Unimolecular Isomerization of CH₂FCD₂Cl via the Interchange of Cl and F Atoms: Assignment of the Threshold Energy to the 1,2-Dyotropic Rearrangement

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.

Analysis of the Five Unimolecular Reaction Pathways of CD2ClCHFCl with Emphasis on CD2Cl(F)C: and CD2Cl(Cl)C: Formed by 1,1-HCl and 1,1-HF Elimination

“…An uncertainty of ± 2 kcal mol –1 is typically assigned − to threshold energies determined from matching calculated rate constants to an experimental rate constant measured for a single energy, and this level of confidence should apply to four-centered reactions of CD 2 ClCHFCl. The sum of states for 1,1-HCl and 1,1-HF transition states depends on the model selected to represent the torsional mode and one other low frequency.…”

“…Because [CD 2 Cl] > [CHFCl], the formation of meso - and d , l -CHFClCHFCl* is not important, and because the decomposition reaction of CD 2 ClCD 2 Cl* has been thoroughly studied, , attention can be focused on CD 2 ClCHFCl*. Fortunately, disproportionation reactions between CD 2 Cl and CHFCl radicals are much less important than recombination, and disproportionation can be ignored.…”

“…Our laboratory has investigated unimolecular reactions of halogenated alkanes using the chemical activation technique coupled with electronic structure calculations; 1,2-HX (X = F, Cl, Br) elimination reactions − and halogen interchange reactions − have been characterized and threshold energies assigned for several systems. Our recent efforts have been directed toward 1,1-HX elimination reactions − of 1,1-dihalomethanes, -ethanes, and -propanes.…”

Theoretical Kinetic and Mechanistic Studies on the Reactions of CF3CBrCH2 (2-BTP) with OH and H Radicals

“…This special reaction proceeds by the interchange of F and Br atoms at two adjacent carbons in a single step, leading to the formation of Ma4 isomer via Ma2-TS4. Although this type of isomerization plays an important role in the saturated halogenated systems [19,20,21], it is unlikely to happen here for Ma2 considering the high barrier of 62.66 kcal/mol.…”

Computational study of the threshold energy for the 1,2-interchange of X and R (X, R = halogens, pseudohalogens, and monovalent hydrocarbon groups) on CH2XCH2R

“…The calculated E 0 s in Tables 1 and 2 suggest that when Cl, Br or SH are attached to carbon atoms the dominant unimolecular reaction will be interchange with themselves or each other and this has been verified for Cl-Br interchange in CH 2 BrCD 2 Cl. 11 Our experience is that when the E 0 (interchange) ≤ 290 kJ/mol the interchange reaction will be competitive with other decomposition processes for our chemical activation method that forms reactants with 350-420 kJ/mol of internal energy. [1][2][3][4][5][7][8][9][10][11] Scanning Tables 1 and 2 suggests that the interchange of Cl or Br might be observed with all of the groups in Table 1 results confirm E 0 (Cl-CH 3 ) = 259 kJ/mol for the CH 3 -Cl interchange reaction for 2,2-dimethyl-1chloropropane; i.e., two methyl groups replace two hydrogen atoms on C-2 of 1-chloropropane.…”