Reactions of trapped ions with state- and conformer-selected neural molecules

Abstract

Understanding gas-phase ion-molecule reactions is of profound importance to gain knowledge about chemical processes taking place in the atmosphere and in space. Moreover, gas-phase studies can help to elucidate the mechanisms of bond activation in catalysis. Due to their high selectivity, reactions involving different conformational isomers of organic molecules are particularly intriguing. Recent progress in manipulating polar molecules using electrostatic fields has made it possible to select and spatially separate different conformers and rotational states of molecules in supersonic molecular beams. Combining this technology with a stationary reaction target of trapped and Coulomb-crystallized ions allowed the study of conformer-selected molecule-ion reaction dynamics and it was observed that reaction-rate constants can strongly depend on molecular conformation. Here, this concept was first applied to the proton-transfer reaction of the spatially separated rotational ground states of para- and ortho-water with cold diazenylium ions. A 23(9)$\%$ higher reactivity for the para nuclear-spin isomer was observed and attributed to the smaller degree of rotational averaging of the ion-dipole long-range interaction compared to the ortho-species. To investigate the role of molecular conformation in more complex organic ion-molecule reactions in the gas-phase, 2,3-dibromobutadiene (DBB) was identified as a promising model system. Its gauche- and s-trans-conformers were successfully separated in a molecular beam, verifying theoretical predictions. Subsequently, the reaction kinetics of conformer-selectd gauche and s-trans DBB with Coulomb crystals of laser-cooled $\mathrm{Ca^{+}}$ ions in an ion trap were investigated. It was found that the reaction rate constant strongly depends on both the conformation of DBB as well as the electronic state of $\mathrm{Ca^{+}}$. In the excited states of $\mathrm{Ca^{+}}$ ($\mathrm{^{2}P_{1/2}}$ and $\mathrm{^{2}D_{3/2}}$), the rate constants are capture-limited and enhanced for the gauche conformer due to its permanent dipole moment. With $\mathrm{Ca^{+}}$ in the ground state ($\mathrm{^{2}S_{1/2}}$), the rate for s-trans DBB stays unchanged, while that for gauche DBB is strongly suppressed, pointing to a strong conformational effect at the level of the short-range ion-molecule potential energy surface. Finally, as a prototypical $[4+1^{+}]$ polar cycloaddition, the reaction of DBB with trapped and sympathetically cooled propene ions was explored. Both conformers of DBB were found to exhibit capture-limited rate constants towards propene ions, which implies the contribution of a stepwise and barrierless reaction pathway in parallel to the canonical concerted mechanism. With these results, the present work marks the first step towards a rigorous systematic investigation of conformational effects in polar cycloadditions.