A unimolecular reaction ABC→A+B+C involving three product molecules and a single transition state. Photodissociation of glyoxal: HCOHCO→H2+CO+CO

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Abstract
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 C2v) 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.