Halomethylenes: Effects of Halogen Substitution on Absolute Heats of Formation

Article abstract of DOI:10.1021/ja00307a044

New values for the heats of formation of CF2, CCl2, CClF, CFH, and CClH have been derived from estimations of the thermochemistry of the reaction(s) CXYH⁺ + B -> CXY + BH⁺ where X and Y are F and/or Cl and B is a molecule for which an absolute value of the gas-phase basicity (or proton affinity) is available.The experiments, carried out in an ion cyclotron resonance spectrometer, lead to the following values for the heats of formation (in kcal/mol) of the ground-state singlet carbenes: CF2, -49 +/- 3; CCl2, 39 +/- 3; CFCl, -2 +/- 7; CFH, 26 +/-3; CClH, 71 +/-5.The value for CF2 is lower by about 5 kcal/mol than the previously accepted value, but in good agreement with values derived from previous "bracketing" results and also in agreement with values derived from the observed threshold energies for ionic dissociation processes.The value for CCl2 is significantly lower than the 1976 value of 47 +/-3 kcal/mol recommended by S.W.Benson, but in good agreement with an earlier value (40 +/- 5 kcal/mol) recommended by this author with more recent experimental results on the onset energy of formation of Cl2^- from CCl4.The values for CFH, CClH, and CFCl are all approximately equal to the averages of the heats of formation of the corresponding CX₂ and CY₂ species, in agreement with assumptions made in previous estimates of these quantities.Values for the C-X bond energies in the halomethylenes, the heats of formation of the corresponding CXY⁺ ions, and the ionization potentials of the CXY species are derived from the results.From the most recent calculations of the energy differences between the ground-state singlet halomethylenes and the first triplet state, values for the heats of formation of the triplet halomethylenes are obtained; an analysis of trends in these values indicates that ³CF2 is substantially destabilized.