Photoelectron Spectroscopy of Biacetyl and Its Cluster Anions

Abstract

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.

Identification of Isoprene Oxidation Reaction Products via Anion Photoelectron Spectroscopy

“…These spectra are identical to that of the methylglyoxal and dimethylglyoxal spectra measured previously by Sanov and co-workers. 70,71 Bands X and a correspond to the X 1 A′ ← X 2 A″ and a 3 A″ ← X 2 A″ transitions in methylglyoxal and the X 1 A g ← X 2 A u and a 3 A u ← X 2 A u in dimethylglyoxal. We further confirmed the latter by measuring the spectrum of the anion generated by expanding dimethylglyoxal seeded in O 2 and Ar; this spectrum is shown superimposed on the isoprene/O 2 /UVgenerated m/z 86 spectrum in the Supporting Information.…”

“…As a first step in identifying these species, we consider the anion PE spectra of species with m / z 72 and 86, as these m / z values coincide with methylglyoxal and dimethylglyoxal (biacetyl), saturated species that are known to bind an electron, forming radical anions. , The spectra obtained using 3.495 eV photon energy are shown in Figure a,b. Raw and reconstructed images are included in the Supporting Information.…”

Anion of Oxalyl Chloride: Structure and Spectroscopy

“…Comparison to Other Anions in the Glyoxal Family. Figure 7 compares the published anion photoelectron spectra of biacetyl (532 nm), 12 methylglyoxal (612 nm), 11 and glyoxal (532 nm) 10 to the new results for oxalyl chloride (355 nm). Each spectrum consists of a single band due to the photodetachment from the ground state of the anion to the ground state of the neutral.…”

“…Using photoelectron imaging spectroscopy and ab initio calculations, we illuminate some of the anion’s features, including its photoelectron spectrum and detachment energy, as well as the electron affinity (EA) of neutral oxalyl chloride. The spectroscopic results for this anion are compared to the recently reported studies of glyoxal, methylglyoxal, and biacetyl …”

“…The spectroscopic results for this anion are compared to the recently reported studies of glyoxal, 10 methylglyoxal, 11 and biacetyl. 12…”