Experimental and Computational Studies of Unimolecular 1,1-HX (X = F, Cl) Elimination Reactions of C2D5 CHFCl: Role of Carbene:HF and HCl Adducts in the Exit Channel of RCHFCl and RCHCl2 Reactions

The gas-phase unimolecular reactions of C2D5CHFCl molecules with 94 kcal mol(-1) of vibrational energy have been studied by the chemical-activation experimental technique and by electronic-structure computations. Products from the reaction of C2D5CHFCl molecules, formed by the recombination of C2D5 and CHFCl radicals in a room temperature bath gas, were measured by gas chromatographymass spectrometry. The 2,1-DCl (81%) and 1,1-HCl(17%) elimination reactions are the principal processes, but 2,1-DF and 1,1-HF elimination reactions also are observed. Comparison of experimental rate constants to calculated statistical rate constants provides threshold energies. The potential surfaces associated with C2D5(F)C: + HCl andC(2)D(5)(Cl)C: + HF reactions are of special interest because hydrogen-bonded adducts with HCI and HF with dissociation energies of 6.4 and 9.3 kcal mol(-1), respectively, are predicted by calculations. The relationship between the geometries and threshold energies of transition states for 1,1-HClelimination and carbene:HCl adducts is complex, and previous studies of related molecules, such as CD3CHFCl,CD2ClCHFCl,C2D5 CHCl2, and halogenated methanes are included in the computational analysis. Extensive calculations for CH3CHFCl as a model for 1,1-HCl reactions illustrate properties of the exit-channel potential energy surface. Since the 1,1-HCl transition state is submerged relative to dissociation of the adduct, inner and outer transition states should be considered for analysis of rate constants describing 1,1-HCl elimination and addition reactions of carbenes to HCl.