Absolute Heats of Formation of CHCl, CHF, and CClF. A Gas-Phase Experimental and G2 Theoretical Study

Abstract

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, ΔHacid, 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): ΔHf,298(CHCl) = 78.0 ± 2.0, ΔHf,298(CHF) = 34.2 ± 3.0, ΔHf,298(CFCl) = 7.4 ± 3.2, ΔHf,298(CCl2) = 55.0 ± 2.0, and ΔHf,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, ΔHacid(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.