Using photoelectron imaging spectroscopy, we characterized the anion of methylglyoxal (XA″ electronic state) and three lowest electronic states of the neutral methylglyoxal molecule: the closed-shell singlet ground state (XA'), the lowest triplet state (aA″), and the open-shell singlet state (AA″). The adiabatic electron affinity (EA) of the ground state, EA(XA') = 0.87(1) eV, spectroscopically determined for the first time, compares to 1.10(2) eV for unsubstituted glyoxal. The EAs (adiabatic attachment energies) of two excited states of methylglyoxal were also determined: EA(aA″) = 3.27(2) eV and EA(AA″) = 3.614(9) eV. The photodetachment of the anion to each of these two states produces the neutral species near the respective structural equilibria; hence, the aA″ ← XA″ and AA″ ← XA″ photodetachment transitions are dominated by intense peaks at their respective origins. The lowest-energy photodetachment transition, on the other hand, involves significant geometry relaxation in the XA' state, which corresponds to a 60° internal rotation of the methyl group, compared to the anion structure. Accordingly, the XA' ← XA″ transition is characterized as a broad, congested band, whose vertical detachment energy, VDE = 1.20(4) eV, significantly exceeds the adiabatic EA. The experimental results are in excellent agreement with the ab initio predictions using several equation-of-motion methodologies, combined with coupled-cluster theory.

We discuss the formation of weak covalent bonds leading to anionic charge-sharing dimerisation or polymerisation in microscopic cluster environments. The covalent bonding between cluster building blocks is described in terms...

Photoelectron spectroscopy of the biacetyl (dimethylglyoxal) anion reveals the properties of the ground singlet and lowest triplet electronic states of the neutral biacetyl (BA) molecule. Due to the broad and congested nature of the singlet transition, which peaks at a vertical detachment energy VDE = 1.12(5) eV, only an upper bound of the adiabatic electron affinity of BA could be determined: EA­(BA) < 0.7 eV. A narrower and more structured triplet band peaking at VDE = 3.17(2) eV reveals the adiabatic electron binding energy of the triplet to be 3.05(2) eV. These results are in good agreement with ab initio (coupled-cluster) calculations. The lowest-energy structures of the anion, singlet, and triplet states of biacetyl are characterized by different orientations of the methyl groups within the molecular frame. In the ground singlet state of neutral BA, the methyl torsion is offset by ∼60° compared to that of the anion, while in the triplet the methyl orientation is similar to that of the anion. Photoelectron spectra of the cluster anions reveal that the intermolecular interactions in the homogeneously solvated (BA) n – clusters are significantly stronger than the interactions of BA– with N2O or even of BA– with H2O. To account for these observations, π–π bonded structures of the dimer and trimer anions of biacetyl are proposed based on density-functional theory calculations. The analysis of the proposed structures indicates that the negative charge in the (BA) n – cluster anions, at least in the dimer and the trimer, is significantly delocalized between all BA moieties present and there is a significant degree of covalent bonding within the cluster.

We report a photoelectron imaging study of gas-phase deprotonation of isoxazole in which spectroscopic data are compared to the results of electronic structure calculations for the anion products corresponding to each of three possible deprotonation sites. The observed photoelectron spectra are assigned to a mixture of the anion isomers. Deprotonation at the most acidic (C5) and the least acidic (C4) positions yields the respective C5- and C4-isoxazolide anions, while the reaction at the intermediate-acidity C3 site leads to a cleavage of the O–N bond and an opening of the ring in the anion. Following photodetachment, the ground states of neutral C5- and C4-isoxazolyl are assigned to be σ radicals (X 2A′), while the ground-state neutral derived from the ring-open C3-anion is a π radical (X 2A″). The relative intensities of the spectral bands exhibit sensitivity to the ion source conditions, giving evidence of competing and varying contributions of the dominant C5 and C3, as well as possible C4, deprotonation pathways.