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The 298 K heats of formation of the singlet halocarbenes CHCl (1), CHF (2), and CClF (3) have been determined from measurements of the chloride dissociation energies of CHCl2 - (1a), CHClF- (2a), and CCl2F- (3a) and the gas-phase acidities of CH2Cl2 (1b), CH2ClF (2b) and CHCl2F (3b), respectively. Analysis of the energy-resolved collision-induced dissociation cross sections for 1a, 2a, and 3a obtained with a flowing afterglow−triple quadrupole instrument gives the 298 K chloride dissociation enthalpies:  37.0 ± 2.7, 22.7 ± 2.2, and 25.2 ± 1.5 kcal/mol, respectively. Proton transfer equilibrium and acid−base bracketing measurements carried out in the flow tube give gas-phase acidities, ΔH acid, for 1b, 2b, and 3b of 377.6 ± 0.7, 385.9 ± 0.3, and 361.3 ± 2.0 kcal/mol, respectively. The chloride dissociation enthalpies and gas-phase acidities are combined in simple thermochemical cycles to derive 298 K heats of formation for 1, 2, and 3 of 80.4 ± 2.8, 34.2 ± 3.0, and 7.4 ± 3.2 kcal/mol, respectively. Critical comparisons of these results with the results of prior experimental measurements and with the results of G2 molecular orbital calculations lead to the following recommended heats of formation (in kcal/mol):  ΔH f,298(CHCl) = 78.0 ± 2.0, ΔH f,298(CHF) = 34.2 ± 3.0, ΔH f,298(CFCl) = 7.4 ± 3.2, ΔH f,298(CCl2) = 55.0 ± 2.0, and ΔH f,298(CF2) = −44.0 ± 2.0. The recommended heats of formation are used to derive other thermochemical data, including halocarbene proton affinities, PA(CXY), halomethyl radical acidities, ΔH acid(CHXY), and C−H bond strengths for halomethyl radicals, DH298(H−CXY). An excellent linear correlation is found to exist between the divalent state stabilization energy (DSSE) of the halocarbenes and the measured or calculated singlet−triplet splittings.

5489The data in Table I as well as the data for allyl vinyl ether 1 suggest that the [ 1,3] rearrangement products observed may arise via dissociated ions followed by recombination. Note that the vinyl ether derived from verbenol (entry 5) gives rise to the [1,3]-rearranged product with exclusive inversion of configuration at the allylic carbon atom, suggesting that the [1,3] rearrangement is concerted. However, the formation of the observed product can also be rationalized via dissociated ions followed by recombination from the least hindered face of the molecule. In order to determine to what extent allyl vinyl ethers ionize in the presence of 3.0 M lithium perchlorate-diethyl ether, a crossover experiment was carried out employing allyl vinyl ethers 4 and 5, each 0.1 M in 3.0 M lithium perchloratediethyl ether. After 1.5 h a t ambient temperature, there was obtained, in excellent yield, a mixture of aldehydes 6 and 7 and ketones 8 and 9 in a ratio of 1.0:1.8:1.5:1.6, suggesting that dissociated ions are involved. 4 5 6 7 8 9(6) Acetone was employed as the solvent because in diethyl ether the reaction rates for the [1,3] rearrangement were too fast to be measured by 'H NMR. In addition, tetra-n-butylammonium perchlorate is insoluble in ether. Reactions were carried out 0.2 M in substrate in sealed NMR tubes at 22 OC. Note that the reaction of 10 in 3.0 M LiCI04-Et20 gave 14% after 1 h to an 86% isolated yield of aldehyde 11.