Characterization of the 1,1-HCl Elimination Reaction of Vibrationally Excited CD₃CHFCl Molecules and Assignment of Threshold Energies for 1,1-HCl and 1,2-DCl plus 1,1-HF and 1,2-DF Elimination Reactions

Abstract

Vibrationally excited CD3CHFCl molecules with 96 kcal mol(-1) of energy were generated by the recombination of CD3 and CHFCl radicals in a room-temperature bath gas. The four competing unimolecular decomposition reactions, namely, 1,1-HCl and 1,2-DCl elimination and 1,1-HF and 1,2-DF elimination, were observed, and the individual rate constants were measured. The product branching fractions are 0.60, 0.27, 0.09, and 0.04 for 1,2-DCl, 1,1-HCl, 1,2-DF, and 1,1-HF elimination, respectively. Electronic structure calculations were used to define models of the four transition states. The statistical rate constants calculated from these models were compared to the experimental rate constants. The assigned threshold energies with ±2 kcal mol(-1) uncertainty are 60, 72, 65, and 74 kcal mol(-1) for the 1,2-DCl, 1,1-HCl, 1,2-DF, and 1,1-HF reactions, respectively. The loose structure of the 1,1-HX transition states, which is exemplified by the order of magnitude larger pre-exponential factor relative to the 1,2-HX elimination reactions, compensates for the high threshold energy; thus, the 1,1-HX elimination reaction rates can compete with the 1,2-HX elimination reactions for high levels of vibrational excitation in CD3CHFCl. The 1,1-HCl and 1,1-HF reactions are observed via the CD2═CDF and CD2═CDCl products formed from isomerization of the CD3CF and CD3CCl carbenes. These D-atom migration reactions are discussed, and the possibility of tunneling is evaluated. The transition states developed from the 1,1-HCl and 1,1-HF reactions of CD3CHFCl are compared to models for the HCl and HF elimination reactions of CHF2Cl, CHFCl2, and CH2FCl.

Experimental and Computational Studies of Unimolecular 1,1-HX (X = F, Cl) Elimination Reactions of C2D5CHFCl: Role of Carbene:HF and HCl Adducts in the Exit Channel of RCHFCl and RCHCl2 Reactions

“…Adduct energies calculated by M06-2X relative to the molecules are usually within 1–2 kcal mol –1 of the energy given by Δ H 0 – D 0 (adduct). d M06-2X/aug-cc-pVTZ calculations for the threshold energy, E 0 , and for the normal mode associated with the HCl stretch vibration. For reference, ω e 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 . g Since the geometry optimization in CBS-QB3 calculations failed for CH 3 (Cl)­C:HCl or for C 2 H 5 (Cl)­C:HCl, D 0 was obtained by subtracting the M06-2X adduct energy (CH 3 CHCl 2 ) or E 0 (1,1-HCl) energy (C 2 H 5 CHCl 2 ) from the enthalpy of reaction. We also reinvestigated the MP2/6-311+G­(2d,p) results for C 2 D 5 CHCl 2 ; convergence was obtained for both the transition state and the adduct, but the energies were nearly identical. h G3 calculations by Zhu and Bozzelli (they did not report a Cl 2 C:HCl adduct). …”

“…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. − , The statistical rate constant at energy E was obtained in the usual way from eq : …”

“…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 C 2 D 5 CHFCl*. 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 Cl-atom ,, remains to be defined. The competing 2,1-DF and 1,1-HF reactions are less important, but they are observed.…”

“…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. ,, 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 complication , in the exit channels.…”

“…The current study of C 2 D 5 CHFCl 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. − 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. − According to computational results, , hydrogen-bonded carbene:HF adducts with dissociation energies between 6 and 10 kcal mol –1 exist in the exit channels.…”