The anomalous behavior of the Zeeman anticrossing spectra of à 1Au acetylene: Theoretical considerations

P. Dupre, R. Jost, M. Lombardi, P. G. Green, E. Abramson, and R. W. Field have observed anomalous behavior of the anticrossing density in the Zeeman anticrossing (ZAC) spectra of gas phase A 1Au acetylene in the 42 200 to 45 300 cm−1 energy range. To best explain this result, they hypothesize a large singlet–triplet coupling due to the existence of a linear isomerization barrier connecting a triplet‐excited cis‐ and trans‐acetylene in the vicinity of the studied energy range (∼45 500 cm−1). Theoretically such a linear stationary point, however, must have two different degenerate bending vibrational frequencies which are either imaginary or exactly zero. Neither case has yet been experimentally detected. Here, we have studied the two lowest‐lying linear triplet‐excited‐state stationary points of acetylene, 3Σ+u and 3Δu, to see if they fit Dupre et al.’s hypothesis. We have completed geometry optimization and harmonic vibrational frequency analysis using complete‐active‐space self‐consistent field (CASSCF)...

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