Thermochemistry of gas phase CF2 reactions:: A density functional theory study

Enthalpies of formation and enthalpies of reaction at 298 K for a set of fluorocarbon species derived from the pyrolysis of hexafluoropropylene oxide (HFPO) were computed using the B3LYP (Becke three-parameter Lee-Yang-Parr) density functional theory. Total energies were calculated at the B3LYP/6-311+G(3df )//B3LYP/6-31G(d) level. Zero-point energies and thermal corrections were calculated using B3LYP/6-31G(d)//B3LYP/6-31G(d) vibrational frequencies scaled by a factor of 0.96. The average absolute deviation of enthalpies of formation and reaction were 2.33 and 1.42 kcal/mol, respectively. The pyrolysis of HFPO to produce difluorocarbene, CF2, and trifluoroacetyl fluoride, CF3CFO, was predicted to be endothermic at 23.6 kcal/mol. The singlet state of CF2 was predicted to be more stable than its triplet state by 52.4 kcal/mol. CF2 polymerization through the addition of a singlet CF2 to an existing unterminated chain was calculated to be more favorable than through the addition of a singlet CF2 to an existing perfluoroalkene molecule. For the former pathway, a linear relationship was found between the enthalpy of formation of the CF2 chains and the number of chain carbons (n) for n greater than or equal to 2. The reaction enthalpy for each successive CF2 extension was found to be -48.7 kcal/mol, and the carbon-carbon bond dissociation energy was found to be 75.5 kcal/mol. For the latter pathway, the stability of CF2=CF2 hindered chain polymerization but provided theoretical support to CF2=CF2 as a primary product in HFPO pyrolysis experiments. (C) 2000 American Institute of Physics. [S0021-9606(00)31134-5].