CARBENE THERMOCHEMISTRY FROM COLLISION-INDUCED DISSOCIATION THRESHOLD ENERGY MEASUREMENTS - THE HEATS OF FORMATION OF X1A1 CF2 AND X1A1 CCL2

The heats of formation of X1A1 CF2 and X1A1 CCl2 have been determined from measurements of the threshold energies for collision-induced halide ion dissociation from CF3- and CCl3- in a flowing afterglow-triple quadrupole apparatus. The derived values DELTA-H-degrees(f,298)(CF2, g) = -39.4 +/- 3.4 kcal/mol and DELTA-H-degrees(f,298)(CCl2, g) = 52.1 +/- 3.4 kcal/mol are significantly higher than the heats of formation recently obtained for these carbenes from proton affinity bracketing experiments by Lias, Karpas, and Liebman (J. Am. Chem. Soc. 1985, 107, 6089) but are in good agreement with measurements from other laboratories and with ab initio molecular orbital calculations. The gas-phase acidity (DELTA-H(acid)) of CHCl3 has also been determined from a proton-transfer bracketing procedure to be 357.6 +/- 2.0 kcal/mol, from which EA(CCl3) = 2.25 +/- 0.10 eV is obtained. The carbon-carbon bond energies in CF2=CF2 (79.1 +/- 3.8 kcal/mol) and C12C=CCl2 (106.3 +/- 4.9 kcal/mol) are derived from the present results and critically compared with the predicted values obtained from the proposed relationship between C = C bond strengths and carbene singlet-triplet energy gaps.