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

Abstract

The gas-phase unimolecular reactions of C 2 D 5 CHFCl 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 C 2 D 5 CHFCl molecules, formed by the recombination of C 2 D 5 and CHFCl radicals in a room temperature bath gas, were measured by gas chromatography− mass 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 C 2 D 5 (F)C: + HCl and C 2 D 5 (Cl)C: + HF reactions are of special interest because hydrogen-bonded adducts with HCl 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-HCl elimination and carbene:HCl adducts is complex, and previous studies of related molecules, such as CD 3 CHFCl, CD 2 ClCHFCl, C 2 D 5 CHCl 2 , and halogenated methanes are included in the computational analysis. Extensive calculations for CH 3 CHFCl 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.

Unimolecular HBr and HF Elimination Reactions of Vibrationally Excited C2H5CH2Br and C2D5CHFBr: Identification of the 1,1-HBr Elimination Reaction from C2D5CHFBr and Search for the C2D5(F)C:HBr Adduct

“…10 The IRC confirmed the MP2 transition state for 1,1-HBr elimination, as well as for the previously studied 1,1-HCl transition states. − Higher level, single-point calculations were done for the transition state and CD 2 Cl­(F)­C:HCl adduct . Based upon our extensive studies of the 1,1-HCl reactions − and their similarity to the transition state and adduct structures for 1,1-HBr elimination, we are confident that the geometries shown in the Supporting Information and in Figure are both conformers of the transition-state for the 1,1-HBr elimination reaction for C 2 D 5 CHFBr.…”

“…As shown in Table 2, the HF and DF rate constants, including the branching ratio, for C 2 D 5 CHFBr* are similar to those for C 2 D 5 CHFCl*. 8 Assignment of Threshold Energies. For a unimolecular reaction that proceeds by crossing a single potential barrier, the The units of the slopes of the D/S plots are Torr; these values are converted to rate constants with units of s statistical rate constant at energy E is given by eq 14; this rate constant is often identified as the RRKM rate constant.…”

“…Geometries of transition states and adducts for 1,1-HF and 1,1-HBr reactions are shown in Figure , and calculated energies are provided in Figure . The transition state and adduct for 1,1-HF elimination closely resemble several other examples of 1,1-HF elimination. − The transition-state geometry clearly differs from the adduct, and E 0 (1,1-HF) is several kcal mol –1 above the energy of the adduct. Also, low vibrational frequencies of the transition state are much higher than those for the adduct (see Supporting Information).…”

“…A similar trend exists for HCl carbene adducts. 8 Table 3 includes results from M06-2X calculations for 2,1-HBr and 2,1-HF threshold energies. Since M06-2X did not find 1,1-HBr transition states, those values are not included in Table 3.…”

“…As previously noted, the E (cis) and Z (trans) isomers are combined so there are only four rate constants. In fact, the cis isomers are always preferred for 1-halopropene products ,, because the cis transition states reflect the slightly greater stability of the cis 1-halopropenes. The plot of 1-fluoropropene- d 5 and 1-fluoropropene- d 4 divided by the stabilized product C 2 D 5 CHFBr versus 1/pressure is shown in Figure .…”