Potential energy surface for unimolecular dissociation and rearrangement reactions of the ground electronic state of HFCO

The potential energy surface of the HFCO molecule in its electronic ground state has been investigated with ab initio method, at levels up to MP4(SDTQ)/6-311G**//MP2/6-31G*. At the highest level, the barrier height for molecular dissociation (HFCO→HF+CO) was calculated to be 46.9 kcal/mol with a zero-point energy correction, in good agreement with an experimental estimate and a recent theoretical result. The intrinsic reaction coordinate (IRC) for molecular dissociation was traced and the coupling between the IRC and normal modes as well as that among the normal modes was analyzed along the IRC. The analysis is consistent with the mode specificity of recently observed quasistable vibrational states of HFCO above the dissociation limit. Almost all possible stationary points on the potential surface of the HFCO system have been located, including the rearrangement and atomic dissociation products and transition states, as well as van der Waals complexes. These are compared with the H2CO system. All the species that have an FO bond are unstable or nonexistent, reflecting the weakness of the bond.