A unimolecular reaction ABC→A+B+C involving three product molecules and a single transition state. Photodissociation of glyoxal: HCOHCO→H ₂+CO+CO

Following an earlier proposal [Y. Osamura and H. F. Schaefer, J. Chem. Phys. 74, 4576 (1981)], the unimolecular reaction HCOHCO→H₂ + CO + CO has been examined via nonempirical molecular electronic structure theory. Specifically, the constrained symmetric (point group C_2v) transition state for this ABC→A + B + C reaction has been located at several levels of self-consistent-field (SCF) theory. Four different basis sets of contracted Gaussian functions were used: an STO-3G minimum basis, the small split valence 3-21G basis, the standard C(9s 5p/4s 2p) double zeta (DZ) set, and a double zeta plus polarization (DZ + P) basis. Vibrational analyses of the four stationary point structures (all of which are geometrically similar) yield a remarkable variety of results. The STO-3G stationary point has three imaginary vibrational frequencies, 3-21G has one imaginary frequency (and thus is a genuine transition state), while the DZ and DZ + P structures yield two imaginary vibrational frequencies. For the latter two cases, one of the two imaginary vibrations is a very small bending frequency, while the larger frequency clearly connects glyoxal with the three products H₂ + CO + CO. This suggests the existence of a slightly nonplanar true transition state. To our knowledge such a unimolecular transition state is without precedent. Configuration interaction (CI) suggests that the barrier for this ABC→A + B + C reaction is competitive with that for HCOHCO→H₂CO + CO.