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Chemical-activation experiments and computational methods have been used to study the unimolecular reactions of CHCHBr and CDCHFBr with 90 and 93 kcal mol of vibrational energy, respectively. The four-centered elimination of HBr and DBr are the dominant reactions; however, 2,1-DF, 1,1-HBr and 1,1-HF reactions are also observed from CCDCHFBr. The main focus was a search for the role of the CD(F)
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, which is in basic agreement with thermal-activation experiments. Comparison of the 2,1-DBr and 2,1-HBr rate constants permit 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 CDCHFBr followed by D atom migration is an experimentally observed product, dissociation of the CD(F)
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 CH(X)
HF adducts are 9.9, 9.3, and 9.0 kcal mol for X = F, Cl, and Br, and the values for CH(F)
HX are 9.9, 6.4 and ≈ 4.9 kcal mol.
This article was published in the following journal.
Name: The journal of physical chemistry. A
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