Photoelectron Imaging Spectroscopy of Intermolecular Interactions in Cluster Anions

Abstract

The heart of this dissertation is an experimental study of biacetyl (BA) and its smaller cluster an-ions, (BA_n)− where n = 2−4, conducted using photoelectron imaging spectroscopy. Large band shifts between photoelectron spectra of cluster anions of nonpolar BA, comparable or some- times even larger than the shift caused by solvation of BA by very polar water molecule, led us hypothesize that interactions between BA molecules are of weak covalent nature. In other words, in (BA_n)− there is a significant degree of charge delocalization, in contrast to a typical cluster anion, that is held together due to electrostatic interactions between the cluster core, holding the excess charge, and surrounding neutral solvent molecules. In order to support ex- perimental findings, we performed geometry optimization calculations using ab initio methods for BA, and DFT method for the cluster anions. The results were quite successful in explaining the features of photoelectron spectra. Especially for BA the agreement between the experiment and theory was excellent. Furthermore, the ab initio calculations showed that the HOMO of BA anion has high charge density with π bonding character along the central C-C bond, which facilitates charge delocalization in the cluster anions, as revealed by DFT results. Specifically, sandwich-like structures of (BA_2)− and (BA_3)−, enabled by flat geometry of BA, are held together by π−π intermolecular interactions. The study of BA and its cluster anions is complemented by the photoelectron imaging spec- troscopy studies of electronic structures of methylglyoxal and oxalyl chloride. We characterize their several lowest electronic states and determine their electron affinities (EA) for the first time. The results are discussed in the context of the substitution series (Figure 1.1), which includes glyoxal, methylglyoxal (single methyl substitution), biacetyl (double methyl substi- tution), and oxalyl chloride (double chlorine substitution). The EAs and anion detachment energies follow the trend: biacetyl < methylglyoxal < glyoxal ≪ oxalyl chloride. The electron- donating character of the methyl group has a destabilizing effect on the substituted anions, reducing the EA from glyoxal to methylglyoxal to biacetyl. In contrast, the strong electron- withdrawing power of Cl lends additional stabilization to the oxalyl chloride anion, resulting in a large (∼ 1 eV) increase in its detachment energy compared to glyoxal.