the M06-2X calculation) from the enthalpy of reaction for CH3(Cl)C:HCl and subtraction of the transition state energy (M06-2X calculation) from the enthalpy of reaction for C2H5(Cl)C:HCl.


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The estimate for C2H5(Cl)C:HCl assumes that the energies of the adduct and transition state are similar. See footnote (g) of Table 2 for more detail. A significant difference exists between geometries of 1,1-HCl transition states and HCl-adducts for RCHFCl and RCHCl2 systems, as displayed in Figure 7 for CH3CHCl2 and CH3CHFCl. The C-H and H-Cl distances in the transition state and adduct for CH3CHCl2 are more similar to one another than for those of CH3CHFCl. The C-H-Cl plane also has a different inclination for the two transition states. We conclude that replacing the F-atom by a Cl-atom causes a qualitative difference in the 1,1-HCl transition state and HCl-adduct. With substitution of F-atoms in the methyl group, CBS-QB3 calculations for CHn-xFx(Cl)C:HCl showed that dissociation energies declined and E0 increased; but the E0 were always less than the enthalpy of reaction (see Table 2). The dependence on computational method was investigated for the four reactions of CH3CHFCl as a model system without complications from conformers or isotope effects. Since geometries of adducts and transition states differ slightly, see footnote of Table 3, for MP2 and M06-2X methods, MP2 calculations were included in the comparison. Results from higher-level calculations at fixed-points also are in Table 3. Differences in energy for the transition state and the CH3(F)C:HCl were close to the same value in both the CCSD(T) and MP4 calculations. Therefore, the submerged nature, by about 6 kcal mol-1, of the transition state for 1,1-HCl elimination from CH3CHFCl seems to be confirmed. The higher-level calculations place the 1,1-HCl transition state 1-2 kcal mol-1 above the adduct. The adjustments given by fixed-point calculations from CCSD(T) and MP4 to energies of MP2 calculations for processes involving Fatoms are larger than those with Cl-atoms. In particular, MP2 with and without the MP4 adjustment and M06-2X with MP4 adjustment place CH3(Cl)C:HF above the energy of CH3(F)C:HCl, which is unreasonable given the CBS-QB3 calculated dissociation energies of 9.9 and 5.8 kcal mol-1 and similar enthalpies of reaction . In general, dissociation energies for carbene:HF adducts are about 2 times larger than carbene:HCl adducts for R-CHFCl systems. Experimental measurements are needed to identify the best computational methods for carbene-HX complexes. Since our focus is carbene-HCl adducts, no further attention was given to the question implied by the MP2 treatment of CH3(Cl)C:HF in Table 3. The adductassisted transition states for H-atom isomerization were 15 and 11 kcal mol-1 above the energies of CH3(F)C:HCl and CH3(Cl)C:HF, respectively, and dissociation would be favored over adduct-assisted isomerization.


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With benefit of the survey presented in Tables 2 and 3, exit channels for 1,1-HCl elimination reactions must have inner and outer transition states corresponding to minimum energy pathways for formation and dissociation of the adducts. Details associated with a given potential energy surface may depend on computational method, see Figure 6 as an example for C2D5CHFCl, but the main features of the exit channel for 1,1-HCl elimination can be accepted. For cases with inner and outer transition states, the observed rate constant is given by eq. 7;33-35 kHX , kDIS and kR are elementary RRKM rate constants; kHX refers to HCl elimination and kDIS and kR refer to dissociation and reverse reaction of the adduct. kobs = kHX [kDIS/(kDIS + kR)]

(7)

If kDIS is larger than kR, kobs, the observed rate constant, is just kHX, which is the case for the majority of 1,1-HF elimination reactions, because E0(1,1-HF) is larger than the enthalpy of reaction. However, for 1,1-HCl reactions kDIS may be comparable to kR, because E0(1,1-HCl) is less than the enthalpy, and it is necessary to have an estimate for kDIS. The density of states of the adduct is the same for both kDIS and kR, so the term in brackets is just the ratio of sums of states for the two transition states. The transition state for kR is the same as for kHCl. Since - E0(DIS) is smaller than - E0(HCl), the sum of states for ΣP‡( - E0(HCl)) can be comparable to ΣP‡( - E0(DIS)) in spite of the low frequencies associated with the transition state for dissociation. In addition to the possible competition between dissociation and reverse reaction implied by eq. 7, the sensitivity of kE to E (and to E0) should be remembered, since k would be reduced by a factor of 2 for a reduction of by 3 kcal mol-1. Given the uncertainty in and the calculated threshold energies, discussion will be based on CH3(F)C:HCl as a model. Dissociation belongs to the class of barrier-less unimolecular reactions, and variation transition-state theory is needed for analysis.34,35 For our model system, 5 frequencies of CH3(F)C:HCl will be converted to rotational and translational motions of products upon dissociation. These transitional frequencies are 4 bending modes of HCl relative to CH3(F)C: plus the C:--(HCl) stretch mode, which becomes the reaction coordinate. Variation transition-state theory requires evaluation of these 4 frequencies as the HCl distance increases. These transitional frequencies are very low and dominate the sums of states. The remaining frequencies are just those of the carbene and HCl fragments. Two of the four frequencies are already very low in the adduct (40-80 cm-1), and they depend somewhat on the computational method. Finding the best frequencies for a variation-theory treatment of the transition state for dissociation would require expertise in dealing with barrier-less potential surfaces. Our goal is more modest.


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We just attempt to show that using eq. 7 is necessary for 1,1-HCl elimination from CH3CHFCl as a model. We first employed CASSCF methods to find frequencies as HCl and CH3(F)C: were separated. The four frequencies in question declined and the C:--(HCl) stretching frequency became imaginary. However, at 3.5 Å separation of the carbon and hydrogen atoms another frequency became imaginary, and the HCl stretch frequency did not change in the expected way. Therefore, the calculation was repeated with M06-2X; those results were more realistic and frequencies associated with CH3(F)C: and HCl for 3.5 Å separation of the carbon and hydrogen atoms were close to those of free products from the CBS-QB3 calculation. The transitional frequencies were 31, 73, 207 and 351 cm-1, the CH3 torsional frequency (128 cm-1) was replaced with a free rotor. The models of the transition states for dissociation and HCl elimination are in the Supporting Information. At a common excess energy of 25 kcal mol-1, the sums of states for dissociation exceeds that for elimination by a factor of ≈ 12. For a total energy of 90 kcal mol-1 and for E0 = 65 and 70 kcal mol-1 for elimination and dissociation, respectively, the ratio is reduced to ≈ 3 and the factor in brackets of eq. 7 is ≈ 0.8. If the difference in threshold energies is increased to 6 kcal mol-1 the factor becomes 0.7. This model calculation strongly suggests that eq. 7 should be included in analysis of 1,1-HCl elimination reactions from RCHFCl molecules. According to M06-2X calculations for C2D5CHFCl, the excess energies for 1,1HCl and dissociation transition states are 31 and 24 kcal mol-1, respectively, and eq. 7 with a 7 kcal mol-1 energy difference is in qualitative accord with the experimental rate constant considering the uncertainty in kDIS, the calculated energies and the experimental rate constant. A similar conclusion holds for 1,1HCl elimination from CD2ClCHFCl.8 Application of the CH3CHFCl model using eq. 7 with an energy difference of 6-8 kcal mol-1 to CD3CHFCl7 overestimates the 1,1-HCl experimental rate constant, which may be anomalously small. A similar model calculation for CH3CHCl2 is more difficult because low frequencies of both transition states are less reliable (and small changes affect the sums of states) and depend on computational method. Also, calculated energies of adducts and 1,1-HCl transition states are less certain. The M06-2X calculation at a separation of 3.0 - 3.5 Å provided 3 of the desired 4 transitional frequencies, which were 41, 177 and 183 cm-1, plus frequencies corresponding to the HCl and carbene moieties. However, the fourth frequency was imaginary (in addition to the imaginary frequency corresponding to the reaction coordinate). Therefore, the fourth frequency was assigned as 30 cm-1 from comparison to CH3(Cl)C:HCl and the transition state for CH3(F)C:HCl dissociation. As already discussed, the geometries and energies of the adduct and 1,1-HCl transition states are similar and E0(1,1-HCl) = 64 kcal mol-1 may be uncertain. The available energy from ACS Paragon Plus Environment

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recombination of CH3 and CHCl2 is 89 kcal mol-1 and the calculated excess energy, i. e. the energy above the transition states, are 25 and 17 kcal mol-1. For these parameters, the sums of states for 1,1-HCl was 10% larger than for dissociation, i.e., dissociation could be the rate limiting step. For this situation kobs = k'DIS [kR/ kDIS + kR]; the ratio of the sums of states is [3.8 x 108/((3.4 + 3.8) x 108)] and kobs = 0.57 k'DIS, where k'DIS is the RRKM rate constant and includes the density of states of the molecule. This approximate model and the experimental results10 suggests that dissociation may be the rate-limiting step for the 1,1-HCl reaction of C2D5CHCl2. In fact, precise specification of geometries and energies of the adduct and HCl transition state requires further work for C2D5CHCl2 because results depend on the computational method. The only other experimental example is CD2ClCHCl2. The adduct was identified, see Table 2, by the CBS-QB3 calculation; D0 is smaller than for C2D5CHCl2, and the competition implied by eq. 7 probably applies to 1,1-HCl elimination. Chloroform is an interesting case because the reaction has been studied in the forward and reverse directions. The calculated D0 is 4.2 kcal mol-1 and inner and outer transition states should be included in evaluating the kinetic data. The rather small rate constant36 for the :CCl2 + HCl reaction implies that such a treatment is needed. 7.

CONCLUSIONS.

The unimolecular reactions of C2D5CHFCl* were characterized by experimental and computational methods. Molecules were activated by recombination of C2D5 and CHFCl radicals in the gas phase. This study is the last of a series that included CD3CHFCl* and CD2ClCHFCl* For each member ΔHf0(CHFCl) = -14.5 kcal mol1 was used to obtain . After the present study was completed, a new high accuracy compilation37 was published that strongly supports -16.2 rather than -14.5 kcal mol-1 for ΔHf0(CHFCl). The -16.2 kcal mol-1 value would lower the average energy of all three molecules by 1.7 kcal mol-1, which would result is a reduction of the assigned threshold energies by about 1 kcal mol-1. For consistency with the published work, we have used -14.5 kcal mol-1, which corresponds to = 94 ± 2 kcal mol-1. Elimination of DCl and HCl are the most important unimolecular reactions of C2D5CHFCl*. Since 1,1-HX(Y) elimination reactions compete with 2,1-DX(Y) reactions of C2D5CHXY type molecules, the major emphasis of this study was to characterize the transition states and hydrogen-bonded adducts associated with 1,1-elimination reactions. The C2D5(F)C:HCl and C2D5(Cl)C:HF adducts have calculated


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dissociation energies of 6.4 and 9.3 kcal mol-1, respectively. The threshold energy for 1,1-HF elimination is above the enthalpy of reaction and the rate constant can be treated in the usual way; however, the transition state for 1,1-HCl elimination is submerged relative to the enthalpy of reaction. Calculations were done for CH3CHFCl as a model to illustrate the need to consider inner and outer transition states for 1,1-HCl elimination reactions of RCHFCl molecules. For RCHCl2 molecules, the apparent 1,1-HCl elimination may correspond to dissociation of the adduct as the rate limiting step. According to calculations, adduct-assisted transition states for D-atom migration exist and such reaction pathways could be of importance for adducts of carbenes with reagents that are good hydrogen-bond donors. CBS-QB3 and M06-2X/aug-cc-pVTZ methods mainly were used for computations. A computational survey of seventeen 1,1-HCl elimination reactions from 1,1-chlorofluoro- and 1,1-dichloro-alkanes is presented to illustrate hydrogen-bonded adducts between fluoro- and chloro-carbenes with HCl. 8. AUTHOR INFORMATION Corresponding Author.* BEH E-mail: [email protected] Telephone: 828-2325168. 9. ACKNOWLEDGEMENTS Financial support from the National Science Foundation (CHE-1111546 and CHE1229406) is gratefully acknowledged. BRG thanks the Goldwater Foundation for a research scholarship. BRG thanks the North Carolina Space Grant program (NASA Grant NNX15AH81H) and MMR thanks the Council on Undergraduate Research (the CUR Fellows Award) for funding summer research stipends. SUPPORTING INFORMATION. Supporting Information contains the vibrational frequencies and moments of inertia for the calculated molecular and transition state geometries using M06-2X/aug-cc-pVTZ for the molecule, transition states and adducts with HX for C2D5CHFCl and of CH3CHFCl. The calculated geometries for the 2,1-DX (X = Cl, F) elimination transition states are also shown for C2D5CHFCl.


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6. Smith, C. A.; Gillespie, B. R.; Heard, G. L.; Setser, D. W.; Holmes, B. E. The Unimolecular Reactions of CF3CHF2 Studied by Chemical Activation: Assignment of Rate Constants and Threshold Energies to 1,2-H Atom Transfer, 1,1-HF and 1,2-HF Elimination Reactions. J. Phys. Chem. A 2017, 121, 87468756. 7. 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 and 2,1-DCl plus 1,1-HF and 2,1-DF Elimination Reactions. J. Phys. Chem. A 2015, 119, 9441-9451. 8. Brown, T. M.; Smith, C. A.; Gillespie, B. R.; Heard, G. L.; Setser, D. W.; Holmes, B. E. Analysis of Five Unimolecular Pathways of CD2ClCHFCl with Emphasis on CD2Cl(F)C: and CD2Cl(Cl)C: Formation by 1,1-HCl and 1,1-HF Elimination. J. Phys. Chem. A 2018, 122, 8446-8457. 9. Kim, K. C.; Setser, D. W. Unimolecular Reactions and Energy Partitioning. Three- and Four-Centered Elimination Reactions of Chemically Activated 1,1,2-Trichloroethane-d0, d1, and d2. J. Phys. Chem. 1974, 78, 2166-2179. 10. Larkin, A. C.; Nestler, M. J.; Smith, C. A.; Heard, G. L.; Setser, D. W.; Holmes, B. E. Chemical Activation Study of the Unimolecular Reactions of


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19. Haworth, N. L.; Smith; M. H.; Bacskay, G. B.; Mackie; J. C. Heats of Formation of Hydrofluorocarbons Obtained by Gaussian-3 and Related Quantum Chemical Computations. J. Phys. Chem. A 2000, 104, 7600-7611. 20. Khursan, S. L. The Standard Enthalpies of Formation of Fluorinated Alkanes: Nonempirical Quantum-Chemical Computations. Russ. J. Phys. Chem. 2004, 78, Suppl. 1, S34-S42 (English Translation). 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. Colegrave, B. T.; Thompson, T. B. Ab Initio Heats of Formation for Chlorinated Hydrocarbons: Allyl Chloride, Cis-and Trans-1-Chloropropene and Vinyl Chloride. J. Chem. Phys. 1997, 106, 1480-1490. 23. 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-Trifluor-1,1,1-trichloropropane: Determination of Threshold energies by Chemical Activation. J. Phys. Chem. A 2014, 118, 9347-9356. 24. Barker, J. R. Multiple Well, Multiple-Path, Unimolecular Reaction Systems, Multi-Well Suite. Int. J. Chem. Kinet. 2001, 33, 232-245. 25. Duncan, J. R.; Solaka, S. A.; Setser, D. W.; Holmes, B. E. Unimolecular HCl and HF Elimination Reactions of 1,2-Dichloroethane, 1,2-Difluoroethane, and 1,2-Chlorofluoroethane: Assignment of Threshold Energies. J. Phys. Chem. A 2010, 114, 794-803. 26. 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 HF and HCl Elimination. J. Phys. Chem. A 2005, 109, 4540-4551. 27. Sexton,T. M.; Howard, J. C.; Tschumper, G. S. Dissociation Energy of H2OHF Dimer. J. Phys. Chem. A 2018, 122, 4902-4908. 28. Sendt, K.; Bacskay, B. Spectroscopic Constants of the X(1A1), a(3B1) and A(1B1) States of CF2, CCl2, and CBr2 and Heats of Formation of Selected Halocarbenes: Ab Initio Quantum Chemical Study. J. Chem. Phys. 2000, 112, 2227-2238. 29. Michael, J. V.; Kumaran, S. S. Thermal Decomposition Studies of Halogenated Organic Compounds. Combust. Sci. and Tech. 1998, 134, 31-44. 30. Zhu, L.; Bozzelli, J. W. Kinetics and Mechanisms for the Thermal Chlorination of Chloroform in the Gas phase: Inclusion of HCl Elimination from CHCl3. Int. J. Chem. Kinet. 2003, 35, 647-660.


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31. Montgomery, J. P.; Frisch, M. J.; Ocherski, J. W.; Petersson, G. A. A Complete Basis Set Model Chemistry. VI. Use of Density Functional Geometries and Frequencies. J. Chem. Phys. 1999, 110, 2822-2827. 32. Biswal, H. S.; Bhattacharyya, S.; Bhattacherjee, A.; Wategaonkar, S. Nature and Strength of Sulfur-centered Hydrogen Bonds: Laser Spectroscopic Investigations in the Gas Phase and Quantum-chemical Calculations. Int. Rev. Phys. Chem. 2015, 34, 99-160. 33. Miller, W. H. Unified Statistical Model for "Complex" and "Direct" Reaction Mechanisms. J. Chem. Phys. 1976, 65, 2216-2223. 34. Baer, T; Hase, W. L. Unimolecular Reaction Dynamics: Theory and Experiments; p. 275-277, Oxford University Press: Oxford, U. K. 1996. 35. Kaur, R.; Vikas, Conflict in the Mechanism and Kinetics of the Barrierless Reaction between SH and NO2 Radicals. J. Phys. Chem. A 2018, 122, 19261937. 36. Gomez, N. D.; D’Accurso, V.; Manzano, F. A.; Codina, J.; Azcarate, M. J. Determination of the Rate Constant of the Reaction of CCl2 with HCl. Int. J. Chem. Kinet. 2014, 46, 382-388. 37.Ganyecz, A.; Kallay, M.; Csontos, J. High Accuracy Quantum Chemical and Thermochemical Network Data for the Heats of Formation of Fluorinated and Chlorinated Methanes and Ethanes. J. Phys. Chem. A 2018, 122, 5993-6006.


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FIGURE CAPTIONS. Figure 1. Energy profile in kcal mol-1 for reactions in the CD3CD2CHFCl system. Calculated values are for the cis-conformers and all calculated values include zero-point energy. The experimentally derived threshold energies for the C2D5CHFCl reactions are in parentheses; see text and Figure 6 for information about assignment of the 1,1-HCl threshold energy. The transition state energies for D-atom migration are shown on the left side of the diagram; the threshold energy for D-atom migration by C2D5(Cl)C:HF is 4 kcal mol-1 below the energy of the free carbene + HF. Figure 2. Plots of the decomposition to stabilization ratios versus inverse pressure and [R = correlation coefficient, m = slope and b = intercept]: (〇) CD3CD=CHF/ C2D5CHFCl [R = 0.964, m = 2.51 and b = 0.07]; () CD3CD=CDF/C2D5CHFCl [R = 0.910, m = 0.534 and b = 0.023]; ()CD3CD=CHCl/C2D5CHFCl [R = 0.929, m = 0.039 and b = -0.005] and ()CD3CD=CDCl/C2D5CHFCl [R = 0.868, m = 0.0105 and b = -0.0007]. The cisand trans-isomers of 1-fluoropropene-d4 and -d5 and 1-chloropropene-d4 and -d5 were combined for these plots. Figure 3. (A) Plots of the ratio of cis-/trans-isomers versus 1/pressure for ()--1,1-HCl elimination and for ()-- 1,1-HF elimination. (B) Plots of the ratio of cis-/trans-isomers versus 1/pressure for ()--1,2-DCl elimination and for ()-- 1,2-DF elimination. Figure 4. Plots of ()-- 2,1-DCl (CD3CD=CHF)/1,1-HCl (CD3CD=CDF), () - 2,1-DF (CD3CD=CHCl)/1,1-HF(CD3CD=CDCl) and ()--total hydrogen chloride/total hydrogen fluoride versus 1/pressure (Torr). Figure 5. Diagram of geometries for the cis-conformers of 1,1-HF and 1,1-HCl transition states, adducts, and adduct-assisted D-migration transition states for C2D5CHFCl. Figure 6. Energy summary from M06-2X calculations for the exit channels of 1,1-HCl and 1,1-HF elimination reactions from C2D5CHFCl. The listed energies include zero-point energy. The energy of the trans-conformers is in parenthesis. PTSC, which stands for post transition-state complex, is a synonym for adduct. The zero-point energy for adducts is larger than that for 1,1-HX transition ACS Paragon Plus Environment

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states, and the electronic energy difference between adducts and transition states is 2.0 (HF) and 0.7 (HCl) kcal mol-1 larger than the numbers shown, which include zero-point energy. The asterisk for C2D5(F)C:HCl indicates that the 1,1-HCl transition state is slightly above the adduct, if zero-point energy is excluded. Figure 7. Comparison of geometries of 1,1-HCl transition states and adducts from M06-2X calculations for CH3CHFCl and CH3CHCl2. The geometry of the transition state for CH3CHCl2 is closer to that of the adduct than CH3CHFCl. D0 for CH3(Cl)C:HCl is 1.5 kcal mol-1 larger than for CH3(F)C:HCl and the imaginary frequency of 99 cm-1 is lower than that for CH3CHFCl, which is 184 cm-1. These low frequencies indicate that the reaction coordinate mainly involves motion of the Cl atom.


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Table 1. Rate constants and threshold energies for CD2CD3CHFCl* and related molecules. Reactions C2D5CHFCl* 2,1-DCl

Experimental resultsa,b Slope of D/S plot kexp, s-1 Torr = 94 kcal/mol 2.5 ± 0.1 3.7 ± 0.5 x 107

Calculated resultsc k , s-1 E0, kcal/mol 5.4 x 107 3.7 x 107 0.92 x 107 0.58 x 107 0.060 x 107 0.046 x 107 0.019 x 107 0.012 x 107

55 56 66 67

1,1-HCl

0.53 ± 0.04

0.78 ± 0.10 x 107

2,1-DF

0.039 ± 0.003

0.059 ± 0.005 x 107

1,1-HF

0.011 ± 0.001

0.017 ± 0.002 x 107

C2D5CHCl2* 2,1-DCl 1,1-HCl

= 88 kcal/mol 1.83 ± 0.09 0.32 ± 0.02

2.7 ± 0.3 x 107 0.47 ± 0.05 x 107

3.3 x 107 0.33 x 107

54 66

C2D5CHF2* 2,1-DF 1,1-HF

= 96 kcal/mol 0.086 ± 0.007 0.021 ± 0.002

1.28 ± 0.24 x106 0.32 ± 0.06 x106

1.2 x 106 0.29 x 106

65 73

CD3CHFCl* 2,1-DCl 1,1-HCl 2,1-DF 1,1-DF

= 96 kcal/mol 62 29 9.4 4.6

8.0 x 108 3.7 x 108 1.2 x 108 0.58 x 108

8.0 x 108 4.4 x 108 1.3 x 108 0.70 x 108

60 71 64 73

63

64 71 72

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 constants4, 7, 10 for CD3CHFCl*, C2D5CHF2 and C2D5CHCl2* are included for comparison to C2D5CHFCl*. The uncertainty in the rate constants for C2D5CHFCl* was increased at least to 10% to account for the uncertainty in kM. b. The collision rate constants of the bath gases with C2D5CHFCl* were calculated from the following collision diameters, Å, and ε/k,oK, values: CF3CH=CH2 (4.90 and 250), C2D5l (5.0 and 394), CHFCl2 (4.6 and 405), C2D5CHFCl (5.3 and 370). The collision rate constants were 4.9 x 10-10, 4.7 x 10-10 and 5.1 x 10-10 cm3 molecule-1 s-1 for C2D5CHFCl* with CHFCl2, CF3CH=CH2 and C2D5I, respectively, as bath gases. The effective collision frequency with C2D5CHFCl* at 1 Torr pressure of the bath gas mixture is 1.5 x 107 s-1. c. Two calculated rate constants are given to illustrate the dependence of kE on E0. All calculated rate constants for C2D5CHFCl* were obtained from models defined by M062X/aug-cc-pVTZ calculations.


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Table 2.

Properties of Exit Channels for Seventeen 1,1-HCl Elimination Reactions.a,b

Molecule

ΔH0 (reaction)c

D0(adduct)c

E0(calc)d

Frequencyd HCl, cm-1

E0(expt)a

CH3CHFCl = 96 C2H5CHFCl = 94 CH2ClCHFCl = 92 CF3CHFCl CF3CHFCHFCl

69

5.8

63.3 (64)e

2062

71

70

6.4

63.0

1863

65

70

4.9

66.5 (67)e

2334

70

76 72

2.8 3.4

75.4 69.2

2750 2655

CH2FCl CHF2Cl CHFCl2

76 49 54

3.8 2.1 3.1

73.3 57.6 (58)f 58.0 (59)f

2414 2805 2667

CH3CHCl2 C2H5CHCl2 = 88 CH2ClCHCl2 = 86 CH2FCHCl2 CHF2CHCl2 CF3CHCl2 CF3CH2CHCl2 CH2Cl2 CHCl3

72 72

≈ 7.2g ≈ 8.0g

63.6 64.1

1758

70

5.9

64.5

2178

69

71 77 77 73 78 58

5.7 4.6 4.0 5.9 4.9 4.2

64.5 73.2 75.3 66.5 73.0 57.0 (58)h

2250 2459 2595 2019 2285 2483

≈ 73 ≈ 56

≈ 56

66

a. All energies in the table are in kcal mol-1 and all include zero-point energy. Entries with specified were studied experimentally with deuterated compounds by the chemicalactivation method; the listed E0 were assigned by assuming that the 1,1-HCl transition state was the rate limiting factor. The E0(expt) values for CHF2Cl, CH2Cl2 and CHCl3 are from thermal experiments29. b. If conformers exist for the molecule or transition-state, the calculated entries are for the cisconformer. c. CBS-QB3 calculations for the dissociation energy of adduct and enthalpies of reaction at 0 K. Calculated enthalpies are in close agreement with values derived from known28 enthalpies of formation of CF2, CFCl, CHF and CHCl. Adduct energies calculated by M06-2X relative to the molecules, are usually within 1-2 kcal mol-1of the energy given by ΔH0 -D0(adduct). d. M06-2X/aug-cc-pVTZ calculations for the threshold energy, E0, and for the normal mode associated with the HCl stretch vibration. For reference, ωe for HCl is 2991 cm-1. e. CCSD(T) with M06-2X /aug–cc-pVTZ calculation of the threshold energy. f. MP2 calculation from ref. 7. g. Since the geometry optimization in CBS-QB3 calculations failed for CH3(Cl)C:HCl or for C2H5(Cl)C:HCl, D0 was obtained by subtracting the M06-2X adduct energy(CH3CHCl2) or E0(1,1-HCl) energy (C2H5CHCl2) from the enthalpy of reaction. We also reinvestigated the MP2/6-311+G(2d,p) results10 for C2D5CHCl2; convergence was obtained for both the transition state and the adduct, but the energies were nearly identical.


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h. G3 calculations by Zhu and Bozzelli30 (they did not report a Cl2C:HCl adduct).

Table 3. Summary of calculated energies (kcal mol-1) for reactions of CH3CHFCl a,b,c Expt.b 59 63 72 74 63d 60d 69d 70d

M06-2X 54.8 63.9 63.4 73.9 63.5 62.0

+CCSD(T)e 56.4 62.5 63.7 70.4 62.5 60.0

+MP4e 56.6 64.1 62.8 73.1 61.3 65.5

Reaction 2,1-HCl 2,1-HF 1,1-HCl 1,1-HF HCl-add HF-add ΔH(HCl) ΔH(HF)

MP2 56.7 65.1 65.1 76.4 64.9 67.7

+CCSD(T)e 56.4 62.9 63.4 71.3 62.3 60.2

+MP4 56.7 64.2 62.7 73.3 61.0 65.3

a. The calculated energies of transition states and adducts are relative to CH3CHFCl as zero; all entries include zero-point energy. The basis sets for the MP2 and MP4 calculations were 6-311+G(2d,p) and for M06-2X and CCSD(T) calculations they were aug-cc-pVTZ. b. The experimental E0 are results from CD3CHFCl with adjustment for the kinetic isotope effect for 2,1-HCl and 2,1-HF reactions. The fitting of the rate constants was based on the 1,1-HX transition states being the rate limiting step. c. The C:--(HCl) and (C:)H--Cl distances in the adduct are 1.807 and 1.348 Å , respectively, for M06-2X calculations and 1.882 and 1.326 Å, respectively, for MP2 calculations. The C:-(HF) and (C:)H--F distances are 1.764 and 0.969 Å, respectively, from M06-2X calculations and 1.813 and 0.946 Å from MP2 calculations. The distances from B3LYP geometries, which is used by CBS-QB3 calculations, are between the M06-2X and MP2 values. d. The enthalpies and experimental adduct energies are for the best thermochemistry and CBS-QB3 calculations. The dissociation energies of the HF and HCl adducts are 9.9 and 5.8 kcal mol-1, respectively, from CBS-QB3 calculations. e. The CCSD(T) and MP4 energies in columns 3 and 4 were computed at the M06-2X geometries, while those in columns 7 and 8 were computed at the MP2 geometries.


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TOC Graphics


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Figure 1


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Figure 3A

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Figure 3B


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Figure 4



Figure 5


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Fig. 6

*Electronic energies for the complex are below the 1,1-HCl Elimination energy.



Figure 7


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Experimental and Computational Studies of Unimolecular 1,1-HX (X

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