A study of the singlet and triplet states of vinylidene by photoelectron spectroscopy of H2C=C−, D2C=C−, and HDC=C−. Vinylidene–acetylene isomerization

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Abstract
The X̃ 1 A 1, ã 3 B 2, and b̃ 3 A 2 states of vinylidene are observed in the ultraviolet (351.1–364.0 nm) photoelectron spectra of X̃ 2 B 2 H2CC−, X̃ 2 B 2 D2CC−, and X̃ 2 A’ HDCC−. The X̃ 1 A 1 state exhibits vibrational structure well above the barrier for isomerization to acetylene. A strict lower bound to the lifetime of the singlet state against rearrangement is τ>0.027 ps, with an estimate of τ≊0.04–0.2 ps based on a simulation of the line shapes including rotational broadening. A vibrational analysis of the singlet and lower triplet state bands provides vibrational frequencies and estimates of the changes of molecular geometries between the anion and the neutral species. A qualitative potential energy surface for the CH2 rock mode, which closely corresponds to the reaction coordinate for isomerization, is extracted from the experimental data. The adiabatic electron affinity is EA(X̃ 1 A 1 H2CC)=0.490±0.006 eV and the triplet term energies are T 0(ã 3 B 2 H2CC)=2.065±0.006 eV and T 0(b̃ 3 A 2 H2CC)=2.754±0.020 eV. Experimental values for the bond dissociation energy of vinyl radical, D 0(H2CC–H)=80.0±5.0 kcal/mol, and the acetylene–vinylidene isomerization energy, ΔH I =46.4±5.5 kcal/mol, are derived. Combining the latter value with the upper limit of Field and co‐workers, ΔH I ≤44.1–44.7 kcal/mol, yields ΔH I ≊41–45 kcal/mol.