| Summary: | <p>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.</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>
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