Conformer-Specific Reaction Kinetics
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 . 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).
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).
This project is performed in collaboration with the group of Stefan Willitsch at Basel University, Switzerland.