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, 9 (1), 2096

Observation of Different Reactivities of Para and Ortho-Water Towards Trapped Diazenylium Ions


Observation of Different Reactivities of Para and Ortho-Water Towards Trapped Diazenylium Ions

Ardita Kilaj et al. Nat Commun.


Water is one of the most 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 behavior. Capitalizing on recent advances in the electrostatic deflection of polar molecules, we separate 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 trapped diazenylium ions. 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.

Conflict of interest statement

The authors declare no competing interests.


Fig. 1
Fig. 1
Schematic of the experimental setup. A pulsed molecular beam of water molecules seeded in argon emanates from a room-temperature reservoir through a pulsed gas nozzle and passes an electrostatic deflector. The inhomogeneous electric field inside the deflector (shown in the inset below) spatially separates para- and ortho-water molecules due to their different effective dipole moments. After the deflector, the beam is directed at an ion trap containing a Coulomb crystal of Ca+ and sympathetically cooled N2H+ reactant ions (inset image). The products and kinetics of reactive collisions between N2H+ and H2O are probed using a time-of-flight mass spectrometer (TOF-MS)
Fig. 2
Fig. 2
Molecular-beam deflection profiles of the ground states of para and ortho-water. a Experimental isomer-specific density profiles of o- (blue squares) and p-H2O (red diamonds) in the deflected molecular beam (deflector voltage 15 kV) measured by (2 + 1) REMPI together with the total deflection profile (sum of the ortho- and para-signals, purple circles). The lines represent Monte-Carlo trajectory simulations of the deflection profiles. The contributions from ortho- and para-water are indicated by the blue and red shaded areas, respectively. b Total water deflection profile measured by femtosecond-laser ionization for deflector voltages of 0 kV (yellow triangles) and 15 kV (purple circles). The three vertical lines marked I, II, and III indicate the deflection coordinates at which reaction rates were measured. The red/blue symbols represent the relative populations of the isomers normalized to the total signal at positions I, II, and III as determined from the REMPI spectra shown in c. c REMPI spectra of H2O measured at the three positions I, II, III (purple circles). The two peaks observed at 80,724 cm−1 and 80,747 cm−1 correspond to transitions from the ground states of ortho- and para-water, respectively. The peaks are fitted with a sum of two Lorentzians (solid black line) with contributions from ortho- and para-isomers depicted as blue and red shaded areas, respectively. Error bars correspond to one standard error of at least three independent measurements
Fig. 3
Fig. 3
Reaction-rate measurements at the deflection coordinates I–III indicated in Fig. 2b. The data are normalized to the ion signal at time t = 0. The lines represent fits to the data according to an integrated pseudo-first-order rate law. The black triangles show an example of a measurement of the reaction rate with background gas at position II for comparison. Error bars correspond to one standard error of four independent measurements
Fig. 4
Fig. 4
Theoretical predictions from ab initio calculations and adiabatic capture theory. a Potential-energy profile along the reaction coordinate for the proton transfer reaction between N2H+ and H2O at the CCSD/aug-cc-pVTZ level of theory. The relative energies with respect to the reactants as well as the structures of the stationary points are shown. Blue, red and white spheres represent nitrogen, oxygen, and hydrogen atoms. b, c Rotationally adiabatic, centrifugally corrected long-range interaction potentials for the reaction of the ground states of o- (b) and p- (c) H2O with N2H+ for different values of the total angular momentum quantum number J. In b, the dashed (solid) lines correspond to the |Ω| = 0(1) components of the ortho-ground state. The grey-shaded areas show an estimate of the uncertainty in the experimental collision energy Ecol indicated by the black horizontal line

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