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

A Tsikritea,K Park,T P Softley,P Bertier,B R Heazlewood,J Loreau

doi:10.1039/d1sc01652k

Abstract

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+ (Petralia et 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.

Highlights

Following the detection of CH in the interstellar medium (ISM) in 1937,1,2 spectroscopic methods have successfully identi ed hundreds of different molecular species in interstellar clouds and circumstellar envelopes
We explored the dependence of the potential energy surfaces with respect to the coordinates (R, q, f, r), where R is the length of the vector R describing the position of the rare gas ion with respect to the center of mass of NH3 (NH3+), q is the angle between the vector R and the C3 axis, 4 is the angle of rotation of this vector around the C3 axis, and the angle r is employed to describe the umbrella motion of ammonia
A limitation of the one-dimensional QM model is that it does not take into account the increase in lifetime that occurs due to intramolecular vibrational redistribution (IVR) between different degrees of freedom of the reaction complex

Summary

Introduction

Following the detection of CH in the interstellar medium (ISM) in 1937,1,2 spectroscopic methods have successfully identi ed hundreds of different molecular species in interstellar clouds and circumstellar envelopes. Some widely-adopted examples include the Kinetic Database for Astrochemistry (KIDA),[3] the Meudon model for atomic and molecular interstellar gas (known as the Meudon photon-dominated region, PDR, code)[4] and the UMIST Database for Astrochemistry (UDfA).[5] While these resources include reaction rate coefficients and branching ratios for many thousands of astrochemically-relevant processes, only a small fraction of these processes have been experimentally examined at temperatures below 300 K. As such, these databases are necessarily reliant on predictions. Rate coefficients for ion-neutral collisions, such as those examined in this work, are o en derived from capture theory calculations or extrapolated from measurements taken at room temperature

Methods

Results

Discussion

Conclusion