Undine Erlekam

Selector for Structural Isomers of Neutral Molecules

We have selected and spatially separated the two conformers of 3-aminophenol (C(6)H(7)NO) present in a molecular beam. Analogous to the separation of ions based on their mass-to-charge ratios in a quadrupole mass filter, the neutral conformers are separated based on their different mass-to-dipole-moment ratios in an ac electric quadrupole selector. For a given ac frequency, the individual conformers experience different focusing forces, resulting in different transmissions through the selector. These experiments demonstrate that conformer-selected samples of large molecules can be prepared, offering new possibilities for the study of gas-phase biomolecules.

An experimental value for the B1u C–H stretch mode in benzene

We here present experimental infrared spectra on two (C(6)H(6))(C(6)D(6)) benzene dimer isomers in the gas phase. The spectra show that the two benzene molecules in the dimer are symmetrically inequivalent and have distinct IR signatures. One of the two molecules is in a site of low symmetry, which leads to the IR activation of fundamental modes that are IR forbidden by symmetry in the monomer. In the spectra, all four fundamental C-H stretch modes of benzene are observed. Modes in the dimer are shifted up to 3 cm(-1) to the red, compared to the modes that are known for the monomer. For the nu(13) B(1u) C-H stretch fundamental mode of benzene, a first experimental value of 3015(+2) (-5) cm(-1) is determined, in excellent agreement with anharmonic frequency calculations presented here.

Mid-IR spectra of different conformers of phenylalanine in the gas phase

The experimental mid- and far-IR spectra of six conformers of phenylalanine in the gas phase are presented. The experimental spectra are compared to spectra calculated at the B3LYP and at the MP2 level. The differences between B3LYP and MP2 IR spectra are found to be small. The agreement between experiment and theory is generally found to be very good, however strong discrepancies exist when -NH2 out-of-plane vibrations are involved. The relative energies of the minima as well as of some transition states connecting the minima are explored at the CCSD(T) level. Most transition states are found to be less than 2000 cm(-1) above the lowest energy structure. A simple model to describe the observed conformer abundances based on quasi-equilibria near the barriers is presented and it appears to describe the experimental observation reasonably well. In addition, the vibrations of one of the conformers are investigated using the correlation-corrected vibrational self-consistent field method.

Structure of the Benzene Dimer—Governed by Dynamics†

Author Institution: Center for Free-Electron Laser Science, 22761 Hamburg, Germany; Max-Planck-Institut fur Kernphysik, 69117 Heidelberg, Germany; Fritz-Haber-Institut der Max-Planck-Gesellschaft, 14195 Berlin, Germany; National Research Council of Canada, Ottawa, Ontario K1A 0R6, Canada; Institut fur Physikalische Chemie und Elektrochemie, Gottfried-Wilhelm-Leibniz-Universtat, 30167 Hannover, Germany; Institute for Molecules and Materials, Radboud University, 6525 AJ Nijmegen, The Netherlands

Unraveling the internal dynamics of the benzene dimer: a combined theoretical and microwave spectroscopy study

We report a combined theoretical and microwave spectroscopy study of the internal dynamics of the benzene dimer, a benchmark system for dispersion forces. Although the extensive ab initio calculations and experimental work on the equilibrium geometry of this dimer have converged to a tilted T-shaped structure, the rich internal dynamics due to low barriers for internal rotation have remained largely unexplored. We present new microwave spectroscopy data for both the normal (C6H6)2 and partially deuterated (C6D6)(C6H6) dimers. The splitting patterns obtained for both species are unraveled and understood using a reduced-dimensionality theoretical approach. The hindered sixfold rotation of the stem can explain the observed characteristic 1 : 2 : 1 tunneling splitting pattern, but only the concerted stem rotation and tilt tunneling motion, accompanied by overall rotation of the dimer, yield the correct magnitude of the splittings and their strong dependence on the dimer angular momentum J that is essential to explain the experimental data. Also the surprising observation that the splittings are reduced by 30% for the mixed (C6D6)(C)(C6H6)(S) dimer in which only the cap (C) in the T-shaped structure is deuterated, while the rotating stem (S) monomer is the same as in the homodimer, is understood using this approach. Stark shift measurements allowed us to determine the dipole moment of the benzene dimer, μ = 0.58 ± 0.051 D. The assumption that this dipole moment is the vector sum of the dipole moments induced in the monomers by the electric field of the quadrupole on the other monomer yields a calculated value of μ = 0.63 D. Furthermore, the observed Stark behavior is typical for a symmetric top, another confirmation of our analysis.

Adsorption of Methane and Ethane on RuO2(110) Surfaces

Abstract The adsorption of methane and ethane on the stoichiometric and O-rich RuO2(110) surfaces, respectively, was studied by thermal desorption and high-resolution electron energy loss spectroscopy. The results support weak adsorption (physisorption) of these molecules on the surface, regardless of the presence of undercoordinated Ru and oxygen surface sites. The vibrational spectra recorded at about 90 K are compared with spectra calculated for gaseous CH4 and C2H6. The nearly complete agreement of frequencies and relative intensities is consistent with physisorption. No evidence of adsorption induced molecule activation is found.

Infrared spectroscopic and ab-initio studies of the benzene dimer

Due to its high symmetry and small size, benzene serves as a model molecule for the symmetry properties of molecular vibrations in polyatomic molecules and research on its vibrational modes dates back 70 years. One would expect the fundamental frequencies of its 20 symmetry unique vibrational modes to be well known. However, for at least one mode, the B1u C-H stretch mode, the until recently accepted value [1] was determined only indirectly. Performing experiments on the benzene dimer provides the first direct experimental determination of this mode [2]. Clusters of benzene have also attracted experimental and theoretical attention. Of special interest is the dimer, as it can serve as a prototype for π − π interaction in aromatic systems and serves since long as a benchmark system to test our understanding of intermolecular interactions. There seems to be no clear consensus from theory if the dimer structure is a parallel or T-shaped structure, although recently, mostly (distorted) T-shaped structures are predicted as being the lowest energy structures. Raman experiments clearly indicate that for the observed species, the two moieties are symmetrically inequivalent [3] and microwave studies show the presence of a dipole moment [4]. Both these results thus support a T-shaped structure. However, it is not clear whether such a T-shaped structure is the exclusive structure or whether a parallel displaced and a T-shaped structure are is some equilibrium and only the T-shaped structure is visible to the experiment. Here, we present a mechanism in which higher energy gas-phase conformers can be converted into lower energy structures with rare gas atoms catalyzing the process [5].