A redetermination of the heats of formation of chloro- and dichlorocarbene and the deprotonation of methyl cation, a spin forbidden process?

The reactions of CH₃⁺, CH₂Cl⁺ and CHCl₂⁺ with standard reference bases were examined in a dual cell Fourier transform mass spectrometer. Deprotonation of methyl cation occurs rapidly via a spin forbidden process to afford triplet methylene even when the reaction is nearly thermoneutral. Bracketing results enable us to assign PA(CHCl) = 209.7 ± 2.2 kcal mol^-1 and PA(CCl₂) = 205.2 ± 1.9 kcal mol^-1, the latter value of which is considerably larger than previous determinations. The resulting heats of formation, however, are in good accord with other measurements and lead to recommended values of Δ H_f °(CHCl) = 74.6 ± 2.4 kcal mol^-1 and Δ H_f °(CCl₂) = 53.0 ± 2.6 kcal mol^-1. These values correspond to a 2-3 kcal mol^-1 lowering of these quantities and are within 1-2 kcal mol^-1 of high level G3 and W1 predictions based upon the atomization energies and a series of four isodesmic reactions.