Previous research projects
Conformer- and state-specific reaction kinetics
Artist's impression of the reactivity measurement using sorted molecules and trapped atoms
Large, complex molecule exhibit multiple structural isomers, such as conformers (torsional isomers, rotamers), even at very low temperatures. The isomers react at distinct rates and often with different products. This provides a perspective to control chemical reactions by selecting molecular conformations. The study of conformer-specific reactions can also provide further insight into the chemical shapes of stereomers in chemical reactions.
The spatial separation of conformers can be performed using inhomogeneous electric fields. We investigate the manipulation of chemical reactions in terms of conformations in gas phase by carrying out reactive scattering experiments between conformer-selected neutral molecules and Coulomb crystallized ions, see the figure of experimental setup below. 3-aminophenol served as a prototypical complex molecules exhibiting two conformations (cis and trans), with well-known molecular parameters. The conformers of 3-aminophenol were spatially separated in a molecular beam passing through the electrostatic deflector, based on their distinct electric dipole moments.
Coulomb-crystals of Ca+ ions in an ion trap serve as a suitable stationary target for reactive collisions. They provide high sensitivities to a level of single reaction events. The reaction is monitored, and the reaction-rate determined, by imaging the laser-induced fluorescence signal of non-reactive ions.
Coulomb-crystals of Ca+ ions in an ion trap serve as a suitable stationary target for reactive collisions. They provide high sensitivities to a level of single reaction events [3]. The reaction is monitored, and the reaction-rate determined, by imaging the laser-induced fluorescence signal of non-reactive ions.
Experimental measurements. A) Conformer-specific renounce-enhanced multi-photon ionization (REMPI) time-of-flight mass-spectrometry (TOF-MS) measurements of the vertical molecular beam density profiles. B) Pseudo-first-order reaction rate of the molecule-ion reaction in dependence of the beam position, C) Second-order rate constant as a function of the beam composition. I.e., the rate of cis-3-aminophenol (left) is about two times larger than for trans-3-aminophenol (right).
Selected Results
We have measured reaction rates of two conformers of 3-aminophenol (AP) with laser-cooled Ca+ ions, and we observed that the reactivity of one conformer is about two times larger than that of the other. In the figure below, figure A shows density profiles of deflected beams of two AP conformers (Square: experimental data, dashed lines: simulations). At high deflection coordinates, only cis conformers present. Figure B shows the conformer-specific (red and blue) and total (black) pseudo-first-order rate constants for the reaction Ca+ + cis-/trans-AP. Figure C shows that the reactivity, i.e., second-order rate constant, increases as the predominant beam component changes from the trans to the cis conformer (dash lines: number densities of the two conformers).
We have further experimental demonstrated the state-specific, distinct reactivities of para and ortho water demonstrating that these are even chemically two different molecular species, Furthermore did we investigate cycloaddition reactions, such as the prototypical Diels-Alder reaction, between two organic molecules, which demonstrated non-concerted reaction pathways in ionic cycloadditions.
Relevant publications
Nuclear-spin effects in laser alignment
The coupling of nuclear spins to overall angular momenta can have a profound influence on the rotational dynamics of molecules and, as a result, change the temporal evolution of the laser-excited rotational wavepacket and thus the molecular alignment. These effects are prominent in molecules containing heavy atoms such as Se, Cl, Br, I, Fe, Au, or Pt, which show strong nuclear-quadrupole interactions. Heavy atoms are commonly utilized as strong scattering and absorption centers in x-ray imaging experiments as well as a good leaving group in Coulomb-explosion velocity-map imaging of molecular dynamics. Therefore, it is essential to account for nuclear quadrupole interactions when simulating the laser-induced alignment of molecules with large nuclear quadrupoles.
We developed a robust variational method, which was implemented in the Richmol computational package, to account for the quadrupole as well as weaker spin-spin and spin-rotation interactions in the laser-induced dynamics of molecules. In several studies, we showed that the impact of nuclear quadrupole interactions on postpulse (field-free) molecular dynamics is minimal during the first revivals. However, over longer time scales, the effect is completely detrimental and is highly influenced by laser intensity. Dephasing in the laser-excited rotational wavepacket caused by irregular spacings between the hyperfine-split nuclear spin states across various rotational hyperfine bands may be used to explain this phenomenon.