ABSOLUTE HEATS OF FORMATION OF CHCL, CHF, AND CCIF - A GAS-PHASE EXPERIMENTAL AND G2 THEORETICAL-STUDY

The 298 K heats of formation of the singlet halocarbenes CHCl (1), CHF (2), and CClF (3) have been, determined from measurements of the chlo ride 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 d issociation cross sections for 1a, 2a, and 3a obtained with a flowing afterglow-triple quadrupole instrument gives the 298 K chloride dissoc iation enthalpies: 37.0 +/- 2.7, 22.7 +/- 2.2, and 25.2 +/- 1.5 kcal/m ol, respectively. Proton transfer equilibrium and acid-base bracketing measurements carried out in the flow tube give gas-phase acidities, D elta H-acid, for 1b, 2b, and 3b of 377.6 +/- 0.7, 385.9 +/- 0.3, and 3 61.3 +/- 2.0 kcal/mol, respectively. The chloride dissociation enthalp ies and gas-phase acidities are combined in simple thermochemical cycl es 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 compar isons of these results with the results of prior experimental measurem ents and with the results of G2 molecular orbital calculations lead to the following recommended heats of formation (in kcal/mol): Delta H-f ,H-298(CHCl) = 78.0 +/- 2.0, Delta H-f,H-298(CHF) = 34.2 +/- 3.0, Delt a H-f,H-298(CFCl) = 7.4 +/- 3.2, Delta H-f,H-298(CCl2) = 55.0 +/- 2.0, and Delta H-f,H-298(CF2) = -44.0 +/- 2.0. The recommended heats of fo rmation are used to derive other thermochemical data, including haloca rbene proton affinities, PA(CXY), halomethyl radical acidities, Delta H-acid(CHXY), and C-H bond strengths for halomethyl radicals, DH298(H- CXY). An excellent linear correlation is found to exist between the di valent state stabilization energy (DSSE) of the halocarbenes and the m easured or calculated singlet-triplet splittings.