Strong inverse kinetic isotope effect observed in ammonia charge exchange reactions

Abstract

Isotopic substitution has long been used to understand the detailed mechanisms of chemical reactions; normally the substitution of hydrogen by deuterium leads to a slower reaction. Here, we report our findings on the charge transfer collisions of cold Xe þ ions and two isotopologues of ammonia, NH 3 and ND 3. Deuterated ammonia is found to react more than three times faster than hydrogenated ammonia. Classical capture models are unable to account for this pronounced inverse kinetic isotope effect. Moreover, detailed ab initio calculations cannot identify any (energetically accessible) crossing points between the reactant and product potential energy surfaces, indicating that electron transfer is likely to be slow. The higher reactivity of ND 3 is attributed to the greater density of states (and therefore lifetime) of the deuterated reaction complex compared to the hydrogenated system. Our observations could provide valuable insight into possible mechanisms contributing to deuterium fractionation in the interstellar medium.

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

“…These findings build upon an earlier study, where we identified a significant KIE in the charge transfer of Xe + with ammonia isotopologues. 1 Secondary inverse KIEs are relatively uncommon, having only been identified in a handful of previous studies. In systems where secondary inverse KIEs have been seen, they are typically modest effects (with k H /k D = 0.8 − 0.9).…”

“…For the recording of rate coefficients, we follow the protocol described in a previous study involving Xe + charge transfer reactions. 1 Kr is introduced through a high-precision leak valve, with Kr + ions formed using a (2 + 1) REMPI scheme at 212.6 nm (utilising the tripled output of a Nd:YAG-pumped dye laser). A subset of Kr + ions are produced in the higher energy 2 P 1/2 spin state.…”

“…7 Indeed, in the absence of experimental data, capture theory models can provide a useful estimate of the rate coefficient for ion-neutral reaction systems. However, as demonstrated in a recent publication, 1 there are also examples of reaction systems where capture theories fail to adequately describe the likelihood of product formation.…”

“…TableIsets out the experimentally measured and predicted rate coefficients and KIEs for the systems of interest in this work. Details on the properties of the previously investigated Xe + + NH 3 and Xe + + ND 3 charge transfer reactions (studied under comparable experimental conditions to this work) are also included, and are plotted in Figure6 1. Finally, rate coefficients from ion cyclotron resonance (ICR) cell and selected-ion flow tube (SIFT) Ar + + ND 3 from this work, and Ar + + NH 3 recorded using an ICR cell at 300 K) yield rate coefficients that are within the uncertainty range of capture theory predictions-and even then, the (more robust) ACCSA model predicts a higher rate coefficient.…”

“…Of the six reactions considered, only those FIG.6. Charge transfer rate coefficients for the Xe + + NH 3 (blue) and Xe + + ND 3 (orange) reaction systems, as established from capture theory models (ACCSA and ADO), from a QM model, and from experimental measurements 1. involving argon ions (…”