Reaction kinetics of trapped molecular ions with conformer- and isomer-selected neutral molecules
The experimental challenges in preparing pure samples of individual molecular isomers and conformers have thus far precluded a characterization of their distinct chemical behavior. 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 [1,2]. By combining this technology with a stationary reaction target of Coulomb-crystallized ions in a linear quadrupole ion trap [3] we have recently explored conformer selected molecule-ion reaction dynamics and observed that reaction-rate constants can strongly depend on molecular conformation [4,5]. More recently, we have extended this method to the separation of different nuclear-spin isomers for studies of ion-molecule reactions with control over the rotational and nuclear-spin state of the neutral reaction partner.
Water is one of the fundamental molecules in chemistry, biology and astrophysics. It exists as two distinct nuclear-spin isomers, para- and ortho-water, which do not interconvert in isolated molecules. We have successfully studied the proton-transfer reaction of the spatially separated ground states of para- and ortho-water with cold ionic diazenylium (N2H+), an important molecule in astrochemistry. We found a 23(9)% higher reactivity for the para nuclear-spin isomer which we attribute to the smaller degree of rotational averaging of the ion-dipole long-range interaction compared to the ortho-species [6]. This finding is in quantitative agreement with a modelling of the reaction kinetics using rotationally adiabatic capture theory [7] and highlights the ramifications of nuclear-spin symmetry on chemical reactivity.
Despite their significance in organic synthesis, the mechanistic details of Diels-Alder cycloadditions, in which a diene and a dienophile react to form a cyclic product, still remain an extensively discussed question. The ionic variants, polar cycloadditions, have proven to be a particularly efficient route to form cyclic compounds, but it has proven difficult to determine whether only the cis conformer (concerted mechanism) or both cis and trans conformers (stepwise mechanism) of the involved diene react to form the cyclic product [8]. In order to shed light on these questions we are currently investigating the ionic cycloaddition reaction of 2,3-dibromo-1,3-butadiene with ionic propene. Having successfully verified the separation of the two conformers using soft vacuum-ultraviolet ionization we are now able to perform experiments that directly test the underlying mechanism of polar cycloadditions.
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[4] Y.-P. Chang, K. Dlugolecki, J. Küpper, D. Rösch, D. Wild, S. Willitsch, Science, 2013, 342, 98.
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[6] A. Kilaj, H. Gao, D. Rösch, U. Rivero, J. Küpper and S. Willitsch, Nat. Commun., 2018, 9, 2096.
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