Inverse kinetic isotope effects in the charge transfer reactions of ammonia with rare gas ions

In the absence of experimental data, models of complex chemical environments rely on predicted reaction properties. Astrochemistry models, for example, typically adopt variants of capture theory to estimate the reactivity...</p

Charge transfer reactions between rare gas ions and polar molecules

Charge transfer reactions between rare gas Xe+, Kr+ or Ar+ ions and polar molecules are investigated. The reactions are undertaken within the cold and controlled environment of Ca+ Coulomb crystals at temperatures lower than 300 K, spanning 194-260 K. Both fully hydrogenated and fully deuterated ammonia and water reactants are employed, with all of these neutral species known to be present in interstellar environments. Inverse kinetic isotope effects (KIEs) are reported for the ammonia charge transfer reactions, with the deuterated molecules reacting at a faster rate than the hydrogenated species. The magnitude of the effect depends on the identity of the rare gas species; the strongest effect is observed for the Xe+ ions. Capture theory models, which are frequently used to describe ion-molecule interactions, fail to predict these findings. Further, the experimentally measured rate coefficients are suppressed in comparison to classical and quantum capture theory predictions, indicating that the reactions are not capture limited. With the aid of high-level ab initio calculations, a tentative explanation is proposed, suggesting that properties of the reaction complex (such as its lifetime) may give rise to the observed inverse KIEs. No inverse KIE-indeed, no isotope effects at all-are observed for the water charge transfer reactions, with the measured reaction rate coefficients being in good agreement with capture theory models. As this thesis will discuss, capture theory models do not always successfully predict the behaviour of ion-molecule reactions. In the absence of experimental data, capture theory models are often employed in astrochemical databases to predict the behaviour of ion-molecule reactions. This work highlights the assumptions underpinning the widespread use of capture theories in astrochemical modelling, and proposes that there is a need for further work to establish when capture theory models can be reliably used to predict the behaviour of ion-molecule reactions

Strong inverse kinetic isotope effect observed in ammonia charge exchange reactions

Accompanying data for publication in Nature Communication

Charge Transfer Reactions between Water Isotopologues and Kr⁺ ions.

Astrochemical models often adopt capture theories to predict the behavior of experimentally unmeasured ion-molecule reactions. Here, reaction rate coefficients are reported for the charge transfer reactions of H2O and D2O molecules with cold, trapped Kr+ ions. Classical capture theory predictions are found to be in excellent agreement with the experimental findings. A crossing point identified between the reactant and product potential energy surfaces, constructed from high-level ab initio calculations, further supports a capture-driven mechanism of charge transfer. However, ion-molecule reactions do not always agree with predictions from capture theory models. The appropriateness of using capture theory-based models in the absence of detailed experimental or theoretical studies is discussed, alongside an analysis of why capture theory is appropriate for describing the likelihood of charge transfer between Kr+ and the two water isotopologues

Capture theory models: An overview of their development, experimental verification, and applications to ion–molecule reactions

Since Arrhenius first proposed an equation to account for the behavior of thermally activated reactions in 1889, significant progress has been made in our understanding of chemical reactivity. A number of capture theory models have been developed over the past several decades to predict the rate coefficients for reactions between ions and molecules—ranging from the Langevin equation (for reactions between ions and non-polar molecules) to more recent fully quantum theories (for reactions at ultracold temperatures). A number of different capture theory methods are discussed, with the key assumptions underpinning each approach clearly set out. The strengths and limitations of these capture theory methods are examined through detailed comparisons between low-temperature experimental measurements and capture theory predictions. Guidance is provided on the selection of an appropriate capture theory method for a given class of ion–molecule reaction and set of experimental conditions—identifying when a capture-based model is likely to provide an accurate prediction. Finally, the impact of capture theories on fields such as astrochemical modeling is noted, with some potential future directions of capture-based approaches outlined. </jats:p

Inverse kinetic isotope effects in the charge transfer reactions of ammonia with rare gas ions

In the absence of experimental data, models of complex chemical environments rely on predicted reaction properties. Astrochemistry models, for example, typically adopt variants of capture theory to estimate the reactivity...</p

Spatial and Temporal Detection of Ions Ejected from Coulomb Crystals.

Coulomb crystals have proven to be powerful and versatile tools for the study of ion-molecule reactions under cold and controlled conditions. Reactions in Coulomb crystals are typically monitored through a combination of in situ fluorescence imaging of the laser-cooled ions and destructive time-of-flight mass spectrometry measurements of the ejected ions. However, neither of these techniques is able to provide direct structural information on the positions of nonfluorescing "dark" ions within the crystal. In this work, structural information is obtained using a phosphor screen and a microchannel plate detector in conjunction with a Timepix3 camera. The Timepix3 camera simultaneously records the spatial and temporal distribution of all ions that strike the phosphor screen detector following crystal ejection at a selected reaction time. A direct comparison can be made between the observed Timepix3 ion distributions and the distributions established from SIMION simulations of the ion trajectories through the apparatus and onto the detector. Quantitative agreement is found between the measured Timepix3 signal and the properties of Coulomb crystals assigned using fluorescence imaging─independently confirming that the positions and numbers of nonfluorescing ions within Coulomb crystals can be accurately determined using molecular dynamics simulations. It is anticipated that the combination of high-resolution spatial and temporal data will facilitate new measurements of the ion properties within Coulomb crystals