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The competition between bimolecular nucleophilic substitution and base-induced elimination is of fundamental importance for the synthesis of pure samples in organic chemistry. Many factors that influence this competition have been identified over the years, but the underlying atomistic dynamics have remained difficult to observe. We present product velocity distributions for a series of reactive collisions of the type X −  + RY with X and Y denoting the halogen atoms fluorine, chlorine and iodine. By increasing the size of the residue R from methyl to tert-butyl in several steps, we find that the dynamics drastically change from backward to dominant forward scattering of the leaving ion relative to the reactant RY velocity. This characteristic fingerprint is also confirmed by direct dynamics simulations for ethyl as residue and attributed to the dynamics of elimination reactions. This work opens the door to a detailed atomistic understanding of transformation reactions in even larger systems. The competition between chemical reactions critically affects our natural environment and the synthesis of new materials. Here, the authors present an approach to directly image distinct fingerprints of essential organic reactions and monitor their competition as a function of steric substitution.

In addition to the nucleophile and solvent, the leaving group has a significant influence on S N 2 nucleophilic substitution reactions. Its role is frequently discussed with respect to reactivity, but its influence on the reaction dynamics remains unclear. Here, we uncover the influence of the leaving group on the gas-phase dynamics of S N 2 reactions in a combined approach of crossed-beam imaging and dynamics simulations. We have studied the reaction F −  + CH 3 Cl and compared it to F −  + CH 3 I. For the two leaving groups, Cl and I, we find very similar structures and energetics, but the dynamics show qualitatively different features. Simple scaling of the leaving group mass does not explain these differences. Instead, the relevant impact parameters for the reaction mechanisms are found to be crucial and the differences are attributed to the relative orientation of the approaching reactants. This effect occurs on short timescales and may also prevail in solution-phase conditions. Little is known about how the identity of a leaving group affects the dynamics of a bimolecular nucleophilic substitution reaction. A study of the reaction of F − with CH 3 Cl, and comparison to its reaction with CH 3 I, now reveals key insights into such effects, with reactant orientation considered a key factor in understanding the behaviour observed.

The influence of quantum mechanics on the dynamics of chemical reactions is unknown for many processes in chemistry. Chemical reaction dynamics are often well described by quasiclassical motion of the atoms on quantum mechanical Born-Oppenheimer potential energy surfaces. Here we present a dynamic isotope effect in a nucleophilic substitution reaction experiment that can only be explained by quasiclassical trajectory simulations for reactants containing deuterium atoms, but not when hydrogen atoms are involved. The calculated energy- and angle-differential cross sections are compared to experimental crossed-beam velocity map imaging data, which show significantly more forward scattering for hydrogenated compared to deuterated reactants. Quantum scattering calculations in reduced dimensions explain this by an increased reaction probability for large total angular momentum, a feature that is not captured in the quasiclassical approach. Crossed-beam reactive scattering experiments of a nucleophilic substitution reaction show a dynamic isotope effect with a remarkable deviation from quasiclassical calculations, which is evidence for quantum dynamics in this ion-molecule reaction.

Associative electronic detachment (AED) between anions and neutral atoms leads to the detachment of the anion’s electron resulting in the formation of a neutral molecule. It plays a key role in chemical reaction networks, like the interstellar medium, the Earth’s ionosphere and biochemical processes. Here, a class of AED involving a closed-shell anion (OH − ) and alkali atoms (rubidium) is investigated by precisely controlling the fraction of electronically excited rubidium. Reaction with the ground state atom gives rise to a stable intermediate complex with an electron solely bound via dipolar forces. The stability of the complex is governed by the subtle interplay of diabatic and adiabatic couplings into the autodetachment manifold. The measured rate coefficients are in good agreement with ab initio calculations, revealing pronounced steric effects. For excited state rubidium, however, a lower reaction rate is observed, indicating dynamical stabilization processes suppressing the coupling into the autodetachment region. Our work provides a stringent test of ab initio calculations on anion-neutral collisions and constitutes a generic, conceptual framework for understanding electronic state dependent dynamics in AEDs. Associative electronic detachment (AED) reactions of anions play a key role in many natural processes. Here, Hassan and colleagues investigate AED reactions between hydroxyl anions and ultracold rubidium atoms in a hybrid atom-ion trap, revealing different dynamics for collisions with ground and electronically excited state rubidium.

We present a characterization of the ions’ translational energy distribution in a multipole ion trap. A linear mapping between the energy distribution of the trapped ions onto the ions’ time-of-flight (TOF) to a detector is demonstrated. For low ion temperatures, a deviation from linearity is observed and can be attributed to the emergence of multiple potential minima. The potential landscape of the trapped ions is modeled via the finite element method, also accounting for subtleties such as surface-charge accumulation. We demonstrate the validity of our thermometry method by simulating the energy distribution of the ion ensemble thermalized with buffer gas using a Molecular Dynamics (MD) simulation. A comparison between the energy distribution of trapped ions in different multipole trap configurations—i.e., with hyperbolic rods, cylindrical rods, and cylindrical wires—is provided. With these findings, one can map the temperature of the trapped ions down to the Kelvin regime using their TOF distributions. This enables future studies on sympathetic cooling and chemical reactions involving ions in multipole traps.

Light is often used to trigger reactions, energetically exciting the reactant(s) to kick them over the intrinsic reaction barrier. Now, however, the reaction between an excited atom and a charged molecule at very low temperatures has been shown not to adhere to this paradigm, instead undergoing a reaction blockading effect.

Cold collisions between hydrogen molecules and helium atoms reveal how the change from spherical to non-spherical symmetry creates a quantum scattering resonance.

We have studied the three-body complex formation rate of the hydroxyl anion with molecular hydrogen at low temperatures. The formed cluster is found to quickly undergo internal proton transfer to a hydrogen anion-water complex. This is probed by photodetachment spectroscopy, which clearly distinguishes the two isomeric structures. The product cluster is the only isomer found to be stably formed at the temperature and densities employed in the experiment. The cluster then binds an additional hydrogen molecule by a second three-body collision, which appears at a rate comparable to the first formation process. This is followed by a rapid growth to larger clusters.

Quantum tunnelling reactions play an important role in chemistry when classical pathways are energetically forbidden 1 , be it in gas-phase reactions, surface diffusion or liquid-phase chemistry. In general, such tunnelling reactions are challenging to calculate theoretically, given the high dimensionality of the quantum dynamics, and also very difficult to identify experimentally 2 – 4 . Hydrogenic systems, however, allow for accurate first-principles calculations. In this way the rate of the gas-phase proton-transfer tunnelling reaction of hydrogen molecules with deuterium anions, H 2  + D −  → H −  + HD, has been calculated 5 , but has so far lacked experimental verification. Here we present high-sensitivity measurements of the reaction rate carried out in a cryogenic 22-pole ion trap. We observe an extremely low rate constant of (5.2 ± 1.6) × 10 −20  cm 3  s − 1 . This measured value agrees with quantum tunnelling calculations, serving as a benchmark for molecular theory and advancing the understanding of fundamental collision processes. A deviation of the reaction rate from linear scaling, which is observed at high H 2 densities, can be traced back to previously unobserved heating dynamics in radiofrequency ion traps. The proton-transfer tunnelling reaction rate between H 2 and D – has been measured as about 1 out of 10 11 collisions, making it the slowest rate constant ever measured for an ion–molecule reaction in the gas phase.