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

Abstract

The X̃ 1A1, ã 3B2, and b̃ 3A2 states of vinylidene are observed in the ultraviolet (351.1–364.0 nm) photoelectron spectra of X̃ 2B2 H2CC−, X̃ 2B2 D2CC−, and X̃ 2A′ HDCC−. The X̃ 1A1 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 1A1 H2CC)=0.490±0.006 eV and the triplet term energies are T0(ã 3B2 H2CC)=2.065±0.006 eV and T0(b̃ 3A2 H2CC)=2.754±0.020 eV. Experimental values for the bond dissociation energy of vinyl radical, D0(H2CC–H)=80.0±5.0 kcal/mol, and the acetylene–vinylidene isomerization energy, ΔHI=46.4±5.5 kcal/mol, are derived. Combining the latter value with the upper limit of Field and co-workers, ΔHI≤44.1–44.7 kcal/mol, yields ΔHI≊41–45 kcal/mol.