Strong inverse kinetic isotope effect observed in ammonia charge exchange reactions

L S Petralia,A Tsikritea,B R Heazlewood,J Loreau,T P Softley

doi:10.1038/s41467-019-13976-8

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 {{rm{Xe}}}^{+} ions and two isotopologues of ammonia, {{rm{NH}}}_{3} and {{rm{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 {{rm{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.

Highlights

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
As long ago as 1932—the year that spectroscopic evidence confirmed the existence of deuterium1—Cremer and Polanyi postulated that species containing hydrogen and deuterium would exhibit different reactivities owing to their differences in zero point energy (ZPE)[2]
Xeþ ions are created by laser ionisation, and are sympathetically cooled into the framework of a Coulomb crystal that consists of a few hundred laser-cooled Caþ ions held within a linear Paul ion trap

Summary

Introduction

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. As a result of these factors, one expects a bond involving a D atom to be less reactive than the analogous bond involving an H atom; deuterium-containing species are typically more stable and exhibit lower reaction rate constants than comparable hydrogen-containing isotopologues. Polanyi himself identified a hypothetical scenario where an exception to the expected deuterium KIE might arise: an inverse isotope effect could occur in processes involving the reaction of atomic H or D3. This is because the ZPE of the reactants will be the same, but the deuterium-containing reaction complex will have a lower ZPE. The reductive elimination of transition metal hydride complexes has seen a number of inverse primary

Methods

Results

Discussion

Conclusion