Electronic structure of protonated aromatic and chiral molecules

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This project aims at the laser spectroscopic, mass spectrometric, and quantum chemical characterization of size-selected protonated aromatic molecules and their clusters isolated in the gas phase, to determine their geometric and electronic structure in the ground and excited electronic states. Target molecules are protonated (polycyclic) aromatic hydrocarbon molecules ranging from benzene to coronene, as well as simple derivatives and clusters with nonpolar and polar ligands (rare gas atoms, nitrogen, water) to mimic both hydrophobic and hydrophilic solvation. The considered molecules are of fundamental relevance in organic chemistry, combustion, biochemistry, and astrochemistry. Protonation has a huge impact on the electronic and geometric structure, because it reduces the symmetry and aromaticity of the aromatic pi-electron system and generates low-lying charge-transfer states. Hence, protonation largely changes the photochemistry and photophysics of these aromatic molecules, which are largely unexplored both experimentally and theoretically. In addition, the interaction with solvent molecules changes drastically upon protonation and electronic excitation, and a particular focus will be on proton transfer to/from solvent in microhydrated clusters. Fundamental questions to be addressed include the energy, structure, charge distribution, molecular orbitals, protonation site and proton affinity, relaxation and fragmentation mechanisms, as well as the effects of protonation, electronic excitation, microsolvation, and substitution of functional groups. The analysis of the spectroscopic and quantum chemical results will provide a detailed understanding of the photophysical and chemical properties of these fundamental species at the molecular level. In addition, we extend these studies along a new research line toward protonated chiral molecules and their interactions. Chirality and chiral discrimination play a fundamental role in a plethora of areas in life and materials sciences, and many of these phenomena are not understood at the molecular level. Although in charged complexes the interactions and resulting chiral discrimination are strong, little spectroscopic information is available for chiral ionic complexes. To this end, we will explore interactions in chiral ionic complexes by considering prototypical protonated (aromatic) complexes of (i) 1-amino-2-indanol (used in asymmetric synthesis) and (ii) glutamic acid (a proteinogenic amino acid and neurotransmitter). Fundamental chirality-specific questions to be addressed include the effects of protonation on the strength of chiral discrimination, the importance of the individual forces (repulsion, electrostatic, induction, dispersion), and the effects of polar and nonpolar solvation, with the ultimate goal of providing a molecular-level understanding of the intermolecular interactions and chiral discrimination in charged chiral complexes.