Isotope Exchange Reaction of OH⁻ Anion with HD at Temperatures from 15 K up to 300 K: Ion Trap Study

Abstract

This paper presents the results of an experimental study of the reaction of OH− anions with HD molecules leading to the formation of OD− anions. The study’s main goal was to obtain the temperature dependence of the reaction rate coefficient and determine the reaction’s enthalpy. This study was carried out at astrophysically relevant temperatures from 15 to 300 K. The reaction was studied using a temperature-variable cryogenic linear 22-pole radio-frequency ion trap. The rotational temperature of the OH− anions in the ion trap was characterized by near-threshold photodetachment spectroscopy. At 15 K, the measured reaction rate coefficient is 5 × 10−10 cm3 s−1. With increasing temperature, the reaction rate coefficient decreases monotonically to 5 × 10−11 cm3 s−1 at 300 K. Comparing with the previously determined rate coefficient of the reverse reaction of OD− anions with H2, we obtained the temperature dependence of the equilibrium constant. The enthalpy and entropy of the title reaction were determined in the studied temperature range as ΔH = (−23.9 ± 0.7 ± 2.6sys) meV and ΔS = (−8.5 ± 1.2 ± 1.4sys) J mol−1 K−1, respectively.

Zero-Point-Energy Driven Isotopic Exchange of the [H3O]− anion Probed by Mid-Infrared Action Spectroscopy

“…Moreover, the experiment with pure para-H 2 , the spin isomer of H 2 with zero excess energy, for the hydrogenation of OD – revealed that the rate is indeed lower, down to <10 –14 cm 3 s –1 below 20 K . The same nature of the isotope exchange rates has recently been confirmed also for HD exchange with OH – and OD – . Although the absolute values of H/D exchange reaction rates for the [X 3 O] − anion with H 2 /D 2 play no role in the current study, our experiment assumed and confirms the comparable order of magnitude as for the H/D exchange rates for the [OX] − .…”

Enthalpy of the N+ + H2 → NH+ + H Reaction—Experimental Study of the Reverse Process

“…After mass selection, the ions were injected into a linear 22-pole radiofrequency trap, where they were cooled by collisions with helium buffer gas. To freeze out the N 2 or NH 3 leaking from the ion source and to avoid parasitic reactions with these species, the trap was kept at temperatures T 22PT < 30 K. Based on our previous studies of endothermic reactions (Mulin et al 2015;Roučka et al 2018;Plašil et al 2022) and photodetachment spectroscopy (Plašil et al 2023), we assume that neither the translational nor the internal temperatures of NH + exceed the trap temperature by more than 10 K under the present operating conditions. We therefore denote both temperatures as T NH + , which is given as…”