“Conformational dynamics and structures of gas phase molecules and clusters”

The combination of molecular beams with spectroscopic techniques is a widely used tool to reveal intrinsic properties of molecules and clusters. Using Infrared ion dip spectroscopy in the C-H-stretch region, we investigate the vibrational properties of the isotopically mixed benzene dimer (C6H6)(C6D6) in the gas phase. Playing on the reduced symmetry in the benzene dimer, Infrared inactive vibrations of the monomer gain intensity due to the dimerization and thus become indirectly accessible. Together with anharmonic DFT calculations, our experimental results allow us to assign the first experimental value to the B1u fundamental C-H-stretch mode of the benzene monomer. Furthermore the REMPI spectrum of (C6H6)(C6D6), that exists in two different isomers, is measured using two different carrier gases (helium and neon) in the molecular beam. A dramatic dependence of the conformer distribution on the choice of the carrier gas is observed. Based on these results a new general model describing the rare gas atom catalyzed interconversion between different isomers in a molecular beam will be presented. This model is applied to observations on the amino acid phenylalanine showing the general importance of the catalytic activity of rare gas atoms in molecular beams. Besides the sequence of amino acids, the secondary structure of a peptide determines significantly its function and activity in organisms. We investigate two different isomers of the peptide Gramicidine: a linear form consisting of 15 aminoacids and a cyclic form consisting of 10 aminoacids. Using FELIX (Free Electron Laser for Infrared Experiments) in the region of the so called Amide I and Amide II modes and a FT-ICR spectrometer for detection, different folding motifs can be assigned to the two isomers of Gramicidine. The first IR spectra of gas phase Gramicidine in the fingerprint region will be presented.