Observation of different reactivities of para- and ortho-water towards cold diazenylium ions

TL;DR

This study demonstrates that para- and ortho-water, two nuclear-spin isomers, exhibit different reactivities with cold diazenylium ions, revealing how nuclear-spin and rotational symmetry influence chemical behavior.

Contribution

The paper provides the first experimental separation and individual reactivity analysis of para- and ortho-water molecules, linking nuclear-spin states to chemical reactivity.

Findings

Para- and ortho-water show different reactivities with N₂H⁺ ions.

Rotational averaging affects ion-dipole interactions during reactions.

Nuclear-spin and rotational symmetry influence chemical reactivity.

Abstract

Water, H$_2$O, is one of the fundamental molecules in chemistry, biology and astrophysics. It exists as two distinct nuclear-spin isomers, para- and ortho-water, which do not interconvert in isolated molecules. The experimental challenges in preparing pure samples of the two isomers have thus far precluded a characterization of their individual chemical behaviour. Capitalizing on recent advances in the electrostatic deflection of polar molecules, we separated the ground states of para- and ortho-water in a molecular beam to show that the two isomers exhibit different reactivities in a prototypical reaction with cold diazenylium ions (N$_2$H$^+$). Based on ab initio calculations and a modelling of the reaction kinetics using rotationally adiabatic capture theory, we rationalize this finding in terms of different rotational averaging of ion-dipole interactions during the reaction. The present results highlight the subtle interplay between nuclear-spin and rotational symmetry and its ramifications on chemical reactivity.