Electron affinity and excited states of methylglyoxal.

Using photoelectron imaging spectroscopy, we characterized the anion of methylglyoxal (X2 A″ electronic state) and three lowest electronic states of the neutral methylglyoxal molecule: the closed-shell singlet ground state (X1 A'), the lowest triplet state (a3 A″), and the open-shell singlet state (A1 A″). The adiabatic electron affinity (EA) of the ground state, EA(X1 A') = 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(a3 A″) = 3.27(2) eV and EA(A1 A″) = 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 a3 A″ ← X2 A″ and A1 A″ ← X2 A″ 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 X1 A' state, which corresponds to a 60° internal rotation of the methyl group, compared to the anion structure. Accordingly, the X1 A' ← X2 A″ 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.