Derivation of activation energies from experiments on chemical activation of alkyl fluorides

Cadman, P.; Kirk, A. W.; Trotman‐Dickenson, A. F.

doi:10.1039/f19767200996

Cited by 17 publications

(13 citation statements)

References 4 publications

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“…b As in Table IV; A: PMP2/6-31G(d,p), B: ROMP2/6-311G(2d,2p), C: CCSD(T)/6-311++G(2d,p), D: CCSD(T)/AUG-cc-pVTZ. c Δ H R (expt) = ∑Δ H f = −18.8 ± 7.5 at 298 K; Δ H f 298 (CH 3 F) = −56.0 ± 6.9 (ref ), Δ H f 298 (OH) = 9.3 ± 0.3 (ref ), Δ H f 298 (H 2 O) = −57.7 ± 0.0 (ref ), Δ H f 298 (CH 2 F) = −6.9 (ref ), −7.8 (ref ), −7.6 (ref ).…”

Section: Resultsmentioning

confidence: 99%

Reaction-Path and Dual-Level Dynamics Calculations of the CH₃F + OH Reaction

et al. 1998

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Using two approaches, reaction-path dynamics (which uses information on electronic structure energy and energy derivatives calculated ab initio along the minimum energy path) and dual-level dynamics (in which the reaction-path is calculated at a low level of theory, and stationary point information from a high level of theory is used to interpolate corrections to energy quantities, vibrational frequencies, and moments of inertia), the reaction-path for the CH3F + OH reaction was traced. Qualitatively, both methods yield similar results, showing the usefulness of the more economical dual-level approach. The H2O product may be vibrationally excited due to the nonadiabatic flow of energy between the reaction coordinate and the O−H bound mode. The rate coefficents were calculated for the temperature range 200−1000 K using the variational transition-state theory. The tunneling effect was included in the first approach using the small-curvature tunneling method, and in the second using the microcanonical optimized multidimensional tunneling method. Tunneling plays an important role in this reaction and the calculated rate coefficients show a more pronounced curvature in the Arrhenius plot than the experimental data.

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Section: Resultsmentioning

confidence: 99%

Reaction-Path and Dual-Level Dynamics Calculations of the CH₃F + OH Reaction

et al. 1998

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“…Models were developed for CF 3 CF 2 CH 3 and CH 3 -CF 2 CH 3 . An earlier version 14 for CH 3 CF 2 CH 3 , giving a value that was too high for the preexponential factor, was reevaluated because vibrational frequencies for the molecule are now available. 22 Vibrational frequencies have not been reported for CF 3 CF 2 CH 3 , so these were estimated from CH 3 CF 2 CH 3 , 22,23 CF 3 CF 2 CF 3 , 24 and CF 3 CH 2 CF 3 .…”

Section: Resultsmentioning

confidence: 99%

“…The rate constant for loss of HF from CF 3 CF 2 CH 3 will be measured and matched to rates calculated using RRKM theory to determine the E o (HF). The threshold energy for CHF 2 CH 3 5 will be compared to those for CF 3 CF 2 CH 3 , CH 3 CF 2 CH 3 , CClF 2 CH 3 , and CF 3 CH 3 3,6 to ascertain the effect of replacing the H by the CF 3 , CH 3 , Cl, and F substituents, respectively, at the α-carbon. These trends should determine whether Cl or F substituents remove or release electron density at the α-carbon by comparison to the effect of an electron-withdrawing substituent, CF 3 , and to the effect of a CH 3 substituent, which provides electron density through hyperconjugation.…”

Section: Introductionmentioning

confidence: 99%

Threshold Energy and Unimolecular Rate Constant for Elimination of HF from Chemically Activated CF₃CF₂CH₃: Effect of the CF₃ Substituent on the α-Carbon

McDoniel

Holmes

1997

J. Phys. Chem. A

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Methyl and CF3CF2 radicals were combined to form chemically activated CF3CF2CH3 with 104 kcal/mol of internal energy, and the experimental rate constant for unimolecular 1,2-dehydrofluorination was 4.5 × 105 s-1. Fitting the calculated rate constant for HF elimination from RRKM theory to the experimental value provided a threshold energy, E o, of 68.5 kcal/mol. Comparing this threshold energy to those for CF2HCH3, CH3CF2CH3, CF2ClCH3, and CF3CH3 shows that replacing the α-H of CF2HCH3 with CH3 lowered the E o by 7 kcal/mol and replacing with CF3, Cl, or F raises the E o about 8 kcal/mol. The CF3 substituent, an electron acceptor, increased the E o by an amount nearly equal to that with F and Cl substituents, suggesting that halogen substituents exert a similar inductive effect at the α-carbon that loses electron density as the transition state forms. These proposals will be compared to recent calculations of the carbon's atomic charges in the reactant and transition state.

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“…On the basis of this analysis, we estimate the uncertainty in E 0 's is ±2 kcal/mol. Cadman et al also developed an RRKM model for CF 3 CH 2 CH 3 and compared the calculations to unpublished experimental work for the formation of CF 3 CH 2 CH 3 by radical combination and by insertion of a CH 2 into the C−H bond of CF 3 CH 3 . Their RRKM model is based on a thermal preexponential factor considerably outside the range we adopted, and their thermochemistry gives 〈 E 〉's that differ by 5 kcal/mol from the present results, so their higher E 0 (68 kcal/mol) is not unexpected.…”

Section: Resultsmentioning

confidence: 99%

Threshold Energies and Unimolecular Rate Constants for Elimination of HF from Chemically Activated CF₃CH₂CH₃ and CF₃CH₂CF₃: Effect of CH₃ and CF₃ Substituents at the β-Carbon and Implications about the Transition State

Ferguson

Holmes

1998

J. Phys. Chem. A

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Chemically activated CF 3 CH 2 CF 3 was prepared with 104 kcal/mol of internal energy by the combination of CF 3 CH 2 and CF 3 radicals, and chemically activated CF 3 CH 2 CH 3 was prepared with 101 and 95 kcal/mol by combination of CF 3 and CH 2 CH 3 radicals and by combination of CF 3 CH 2 and CH 3 radicals, respectively. The experimental rate constants for unimolecular 1,2-dehydrofluorination were 1.2 × 10 5 s -1 for CF 3 CH 2 CF 3 and 3.2 × 10 6 s -1 for CF 3 CH 2 CH 3 with 95 kcal/mol and 2.0 × 10 7 s -1 with 101 kcal/mol of energy. Fitting the calculated rate constants for HF elimination from RRKM theory to the experimental values provided threshold energies, E 0 , of 73 kcal/mol for CF 3 CH 2 CF 3 and 62 kcal/mol for CF 3 CH 2 CH 3 . Comparing these threshold energies to those for CF 3 CH 3 and CF 3 CH 2 Cl illustrates that replacing the hydrogen of CF 3 CH 3 with CH 3 lowers the E 0 by 6 kcal/mol and replacing with CF 3 or Cl raises the E 0 by 5 and 8 kcal/mol, respectively. The CF 3 substituent, an electron acceptor, increases the E 0 an amount similar to Cl, suggesting that chlorine substituents also prefer to withdraw electron density from the β-carbon. As the HF transition state forms, it appears that electron density flows from the departing hydrogen to the β-carbon and from the β to the R-carbon, to the R-carbon from its substituents, but the R-carbon releases most of the incoming electron density to the departing fluorine. The present work supports this scenario because electron-donating substituents, such as CH 3 , on either carbon would reduce the E 0 as they aid the flow of negative charge, while electron-withdrawing substituents such as Cl, F, and CF 3 would raise the E 0 for HF elimination because they hinder the flow of electron density.

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Derivation of activation energies from experiments on chemical activation of alkyl fluorides

Cited by 17 publications

References 4 publications

Reaction-Path and Dual-Level Dynamics Calculations of the CH₃F + OH Reaction

Reaction-Path and Dual-Level Dynamics Calculations of the CH₃F + OH Reaction

Threshold Energy and Unimolecular Rate Constant for Elimination of HF from Chemically Activated CF₃CF₂CH₃: Effect of the CF₃ Substituent on the α-Carbon

Threshold Energies and Unimolecular Rate Constants for Elimination of HF from Chemically Activated CF₃CH₂CH₃ and CF₃CH₂CF₃: Effect of CH₃ and CF₃ Substituents at the β-Carbon and Implications about the Transition State

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Derivation of activation energies from experiments on chemical activation of alkyl fluorides

Cited by 17 publications

References 4 publications

Reaction-Path and Dual-Level Dynamics Calculations of the CH3F + OH Reaction

Reaction-Path and Dual-Level Dynamics Calculations of the CH3F + OH Reaction

Threshold Energy and Unimolecular Rate Constant for Elimination of HF from Chemically Activated CF3CF2CH3: Effect of the CF3 Substituent on the α-Carbon

Threshold Energies and Unimolecular Rate Constants for Elimination of HF from Chemically Activated CF3CH2CH3 and CF3CH2CF3: Effect of CH3 and CF3 Substituents at the β-Carbon and Implications about the Transition State

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Reaction-Path and Dual-Level Dynamics Calculations of the CH₃F + OH Reaction

Reaction-Path and Dual-Level Dynamics Calculations of the CH₃F + OH Reaction

Threshold Energy and Unimolecular Rate Constant for Elimination of HF from Chemically Activated CF₃CF₂CH₃: Effect of the CF₃ Substituent on the α-Carbon

Threshold Energies and Unimolecular Rate Constants for Elimination of HF from Chemically Activated CF₃CH₂CH₃ and CF₃CH₂CF₃: Effect of CH₃ and CF₃ Substituents at the β-Carbon and Implications about the Transition State