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 of ionic species present in interstellar environments. In this work, we examine astrochemically-relevant charge transfer reactions between two isotopologues of ammonia, NH3 and ND3, and two rare gas ions, Kr+and Ar+. An inverse kinetic isotope effect is observed; ND3 reacts faster than NH3. Combining these results with findings from an earlier study on Xe+(Petraliaet al.,Nat. Commun., 2020, 11, 1), we note that the magnitude of the kinetic isotope effect shows a dependence on the identity of the rare gas ion. Capture theory models consistently overestimate the reaction rate coefficients and cannot account for the observed inverse kinetic isotope effects. In all three cases, the reactant and product potential energy surfaces, constructed from high-level ab initio calculations, do not exhibit any energetically-accessible crossing points. Aided by a one-dimensional quantum-mechanical model, we propose a possible explanation for the presence of inverse kinetic isotope effects in these charge transfer reaction systems.
|Number of pages||9|
|Early online date||22 Jun 2021|
|Publication status||Published - 7 Aug 2021|
Bibliographical noteFunding Information:
B. R. H. and T. P. S. acknowledge funding provided by the EPSRC (projects EP/N004647/1 and EP/N032950/1). B. R. H. also acknowledges the ERC (Starting Grant project 948373) and the Royal Society (RGS\R2\192210) for funding. A. T. thanks the Clarendon Fund for providing her with a scholarship and acknowledges that this paper was supported by the Onassis Foundation, Scholarship ID: F ZP 055-1/2019-2020. J.L. acknowledges support from Internal Funds KU Leuven through grant STG-19-00313.
© The Royal Society of Chemistry 2021.
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