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Emel’yanova NS, Pokidova TS. Quantum-Mechanical Modeling of the Concerted Ring Cleavage Reactions. RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A 2020. [DOI: 10.1134/s0036024420050052] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
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Brown TM, Gillespie BR, Rothrock MM, Ranieri AJ, Schueneman MK, Heard GL, Setser DW, Holmes BE. Unimolecular HBr and HF Elimination Reactions of Vibrationally Excited C 2H 5CH 2Br and C 2D 5CHFBr: Identification of the 1,1-HBr Elimination Reaction from C 2D 5CHFBr and Search for the C 2D 5(F)C:HBr Adduct. J Phys Chem A 2019; 123:8776-8786. [PMID: 31513404 DOI: 10.1021/acs.jpca.9b07029] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Chemical activation experiments and computational methods have been used to study the unimolecular reactions of C2H5CH2Br and C2D5CHFBr with 90 and 93 kcal mol-1 of vibrational energy, respectively. The four-centered elimination reactions of HBr and DBr are the dominant reactions; however, 2,1-DF, 1,1-HBr, and 1,1-HF reactions are also observed from C2D5CHFBr. The main focus was to search for the role of the C2D5(F)C:HBr adduct in the 1,1-HBr elimination for comparison with carbene adducts in 1,1-HX(Y) elimination from RCHXY (X,Y = Cl and F) molecules. Models of transition states and molecules from electronic structure calculations were used in statistical calculations of the rate constants to assign threshold energies for each reaction based on the experimental rate constants. The threshold energy for 2,1-HBr elimination from 1-bromopropane is 50 kcal mol-1, which is in basic agreement with thermal activation experiments. Comparison of the 2,1-DBr and 2,1-HBr rate constants permits discussion of the kinetic isotope effects and the effect of F atom substitution on the threshold energy for 2,1-HBr elimination. Although CD3CD═CDF from 1,1-HBr elimination of C2D5CHFBr followed by D atom migration is an experimentally observed product, dissociation of the C2D5(F)C:HBr adduct may be the rate-limiting step rather than crossing the barrier associated with the transition state for 1,1-HBr elimination. The calculated dissociation energies of C2H5(X)C:HF adducts are 9.9, 9.3, and 9.0 kcal mol-1 for X = F, Cl, and Br, and the values for C2H5(F)C:HX are 9.9, 6.4, and ∼4.9 kcal mol-1.
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Affiliation(s)
- Timothy M Brown
- Department of Chemistry , University of North Carolina-Asheville , One University Heights, Asheville , North Carolina 28804-8511 , United States
| | - Blanton R Gillespie
- Department of Chemistry , University of North Carolina-Asheville , One University Heights, Asheville , North Carolina 28804-8511 , United States
| | - Mallory M Rothrock
- Department of Chemistry , University of North Carolina-Asheville , One University Heights, Asheville , North Carolina 28804-8511 , United States
| | - Anthony J Ranieri
- Department of Chemistry , University of North Carolina-Asheville , One University Heights, Asheville , North Carolina 28804-8511 , United States
| | - Melinda K Schueneman
- Department of Chemistry , University of North Carolina-Asheville , One University Heights, Asheville , North Carolina 28804-8511 , United States
| | - George L Heard
- Department of Chemistry , University of North Carolina-Asheville , One University Heights, Asheville , North Carolina 28804-8511 , United States
| | - Donald W Setser
- Kansas State University , Manhattan , Kansas 66506 , United States
| | - Bert E Holmes
- Department of Chemistry , University of North Carolina-Asheville , One University Heights, Asheville , North Carolina 28804-8511 , United States
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Smith CA, Gillespie BR, Heard GL, Setser DW, Holmes BE. The Unimolecular Reactions of CF 3CHF 2 Studied by Chemical Activation: Assignment of Rate Constants and Threshold Energies to the 1,2-H Atom Transfer, 1,1-HF and 1,2-HF Elimination Reactions, and the Dependence of Threshold Energies on the Number of F-Atom Substituents in the Fluoroethane Molecules. J Phys Chem A 2017; 121:8746-8756. [PMID: 28926250 DOI: 10.1021/acs.jpca.7b06769] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The recombination of CF3 and CHF2 radicals in a room-temperature bath gas was used to prepare vibrationally excited CF3CHF2* molecules with 101 kcal mol-1 of vibrational energy. The subsequent 1,2-H atom transfer and 1,1-HF and 1,2-HF elimination reactions were observed as a function of bath gas pressure by following the CHF3, CF3(F)C: and C2F4 product concentrations by gas chromatography using a mass spectrometer as the detector. The singlet CF3(F)C: concentration was measured by trapping the carbene with trans-2-butene. The experimental rate constants are 3.6 × 104, 4.7 × 104, and 1.1 × 104 s-1 for the 1,2-H atom transfer and 1,1-HF and 1,2-HF elimination reactions, respectively. These experimental rate constants were matched to statistical RRKM calculated rate constants to assign threshold energies (E0) of 88 ± 2, 88 ± 2, and 87 ± 2 kcal mol-1 to the three reactions. Pentafluoroethane is the only fluoroethane that has a competitive H atom transfer decomposition reaction, and it is the only example with 1,1-HF elimination being more important than 1,2-HF elimination. The trend of increasing threshold energies for both 1,1-HF and 1,2-HF processes with the number of F atoms in the fluoroethane molecule is summarized and investigated with electronic-structure calculations. Examination of the intrinsic reaction coordinate associated with the 1,1-HF elimination reaction found an adduct between CF3(F)C: and HF in the exit channel with a dissociation energy of ∼5 kcal mol-1. Hydrogen-bonded complexes between HF and the H atom migration transition state of CH3(F)C: and the F atom migration transition state of CF3(F)C: also were found by the calculations. The role that these carbene-HF complexes could play in 1,1-HF elimination reactions is discussed.
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Affiliation(s)
| | | | | | - D W Setser
- Kansas State University , Manhattan, Kansas 66506, United States
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Duncan JR, Heard GL, Holmes BE. Theoretical investigation of 1,2-interchange of a chlorine atom and methyl group in 1,1-dichloroacetone. J Phys Chem A 2010; 114:12992-7. [PMID: 21105681 DOI: 10.1021/jp1072582] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
A recent photofragment translational spectroscopy study of 1,1-dichloroacetone at 193 nm reported two primary unimolecular decomposition channels: C-Cl bond cleavage and elimination of HCl in a 9:1 ratio, respectively. The HCl translational energy distribution was bimodal suggesting two distinct decomposition pathways that were assumed to be 1,1-HCl loss forming a carbene and a 1,3-HCl elimination reaction forming a biradical ( Butler , L. J. ; Liu , Y. ; Lau , K. ; McCunn , L. R. ; Fitzpatrick , B. L. ; Bell , J. M. ; Krisch , M. J. J. Phys. Chem. A 2007 , 111 , 5968. ). An alternative two-step mechanism for HCl loss has been proposed involving interchange of a chlorine atom and a CH(3) group converting 1,1-dichloroacetone into 2-chloropropanoyl chloride followed by either a 1,2-HCl or 2,3-HCl elimination reaction. This alternative mechanism was computationally explored with density functional theory using B3PW91/6-31G(d',p') and unimolecular rate constants were calculated. The theoretical rate constant ratio for loss of HCl and the mean HCl translation energy for each elimination channel were in excellent agreement with the experimental results.
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Affiliation(s)
- Juliana R Duncan
- Department of Chemistry, University of North Carolina-Asheville, One University Heights, Asheville, North Carolina 28804-8511, United States
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